Pd and Pd based alloy nanoparticles as

visible light photocatalysts for coupling

reactions under ambient conditions

Gallage Sunari Peiris

B.Sc. (Hons) Chemistry, University of Sri Jayewardenepura, Sri Lanka 2010

Thesis completed under the supervision of Prof. Huai-Yong Zhu and Dr. Sarina Sarina,

and submitted to Queensland University of Technology, in fulfilment of the

requirements for the degree of

Doctor of Philosophy

School of Chemistry, Physics and Mechanical Engineering

Science and Engineering Faculty

Queensland University of Technology

2017

ii

Keywords

Photocatalysis; Visible light; Localized surface plasmon resonance; Plasmonic

photocatalysts; Plasmonic metal nanoparticles; Non-plasmonic metal nanoparticles;

Alloy nanoparticles; Palladium nanoparticles; Cross-coupling reactions;

Nitrobenzene reduction; Reductive N-alkylation; Organic synthesis

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Abstract

Photocatalysis is a rapidly emerging research field, with great potential for a

wide range of applications, since it can utilize solar energy. Solar light has received

much attention as it is the most abundant and cleanest renewable energy source,

which produces no pollution. Therefore, synthesis of fine chemicals with solar light

at ambient temperature is of the utmost interest. Nonetheless, it is still a challenge to

devise new catalysts, which exhibit high activity under the full solar spectrum and

moderate conditions. This project aimed to develop novel metal nanoparticle

photocatalysts for several important organic reactions under visible light irradiation.

The prospect of visible light irradiation driving chemical synthesis may extend the

scope of organic synthesis via a more controlled, simplified, and greener process.

Firstly, we focused on a systematic study of palladium nanoparticle-catalysed

cross-coupling and homo-coupling reactions under visible light irradiation. These

metal nanoparticles strongly absorb the light primarily through interband electronic

transitions. The excited electrons interact with the reactant molecules adsorbed on

the metal particle surface to accelerate coupling reactions. Therefore, the rate of the

catalysed reaction depends on the concentration and energy of the excited electrons,

which can be increased by increasing the light intensity. Nevertheless, mild reaction

conditions, such as ambient temperatures and pressures in the reaction systems make

it more environmentally benign.

Secondly, we incorporated palladium metal component with silver

nanoparticles to obtain silver-palladium alloy nanoparticles (Ag-Pd alloy NPs):

which can catalyse the reductive coupling of nitroarenes reactions by light irradiation

at ambient conditions. This provided a general indication for the possibility of the

design of an alloy nanoparticle photocatalysts using silver with other transition

metals, such as nickel, cobalt. This photocatalytic process is a more efficient and

greener approach than thermal reactions for the reductive coupling of nitroarenes,

and this improving the product yield by avoiding over-reduced products. The alloy

nanoparticles strongly absorb light, energizing the conduction electrons of silver,

which migrate to palladium sites at the alloy nanoparticle surface because of charge

redistribution between the two metals. The alloying affects the charge redistribution

iv

between silver and palladium, which enhances interaction between reactant

molecules and the nanoparticles. The reduction activity is sensitive to the intensity of

the irradiation, the wavelength of the incident light, metal molar ratio and

atmosphere of the reaction. When the molar ratio of silver and palladium in alloy

nanoparticles is nearly equal, the catalysts exhibited the best performance.

Finally, supported gold-palladium alloy nanoparticles (Au-Pd alloy NPs) on

zirconium dioxide (ZrO2) can act as efficient visible light photocatalysts for

reductive N-alkylation of nitrobenzene with benzyl alcohol. Here in, we studied the

possibility of gold-palladium alloy nanoparticles usage as a photocatalyst for amine

synthesis under mild conditions. The performance of the alloy nanoparticle

photocatalyst mainly depends on the alloy composition, light intensity and reaction

temperature. These heterogeneous catalysts can be easily recycled, which is

significant in the development of practical and cost-effective catalytic processes.

This finding provides a useful guideline for green amine synthesis driven by solar

energy and further reveals the possibility of designing efficient photocatalysts for a

number of organic syntheses using transition metals alloying with gold metal.

v

List of Publications

Journal Publications

1. Sunari Peiris, John McMurtrie and Huai-Yong Zhu*. Metal nanoparticle

photocatalysts: emerging processes for green organic synthesis. Catalysis

Science & Technology, 2016, 6, 320-38. - 2016 most accessed Catalysis

Science and Technology articles

2. Sunari Peiris, Sarina Sarina*, Chenhui Han, Qi Xiao and Huai-Yong Zhu.

Silver and Palladium Alloy Nanoparticles Catalysts: Reductive coupling of

Nitrobenzene through Light Irradiation. Dalton Transactions, 2017. DOI:

10.1039/C7DT00418D

3. Sunari Peiris, Sarina Sarina*, Chenhui Han, Xiayan Wu, Qi Xiao and Huai-

Yong Zhu. Non-plasmonic Palladium nanoparticles for homo-coupling and

cross-coupling reactions under visible light irradiation. Manuscript ready to

submit to Chemistry – An Asian Journal.

4. Sunari Peiris, Sarina Sarina*, and Huai-Yong Zhu. Reductive N-alkylation of

nitrobenzene with benzyl alcohol by Au-Pd alloy nanoparticles under light

irradiation. Manuscript ready to submit to RSC Advances.

5. Sarina Sarina, Sunari Peiris and Huai-Yong Zhu*. Silver metal on different

supports as a photocatalyst for reductive coupling of Nitroaromatics.

Manuscript ready to submit.

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Conference Presentations

1. Sunari Peiris, Sarina Sarina, Qi Xiao and Huai-Yong Zhu*. Non-plasmonic

palladium nanoparticles for homo-coupling and cross-coupling reactions

under visible light irradiation. 9th

European Meeting on Solar Chemistry and

Photocatalysis: Environmental Applications, 13-17th

June 2016, Strasbourg,

France. - Oral presentation

2. Sunari Peiris and Huai-Yong Zhu*. Ag-Pd alloy nanoparticles catalysts:

coupling of nitroaromatics through light irradiation, Nanotechnology and

Molecular Science HDR Symposium, QUT, 12-13th

Feb 2015.- Oral

presentation

3. Sunari Peiris and Huai-Yong Zhu*. Non-Plasmonic Palladium Nanoparticles

for Homo-Coupling and Cross-Coupling Reactions under Visible Light

Irradiation. Nanotechnology and Molecular Science HDR Symposium, QUT,

16-17th

Feb 2016.- Oral presentation

QUT Verified Signature

viii

Acknowledgements

First and foremost, I would like to sincerely acknowledge my principal supervisor,

Prof. Huai-Yong Zhu, for his expert guidance and encouragement throughout my

PhD. His wide knowledge and logical approach for research work has been

extremely useful for my research.

I would also like to extend my gratitude to my associate supervisor, Dr. Sarina Sarina

for her guidance, advice and for her invaluable assistance with research. Dr. Qi Xiao

is gratefully acknowledged, for his guidance in numerous ways.

Many thanks to A/ Prof. John McMurtrie, Dr. Sarina Sarina and Dr. Qi Xiao for

collaborations and valuable suggestions particularly in the method of conducting

research.

Thanks to late Dr. Chris Carvalho, Dr. Lauren Butler, Mr. Tony Raftery, Ms. Rachel

Hancock, Dr. Natalia Danilova, Mr. Peter Hegarty, Dr. Lorraine Calwell, Dr. Llew

Rintoul and Dr. Jamie Riches for giving me training and providing me assistance

with the instruments when necessary.

Warm thanks go to my lab mates, initially helping me out to find my way around the

lab, for all the good times we had and the friendship. Many thanks to all my friends

around QUT and throughout my entire life.

Thanks to QUT for the scholarship, this made my stay in Australia possible, and

ARC for the research funding.

My deepest gratitude must indeed go to my parents, for bringing me up into such

happiness and such love and having trust in me, more than I do. Finally to my

husband Dilan, for everything we have shared from the start of our wonderful

journey in life together, no words could ever express my appreciation for all that he

has done for me.

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Table of Contents

Keywords ................................................................................................................................. ii

Abstract ................................................................................................................................... iii

List of Publications ...................................................................................................................v

Statement of Original Authorship .......................................................................................... vii

Acknowledgements ............................................................................................................... viii

Table of Contents .................................................................................................................... ix

List of Abbreviations ................................................................................................................x

Introductory Remarks ............................................................................................................. xi

Chapter 1: Introduction and literature review ...................................................... 1

1.1 Introductory Remarks .....................................................................................................1

1.2 Article 1 ..........................................................................................................................2

Chapter 2: Supported non-plasmonic metal nanoparticles for organic synthesis

under visible light irradiation ..................................................................................... 52

2.1 Introductory Remarks ...................................................................................................52

Article 2 ..................................................................................................................................53

Chapter 3: Supported silver based alloy nanoparticle photocatalysts for organic

synthesis under visible light irradiation ..................................................................... 77

3.1 Introductory Remarks ...................................................................................................77

3.2 Article 3 ........................................................................................................................78

Chapter 4: Supported gold based alloy nanoparticle photocatalysts for organic

synthesis under visible light irradiation ................................................................... 114

4.1 Introductory Remarks .................................................................................................114

4.2 Article 4 ......................................................................................................................115

Chapter 5: Conclusions & Future work ........................................................... 139

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List of Abbreviations

DFT : Density Functional Theory

EDS : Electron Diffraction Pattern

GC : Gas Chromatography

GC-MS : Gas Chromatography-Mass Spectrometry

LSPR : Localised Surface Plasmon Resonance

NPs : Nanoparticles

PNPs : Plasmonic Nanoparticles

SEM : Scanning Electron Microscopy

TEM : Transmission Electron Microscopy

TOF : Turnover Frequency

TON : Turnover Number

UV/Vis : Ultraviolet-Visible Spectroscopy

VOCs : Volatile Organic Compounds

XPS : X-ray Photoelectron Spectroscopy

XRD : X-ray Diffraction

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Introductory Remarks

“Pd and Pd based alloy nanoparticles as visible light photocatalysts for coupling

reactions under ambient conditions” investigated non-plasmonic and alloy metal

nanoparticles and their applications in fine chemical synthesis using visible light

irradiation. The object of this thesis is to develop photocatalysts using metal

nanoparticles to utilize visible light to fine chemical synthesis under mild conditions.

This thesis investigates the photocatalytic activity of the non-plasmonic palladium

metal nanoparticles, which can drive many useful organic chemical reactions under

traditional thermal catalysis conditions. Moreover, this thesis shows that the

activities of plasmonic metal nanoparticles (such as silver, gold) are enriched by

alloying them with non-plasmonic metals and enhances the efficiency of organic

reactions under visible light irradiation. The metal nanoparticle photocatalysts

function via different reaction mechanisms. The different reaction mechanisms for

the metal nanoparticle photocatalysts are highlighted. The findings of this study

demonstrate the use of visible light or sunlight to drive chemical reactions, which is

an important aspect in the view of a sustainable and green chemistry.

This thesis is a collection of published, submitted and prepared works by the author

to various scientific journals. Thus, the general formatting follows the style of the

specific journals. Repetition and redundancy in the introductory sections of each

paper are unavoidable owing to the close relationships between the subject matter

published. The following Figure 1 is a graphical representation of the structure of the

thesis.

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Figure 1:.Schematic illustration of the thesis structure.

CHAPTER 1: INTRODUCTION AND LITERATURE REVIEW

A review of the literature relating to the latest developments in

direct photocatalyst using plasmonic, non-plasmonic and alloy

metal nanoparticles for organic synthesis.

CHAPTER 2: SUPPORTED NON-PLASMONIC METAL

NANOPARTICLES FOR ORGANIC SYNTHESIS UNDER

VISIBLE LIGHT IRRADIATION

A study on non- plasmonic palladium nanoparticle photocatalysts

for homo-coupling and cross-coupling reactions.

CHAPTER 3: SUPPORTED SILVER BASED ALLOY

NANOPARTICLE PHOTOCATALYSTS FOR ORGANIC

SYNTHESIS UNDER VISIBLE LIGHT IRRADIATION

A study on the Ag-Pd alloy nanoparticle photocatalysts for

reductive coupling of nitrobenzene.

CHAPTER 4: SUPPORTED GOLD BASED ALLOY

NANOPARTICLE PHOTOCATALYSTS FOR ORGANIC

SYNTHESIS UNDER VISIBLE LIGHT IRRADIATION

A study on the Au-Pd alloy nanoparticle photocatalysts for

reductive N-alkylation of nitrobenzene with benzyl alcohol.

CHAPTER 5: CONCLUSION AND FUTURE WORK

Conclusions are derived based on the scientific work presented in

this thesis with respect to each chapter and the avenues for future

work are noted.

1

Chapter 1: Introduction and literature

review

1.1 INTRODUCTORY REMARKS

This chapter includes one review article:

This article is an invited perspective by Catalysis Science & Technology (2016

most accessed Catalysis Science and Technology articles). This Perspective

summarizes the overview of recent research on direct photocatalysis of supported

metal nanoparticles (plasmonic, non-plasmonic and alloy metal nanoparticles) for

organic synthesis under light irradiation and discusses the significant reaction

mechanisms that occur through light irradiation. The progress in this new burgeoning

research area is of great interest. This perspective provides a comprehensive

backdrop of the unique features of the localized surface plasmon resonance effect in

plasmonic metals and their applications in organic transformations. Herein, we

reviewed a number of different reactions carried out using metal NP photocatalysts,

including selective oxidation, selective reduction, coupling, addition and degradation

reactions. The role of light irradiation in the catalysed reactions and the light-excited

energetic electron reaction mechanisms were highlighted and discussed each reaction

mechanism individually. Further, this provides a discussion on the outlook and future

directions of this exciting new field.

QUT Verified Signature

QUT Verified Signature

3

Metal nanoparticle photocatalysts: emerging processes for

green organic synthesis

Sunari Peiris, John McMurtrie and Huai-Yong Zhu*

ABSTRACT: Metal nanoparticle photocatalysts have attracted recent interest due to

their strong absorption of visible and ultraviolet light. The energy absorbed by the

metal conduction electrons and the intense electric fields in close proximity, created

by the localized surface plasmon resonance effect, makes the crucial contribution of

activating the molecules on the metal nanoparticles which facilitates chemical

transformation. There are now many examples of successful reactions catalysed by

supported nanoparticles of pure metals and of metal alloys driven by light at ambient

or moderate temperatures. These examples demonstrate these materials are a novel

group of efficient photocatalysts for converting solar energy to chemical energy and

that the mechanisms are distinct from those of semiconductor photocatalysts. We

present here an overview of recent research on direct photocatalysis of supported

metal nanoparticles for organic synthesis under light irradiation and discuss the

significant reaction mechanisms that occur through light irradiation.

1. INTRODUCTION

Many syntheses of organic compounds use catalysts at elevated temperatures

(thermal catalysis) to achieve higher efficiencies. Nevertheless, it will be especially

valuable to drive these reactions by light irradiation at ambient temperatures, which

will avoid unwanted by-products formed at elevated temperatures.1 Throughout the

last decade; the area of heterogeneous photocatalysis has grown rapidly with the

development of new photocatalysts, which are active in visible light and suitable for

organic synthesis. Moreover, sunlight has received much attention, as it is the

cleanest and most abundant energy source. Solar light is a combination of 5% UV

(wavelength 200–400 nm), 43% visible (wavelength 400– 800 nm), and 52%

infrared (wavelength >400 nm) radiation. Given that visible and infrared light

constitutes most of the available solar emission (approximately 95%),2 developing

novel catalysts that exhibit high activity with irradiation in the solar spectrum is a

significant challenge in photocatalysis.

4

Plasmonic-metal nanoparticles (PNPs) have been recognized as a novel class of

material that is specifically efficient in harvesting light energy for chemical synthesis

due to their intense optical absorption over a wide range of the sunlight spectrum.3–7

The characteristic feature of the PNPs is their strong interaction with resonant

incident light through excitation of localized surface plasmon resonance (LSPR). The

energy of the incident light can be gained by the conduction electrons of the metal

nanostructures. The optical properties of the PNPs strongly depend upon the size and

geometry of the nanoparticle.8 This property was utilized for visible light

photocatalysis to enhance the semiconductor photocatalytic activity on water

splitting or dye degradation.9–11

In year 2010, we reported for the first time that gold

(Au) NPs on photocatalytically inert supports could be used as photocatalysts for

chemical synthesis.12

We found that nitroarenes can be directly reduced to azo

aromatic compounds using photocatalysts made of Au NPs on ZrO2 at 40 °C under

visible light, achieving high conversion rates and product selectivity.12

The work

demonstrates the potential of direct photocatalysis of PNPs for organic synthesis.

Moreover, the role of the PNPs as light energy harvesters can be utilized in alloy NPs

of a plasmonic metal and a metal with intrinsic catalytic activity for the specific

reaction under investigation. In these novel systems no electron transfer between the

NPs and support material was observed and the metallic NPs serve as both the light

absorber and host to the catalytic sites.13

Since the metal NPs serve as both light

absorber and host the catalytic sites; many potential materials (insulating solids,

porous solids, polymers and carbon-containing materials) could potentially be used

to create better photocatalysts. It follows that coupling light harvesting and catalytic

functions greatly broadens the potential applications of metal nanoparticle

photocatalysis for fine chemical synthesis.12–14

The photocatalysts of Au NPs were initially used for environmental

remediation via oxidation of volatile organic compounds (VOCs).14

Compared to

conventional oxidation methods, which involve heating, the visible light driven

catalysis process has a significant benefit as reactions can occur at ambient

temperatures. For complete oxidation of VOCs to CO2 large oxidation power of the

catalytic process is a priority as no product selectivity is required. However,

reactions involving catalytic synthesis of fine organic chemicals often favour

partially oxidized or reduced products or a specific compound from several possible

5

products. Thus, product selectivity is at least of equal importance as a fast reaction

rate. An important feature of metal NP photocatalysis is lower reaction temperatures,

which in turn leads to formation of fewer side products and hence increased product

selectivity. Ultimately, this will be reduces equipment costs and expenditure on

energy required for the catalytic process. Since 2010, we have focused on synthesis

of fine chemicals using supported metal NP catalysts under visible light radiation.12,

15 During that time, a variety of new photocatalysts comprising supported metal NPs

have also been developed for organic synthesis by other research groups.16

This review concentrates on direct photocatalysis using supported metal NPs

for organic synthesis and the distinct mechanisms of these photocatalytic processes.

We begin with a brief discussion of semiconductor photocatalysis and associated

drawbacks followed by an overview of PNPs and the characteristic features of the

LSPR effect in plasmonic metals. This is followed by an examination of the use of

metal NP photocatalysts for organic transformations and the differences in reaction

mechanisms while under light irradiation. Throughout the review we also focus on

the critical mechanisms of direct light induced energetic electron transfer from the

metal NP surface to the adsorbed reactant molecules. Finally, we provide some

discussion about the outlook and future directions of this exciting new field.

1.1. Semiconductor photocatalysis and associated drawbacks

The discovery of water splitting on a TiO2 electrode by Fujishima and Honda in 1972

ushered in a new era for heterogeneous photocatalysis.17

Semiconductor

photocatalysts such as TiO2 are promising materials for other photochemical

applications too, for instance the degradation of VOCs, dye sensitized solar cells and

super-hydrophilic materials.18–21

Semiconductors are only able to absorb photons

with energy greater than or equal to their specific band gap energy. The valence band

electrons are excited to the conduction band by leaving holes (positively charged) in

the valence band. The separated charges migrate to the particle surface and these

charges (electrons and holes) can reduce and oxidize species absorbed on the solid

surface. Unfortunately, due to its wide band gap (3–3.2 eV); TiO2 can only utilize

ultraviolet (UV) radiation, which accounts for no more than 3–5% of the total solar

energy available on the earth's surface.22, 23

Generally, chemically stable

semiconductors have wide band gaps while materials that have narrow bandgaps are

6

unstable under most photocatalytic reaction conditions and as such both are limited

in their practical usage.23

To develop novel photocatalysts which can catalyse reactions under the full

solar spectrum, a number of approaches have been developed, such as doping TiO2

with metal ions or metal atom clusters,22, 23

integrating nitrogen24

and carbon25

into

TiO2 and using other metal oxides as catalyst materials.22, 26, 27

Nevertheless,

semiconductor photocatalysts still have disadvantages, such as the high probability of

electron–hole recombination, which decreases the quantum efficiency; energy lost

during charge transfer (charges need to move on to the surface for interaction with

the reactant molecules) and relatively low charge density on the TiO2 surface. More

importantly, semiconductor photocatalysts possess a weak affinity toward many

organic reactants and low surface concentrations of active sites for catalysing the

reactions.13

Surface reactions are the slowest step, and take much longer than

generating charges and charge migration inside the semiconductor particles. Hence,

most of the light-generated charges are quenched rather than participating in

reactions on the particle surface. Given that the limitations of light absorption and

photon efficiency are associated with the intrinsic nature of semiconductor

photocatalysts, the search is on for new materials that can work under the full solar

spectrum.

1.2. Direct photocatalysis of metal nanoparticles

Metal NPs have unique properties, which are different from those of bulk materials.

For example, NPs of gold and silver can intensely absorb visible light owing to the

localized surface plasmon resonance (LSPR) effect3, 28–30

and the light absorbance

depends on their particle size and shape. There is another mechanism by which metal

NPs absorb light: one electron absorbs one photon.

The properties of metals can be explained reasonably well by the electron-sea

model, (Fig. 1A) which defines the bonding in metals as resulting from positively

charged metal atoms in fixed positions, surrounded by delocalized conduction

electrons. The mobility of the electrons in the electron sea is used to explain the high

electrical and thermal conductivity of metals. In a solid metal, a large number of

7

electronic orbitals overlap, resulting in a large number of orbitals within continuous

bands.

Figure 1: A) Schematic illustration of the electron-sea model. B) The molecular

orbital energy spacing decrease as the number of interacting atoms increases.

According to Einstein's explanation of the photoelectric effect; when a photon

of incident light collides with an electron in a metal, if the photon has enough energy

(i.e. greater than the work function (ϕ) of the metal) then the electron is ejected from

the atom (Fig. 2). When a photon has less energy than the work function, it is unable

to eject electrons, but is able to excite a metal electron to the energy levels between

the vacuum and Fermi levels of the metal. The subsequent electron–electron collision

generates more hot electrons, but with lower energy (as the overall energy is

conserved during this relaxation process).

Figure 2: Schematic illustration of light absorption in metal NPs and the

photoelectric effect.

The collision between two electrons occurs in the order of 10 fs according to

the Fermi-liquid theory.31

The energy dissipating process in the NP by electron–

8

electron collisions continues for about 500 fs,32

and subsequent electron–phonon

interaction33

completes in several picoseconds and then the NP achieves an

equilibrium state with a slight temperature increase. This process is universal for the

NPs of all metals under all wavelengths of irradiation when the energy exchange of

the NP with the surrounding medium is ignored. We have found that light absorption

by this process is more effective in driving reactions when the wavelength is

shorter.13

1.2.1 LSPR – localized surface plasmon resonance. Localized surface plasmon

resonance is another light absorption mechanism, which is observed in the visible

light range for the NPs of a few metals such as Au, Ag and Cu. A localized surface

plasmon is an optical phenomenon that arises when light is incident on a metal NP

that is smaller than the wavelength of incident light. This produces a strong

interaction between the incident electric field and the free conduction electrons of the

metal NPs (Fig. 3).34

Figure 3: Schematic illustration of a localized surface plasmon resonance.

When the frequency of the free electron oscillation is the same as that of the

incident light, constructive interference results in the strongest possible oscillation as

well as localized field strength. Moreover, the frequency and strength of the plasmon

resonance also depends on the intrinsic dielectric properties of the metal NP, the

surrounding medium and the surface polarization, which can be influenced by the

particle size and shape. It is possible to modify the LSPR of metal NPs simply by

synthesizing the preferred NP size and shapes.35

There are many findings on the

association of the LSPR effect and NP size, and it has been reported that small Au

9

NPs with diameter <5 nm do not show any LSPR absorption.13

Nonetheless, a very

recent study shows that Au clusters of about 300 atoms (which have size <2 nm)

exhibit LSPR absorption.36

This is significant because when photocatalysis takes

place on the NP surface, smaller NPs have larger specific surface area and therefore

more sites for reaction. On the other hand, large NPs exhibit stronger light absorption

which is the driving force of photocatalytic reactions. Generally, Au NPs of 5–50 nm

size exhibit a sharp absorption peak in the 520–530 nm range.37, 38

The LSPR

absorbance varies from element to element, for example, the extinction spectra of

Au, Ag, and Cu spherical NPs with diameters of 20 nm show maxima at 530, 400,

and 580 nm, respectively (Fig. 4).39

Furthermore, metal NPs can absorb the incident

light passing in their vicinity at the LSPR wavelengths.40

The PNPs are thus more

efficient light absorbers compared to semiconductors as they can concentrate the

incident photon energy on the NPs (Fig. 5).

Figure 4: Surface plasmon absorption bands for Au, Ag and Cu nanoparticles.39

When the particles grow larger, the absorption band broadens and shifts to

longer wavelengths.41, 42

For Ag NPs, it is observed that there is a shift in the

plasmon absorption band from 400 to 670 nm when the particle shape varied from a

sphere to a cube.4, 43

As the shape and/or size of the NP changes, the intensity of the

electromagnetic field at the NP surface also changes. The local electromagnetic

fields near the rough edges of noble metal NPs can be significantly enhanced due to

these changes.28, 44

In fact, the enhanced local field strength can be over 500 times

greater than the applied field when the noble metal nanostructures have sharp tips,

10

edges and concave curves, such as in nanowires, cubes, triangular plates and NP

junctions.45

Figure 5: A) Schematic illustration of electric fields of incident light and that created

by the electron oscillations near the metal NP;40

B) the extinction cross-section and

corresponding resonant wavelength for isolated metallic NPs.

When two or more metal NPs are in close proximity, they can couple to

generate an enhanced local electromagnetic field that is larger than that produced by

one NP in isolation. The spots of the enhanced field between the NPs are called hot-

spots. Finite difference time-domain (FDTD) simulations revealed that the electric

field intensity of local plasmonic hot-spots can be reached that are up to 106 times

larger than the incident electric field.46, 47

Further, the electron–hole pair generation

rate is 1000 times greater in hot-spot areas than what it generates in incident

electromagnetic field.46

The LSPR absorption of metal NPs is correspondingly

sensitive to the neighbouring environment, including solvent and support materials.

Both light absorption and activation of the reactants takes place on the metal

NPs and it is a distinct feature of plasmonic photocatalysis systems. According to

Kale et al., 48

three routes can transfer light energy into the adsorbed reactants in

direct plasmonic-metal photocatalysis: 1) elastic radiative re-emission of photons, 2)

non-radiative Landau damping (the excitation of energetic charge-carriers in the

11

metal particle causes) and 3) the interaction of excited surface plasmons with

unpopulated adsorbate acceptor states, inducing direct electron injection into the

adsorbate, which is called chemical interface damping (CID; Fig. 6). The processes 1

and 2 are conceivably amplified due to the higher intensity of scattering light and

raised electric fields near PNPs.49, 50

Figure 6: Schematic showing the three dephasing mechanisms of oscillating surface

plasmons.48

The magnitude of the field improvement, the resonant wavelength, and the

proportion of plasmon excitations that decay through these processes depend on

nanostructure geometry, size, composition, dielectric environment and separation

distance.29, 51

The light-induced surface plasmons will ultimately decay and may

produce energetic charge-carriers in the metal NPs (processes 2 and 3 above). These

carriers can transfer the energy gained from irradiation to the surroundings52

or to

heating the NPs.53, 54

The enhanced electric field itself may also accelerate the

transfer of charge carriers from the NPs. The plasmon-assisted transfer of energetic

electrons into the adsorbate can facilitate chemical reactions. There are two possible

ways for this charge carrier driven transformation to take place: direct and indirect

(Fig. 7).55

In the case of the indirect charge transfer mechanism, the optically excited

energetic charge carriers favour transfer towards the adsorbate acceptor orbitals

(LUMO), which have energies closer to the Fermi level (due to the higher

concentration of low energy electrons). On the other hand, in the direct charge

transfer mechanism, charge transfer to adsorbate occurs via plasmon-mediated

charge scattering.56

Light absorption induced NP heating may cause desorption of the

reactant molecules, which has a negative effect on the reaction rate however the

reaction rate can be greater at higher temperatures if desorption is not severe.

12

Moreover, the interaction between a reactant molecule and a metal NP surface may

lead to perturbations in electronic structure and create a polarizability change, which

might facilitate chemical reactions.57

Figure 7: Indirect (a) and direct (b) charge transfer mechanisms.55

Principally, metals have continuous electron energy levels: the conduction

electrons gain energy from light irradiation and could re-distribute this to higher

energy states from the lower levels. The wavelength (energy) of the incident photons

regulate the maximum energetic level that the electrons can reach.58

Since there is no

requirement to overcome a band gap, as there is in semiconductors, the metal NPs

have absorption over a broad wavelength range from UV to infrared. They can

absorb the energy of light and heat simultaneously, exciting their conducting

electrons to higher energy levels. This unique property is useful when the metal NPs

are used to catalyse organic reactions. Moderate heating can further assist in

achieving excellent yields of product in many cases.

2. SELECTIVE OXIDATION REACTIONS

Selective oxidation of compounds is of great interest for both fundamental

research and commercial fine chemical production.

2.1. Alcohol oxidation

Selective oxidation of alcohols to their corresponding aldehydes/ ketones, imines and

esters is an important process for chemistry research and in industrial processes.59–61

These compounds serve as vital and versatile intermediates for fine chemical

13

production,62–64

such as in perfumes, dyes, pharmaceuticals,65–67

and

agrochemicals.64, 68

The challenge is to develop catalytic systems that can achieve

high reaction rates and product selectivity simultaneously, use green oxidation

agents, such molecular oxygen and have significant tolerance of various substituted

functional groups in the alcohols. Catalytic systems under heating can attain high

reaction rates but high temperatures often negatively affect the product selectivity by

over oxidation. Photocatalytic systems working at ambient temperature exhibit great

potential for such reactions.

2.1.1 Selective oxidation and dehydrogenation of aromatic alcohols to

aldehydes/ketones. Among aromatic aldehydes, benzaldehyde is the simplest and

most widely investigated. Usually, benzaldehyde is manufactured by hydrolysis of

benzyl chloride or by oxidation of toluene.69, 70

However, the hydrolysis of benzyl

chloride yields traces of chlorine, and the commercial oxidation of toluene is

inefficient.71

Both processes produce a substantial quantity of waste. In most studies,

long reaction times, high O2 pressures and also elevated temperatures are essential

for the oxidation of alcohols to aldehydes.72, 73

Some chlorine-free catalytic processes

using the oxidants permanganate and dichromate have been employed,74–76

but they

are expensive and/or toxic. Because many of the reactions are conducted at high

temperatures and/or high pressures they can result in significant quantities of

unwanted over-oxidized by-products.

Zhang et al. reported the use of catalysts consisting of Au NPs on zeolite

supports (A, beta, Y, silicalite-1, TS-1 and ZSM-5) for the oxidation of benzyl

alcohol to synthesis benzaldehyde.77

The reactions were carried out under visible

light irradiation at close to ambient temperature (40 °C) and under an O2 atmosphere.

Decent conversions were achieved with excellent product selectivity (99%). In

contrast, much lower benzaldehyde yield (32 %<) achieved under dark at the same

temperature. The LSPR effect of the supported Au-NPs made a vital contribution to

converting aromatic alcohols with greater selectivity. This photocatalytic process

was applicable for the selective oxidation of several aromatic alcohols with different

substituent groups (cinnamyl alcohol, 4-methoxybenzyl alcohol and 4-methylbezyl

alcohol), demonstrating its general applicability. The zeolite support exhibited no

photocatalytic activity itself. However, the zeolite supports favour adsorbing benzyl

14

alcohol. The adsorption of benzyl-type alcohols on zeolite surfaces (zeolite Y and

ZSM-5) with the interaction between the hydroxyl groups of alcohols and zeolites

(Si–O–Al, SiO−, or Na+) being through hydrogen bonds.78

The Au/Y catalyst

performed the best, while Au/silicalite-1 exhibited the lowest activity, with the

overall order of catalytic activities being:

Au/Y > Au/TS-1 > Au/A > Au/beta > Au/ZSM-5 > Au/silicalite-1.77

In 2013, we reported that the yield by selective oxidation of aromatic alcohol to

aldehyde at 45 °C under visible irradiation was excellent, 99% or above, using a

catalyst of alloy NPs containing Au and Pd on ZrO2 (Au–Pd/ZrO2) with an optimal

Au/Pd molar ratio of 1 : 1.86.79

ZrO2 support is photocatalytically inert. In the

mechanism the conduction electrons of the NPs gain the absorbed light energy,

generating energetic conduction electrons on the Pd surface sites, for which the

reactant molecules have an affinity. The charge heterogeneity of the alloy NP surface

is greater than that of Au NP or Pd NP surfaces, leading to a stronger interaction

between the alloy NPs and reactant molecules.79, 80

As the distribution of Pd sites and

charge heterogeneity at the NPs play leading roles in the catalytic reactions; the

Au/Pd molar ratio has an important influence on the catalytic performance of the

alloy NP. The conversion of benzyl alcohol to benzaldehyde with the Au–Pd/ZrO2

catalyst is 100% (in the dark it is 44%). Good performance was also observed when

the aromatic alcohol contained various additional functional groups. Enache et al.

were able to get a good selectivity (96%) for the corresponding aldehydes; yet the

reaction temperatures were comparably high.72

The introduction of Au to Pd

improves selectivity, and they argue that the Au performs the electronic promoter

role for Pd and that the active catalyst has a surface that is significantly enriched in

Pd.81

The results also support the mechanism that the Au nanostructures are the

antenna for light absorption and the catalytic reaction takes place on surface Pd sites

as the hot electrons generated by light migrate to the surface Pd sites thus

accelerating the reaction. To achieve a better benzaldehyde yield there are processes

using H2O2 as a reactant and inorganic–organic hybrid materials (metal anions–

organic esters) as catalyst, in which the reaction mixture is refluxing in the dark.82, 83

Besides, non-plasmonic transition metals are widely used as catalysts for various

catalytic reactions under heating because of their inherent affinity for organic

reactants. Sarina et al. found that the Pd/ZrO2 catalyst is a very good photocatalyst

15

for this reaction (exhibiting a benzyl alcohol conversion of 94% but 29% in the

dark), while Pt/ZrO2 exhibited the poorest activity for selective oxidation of benzyl

alcohol.58

The light absorption by the non-plasmonic metal NPs follows a different

mechanism from that of the plasmon metal NPs as mentioned in section 1. The light

absorption of alloy NP structure is usually more intense compared to that of the non-

plasmonic metal NPs, but the mechanism is complicated and depends on the

composition.

Scheme 1: Proposed mechanism of aromatic alcohol oxidation over Au–Pd alloy

NPs under visible light irradiation.79

The reaction pathway of the photocatalytic process77, 79, 80

has been proposed

based on the analyses of experimental results and information from the literature. An

example is illustrated in Scheme 1. Sarina et al. suggested that Au–Pd alloy NPs

under light irradiation drives the α-H abstraction of aromatic alcohols and

consequently the transformation from alcohol into the corresponding aldehyde is

possible to achieve in an oxidant free environment at ambient temperatures.80

Moreover, it was concluded that the α-H abstraction is the rate-determining step of

the selective oxidation and takes place via a photocatalytic process on the surface of

the supported alloy NPs. Nevertheless, photocatalytic selective oxidation can be

achieved under ambient temperatures and lower oxygen pressures.80

Under visible

light irradiation, the hydrogen abstraction from the α-C atom is facilitated by the

excitation of NP electrons to the benzyl alcohol molecules adsorbed78, 84, 85

and it is

the primary step of the pathway for selective oxidation of aromatic alcohols to the

16

corresponding aldehydes or ketones.86, 87

The reaction was conducted at ambient

temperature on the Au/Y catalyst under argon atmosphere with 24.6% conversion of

benzyl alcohol to benzaldehyde observed in twenty hours under visible light

irradiation, while slight conversion was detected in the dark under the same

conditions. The two α-H atoms on the methylene group (–CH2–) of benzyl alcohol

are more active and electrochemically polarized Au-NPs readily abstracted them to

form an Au–H bond.86, 87

After this abstraction is completed, the subsequent

abstraction of the H atom from the hydroxyl group of the transient anionic species

proceeds readily producing aldehyde as the final product, while the negative charge

of the transient anions returns to the alloy NPs.80, 84, 85

Generally, the charge is

injected into the reactant adsorbed on the metal NPs and then returns to the metal

NPs after reaction completion,84, 85

in which molecular oxygen is the oxidant in the

photocatalytic oxidation process. In contrast, molecular oxygen can scavenge the hot

electrons on polarized metal-NPs to form activated O2− species.

15 These active

species remove hydrogen from metal–H bonds and yield water as the by-product. To

validate this mechanism, the 2,2′,6,6′-tetramethylpiperidine-N-oxyl (TEMPO – a

stronger hydrogen abstractor than molecular oxygen) was used and yielded

hydroxylamine (TEMPOH) instead of water (benzyl alcohol : TEMPO = 1 : 1

molecular ratio).88

No product was detected under the controlled experiments with

the zeolite Y support as the catalyst (without Au NPs) and TEMPO under otherwise

identical conditions. Based on these results, it is believed that Au–H species are

formed89

during the photocatalytic oxidation of benzyl alcohol, and the surface

hydrogen species can be removed by activated oxygen species (or TEMPO). After

the hydrogen on the NP surface reacted with molecular oxygen, the NP is able to

react with alcohol and gain hydrogen from the alcohol forming Au–H species. In the

meantime, the electron-deficient Au NPs are reloaded with released electrons from

the adsorbed benzyl alcohol compounds which form benzyl alcohol radicals.90

Afterward, these benzyl alcohol radicals may automatically release hydrogen atoms

from the hydroxyl group (–OH) to facilitate formation of the C=O bond. Finally, the

product benzaldehyde desorbs from the support. Conversion of 23% was obtained in

48 h with high benzaldehyde (100%) selectivity.15

When Ag/zeolite Y was used as

the photocatalyst, benzyl alcohol conversion of 11% was achieved in 48 h under UV

light irradiation with the product benzaldehyde (62%).91

Au–Pd alloy NPs can

exhibit better performance under visible light irradiation than pure Au or Pd NPs.

17

This is attributed to two reasons: 1) Pd sites on the alloy NPs are more active for this

selective oxidation (or dehydrogenation), and 2) the surface electronic heterogeneity

of the alloy NPs enhances the interaction with the reactants, which facilitates the

reaction.

2.1.2 Oxidation of aliphatic alcohols to esters. Conventionally, esters are

synthesized by the reaction of activated acid derivatives with alcohols.65

However,

the multi-step reaction procedures used often produce great quantities of undesirable

by-products. The strong acidic or basic conditions also limit the use of this

methodology as not all substrates are stable under such conditions. In recent years,

considerable effort has been made to develop methods for the direct synthesis of

esters by oxidative esterification of aldehydes with alcohols. Usually alcohols are

readily available as bulk materials, more stable than the carbonyl compounds as well

as being inexpensive, less toxic, and easy to use in the laboratory.92–94

Hence, they

represent attractive starting materials for large scale production and the direct

conversion of alcohols into esters represents a significant advance towards green,

economic and sustainable processes.95–99

The selective oxidation of alcohols usually

provides the required aldehydes. However, selective oxidation of aliphatic alcohols

with molecular oxygen is rather challenging, especially with no added base and

under moderate reaction conditions,100

although it is highly desirable from both

economic and environmental points of view. In 2015, our group reported a stable and

reusable catalyst of Au–Pd alloy NPs supported on phosphate anion modified

hydrotalcite suitable for the direct oxidative esterification of aliphatic alcohols.101

This is a one-pot reaction and under visible light irradiation, where we were able to

achieve excellent conversion (94%) and good selectivity (76%) even without any

additional base being added. Compared to the direct esterification in the dark

(conversion of 62%) visible light irradiation resulted in enhanced conversion at

ambient temperatures.

The proposed mechanism for the direct oxidative esterification of alcohols is

depicted in Scheme 2. The oxidative esterification reaction may proceed through an

oxidation of alcohol to aldehyde (step IV) followed by a condensation reaction. The

alcohol molecules first adsorb on the Au–Pd alloy NP surface; as it has a strong

binding affinity. The light excited hot electrons of the metal NP facilitate the

18

cleavage of the C–H bond of the alcohol adsorbed on the NP surface79

and leads to

the formation of metal alkoxide and metal hydride species (step II). Moreover, the

basic sites on the surface of the support can bind the hydrogen atom of the alcohol

and also facilitate the O–H cleavage on the metal surface.102

The basic surface sites

should lower the activation barrier of the C–H bond of the metal alkoxide

intermediate to form the aldehyde.103

Hot electrons can also enhance hydrogen

abstraction from the α-H of the metal alkoxy species and transform into aldehyde

(steps IV–V). The condensation reaction between the aldehyde and another molecule

of alcohol results in the formation of hemiacetal intermediate (step V),104–106

which is

followed by oxidative dehydrogenation to give the corresponding ester. One can

further increase the efficiency of the reaction by adequate heating (over 55 °C).

Scheme 2: Proposed mechanism for synthesis of esters from aliphatic alcohols.101

2.2. Amine oxidation

Synthesis and applications of imines play a critical role in modern organic

synthesis.107

Oxidation of secondary amines,108, 109

self-coupling of amines110, 111

and

coupling of alcohols and amines112, 113

are the alternative approaches for imine

production. Generally, it requires high temperatures (around 100 °C), O2 atmosphere

as well as relatively extended reaction hours (>24 h) to synthesis imines through

oxidation of secondary amines.109, 114

Moreover, some approaches require

stoichiometric or excess amounts of strong oxidants such as o iodooxybenzoic acid,

19

Ph(OAc)2 and MnO2 and they produce large amounts of undesired waste.115, 116

Copper based catalysts,117

Al grafted MCM-41118

ruthenium based catalyst119

have

been developed for synthesis of imines from amines.117

However, only a limited

range of amines can be oxidized into imines and this at relatively high temperatures

(>100 °C). Aschwanden et al. reported Au(OAc)3 anchored CeO2 catalyst can

oxidize benzylamine at 108 °C at 0.98 atm O2 pressure, with a product yield (up to

89%) after 16 h.120

The elevated reaction temperatures and pressure indicate that this

reaction requires demanding conditions when using supported metal NPs as the

catalyst. In 2013, the selective photocatalytic oxidation of benzylamine to the

corresponding imine was achieved at 45 °C on the Au–Pd/ZrO2 catalyst (optimal

Au/Pd molar ratio – 1 : 1.86). The yield was 95% and selectivity was above 96%

(yield at dark – 36%).79

The catalytic activity of Pd and the charge heterogeneity of

the alloy NP enhance the efficiency of photocatalytic oxidation of amine.

Recently Sarina et al. found that the non-plasmonic transition metals NPs (Pd,

Pt, Rh, and Ir) on ZrO2 exhibit good photocatalytic activity for selective oxidation of

amines under visible or UV light irradiation. The Pt/ZrO2 catalyst performed the best

activity (71%), while Ir/ZrO2 and Rh/ZrO2 exhibited the poorest activity (34–35%).58

In these systems, the metal NPs absorb light via the mechanism in which one photon

excites one electron directly and the generated hot electrons drive the reaction. While

low benzylamine conversions (17%<) resulted in the dark.

Scheme 3: Proposed mechanism for the photo oxidation of benzylamine.79

20

The most probable pathway for oxidative coupling of benzylamine is illustrated

in Scheme 3. Initially under light irradiation, benzylamine is oxidized into

benzaldehyde by the abstraction of the α-H from the –CH2– group. Afterwards, the

unreacted amines connect by nucleophilic attack of these nascent aldehydes to yield

the corresponding imines.121, 122

2.3. Aldehyde oxidation

As mentioned earlier, esterification is one of the fundamental transformations in

organic synthesis. Therefore, the development of novel approaches for synthesis of

esters has attracted the interest of chemists owing to the extensive use of these

compounds. The synthesis of ester derivatives under mild conditions comprises the

stoichiometric activation of carboxylic acid as an acyl halide, anhydride, or activated

ester amenable to subsequent nucleophilic substitution.65

However, these one-pot

conventional approaches require the consumption of heavy-metal oxidants such as

KMnO4,123

CrO3,124

and reactive hydrogen peroxide,125, 126

or other transition-metal

catalysts.127–132

However, active graphitic carbon nitride (g-C3N4) catalyst shows

lower yield (<34%) for esterification of benzaldehyde and alcohol under visible light

radiation even with H2O2 as oxidant.126

Zhang et al. reported an exciting, important

alternative transformation method for ester derivative synthesis; which converts

aldehydes in mild conditions using supported Au NP photocatalyst under visible

light.133

A number of substituted benzaldehydes were successfully converted to the

corresponding esters in good to moderate yields. The targeted ester was isolated in

78% yield when benzaldehyde was reacted with ethanol under a visible light source

in atmospheric air at room temperature. In the dark, the reaction conversion at same

temperature was only 4.4%.133

Scheme 4: Proposed mechanisms for the esterification of benzaldehyde with

alcohol.133

21

The proposed mechanism for the esterification of benzaldehyde with alcohol is

illustrated in Scheme 4 and is consistent with the literature where an oxidant converts

a hemiacetal intermediate to the ester.134–136

The Au NP surface has a strong binding

affinity towards aromatic substances; thus the benzaldehyde molecule is readily

adsorbed on to the surface. The oxidative esterification reaction proceeds through a

condensation reaction between benzaldehyde (I) and alcohol (II), which results in the

formation of hemiacetal intermediate(III). The light excited hot electron of Au NPs

facilitates a condensation step possibly through electron transfer and recombination.

Moreover, the local electromagnetic fields enhanced near rough surfaces owing to

LSPR effect of Au NPs increases the reactivity toward nucleophilic attack by the

alcohol to give the hemiacetal. Finally, the oxidative dehydrogenations give the

corresponding ester (IV). When the reactant molecules are adsorbed on the surface of

Au NPs, it is possible to assist the oxidative dehydrogenation.

3. SELECTIVE REDUCTION REACTIONS

Reduction reactions are essential in organic synthesis and biological chemistry

and consequently are under intense study.137–139

3.1. Reduction and reductive coupling of nitroarenes

Aromatic azo compounds are widely used in the textile, food, polymer and

pharmaceutical industries.140–142

Syntheses of azo compounds are often conducted

under high pressures and at high temperatures using transition-metal reducing

agents.140, 142, 143

However, some by-products formed from the reducing agent, are

harmful to the environment.140

In 2008, Grirrane et al. reported that Au NPs/TiO2

could catalyse the production of azobenzene from nitrobenzene through a twostep

and one-pot reaction at 100 °C or above.144

First, the nitroaromatic compounds are

over reduced to corresponding amines under 8.8 atm pressure of H2. Secondly, the

amines are oxidized to aromatic azo compounds under an atmosphere of O2 at a

pressure of 4.9 atm. This is because the azo compounds are unstable under the high

temperatures and high hydrogen pressures, which are required to achieve better

reduction rate.141, 145

In 2010 Wang et al. reduced nitrobenzene mostly to aniline over

silica gel supported nickel but at higher temperatures (70–90 °C) and higher H2

22

pressures (19.7 atm) to achieve the results.146

In the same year, Zhu et al. discovered

that Au/ZrO2 can catalyse a one-step reductive coupling reaction of aromatic

nitroarenes to corresponding azo compounds at 40 °C with high yields

(approximately 100%) under visible light or UV light illumination.12

Recently, Guo

et al. reported that 3 wt% Cu NPs on graphene (Cu/graphene) exhibits excellent

photocatalytic activity for coupling of nitroarenes under visible light and sunlight.

For example, the yield of azoxybenzene from nitrobenzene is 90% at 60 °C and 96%

at 90 °C and a control experiment indicated that a negligible reduction of

nitrobenzene occurred in the dark (28%<).147

The study also demonstrates the

possibility to drive such chemical syntheses with sunlight – the most abundant

energy resource.

Scheme 5: Proposed mechanisms for the reductive coupling of nitrobebzene.12

In the reactions described above, all reactants of the photocatalytic process are

in solution phase and isopropanol performs the role of both hydrogen donor and

solvent. High concentration of the reactants on the solid catalyst surface can be

achieved under ambient pressure (Scheme 5). The presence of KOH improves the

hydrogen release from isopropyl alcohol that is transformed into acetone. The Au-

NPs are able to abstract hydrogen from isopropanol creating transient Au–H species

under visible light irradiation (step II).12, 89

The LSPR effect excited the electrons and

those energetic electrons strongly interact with the electrophilic nitro groups in the

reactant molecules, facilitating the cleavage of N=O bonds by H–Au species to yield

HO–AuNP species (step III). The significant role of the surface Au–H species in the

23

reduction pathway is confirmed by adding an efficient hydrogen-abstracting reagent,

in this case, 2,2′,6,6′-tetramethylpiperidine N-oxyl (TEMPO)—which can abstract

hydrogen from Au surface, into the reaction system while the other experimental

conditions were unchanged.148

The reduced products weren't detected. The presence

of the surface Au–H species is therefore essential for the photocatalytic reaction.

3.2. Deoxygenation of epoxides

The Deoxygenation of epoxides into alkenes is an important synthetic transformation

in organic and pharmaceutical chemistry, as it allows the use of the oxirane ring as a

protective group for C=C double bonds.149–151

However, conventional deoxygenation

requires stoichiometric reagents such as phosphines, silanes, iodides and heavy

metals and produces a large amount of undesirable waste.152–156

Even in the recent

literature, high temperatures and pressures were applied for conversion of styrene

oxide to styrene to achieve better reaction efficiency.157, 158

The proposed mechanism

for the selective reduction of epoxide is also illustrated in Scheme 6. The reduction

was conducted in isopropanol solvent (also the hydrogen donor), and the species with

the hydrogen atom bonded to the Au NP surface possible to form and react with the

epoxide molecules on the Au NPs.12, 15, 89

The active Au–H species attacks the

epoxide bond to facilitate the deoxygenation. The hydrogen atoms of the Au–H

species are able to release hydrogen to reactant molecules to form alkenes.159, 160

Scheme 6: Proposed mechanism for the selective reduction reactions.160

24

3.3. Selective reduction of ketones

The selective reduction of ketones is one possible method for the preparation of

alcohols which are important building blocks in the synthesis of pharmaceutical and

agrochemical products as well as other important materials.161–163

Most catalysts used

for hydrogenation comprise either phosphorus/ nitrogen donor ligands or a

combination of each type with high temperatures and pressures being required.164–167

Recently, Ke et al. reported the synthesis of benzyl ethanol from acetophenone with

good selectivity (>99%) under visible light and mild reaction conditions.159, 160

Under

light irradiation, the Au NPs are capable of forming transient Au–H species by

abstracting hydrogen from isopropanol (Scheme 6).12, 89

The active Au–H species

attack the C=O double bonds, leading to the hydrogenation, in which the hydrogens

of Au–H species are consumed and acetophenone is transformed into benzyl ethanol.

No reduced products were detected in the control experiment; which was done with

the use of TEMPO to confirm the key role of the Au–H species in the reduction

reactions.159, 160

The enhancement of the local electromagnetic fields near rough

surfaces of the Au NPs, due to the LSPR effect, could also assist with activation of

the double bonds of the reactant molecule.

3.4. Selective reduction of azo compounds

Bacterial strains and their enzymes are used for degradation of azo compounds via

oxidation/reduction processes.168, 169

Moreover, Nam et al. used iron metal to

decolorize azo dyes by reduction of the azo groups, which resulted in formation of

aromatic amines as products.170

However, the above methods are applied for the

purpose of environmental remediation rather than chemical synthesis. Recently, it

was reported that 40% (in dark – 10%) of azobenzene was reduced to

hydroazobenzene in 6 h with a selectivity of 78% at ambient temperatures, under

irradiation with visible light in the presence of Au NPs.160

Photocatalytic abstraction

of hydrogen from isopropanol12, 89

forms the transient Au–H species which attack the

N=N double bonds leading to the hydrogenation. None of the final reductive

products were detected under the control experiment which was done with the use of

TEMPO, once again confirming the significant role of the surface Au–H species in

the reduction reactions.159, 160, 171–173

25

3.5. Selective reduction of alkenes and alkynes

The one-step amination of alkenes or alkynes is called hydroamination and this

enables the synthesis of nitrogen containing organic compounds which are widely

used in many processes in, for example, the cosmetic, pharmaceutical and

agrochemical industries.171–173

This reaction offers an atom efficient route to various

nitrogen contain organic molecules and provides a convenient pathway for the

synthesis of numerous important fine chemicals with limited by-product

formation.172–175

The high activation barrier of hydroamination reactions demands

high temperatures (generally above 100 °C) to achieve decent yields.176, 177

Zhao et

al. were able to synthesise 1-phenethyl-2-phenylacetylene (conversion 90% and

selectivity 91%) successfully, under visible light at 40 °C in 25 h under an argon

atmosphere.178

Scheme 7: Proposed mechanism for the photocatalytic hydroamination of alkynes.178

The proposed mechanism for the photocatalytic hydroamination of alkynes on

the Au NP photocatalysts is illustrated in Scheme 7. The Au NPs absorb visible light

due to the LSPR effect and the energetic conducting electrons of the AuNPs can

migrate to the conduction band of the support (step I).179, 180

The positively charged

Au NPs interact with the nucleophilic aniline, forming a N-centred radical cation

which would lead to initiation for the electrophilic attack of the electron rich sites of

alkynes (e.g. the C atom of ≡C–Ph) (step II). The N doping could form Ti3+

sites;

which have greater coordination capability than Ti4+

sites and that makes the terminal

26

C atom more favourable for the H addition. The interaction of the electron rich C–H

bond of the terminal alkynes with lower oxidation state active Ti3+

sites might

activate the alkenes. Then the N atom links to the C atom of the phenyl ring; whereas

the H atom is added onto the terminal C atom of C≡C. The adsorption of C≡C on the

surface is weaker than C≡C and the product molecules desorb from the support

surface.181, 182

Finally, the activated sites on the photocatalyst return to the desired

initial state to start over the catalytic process.

4. CROSS-COUPLING REACTIONS

The cross-coupling reactions of mediated by organometallic compounds used

for a wide range of bond forming processes, such as C–C, C–N, C–O, C–S or C–P.183

These bond forming cross coupling reactions can produce symmetrical and

unsymmetrical biaryls and have been accepted as convenient one-step methods for

constructing complex structures which are used in the synthesis of natural

materials,184–187

bioactive products,188

agrochemicals,189

medicines190

and advanced

materials.183, 191–195

4.1. C–C coupling

4.1.1 Suzuki–Miyaura coupling. Palladium phosphine complexes are usually used

as catalysts for Suzuki reactions.196, 197

However, they cause major problems in

purification of biaryl compounds and separation of the catalyst is challenging and

leads to toxic waste.198, 199

It is challenging to recycle the catalysts used in

homogeneous catalytic processes, which is an important consideration for industrial

applications and potential impacts on the environment.200

With heterogeneous

catalysts these coupling reactions require elevated reaction temperatures and

prolonged reaction times. For example, Pd NPs supported on carbon nanotubes

resulted in 87–94% yield at 70–100 °C.201

Pd NPs on polystyrene-divinyl benzene

polymer achieved high yield (100%) at 100 °C after 12 h.202

Recently, we found that

Au–Pd alloy NPs on ZrO2 can drive the same reactions under visible light irradiation

at much lower temperatures (only 30 °C) while achieving excellent yields.79, 203

The

best conversion achieved under light illumination at 30 °C for Suzuki– Miyaura cross

coupling was 96% (Au/Pd ratio 1 : 1.86) and showed decent viability on a number of

27

substrates. The contribution of thermal effect was investigated in the dark at same

temperature and 37% conversion of 3-iodotoluene achieved. The combination of

enhanced light absorption of alloy NPs, the improved interaction between the aryl

halides and the alloy NPs, as well as the catalytic activity of the Pd being the

important factors in promoting the coupling reactions.

Scheme 8: Proposed mechanism for Suzuki reaction.79

The significant step of Suzuki–Miyaura coupling is the activation of

iodobenzene on the electron-rich Pd sites on the surface of Au–Pd alloy NPs under

light irradiation (as illustrated in Scheme 8). Generally, the palladium metal could

activate the aryl halides and facilitate the Suzuki–Miyaura coupling.200, 204

The

surface charge heterogeneity is significantly enhanced by light irradiation and

subsequently increases the interaction (absorption of reactant molecules on the metal

NP surface) between reactant molecules and Au–Pd alloy NPs (step II).

Subsequently, the aromatic borate species react with the activated aryl species on the

electron-rich Pd surface sites (step III). The initial alloy surface regenerates through

reductive elimination of the cross coupling product (R1Ph–PhR2 molecule) by

completing the photocatalytic cycle.

4.1.2 Stille coupling. The palladium-catalysed cross coupling of organostannanes

with organic halides and triflates is known as the Stille reaction.205, 206

Generally,

phosphine ligands in combination with palladium precursors provide effective

catalysts for the reaction.207, 208

However, most of the phosphine ligands are

28

expensive, toxic and air sensitive; which places significant limits on their synthetic

applications.209–211

Furthermore, to achieve higher product yields; researchers are

using microwave irradiation as well as elevated temperatures.205, 206, 212

Recently,

Xiao et al. synthesised biaryls through Stille coupling under visible light irradiation

at 45 °C using Au–Pd alloy catalyst (3-methylbiphenyl yield – 81% under light

irradiation, 33% in dark).213

4.1.3 Hiyama coupling. The Hiyama cross-coupling is an effective reaction in

organic chemistry for the synthesis of unsymmetrical biphenyl derivatives. Normally,

the organosilicon reagents are nontoxic, available at low cost, can be prepared easily

and have good stability under a variety of reaction conditions. The Hiyama coupling

reaction of trimethoxyphenylsilane with a range of aryl chlorides under microwave

irradiation conditions have been studied.214

The conversion using Hiyama coupling

with Au–Pd NPs on ZrO2 increased from 55% to 71% upon irradiation with

incandescent light in the presence of Pd NPs on ZrO2 at the same temperature (45

°C).58, 213

The temperature of the reaction mixture in the dark was kept the identical

as the reaction mixture under light by a water bath but only 7% of biphenyl

derivative yield resulted. The light irradiation on Au–Pd alloy NPs extensively

enhanced the intrinsic catalytic activity of Pd even at lower temperatures for

coupling reactions. The ability to efficiently concentrate the photon flux energy to a

very small volume (by the PNPs) and to transfer this energy to the adsorbed

molecules to induce their reaction (by catalytically active metal) are outstanding

features of alloy NPs.

4.1.4 Sonogashira coupling. Among the various cross coupling reactions, the

palladium/copper catalysed Sonogashira coupling of aryl halides with terminal

alkynes provides a convenient method to generate aryl alkynes.215–217

Typically,

Sonogashira reactions require a dry organic solvent, inert atmosphere, strong base,

prolonged reaction time and a phosphine-ligated palladium complex with a copper

co-catalyst. 215, 218

However, the copper derivatives are moisture and air sensitive and

they result unwanted terminal alkyne homocoupling through the Glaser reaction.215,

218 The Pd/SNW1 (melamine-based microporous polymer) showed decent catalytic

29

activity in copper-free Sonogashira coupling in water.219

The coupling reaction was

performed in the presence of base in water at 80 °C. The 100% conversion (98%

selectivity) of iodobenzene was observed in the presence of the Pd/SNW1 as a

catalyst.219

Generally, the phosphine-assisted method is one of the well-established

methods,220, 221

which provides excellent results in the wide range of reactions.

However, phosphine ligands are comparably expensive, toxic, and unrecoverable. In

2014, Sharma et al. synthesised catalyst (SBA-15–EDTA–Pd-11%) by anchoring a

Pd–EDTA complex over the surface of organo-functionalized SBA-15 and achieved

excellent yield (100%) at 120 °C. However, these heterogeneous catalysts used for

Sonogashira coupling reactions require elevated reaction temperatures.218, 222, 223

Xiao

et al. achieved an excellent conversion (yield – 80% under light, 10% in dark) for the

Sonogashira reaction at only 45 °C under visible irradiation, using a catalyst of NP

alloys of Au and Pd on ZrO2 (Au–Pd/ZrO2).213

4.2. C–N coupling

4.2.1 Buchwald–Hartwig coupling. The Pd catalysed Buchwald–Hartwig amination

of aryl and heteroaryl halides is a prominent method for forming C–N bonds in

modern synthetic chemistry.224–226

However, this chemistry often requires elevated

temperatures, microwave heating, refluxing and long reaction times to occur at

reasonable reaction yields.227–229

Buchwald–Hartwig coupling cannot progress in the

dark except when the temperature is over 45 °C, but it can be initiated by light

irradiation to enable 35–39% conversion at 45 °C.58

In contrast, Saikai et al. could

achieve only 54% yield under heating the reaction mixture at 120 °C for 12 hours.229

Scheme 9: Schematic diagram of the pathway for the photocatalytic cross-coupling

reactions.213

30

The proposed mechanism for the cross-coupling reactions by Xiao et al. on the

Au–Pd NP photocatalysts213

is illustrated in Scheme 9. The light irradiation

facilitates the energetic electron transfer to the adsorbed aryl iodide molecule from

the metal NP, yielding a transient negative ion species. The C–X (X = halide) bond

cleavage is the rate determining step for the coupling reactions. Loss of halide atom

affords either an adsorbed phenyl radical or an organometallic aryl palladium iodide

complex on the metal surface and later triggers the coupling reactions.213

However,

in the mechanism for thermal catalysis it is driven through a different Pd2+

intermediate.230

The aryl halide undergoes oxidative addition to the Pd0 centre and

gives unsaturated Pd2+

intermediate; which coordinates with the amine and generates

tetra coordinated Pd2+

. Then it is deprotonated with a base yielding anionic amido

complex; which subsequently gives a 3-coordinate complex. Finally, it forms the C–

N bond by coupling the aryl halide with the amine.

5. ADDITION REACTIONS

5.1. Hydrochlorination

The addition of hydrogen halides to alkenes is one of the fundamental reactions in

organic chemistry.231, 232

Nevertheless, this reaction is slightly limited in scope. For

example, addition of hydrogen halides such as HCl only occurs at satisfactory rates

in the case of strained olefins.233, 234

Gaspar et al. have studied a series of

hydrochlorination reactions using a Co catalyst at room temperature in ethanol

solution.231

The sources of hydrogen and chloride are from PhSiH3 and paratoluene

sulfonyl chloride (TsCl), respectively. The mechanism proposed by Gaspar for

hydrochlorination involves olefin hydrocobaltation and the catalytic cycle is initiated

by formation of a cobalt–hydride complex (Co2+

/Co3+

and silane).

The catalysts of noble metal nanoparticles (Pt, Pd and Au) supported on

zirconia exhibit higher activities under the irradiation of visible light for the

hydrochlorination of alkenes using hydrochloric acid as the sources of chloride and

hydrogen, which is regarded as a reaction that is difficult to occur under usual

thermal conditions.235

The light-generated hot electrons on the surface of the noble

metal NPs increase the activity of the catalyst. Under light irradiation, these hot

31

electrons can interact with protons in the solution. The hydrogen atoms on the

surface of metal NPs can readily add to the terminal carbon of the C=C group in the

molecule of 4-phenyl-1-butene according to the Markovnikov rule.236

A chloride or

hydroxide anion in the solution can be attracted by the positively charged carbon to

form the final products 4-phenylbutyl chloride or 4-phenyl-2-butenol (Scheme 10).

Remarkably higher selectivity for 4-phenylbutyl chloride was observed by all the

noble metal catalysts with Pd/ZrO2 having the best overall selectivity for the

reaction.

Scheme 10: Mechanisms of addition on supported noble metal catalysts under

irradiation of visible light.146

5.2. Acetalization

Acetalization of carbonyl compounds with alcohols to form acetals is one of the most

common methods for protecting aldehydes and ketones in organic synthesis.237–239

Conventionally, this reaction is carried out in the presence of a homogeneous acid

catalyst such as p-toluenesulfonic acid, pyridinium salts or hydrochloric acid.237–239

Although these homogeneous acids show adequate catalytic performance for

acetalization reactions, they cause problems with purification of the product, large

amounts of acidic and toxic wastes and corrosive reagents that lead to severe

environmental pollution.240–243

Acidic ionic liquids have also been used as efficient

catalysts for acetalization.238

However, a number of drawbacks of ionic liquids, such

as high cost and challenging purification of end products limit the scope of practical

industrial applications. Acetalization reactions showed decent performance with a

high selectivity on Au supported MZSM-5 (M = H+, Na

+, Ca

2+, or La

3+) under visible

32

light irradiation and at 60 °C.244

In contrast, other processes employ higher

temperatures238

and refluxing conditions.237, 239

Kawabata et al. proposed a mechanism for acetalization reactions through

hemiacetal intermediates.237

Zhang et al. suggested the plasmon-mediated catalytic

mechanism for the acetalization and discussed how the LSPR effect enhances the

activity of the catalyst.244

Plasmonic Au NPs which are loaded on ion-exchanged

ZSM-5 zeolites (Au/MZSM-5, M = H+, Na

+, Ca

2+ or La

3+) perform as “antennas”

under light irradiation by efficiently absorbing visible light. Au NPs generate active

Au(δ+,δ−)

dipole moments due to the LSPR effect under visible light irradiation, at the

same time the electric nearfield enhancement (ENFE) also induced around the

surface of Au NPs.4, 7

The ENFE of Au NPs might strengthen the polarized

electrostatic fields (PEF) of MZSM-5 hence facilitating the effective activation of the

reactant aldehyde by stretching the C=O bonds.242

The “stretched” reactants with

greater molecular polarities can be activated more efficiently by active H+ catalytic

centres improving the acetalization (Scheme 11).245–248

Scheme 11: The acetalization on Au/MZSM-5 under the irradiation of visible

light.244

33

6. DEGRADATION REACTIONS

6.1. Aldehydes and alcohols

Removal of VOCs by photocatalytic treatment has drawn extensive interest as

an environmentally friendly technique over the last few decades.249

Indoor air quality

has attracted significant attention. Formaldehyde (HCHO) is one of the major

pollutants of indoor air,249, 250

and is frequently used in room decorating, plastics,

paint, glue and refurbishing processes, as well as in furniture production. It is known

to be carcinogenic, mutagenic and teratogenic.251, 252

Catalytic oxidation is an

effective approach for removing low concentrations (at the level of parts per million)

of HCHO in indoor air.253

In general, elevated temperatures and high pressures are

desirable for complete oxidation of formaldehyde.254

Normally, titania/ titania

derivatives254, 255

and peroxone (ozone/hydrogen peroxide) 256, 257

are used under UV

irradiation for catalytic degradation. Ozone forms O3−, which can directly participate

in the reaction and hydroxyl radicals generated by hydrogen peroxide to enhance the

photocatalysis. In 2008, our group reported that Au NPs on ZrO2 support could

decrease HCHO content by 64% within 2 hours with irradiation of blue light at room

temperature and pressure.14

This is the first report on direct photocatalysis of metal

NPs which was conducted under practical reaction conditions using visible

irradiation rather than conducted in ultra-high vacuum chambers or using lasers. In

later research Au NPs on mesoporous ZrO2 nano composite (ZrO2 corporate with

LAPONITE® clay) were found to be catalytically superior for oxidation of HCHO.

During the catalytic oxidation of HCHO on Au/ZrO2- nanocomposite, the higher

oxidation state (Au3+

) of the Au is reduced to metallic Au nano crystals (Au0) and

however the both the Au3+

and Au0 states are activated the HCHO oxidation.

258

Many industrial wastewater streams contain high concentrations of organic

pollutants which are difficult to degrade biologically. Phenol is one of the most

common such pollutants and is extremely toxic to the environment and may cause

harmful effects to human health even at very low concentrations. Catalytic wet air

oxidation (CWAO) is a treatment for organic pollutants in the liquid phase,

effectively oxidising them completely to carbon dioxide and water using air or pure

oxygen.259, 260

This method requires very high temperatures and O2 pressures to

enhance the catalytic activity by formation of oxygen radicals, which can react with

34

water to form hydroxyl radicals that may then oxidize phenol.259

In 2009 Zhu et al.

reported oxidation of phenol in aqueous solution, a process which is potentially

impossible to catalyse under visible light. The Au NPs on zeolite Y, SiO2 and ZrO2

converted 21%, 28% and 45% of phenol respectively under 120 h of UV

irradiation.15

The UV light absorption results in a much greater quantity of the

electron transfer from the AuNPs to the oxygen molecule than the visible light

(LSPR absorption). Therefore, more positive charges are left in 5d band (lower

energy levels) of the Au NPs when they are exposed to UV light and those are able to

oxidize the molecules that are more challenging to oxidize such as phenol. The

photocatalytic process has a significant advantage compared to the conventional

oxidation with heating as it requires much less energy input to activate the reaction

and it is outstanding for indoor air and waste water purification at ambient

temperatures. However, the oxidation power of the metal NP photocatalysts is not as

strong as TiO2, although a much larger number of charge carriers (electrons and

holes) can be generated by light and the metal NPs have a better affinity for

adsorption of organic molecules. From the point of view of practical applications,

they cannot compete (at this stage) with TiO2 for degradation for environmental

purpose, where the concentration of reactant to be decomposed is very low.

6.2. Dyes

The synthetic fabric dyes and industrial dyestuffs constitute one of the largest groups

of chemicals manufactured around the world.261, 262

Azo dyes and fluorone dyes

constitute a significant portion in the overall category of dyestuffs and are more

destructive to the environment than many other common dyes. Moreover, some of

the azo dyes and fluorone dyes and their degradation products are extremely

carcinogenic.263

A number of these dyes are resistant to self-photo degradation,

oxidation and decay by acids and bases.264

Effective utilization of light to degrade

these synthetic dyes in the presence of aqueous titania dispersions265, 266

and metal

doped titania264, 267, 268

provide attractive approaches for minimising energy resources

required for environmental remediation. The dye sulforhodamine-B (SRB) can be

degraded effectively by both sensitized and direct photocatalysis. Both positive holes

and hydroxyl radicals are found as the oxidizing species responsible for initiating the

degradation in direct photocatalysis. 266

In the presence of water, the TiO2 particles

35

absorb UV light to produce electron–hole pairs. Mainly, the dye SRB adsorbed on

TiO2 is oxidized by a photogenerated hole, which is localized at the surface of the

irradiated TiO2. The dye cation radicals formed combine with adsorbed

molecularoxygen which results in formation of degraded fragments.266, 269

Moreover,

SRB water soluble dye molecules could be excited under light irradiation and the

excited SRB molecules (SRB*) are able to inject their electrons into the substrate.166

This “dye sensitization” effect of the excited SRB molecules on the Au-NPs

promotes the formation of O2− species. The photosensitization process under light

irradiation involves initial excitation of the SRB molecules and is helpful for

injecting dye electrons to the holes left in the 5sp band.91, 266, 270

This may combine

with the LSPR effect of the AuNPs, resulting a high rate of dye degradation. Chen et

al. found that dye content can be decreased by 74% within 3 hours under blue light

irradiation with Ag NPs on ZrO2.91

According to Zhao et al., the platinum dopant

acts as an electron sink from which molecular oxygen scavenges the electrons to

yield superoxide radical anions (O2−) first and then ˙OH radicals which are known to

cause the degradation of the SRB dye.270

In 2009, a study was reported describing

the photo degradation of sulforhodamine B using Au@TiO2/ bentonite under UV and

visible light irradiation and the researchers propose that Au was photoexcited owing

to the surface plasmon resonance effect and the photogenerated electrons were

injected into O2 adsorbed on TiO2, which increased the production of superoxide and

hydroxyl radicals.268

7. CONCLUSIONS AND OUTLOOK

The discovery of direct metal NP photocatalysis was a breakthrough for fine

organic chemical synthesis, particularly those that favour “green” synthesis strategies

(using moderate reaction conditions, fewer additives and high chemoselectivity). The

increasing numbers of papers on metal NP photocatalysis in recent years have

significantly advanced knowledge in this area. The novel developments in this

dynamic field have broadened the pathways for the efficient transformation of solar

energy into chemical energy. In direct metal NP photocatalysis, the light harvesting

and catalysing reaction can be effectively coupled on the NPs. Furthermore, these

reaction systems are highly energy efficient since the intensive light absorption is by

the metal NPs, and not by the solvent, the support material, the environment, or the

36

reaction container. The hot electrons created by light irradiation are able to induce

reactions of adsorbed reactant molecules at the surface of the metal NPs. The number

of the hot electrons and energy distribution can be manipulated by tuning the light

intensity and irradiation wavelength to optimize the reaction efficiency

Subsequently, the natural affinity of the metal NP surface to organic reactants, the

high density of conduction electrons on the NP surface and the ability of the NP to

couple the stimuli of light and heat to excite conduction electrons, creates a novel

class of photocatalysts superior to semiconductor or composite photocatalysts for

synthesis of organic compounds. In principle, this photocatalyst structure is likely to

be efficient in driving various chemical reactions with sunlight (or focused sunlight),

especially the reactions of organic molecules. Therefore, it is reasonable to assume

there will be significant focus in the near future on the field of metal NP

photocatalysts. In addition, the research on direct plasmon/non-plasmon driven

photocatalysis is still in its early stages. The development of structure (size and

shape) – function and composition function relationships of the metal photocatalysis

isn't fully revealed yet. A known characteristic feature of PNPs is their tunable LSPR

wavelength based on particle geometry, such as composition, shape and size. In

principle, it is possible to design nanostructures that can absorb the complete solar

spectrum efficiently by controlling these properties through catalyst preparation. The

dependence of direct plasmon-driven photocatalytic characteristics (e.g. efficiency,

wavelength dependence, reaction selectivity) on the structure of the PNPs is expected

to be investigated in future research. Even though some significant improvements

have been made, methods and techniques need further refinement. More exciting

discoveries can be expected in the pursuit of eco-friendly fine chemical synthesis,

and environmental remediation protocols in the very near future.

Acknowledgements

We gratefully acknowledge financial support from the Australian Research Council

(ARC DP150102110).

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52

Chapter 2: Supported non-plasmonic metal

nanoparticles for organic

synthesis under visible light

irradiation

2.1 INTRODUCTORY REMARKS

This chapter includes one article ready to submit to Chemistry – An Asian

Journal.

In this Article, we focused on a systematic investigation of the palladium

nanoparticle catalysed coupling reactions under visible light irradiation at lower

reaction temperatures. Non-plasmonic transition metals, such as palladium, are

widely used as catalysts for the synthesis of important organic compounds. Until

now, knowledge about their photocatalytic ability is limited. In this paper, we

discovered that irradiation with light can significantly enhance the catalytic

performance of non-plasmonic palladium nanoparticles at ambient temperatures for

several types of coupling reactions, including the Sonogashira, Stille, Suzuki-

Miyaura and Ullmann reactions. Palladium nanoparticles strongly absorb light

mainly through interband electronic transitions. The strong interaction facilitates the

transfer of light-excited electrons to reactant molecules adsorbed to the metal

nanoparticles, and electron transfer weakens the C–X (X- Halogen) bond of the

reactant molecules and facilitates the reactions. The rate of the catalysed reaction

depends on the concentration and energy of the excited electrons, which can be

increased by increasing the light intensity or by reducing the irradiation wavelength.

This finding provides a useful guideline for green cross-coupling reactions driven by

solar energy and reveals the possibility of designing efficient photocatalysts for a

number of organic syntheses using non-plasmonic transition metals.

QUT Verified Signature

QUT Verified Signature

54

Non-plasmonic Palladium nanoparticles for homo-coupling

and cross-coupling reactions under visible light irradiation

Sunari Peiris, Sarina Sarina,*

Chenhui Han, Xiayan Wu, Qi Xiao,

and Huai-Yong

Zhu

Abstract: The Palladium nanoparticles are extensively used as thermal catalyst for

cross-coupling reactions; nonetheless, knowledge about their photocatalytic ability is

limited. We report here that the catalytic performance of palladium nanoparticles

under light irradiation can significantly enhance the coupling reactions, such as the

Suzuki-Miyaura, Sonogashira, Stille and Ullmann reactions. The photocatalytic

activity of the palladium nanoparticles depends on the metal content, reaction

temperature, intensity and wavelength of the incident light. Higher reaction rates

were observed with increases in the incident light intensity. We believe that the

conduction electrons of palladium nanoparticles gain energy from photon absorption

and the increasing population of photoexcited electrons at the nanoparticle surface

leads to higher photocatalytic ability. The photoexcited electrons interact with the

reactant molecules on the nanoparticle’s surface and accelerate the chemical reaction.

These findings provide useful guidelines for designing efficient catalysts for a

number of organic syntheses using non-plasmonic, catalytically active transition

metal nanoparticles under UV-Visible light irradiation.

Introduction

Palladium (Pd) is unarguably the most versatile and conventional catalytic

metal in chemical synthesis field.1, 2

Pd catalysed cross-coupling is a dominant and

functional reaction in organic synthesis for the formation of carbon–carbon (C-C)

bonds in industrial and fundamental synthetic chemistry laboratories.2, 3

These

coupling reactions have been extensively applied to the fabrication of synthetic

materials, natural products, and bio-active compounds. 4-8

Owing to their wide

utilization of C–C bond formation, enormous interest continues in developing more

efficient catalysts aimed at industrial applications within environmentally benign

processes. Many of the catalytic cross-coupling reactions are driven by heat using

55

both homogeneous and heterogeneous Pd catalysts to achieve viable efficiency.9-14

However heating has negative side effects such as increasing the extent of the

formation of unwanted by products that compromises the long-term stability of

catalysts.15-19

Catalysis driven by light irradiation instead of heat is particularly

fascinating in the green chemical synthesis field as it combines the efficiency of

catalysis with the possibility for use of solar energy. Photocatalysis is an ideal

process for the future as it has lesser environmental impact and greater sustainability.

The application of direct photocatalysis to drive organic synthesis reactions largely

extends to alloying plasmonic-plasmonic metals NPs and the plasmonic- non-

plasmonic metals NPs.20-24

However, there are many other organic reactions that pure

Au, Ag and Cu NPs simply cannot catalyse. Recently, we discovered that non-

plasmonic metal NPs (Pd, Pt, Rh and Ir) can intensely absorb visible light and

efficiently improve the extent of conversion of a number of reactions at much lower

reaction temperatures. 25-27

It would be of great interest to investigate the impact of

light irradiation on the catalytic performance of non-plasmonic Pd NPs for coupling

reactions in detail. There are several homogeneous catalytic processes catalysed by

Pd complexes with various ligands that have been reported for coupling reactions.10,

11, 14 Although organometallic catalysts commonly exhibit notable activity and

selectivity, the applications in the industry remain challenging owing to their

expense, the problem of aggregation of metallic particles and difficulties of

separation.15, 28

In this context, heterogeneous catalysts are a promising option, due to

their easy separation, recycling, stability, handling and the use of a lesser catalyst

amount in the production procedure. 29

Herein, we envisioned that if the

photocatalytic cross-coupling reaction using supported Pd NPs as catalysts can be

realized, the synthesis of biaryl compounds would be a much more controlled,

simplified, and greener process.

Results and Discussion

In this study, Pd NPs with various metal contents from 1% to 5% on zirconia

(ZrO2) support were prepared via an impregnation–reduction procedure as described

in the Experimental Section. The transmission electron microscopies (TEM) image

shows that the Pd NPs are distributed evenly on the ZrO2 particle surfaces, and the

mean diameters of the Pd NPs are 16 nm (Figure 1). The corresponding Pd metal

56

percentage was obtained via Scanning Electron Microscopy (SEM) with Energy

Dispersive X-Ray Analysis (EDX). Further, energy-dispersive X-ray spectroscopy

(EDS) shows that, the catalyst consists of elemental Pd distributed uniformly on the

support (Figure S1). X-ray photoelectron spectra (XPS) of the samples (Figure 2-a)

confirmed that the metal existed in a metallic state when formed on the ZrO2 support.

Figure 2-b shows the X-ray diffraction (XRD) patterns of the catalyst Pd NPs on

ZrO2 supports. All diffraction peaks can be indexed to monoclinic ZrO2, no

reflection peaks of Pd were observed by XRD, because the metal content is low and

the metal diffraction peaks may interfere with the diffraction peaks of the supporting

ZrO2 structure.

Figure 1. The catalyst characterization. (a) TEM image of the Pd NPs/ZrO2 catalyst,

(b) TEM image of a Pd NP and Particle size distribution.

Figure 2. (a) XPS profile of Pd on the ZrO2; (b) XRD pattern of ZrO2 (black curve)

and Pd/ZrO2 catalyst with different metal content.

57

Diffuse reflectance ultraviolet−visible (DR UV−vis) spectra of Pd NPs on

ZrO2 support was collected and shown in Figure 3. ZrO2 support exhibits a negligible

light absorption at wavelengths longer than 400 nm. Thus, the ZrO2 support itself

does not contribute to photocatalytic activity.30

Generally, the conduction electrons

and the bound electrons determine the light absorptivity of the metal NPs.26, 31

However, the light absorption of a metal can have contributions from the LSPR

effect and interband transitions. These vary from metal to metal. For typical

plasmonic metal NPs, such as Au, Ag etc. light absorption through the LSPR effect

results in a collective oscillation of the free electrons, whereas non-plasmonic metal

NPs absorb light by bound electron excitation to high energy levels through

interband transitions.31, 32

This photogeneration of hot electrons increases the

intrinsic catalytic activity of Pd and makes it possible to apply this non-plasmonic

NP metal catalyst to drive various reactions at ambient temperatures under light

irradiation.

Figure 3. The normalized diffuse reflectance ultraviolet−visible (DR-UV/Vis)

extinction spectra of the Pd/ZrO2 with different metal content.

Pd has the required orbital energies to associate with a carbon–carbon double

bond.33

The characteristic ground-state electronic structure (4d10

5s0) of Pd confers

outstanding catalytic activity, because it is the only transition metal having a filled d

58

orbital with an empty frontier s orbital.2 The reported cross-coupling reactions

catalysed via homogeneous and/or heterogeneous processes need elevated

temperatures as well as high pressures to achieve higher yields.12, 34-37

However, this

study demonstrates that visible light irradiation can drive the same reactions on the

supported Pd NPs under much milder reaction conditions (< 45 °C and atmospheric

pressure), achieving great yields with negligible conversion in the dark. This reveals

the photocatalytic capability of a famed thermal catalyst. Moreover, photocatalysis

active in water can offer great advantages in terms of green chemistry, and has been

actively investigated.38, 39

In comparison to the results reported in the past literature,

this photocatalytic process without a surfactant (Suzuki coupling reaction) shows a

significant yield of biphenyl and it leads to easy product separation in the industry.

Iodobenzene was used as the aryl halide substrate for all the cross-coupling

and homo-coupling reactions. The visible light irradiation increased activity of Pd

NPs photocatalyst compared with the same reactions conducted in the dark. The

visible and UV light absorption could promote the electron interband transition, these

energetic electrons at the metal NPs surface enhance the catalytic activity of the Pd

NPs. Control experiments were carried out using the support ZrO2 as the catalyst and

nil or negligible conversion was observed with light irradiation or in the dark

condition. Generally, ZrO2 exhibits an insignificant light absorption in the visible

range since its large band gap (5 eV).30

This confirms the catalytic activity is due to

the Pd NPs. The highest activity was exhibited by the photocatalyst with a Pd content

of 3 wt% and lower or higher Pd content, 1 wt% or 5 wt% exhibit obviously poorer

performance (Figure S2). The Pd loading higher than 3 wt% resulted aggregation of

the NPs, which reduces the surface area of the Pd NPs, on where the catalytic

reaction took place.

A series of different aryl halide substitutes were used to investigate the wide

applicability of non-plasmonic Pd NPs photocatalyst on all the four coupling

reactions. The light irradiation significantly increased the yield of the desired cross-

coupling product in both electron donor or acceptor substituted halides (Table 1).

59

Table 1. The Pd NP catalysed coupling reactions with different aryl halides under

visible light irradiation and in the dark (in parentheses).

60

The yields were calculated from the product content and the aryl halide conversions

measured by GC. The products were analysed by GC and mass spectrometry.

Generally, activating C−Br or C−Cl bonds are more challenging than

activating C−I bond in heterogeneous catalysis systems due to the higher activation

barrier, and usually requires harsh reaction conditions.8 In this study, bromobenzene

and chlorobenzene were used to investigate the photocatalytic activity of Pd NPs for

activating C−Br or C−Cl bonds under light irradiation. The results (C-Br -24-28%

and C-Cl -10-16%) are shown in Table 2 and Pd NP based photocatalytic process

showing a promising approach to activate the challenging C-Cl and C-Br bond at

near ambient temperatures.

Table 2. Examples of bromobenzene and chlorobenzene as substrates for cross and

homo-coupling reactions using ZrO2 supported Pd NPs under visible light irradiation.

Substrates Product Yield (%)

1 Br + Sn(Bu)3

24 (7)a

2 Br + B(OH)2

28 (17)b

3 Br

<1b

4 Cl + Sn(Bu)3

16 (6)a

5 Cl + B(OH)2

10 (3)b

The yields were calculated from the product content and the aryl halide conversions

measured by GC. The numbers in parentheses are the data for reactions controlled

under the same conditions in the dark. Reaction temperature of (a) 55 °C, (b) 60 °C.

Light intensity is 0.9 W/cm2, and the other reaction conditions were kept the same.

The light irradiation intensity was increased from 0.34 to 0.50, 0.63, and 0.80

W/cm2 and the intensity dependent conversion of the Sonogashira, Susuki, Stille and

Ullmann reactions are depicted in Figure 4. The conversion amount of iodobenzene

on Pd NPs increases gradually with the light intensity increases, with other reaction

conditions unchanged. For example, when the light intensity was 0.34 W/cm2, the

light contributions of Ullmann homo coupling reaction was only 75% and when the

light intensity increased to 0.8 W/cm2, it improved to 92%. This demonstrates that

61

irradiation intensity is a primary factor in direct metal photocatalyst systems. The

coupling reactions are induced via a single photon absorption event and it confirms

by the linear relationship between photocatalytic enhancements with the light

intensity.40

The contribution of the thermal effect studied by conducting the reactions

in the dark, by maintaining the same reaction temperature using an oil bath.

Generally, the Pd is recognized as a good thermal catalyst for the organic synthesis.2

Nevertheless, the photo thermal heating effect is negligible on Pd NPs for all the

reaction systems in the dark. This confirms that the catalytic activity has a positive

relationship on the intensity and it is owing to the Pd NPs. The photocatalytic

reaction rate depends upon the population of excited electrons with sufficient energy

to initiate the reactant molecules and stronger light intensity is able to excite more

energetic electrons of Pd NPs.26

Moreover, the number of photoexcited electrons

with sufficient energy can be increased by tuning the irradiation wavelength and this

study assists understanding of the reaction mechanism.

(a) (b)

(c)

(d)

62

Figure 4. Dependence of the photocatalytic activity of 3% Pd NPs/ZrO2 for (a)

Sonogashira, (b) Suzuki, (c) Stille, and (d) Ullmann coupling reactions on the

intensity of light irradiation. The values with percentages demonstrate the light

irradiation contribution.

The dependence of the catalytic reaction rate on the irradiation wavelength is

illustrated by the action spectrum, which is used to determine whether the reaction

occurs through a photo induced processes or a thermocatalytic process.41, 42

The

reaction rates of the photocatalytic reactions under irradiation with different

wavelengths were determined for Pd NP/ZrO2 catalysts. LED lamps with five

different wavelengths (400 ± 5, 470 ± 5, 530 ± 5, 590 ± 5, and 620 ± 5 nm) were

used, and the rates were converted to the apparent quantum yields (AQYs). The

apparent quantum yield (AQY%) was calculated as follows: apparent quantum yield

= [(Mlight–Mdark)/Np] × 100%, where Mlight and Mdark are the molecules of products

formed under irradiation and dark conditions respectively, Np is the number of

photons involved in the reaction. The irradiance intensity and reaction temperature

were held constant for coupling reactions to ensure the total input energy gained by

the metal NPs was identical, under irradiation at different wavelengths. Moreover,

the number of product molecules formed in the dark (Mdark) was deducted to exclude

the impact of thermal heating. The action spectra of Suzuki and, Stille reactions are

shown in Figure 5 and each of them compared with the light absorption spectrum of

the Pd NPs supported with ZrO2. The highest reaction activity is discovered at

wavelengths at which the catalysts intensely absorb light. Since the ZrO2 support

doesn’t contribute to the photocatalytic activity, when Pd NPs are in the system they

perform as active sites for the coupling reactions.

63

Figure 5. The action spectra for (a) Suzuki and (b) Stille cross-coupling reactions.

The light absorption spectra (left axis) are DR UV−vis spectra of Pd NPs/ZrO2

(black). The AQY values were calculated on the basis of the median of three

experiments.

The impact of the reaction temperature on the photocatalytic activity of Pd

NPs was investigated by changing the reaction temperature from 40 °C to 80 °C

using oil baths while maintaining other experimental conditions unchanged. The

photocatalytic activity of the metal NPs increases with elevated reaction

temperature.43, 44

The reaction temperature increased the product yield of the

Sonogashira coupling at 40 °C (17%) to 80 °C (100%), and the yield of Stille

coupling at 30 °C (25%) to 70 °C (96%), and the yield of Ullmann coupling at 30 °C

(24%) to 70 °C (100%). (Figure 6) The contribution of the irradiation effect was

calculated by the yield difference between the light and the dark reaction divided by

the total yield under light irradiation. The light contribution= [(Ylight–Ydark)/Ylight]

×100%, where Ylight and Ydark are the product yields under irradiation and dark

conditions, respectively. For Sonogashira coupling reaction, the product yield

difference between the light reaction and dark reaction at 40 °C is 64% and it

accounted for 94% of the total product yield. It is noted that the contribution from the

light effect decreases as the reaction temperature was raised. At elevated

temperatures, the light excites more electrons of PdNPs to higher energy levels (these

electrons can further gain energy from the light irradiation), and transfer these

excited electron to the adsorbed reactant molecules to initiate the reaction.43, 44-46

The

relative population of excited vibrational states of the adsorbed reactant molecule

64

increases according to the Bose-Einstein distribution at higher temperatures.47

This

will reduce the energy requirement from irradiation to overcome the reaction

activation barrier of the iodobenzene. On the other hand, at lower reaction

temperatures, the light-excited electrons play a predominant role in photocatalytic

activity and the thermal effect contributes much less.46

The metal NPs can utilize

both thermal energy and photon energy simultaneously and that increases the

potential usage of the solar spectrum to facilitate chemical reactions under mild

conditions.16, 17, 19, 44

(a)

(b)

(c)

(d)

Figure 6. Dependence of photocatalytic activity on different reaction temperatures

for (a) Sonogashira, (b) Suzuki, (c) Stille, and (d) Ullmann coupling reaction. Under

a thermal heating process in the black squares and the light irradiation process

coloured (red, green, blue and black ) hollow squares.

65

Figure 7. Proposed mechanism of the photocatalytic reactions with non-plasmonic

Pd NP photocatalysts. (a) The electrons follow a Fermi–Dirac distribution at the

thermal temperature of the system; electron transfer cannot occur in the dark at low

temperatures. (b) Under higher irradiation, electrons of Pd NPs populate higher

energy levels and light-excited electrons directly injected into the antibonding

orbitals (LUMO) facilitate reaction of the adsorbed reactant molecules. (c) At

elevated reaction temperatures, more excited electrons of Pd NPs populate higher

energy levels of the Pd NPs, which can readily transfer to the LUMO of the absorbed

molecule to facilitate reactions. (d) At shorter wavelengths, electrons of Pd NPs are

excited to higher energy levels, which can also readily transfer to the LUMO of the

absorbed molecule to facilitate reactions. Figure 7 depicting the electronic energy

distribution where the y-axis is energy, E, with Fermi energy, EF. The black shading

indicates filled electronic states; red coloured HOMO & LUMO states for adsorbed

reactant.

(a)

(b) (c) (d)

66

Here, we propose a possible reaction pathway for coupling reaction that uses

the Pd/ZrO2 as photocatalyst. (Figure S3) We utilized the past literature knowledge

and focussed on possible roles light irradiation facilitates the coupling reaction at the

Pd NP surfaces.23, 26, 48-50

When the Pd NPs were irradiated with light, electrons

excited to the high-energy band and increase the energetic electron population at the

metal surface (Figure 7-b).26

The rate-determining step of coupling is the activation

of aryl halide on the Pd NPs by the transfer of electrons from the Pd atoms to the

halogen atoms and this facilitates the carbon–halogen bond cleavage (oxidative

addition).51

There are two possible pathways, which the light irradiation can enhance

the photocatalytic activity.52

The energetic electrons (hot electrons), have sufficient

energy to overcome the energy barrier and can migrate to the unoccupied LUMO of

the reactant molecules under light irradiation.26, 52

Moreover, the transient electron is

able to transfer from metal NP to the chemically adsorbed reactant molecules

inducing the reactions under light irradiation.50, 53, 54

Xiao et al. performed DFT

calculations and found that the cleavage of the C−I bond will be much easier when

one electron enters an unoccupied orbital of the reactant molecule.23

Furthermore,

higher reaction temperatures increased the excited electron population at higher

energy levels of the Pd NPs and readily transferred to the LUMO of the absorbed

molecule and facilitate the bond breaking (Figure 7-c). Following on, activation of

the coupling partner molecules facilitates transmetalation. Finally, the lower energy

electrons return to the Pd and reductive elimination of the cross-coupling product

R1Ph-PhR2 molecule completes the photocatalysis cycle.

Conclusions

In summary, it is found that visible light irradiation can efficiently drive the

cross-coupling and homo-coupling reactions using Pd NPs supported by ZrO2 at low

temperatures and at atmospheric pressure. The supported Pd NPs catalyst is almost

inactive for the coupling reactions at lower temperatures without irradiation. Pd

metal NPs serve as both a visible light harvester and a provider of catalytic sites. The

energetic electrons excited by light irradiation are driving the photocatalytic reaction

and it can be improved by tuning the incident light intensity and wavelength. These

excited electrons at the surface Pd sites interact with the reactant molecules and

initiate the photoreaction. Generally, non-plasmonic metals such as Pd, have been

67

widely used as a thermal catalyst for various industrial chemical synthesis. This

study indicated that the irradiation of metal particles leads to enhanced catalytic

activity. The findings reported here may significantly broaden the application of non-

plasmonic metals as photocatalysts and reveal the possibility of green cross coupling

reactions at mild reaction conditions. The knowledge acquired in this study may

encourage further studies in non-plasmonic metal catalysts for a wide range of

organic syntheses driven by visible light.

Experimental Section

Catalysts Preparation

Photocatalysts with 3% Pd on the ZrO2 powder were prepared: 1.0 g of ZrO2

powder (particle size less than 100 nm) was dispersed in 28 mL of a given

concentration of PdCl2 solution (dissolved in dilute ammonium hydroxide (1M)

solution) with vigorous stirring. To this suspension, 20 mL of 0.05 M NaBH4

solution was added drop wise over 30 min. The mixture was aged for overnight and

then the solid was separated, washed with water and ethanol, and dried at 60 °C. The

dried solid was used directly as a catalyst. Catalysts with other Pd loadings (1, 3 and

5 % of the overall catalyst mass, expressed in wt%) were prepared in a similar

method but using different quantities of PdCl2 aqueous solution.

Characterization of Catalysts

TEM studies were carried out on a JEOL JEM-2100 Transmission Electron

Microscope with an accelerating voltage of 200 kV. The Pd content of the prepared

catalysts was determined by energy dispersion X-ray spectrum (EDS) technology

using the attachment to a FEI Quanta 200 environmental scanning electron

microscope (SEM). Diffuse reflectance UV−visible (DR-UV-vis) spectra of the

sample powders were examined by a Varian Cary 5000 spectrometer with BaSO4 as

a reference. X-ray photoelectron spectroscopy (XPS) data were acquired using a

Kratos Axis ULTRA X-ray Photoelectron Spectrometer incorporating a 165 mm

hemispherical electron energy analyser. X-ray diffraction (XRD) patterns of the

sample powders were collected using a Philips PANalytical X’pert Pro

68

diffractometer. Cu Kα radiation (λ= 1.5418 Å) and a fixed power source (40 kV and

40 mA) were used.

Photocatalytic Reactions

General Procedure for Cross-coupling Reactions: A Pyrex round bottom flask

was used as the reaction container, and after the reactants and catalyst had been

added, the flask was sealed with a rubber septum cap and stirred with a magnetic

stirrer. The flask was irradiated using a halogen lamp (from Nelson, wavelength in

the range of 400−750 nm) as the visible light source, and the light intensity was

measured to be 0.8 W/cm2. The temperature of the reaction system was carefully

regulated with an air conditioner, which attached to the reaction chamber. The

reaction setups under dark condition were maintained at the same temperature as the

corresponding reactions under light irradiation by using oil bath placed above a

magnetic stirrer to the comparison. The reaction flask was wrapped with aluminium

foil to avoid exposure of the reaction mixture to light in the dark. At given irradiation

time intervals, the product was extracted with dichloromethane (CH2Cl2) and 2 mL

aliquots were collected, centrifuged, and then filtered through a Millipore filter (pore

size 0.45 μm) to remove the catalyst particulates. The liquid-phase products were

analysed with an Agilent 6890 gas chromatography (GC) HP-5 column to measure

the change in the concentrations of reactants and products. An Agilent HP5973 mass

spectrometer was used to identify the product. The GC conversion and selectivity

were calculated from the product content and the aryl halide conversions.

Sonogashira cross-coupling Reaction: Aryl halide (1 m mol), alkyl alkyne (1.2 m

mol), photocatalysts (50 mg), cetyltrimethylammonium bromide (CTAB) (1 m

mol), and K3PO4 (2 m mol) were added to 10 mL of H2O. The reaction

temperature was 45 ± 2 °C, under a 1 atm argon atmosphere, with a reaction

time of 24 h.

Suzuki cross-coupling Reaction: Aryl halide (1 m mol), arylboronic acid (1.5 m

mol), photocatalysts (50 mg) and K2CO3 (3 m mol) were added to 10 mL H2O.

The reaction temperature was 30 ± 2 °C, under a 1 atm argon atmosphere, with a

reaction time of 6 h.

69

Stille cross-coupling Reaction: Aryl halide (0.5 m mol), tributylphenylstannane

(0.6 m mol), photocatalysts (30 mg), cetyltrimethylammonium bromide (CTAB)

(0.5 m mol), and NaOH (1.5 m mol) were added to 4 mL of H2O. The reaction

temperature was 45 ± 2 °C, under a 1 atm argon atmosphere, with a reaction

time of 24 h.

Ullmann homo-coupling Reactions: Aryl iodide (0.5 m mol), photocatalysts (30

mg), and NaOH (1.5 m mol) were added to 4 mL of an EtOH/H2O mixture [1/1

(v/v)]. The reaction temperature was 50 ± 2 °C, with a reaction time of 24 h.

Acknowledgements

We gratefully acknowledge financial support from the Australian Research

Council (ARC DP150102110). The electron microscopy work was performed

through a user project supported by the Central Analytical Research Facility

(CARF), Queensland University of Technology.

Keywords: Metal nanoparticles• coupling reaction• photocatalysis• green synthesis•

light irradiation

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Supporting Information

Non-plasmonic Palladium nanoparticles for homo-

coupling and cross-coupling reactions under visible

light irradiation

Sunari Peiris,

a Sarina Sarina,

a* Chenhui Han,

a Xiayan Wu,

a Qi Xiao,

b and Huai-

Yong Zhua

a. School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and

Technology, Queensland University of Technology, Brisbane, QLD 4001, Australia.

b. CSIRO Manufacturing, Bayview Avenue, Clayton, VIC 3168, Australia.

*Corresponding author email: s.sarina@qut.edu.au

74

Figure S1. (a) SEM image of 3% Pd/ZrO2 sample and the corresponding mapping

of Zr, O and Pd elements; (b) EDX spectrum of of 3% Pd/ZrO2 sample.

(a)

(b)

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Figure S2: Dependence of Pd metal NP photocatalytic performance on the metal

amount.

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Figure S3. Proposed catalytic cycle for coupling reactions using the Pd/ZrO2 under

light irradiation.

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Chapter 3: Supported silver based alloy

nanoparticle photocatalysts for

organic synthesis under visible

light irradiation

3.1 INTRODUCTORY REMARKS

This chapter includes one article published (online) on Dalton Transactions,

2017. DOI:10.1039/C7DT00418D

Recently, extend studies have been done using PNPs on nitrobenzene

reductive coupling, for example, gold NPs, copper NPs, Ag-Cu NPs and Au-Cu NPs

and found that the alloying between two plasmonic metals leads to significantly

enhanced photocatalytic performance. However, the NPs of several Ag alloys have

been known to be catalytically active in thermal reaction, there has been a few

reports on the photocatalysis of Ag based alloy catalysts so far. In this article, we

found that by alloying plasmonic metal Ag to a non-plasmonic metal Pd; the

photocatalytic activity in nitrobenzene reduction is increased remarkably compared

to both Ag NPs and Pd NPs. The intrinsic catalytic activity of palladium is

significantly enhanced in the alloy NPs even at ambient temperature under light

irradiation. The Ag-Pd alloy nanoparticles absorb visible light, and the light excited

energetic electrons on the metal alloy NP surface activate the reactants. The

performance of the photocatalysts depends on the metal ratio, light intensity and

wavelength. Notably, these heterogeneous catalysts are easily recycled and can be

conveniently reused, which is an important aspect in the development of practical

and cost-effective catalytic processes. This study provides a general guiding principle

for determining the applicability of the alloy NP photocatalysts as well as a clue for

designing suitable photocatalysts made from transition metal alloyed with silver.

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79

Silver and Palladium Alloy Nanoparticles Catalysts:

Reductive coupling of Nitrobenzene through Light

Irradiation

Sunari Peiris, Sarina Sarina*, Chenhui Han, Qi Xiao, and Huai-Yong Zhu

The silver-palladium (Ag-Pd) alloy nanoparticles strongly absorb visible light

and exhibit significantly higher photocatalytic activity compared to both pure

palladium (Pd) and silver (Ag) nanoparticles. Photocatalysts of Ag-Pd alloy

nanoparticles on ZrO2 and Al2O3 supports are developed for catalyze the

nitoaromatic coupling to the corresponding azo compounds under visible light

irradiation. Ag-Pd/ZrO2 exhibited the highest photocatalytic activity for nitrobenzene

coupling to azobenzene (yield of ~80 % in 3 hours). The photocatalytic efficiency

could be optimized by altering the Ag: Pd ratio of the alloy nanoparticles, irradiation

light intensity, temperature and wavelength. The rate of the reaction depends on the

population and energy of the excited electrons, which can be improved by increasing

the light intensity or by using a shorter wavelength. The knowledge developed in this

study may inspire further studies of Ag alloy photocatalysts and organic syntheses

using Ag-Pd nanoparticles catalyst driven under visible light Irradiation.

Introduction

The aromatic azo compounds are important intermediates for a variety of

specific and fine chemicals in industries, such as dyes, food additives and

pharmaceutical products.1-6 However, conventional methods used in azo compound

synthesis involve the use of transition metal reducing agents and conditions of high

temperature and pressures.3, 6-11 The metal compounds formed from the reducing

agent are of environmental concerns.3, 8, 10 However, these routes always show low

yields and poor selectivity. Therefore, it is highly desirable to develop efficient as

well as environmentally friendly process for the coupling of nitrobenzene.

Photocatalytic reaction is driven by light irradiation, applying photon energy

instead of conventional thermal energy. Thus the photocatalytic reaction is able to be

conducted under much moderate conditions (ambient temperature and pressure),

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which make it possible to achieve a certain unstable intermediate of thermal reaction

as the final product in the photocatalytic reactions.

We reported previously that plasmonic metal nanoparticles (NPs) could

intensively absorb visible light via the localized surface plasmon resonance (LSPR)

effect, and successfully achieved azo aromatic compounds directly from the coupling

of nitroaromatic compounds at room temperature and 1 atm argon atmosphere.12 The

LSPR effect is the collective oscillation of conduction electrons in the NPs, which

resonate with the electromagnetic field of the incident light. These conduction

electrons could gain the light irradiation energy and increase the high energetic

electrons at the metal NP surface, which activates the reactant molecules for the

chemical reactions. Series of studies have been done using plasmonic NPs on

nitrobenzene reductive coupling, for example gold NPs, copper NPs, Ag-Cu NPs and

Au-Cu NPs and found that the alloying between two plasmonic metals show

significantly enhanced photocatalytic performance.11-14 However when alloying

plasmonic metal Au to a non-plasmonic metal Pd, the photocatalytic activity in

nitrobenzene reduction is reduced remarkably compared to the pure Au NP.15 This is

because supported AuNPs are an efficient photocatalyst themselves for coupling of

nitrobenzene.2, 8, 12, 16, 17 The key step of the reduction of nitrobenzene is to cleavage

the N-O bonds.11,12 This N-O bond cleavage by the hydrogen atoms bound to the

metal NP surface (E.g. H–AuNP). The abstraction of a hydrogen atom from

isopropyl alcohol (IPA) plays a critical role in catalytic activity. However, Ag shows

a weaker ability to abstract H from IPA compared to Au. Hence, alloying with some

stronger H absorbing metal such as Pd will improve utilise of Ag for the

photocatalytic chemical synthesis in economical way.18 Moreover, the NPs of several

Ag alloys have been known to be catalytically active in thermal reaction, there has

been a few reports on the photocatalysis of Ag based alloy catalysts so far.14

Therefore, in this study we investigated the possibility of the alloy NPs of Ag and a

transition metal; such as Pd, developed to an efficient photocatalysts for coupling of

Nitrobenzene.

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Experimental Section Chemicals:

Zirconium(IV) oxide (ZrO2, <100 nm particle size), silver nitrate (AgNO3,

≥99.9% trace metal basis), palladium(II) chloride (PdCl2, Reagent Plus, 99%),

sodium borohydride powder (NaBH4, ≥98.0%), nitrobenzene (99.0%), potassium

hydroxide (99.0%, KOH) and isopropanol (99.5%) were purchased from Sigma-

Aldrich (unless otherwise noted) and used as received without further purification.

The water used in all experiments was prepared by being passed through an ultra-

purification system.

Catalysts Preparation:

Ag-Pd NPs/ZrO2: Catalysts with 3 wt % of pure silver NPs on ZrO2 (3%Ag/ZrO2), 3

wt % of pure Pd NPs on ZrO2 (abbreviated 3%Pd/ZrO2) and the catalysts of Ag and

Pd alloy NPs supported by ZrO2 (3%Ag−Pd/ZrO2), with different Ag/Pd ratios were

prepared by impregnation-reduction method. For example, 1.5 wt %Ag−1.5 wt %

Pd/ZrO2 (3%Ag−Pd(1:1)/ZrO2) was prepared by the following procedure: 1.0 g ZrO2

powder (particle size less than 100 nm) was dispersed in 14 mL of 0.01 M AgNO3

aqueous solution and 14 mL of 0.01 M PdCl2 dissolved in dilute ammonium

hydroxide (1M) solution were added while magnetic stirring. To this suspension, 20

mL of 0.05 M NaBH4 solution was added drop wise in 30 min. The mixture was

aged for overnight and then the solid was separated, washed with water and ethanol

by centrifugation, and dried at 60 °C. The dried solid was used directly as a catalyst.

Catalysts with other Ag/Pd ratios were prepared in a similar method, but using

different quantities of AgNO3 aqueous solution and/or PdCl2 aqueous solution.

Ag-Pd NPs/ɣ-Al2O3: Catalysts with 3 wt % of pure silver NPs on ɣ-Al2O3 (3%Ag/ ɣ-

Al2O3), 3 wt % of pure Pd NPs on ɣ-Al2O3 (3%Pd/ ɣ-Al2O3) and the catalysts of Ag

and Pd alloy NPs supported by ɣ-Al2O3 (abbreviated 3%Ag−Pd/ ɣ-Al2O3), with

different Ag/Pd ratios were prepared by impregnation-reduction method. For

example, 1.5 wt %Ag−1.5 wt % Pd/ ɣ-Al2O3 (3%Ag−Pd(1:1)/ ɣ-Al2O3) was prepared

by the following procedure: 1.0 g of ɣ-Al2O3 powder (fiber length around 200 nm)

was dispersed 14 mL of 0.01 M AgNO3 aqueous solution and 14 mL of 0.01 M

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PdCl2 dissolved in dilute ammonium hydroxide (1M) solution were added while

magnetic stirring. To this suspension, 20 mL of 0.05 M NaBH4 solution was added

drop wise in 30 min. The mixture was aged for overnight and then the solid was

separated by centrifugation, washed with water and ethanol, and dried at 60 °C. The

dried solid was used directly as a catalyst. Catalysts with other Ag/Pd ratios were

prepared in a similar method, but using different quantities of AgNO3 aqueous

solution and/or PdCl2 aqueous solution.

ɣ-Al2O3 support: 4.7 g NaAlO2 was dissolved in 12.5 mL water in a 50 mL beaker,

and stirred for 15 min to obtain a homogeneous solution. 15 mL of 5 M acetic acid

solution was added into a 100 mL beaker. Then NaAlO2 solution was poured into the

burette (50 mL) and then added into acetic acid solution drop wise under vigorous

stirring. When finish titrating, continue adding acetic acid solution (5 M) until the pH

value is adjusted to 5.0. The white precipitate of aluminium hydrate was washed with

water and recovered by centrifuge for four times (3000rpm for 15 min). The

collected white precipitate was transferred to a blue cap bottle, to which 10 g PEO

surfactant (T15 S-7) was previously added. The above mixture was kept stirring for

1h. Then the homogenous mixture was transferred into an oven and kept at 100 °C.

Every two days, fresh aluminium hydrate was prepared and the obtained white

precipitate was added into the glass bottle and the mixture was kept stirring for 1 h.

The stirred homogenous mixture was put back into oven and kept at 100 °C. This

circle will continue until we get the desired ɣ-Al2O3 length. The precipitate was kept

in the 450 °C in the furnace for 5 hours and crushed the solid using a mortar. The

powdered solid was used directly as support.

Characterization of Catalysts

Transmission electron microscopy (TEM) images and line profile analysis (By

the energy dispersion X-ray spectrum technique) were acquired on a JEOL JEM-

2100 transmission electron microscope employing an accelerating voltage of 200 kV.

The element line scanning was conducted on a Bruker EDX scanner attached to the

TEM. The composition (Ag and Pd contents) of samples was determined by using

the energy-dispersive X-ray spectroscopy (EDS) attachment of an FEI Quanta 200

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scanning electron microscope (SEM). X-ray diffraction (XRD) patterns of the

catalysts samples were collected using a Philips PANalytical X’pert Pro

diffractometer. CuKα radiation ( =1.5418 Å) and a fixed power source (40 kV and

40 mA) were used. The diffuse reflectance UV-Visible spectra (DR−UV−vis) of the

samples were examined by a Cary 5000 spectrometer. The X-ray photoelectron

spectroscopy (XPS) data were acquired using a Kratos Axis ULTRA X-ray

Photoelectron Spectrometer.

Photocatalytic Reactions

The photocatalyst (30 mg), solvent (IPA-5mL), base (KOH- 0.2 m mol) and

the reactant (nitrobenzene – 0.15 m mol) was placed in a reaction vessel and used

500 W Halogen lamp (from Nelson, wavelength in the range 400−750 nm) as the

visible light source and usual light intensity was kept at 0.80 W/cm2 unless for

investigate the light intensity impact. The temperature of the reaction system was

vigilantly regulated with an air conditioner, which attached to the reaction chamber.

The reaction setups under dark condition were maintained at the same temperature as

the corresponding reactions under light irradiation by using oil bath placed above a

magnetic stirrer to the comparison. Catalytic reduction of nitrobenzene was

conducted under the argon atmosphere. The details of the reaction systems are given

briefly as footnotes in Table for each reaction. At given irradiation time intervals, 0.5

mL aliquots were collected and removed the catalyst particulates by filtering through

a Millipore filter (pore size 0.45 m). The filtrates were analysed by gas

chromatography (HP6890 Agilent Technologies) with a HP-5 column to measure the

concentration change of reactants and products. The products were identified using a

mass spectrometer (Agilent HP5973).

Results and Discussion

The TEM images of ZrO2 and Al2O3 supported sample showed that Ag-Pd alloy

NPs dispersed uniformly on supports and the mean diameter is about 7 nm and 8-13

nm in size respectively (Figure 1-2 and Figure S1). Figure 1-(b) is a line profile

analysis of the energy dispersion X-ray (EDX) spectrum for a Ag−Pd alloy NP,

showing that the NP consists of both Ag and Pd dispersed spherically around a

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common center, which confirms that the two metals exist as an alloy NPs in this

sample. Scanning electron microscopy (SEM) analysis confirms the appearance of

both metals, Ag and Pd, in the elemental mapping in Ag-Pd(1:1)/ZrO2 sample

(Figure S2). The X-ray diffraction (XRD) patterns of the Ag-Pd alloy/ZrO2

photocatalysts is shown in Figure 1-(d). The diffraction peaks of the sample can be

indexed to the monoclinic structure of the ZrO2 crystals. The reflection peaks of Ag-

Pd alloy could not be recognized owing to the low metal content (3 wt%).

Figure 1: Catalyst characterization of Ag-Pd (1:1) /ZrO2. (a) Transmission

electron microscopy (TEM) image; (b) The line profile analysis of EDX spectra for a

typical Ag−Pd NP indicated by the square and the information of the elemental

composition and distribution of the NP; (c) Particle size distribution; (d) The XRD

pattern of the Ag-Pd (1:1) /ZrO2 photocatalysts.

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Figure 2: Catalyst characterization of Ag-Pd (1:1) /Al2O3. (a) TEM image; (b)

Particle size distribution.

UV-visible spectra of these photocatalysts are shown in the Figure 3. ZrO2 exhibits a

weak visible light absorption (band-gap is about 5 eV); consequently, the support by

itself does not contribute to photocatalytic activity.19 However, the Al2O3 support

show light absorption in the 300-400 nm range. The absorption peak at 410 nm in the

spectrum of the samples is due to the LSPR absorption of the Ag/ZrO2 (Not showing

on the graph).20, 21 In the spectrum of the alloy samples, the characteristic Ag NP

LSPR absorption peak at 410 nm is disappeared; and we can assume that Ag NPs

may blend well with the Pd NPs.22-24 The strong light absorption from 450 nm to 600

nm is attributed to scattering caused by closely spaced metal NPs and NP

aggregates.25, 26

Figure 3: UV-Visible diffuse reflectance spectra of the supported Ag-Pd (1:1)

alloy NPs catalysts and the corresponding supports.

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The X-ray photoelectron spectroscopy (XPS) spectra of the catalysts are also

shown in Figure 4. The binding energies of Ag 3d5/2 and 3d3/2 electrons are 368.3 and

374.0 eV, respectively (Figure 4-a).27 In addition, the binding energies of Pd 3d5/2

and 3d3/2 electrons are 335.2 and 340.5 eV, respectively (Figure.4- b).28 These results

confirmed that the alloy NPs are in the metallic state.

Figure 4: X-ray photoelectron spectra (XPS) binding energy of (a) Ag 3d5/2

and Ag 3d3/2 for Ag-Pd NPs on ZrO2 and Al2O3; (b) Pd 3d5/2 and Pd 3d3/2 for Ag-Pd

NPs on ZrO2 and Al2O3.

The alloy photocatalysts with different Ag and Pd contents, pure Ag and pure

Pd catalyst supported by ZrO2/ Al2O3 were also prepared in the similar method for

reference and the corresponding Ag-Pd molar ratios were also calculated and listed

in Table S1. As can be seen in figure 5, the photocatalytic performance of Ag-Pd

alloy catalysts depend on the Ag:Pd molar ratio for the reduction coupling of

nitrobenzene. The results reveal that the highest yield of target products achieved

when the alloys NPs have the Ag:Pd molar ratio of 1:1.01. Alloy NPs with other Ag:

Pd molar ratios exhibited much lesser activity, either under light irradiation or in the

dark. The conversion of nitrobenzene with the pure Ag/ZrO2 and pure Pd/ZrO2

catalysts were below 45%. Similar trends were observed for the pure Ag and pure Pd

on Al2O3 as well. The alloying affects the surface electronic properties of the NPs.

Therefore, the catalytic activity of the alloy NPs are significantly improved compare

to pure Ag NP or Pd NP. The charge heterogeneity is a key factor in the catalytic

reactions and it depends on the Ag:Pd molar ratio. Thus, molar ratio has a significant

impact on the catalytic performance of the alloy NPs.15 The electronegativity of Pd

(2.20) is higher than that of Ag (1.9) and there will be charge heterogeneity at the

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alloy NP surface, with both negatively charged (electron rich) sites and positively

charged (electron poor) sites present. Hence, the conduction electrons will flow

electron rich to poor, until equilibrium is reached and the electron chemical potential

is equal everywhere in the alloy NP. The heterogeneous charge distribution increases

the interaction between reactant molecules and it reduces the activation energy of the

reaction, hence increases the catalytic activity.

Figure 5: Dependence of Ag−Pd metal NP photocatalytic performance on the

Ag/Pd molar ratio of the alloy NPs.

The Ag-Pd alloy NP/ZrO2 photocatalysts exhibited significantly higher activity

for reductive coupling of nitrobenzene under light irradiation at 1 atm of Ar and at

60°C. In contrast, both pure Ag/ZrO2 and Pd/ZrO2 exhibited relative low activity for

the reaction under similar reaction conditions. Table 1 shows the results of the

reduction of nitrobenzene with two catalysts. A notable feature of the photocatalyst is

its selectivity towards the nitro groups. (GCMS spectras were included in supporting

information) When the reactant contains multiple reducible groups; nitro groups are

the only one reduced under light irradiation. It is difficult to achieve this by

conventional reduction at high temperatures using pressured H2, where the reducible

groups are often reduced indiscriminately.29, 30 Moreover; the control experiment

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(under dark condition) at same temperature shows significantly poor conversions.

The blank experiments without metal NPs were also conducted and negligible

conversion was observed under light irradiation or in the dark. These observations

strongly suggest that the reductive coupling catalyzed effectively by Ag-Pd NPs and

that nitroaromatic conversion are achieved under light illumination.

Table2: Photocatalytic reductive coupling of aromatic nitro compounds

photocatalyst.

The yields were calculated from the product content and the Nitrobenzene conversions

measured by GC. The products were analysed by GC and mass spectrometry. Reaction time of 8 h, [a]

Reaction time 3 h, [b] Reaction time of 6 h- rest Azoxybenzene

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The photocatalytic reductive coupling of nitrobenzene achieved a high

conversion rate of nitrobenzene and a high selectivity to the target product,

azobenzene, when illuminated with light and intensity of 0.80 W/cm2. In 3 hours,

more than 98 % of nitrobenzene was reduced and more than 80 % of the product was

azobenzene. The incident light’s wavelength and intensity dependence for the

catalytic activity of the Ag−Pd alloy NPs was investigated. At 60 °C, when the light

intensity decreases from 0.80 to 0.63, 0.5, and 0.34 W/cm2, the conversion of

nitrobenzene also decreases from 98% to 83%, 66% and then to 56% respectively for

Ag-Pd/ZrO2 in 3 hours, while the selectivity of azobenzene remains almost

unchanged. The contribution of the light irradiation to the conversion efficiency was

calculated by difference between the conversion efficiency of the reaction in the dark

and under irradiation at the same temperature. The thermal contribution for the

conversion efficiency was observed in dark condition and both relative contributions

are shown in Figure 6. The greater contribution to the overall conversion rate

achieved under the higher the light intensities. For example (Ag-Pd/ZrO2), when the

light intensity is 0.34 W/cm2, 61% of the conversion results from light irradiation

with 39% attributed to the thermal effects at 60 °C ± 2 °C. When the light intensity is

0.8 W/cm2, 78% of the conversion is due to light irradiation. Similar changes were

observed for the Ag-Pd/Al2O3 as well. The conversion dependence on the light

intensity indicates that the coupling of nitrobenzene is an electron-driven chemical

reaction over the Ag-Pd alloy NPs. Moreover, it indicates that the reaction rate can

be controlled by the intensity of the irradiation. The controlled experiment was

carried out at 60 °C under dark and observed a lower conversion of nitrobenzene.

Moreover, negligible reaction was observed in a blank experiment, which conducted

using supporting powder (ZrO2 and Al2O3) under otherwise identical conditions.

These results further confirm that the coupling of nitrobenzene was driven by light

irradiation.

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Figure 6: Intensity influences on the reductive coupling of nitrobenzene using

(a) Ag:Pd/ZrO2 -3 hours; (b) Ag:Pd/Al2O3 -6 hours.

Additionally, the dependence of photocatalytic activity on incident light

wavelength also investigated. The photocatalytic activity depends on the energetic

electrons excited by light absorb by alloy NPs. Therefore, the reaction rate is

expected to improve by increase the number of electrons with sufficient energy to

initiate the reaction of the reactant molecules. Tuning the irradiation wavelength and

using the higher light intensity could increase the number of energetic electrons. The

action spectrum is a one-to-one mapping between the wavelength-dependent

photocatalytic rate and the light extinction spectrum.31, 32 This can be used for

determine whether the catalytic reaction is driven by light (a photocatalytic process)

or by heat (a thermocatalytic process). In this study, we conduct the photocatalytic

coupling of nitroaromatics over Ag−Pd alloy NPs at 60 ± 2 °C under irradiation with

different wavelengths with wavelengths of 400 ± 5, 470 ± 5, 530 ± 5, 590 ± 5, and

620 ± 5 nm. The reaction rates were converted to the apparent quantum yields

(AQYs).14, 33, 34 The apparent quantum yield (AQY%) was calculated as follow:

apparent quantum yield = [(Mlight–Mdark)/Np] × 100%, where Mlight and Mdark are the

molecules of products formed under irradiation and dark conditions respectively, Np

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is the number of photons involved in the reaction. The action spectrum of coupling of

nitrobenzene by Ag-Pd/ZrO2 (Figure 7) is compared with the light absorption

spectrum of the both Ag−Pd alloy NPs and Ag NPs.

The action spectra of the coupling of nitroaromatics don't match with the

absorption spectrum of the Ag−Pd alloy NPs/ZrO2 catalyst (Figure 7). At longer

wavelengths, the absorption spectrum contains a considerable contribution from

scattering.25, 26 However, the action spectra results indicate that the scattering has

little impact on the catalytic performance. Additionally, the AQY of the

nitrobenzene coupling follows the light absorption of Ag NPs, which show the

characteristic LSPR absorption peak in the range between 380 and 410 nm.20, 21

Generally; Ag NPs absorb visible light via LSPR and excited electrons to high

energy levels. These light-excited electrons can transfer to the surface Pd sites of the

alloy NPs and enhance the catalytic performance of the alloy NPs. The results of the

action spectrum confirm that the enhancement of the catalytic performance is mostly

owing to the LSPR absorption of Ag in the alloy NPs. Therefore, we could argue that

the Ag acts as an antenna, which harvests the visible light and enhance the catalytic

activity of alloy NP.

Figure 7: Photocatalytic action spectrum for reductive coupling of

nitrobenzene using Ag:Pd/ZrO2.

The reaction temperature is also a key parameter of a photocatalytic reaction,

and the activity of alloy NPs photocatalyst can be improved by increasing the

reaction temperature.35-37 The experiment conducted under five different reaction

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temperatures, while maintaining the light intensity in constant value. The

nitrobenzene yield increases clearly with the increasing reaction temperature (from

40 °C to 80 °C) (Figure 8). Nevertheless, the product selectivity changes with the

elevated reaction temperature. Until 70 °C temperature the main product is

azobenzene for Ag−Pd alloy NPs/ZrO2. At 60-70 °C, the selectivity of azobenzene

reaches a maximum for Ag−Pd alloy NPs supported ZrO2 & Al2O3 and then declines

(Figure 8-a, b). However, at higher temperature, the product azobenzene is not as

stable as at low temperature. Under harsh reaction conditions, such as high reaction

temperatures, high gas pressure, or strong base media, azobenzene could be rapidly

reduced to aniline according to known Haber’s mechanism.8,38 It is possible to select

the desired products by controlling the reaction temperature. The number of

conduction electrons at high energy levels increases with the reaction temperature.

These thermally excited electrons still able to gain more energy through the LSPR

effect.33 The thermal and photonic energies could be coupled by electrons of alloy

NPs to drive the chemical reactions effectively.

93

Figure 8: Conversion of nitrobenzene reductive coupling and selectivity of

azobenzene and aniline (a) using Ag:Pd/ZrO2; (b) using Ag:Pd/Al2O3 at different

temperatures.

The proposed mechanism of the photocatalytic coupling of nitrobenzene on the

Ag-Pd/ZrO2 catalyst is similar to the work reported by Zhu et al.12 The key step of

the reduction of nitrobenzene is to break the N-O bonds. The Pd sites are able to

abstract hydrogen from isopropanol, which is a hydrogen donor and form the

transient Pd–H species. The Pd–H species, facilitating the cleavage of N-O bonds

and release azobenzene as the product (Figure 9). Therefore, it is reasonable to

expect that Ag alloying with Pd could enhance the photocatalytic activity than pure

Pd or Ag alone.

Figure 9: Proposed reaction pathway of coupling of nitroaromatics.

The one of the significant properties of photocatalysts is reusability.39 The

activity of the 3 wt% Ag−Pd alloy NPs/ZrO2 catalyst was checked for five successive

rounds (Figure 10). The catalyst was used under light irradiation with each run

reaction conditions were kept identical. The results illustrate that the catalyst can be

recycled without dropping activity considerably. The product yield can be

maintained and Ag–Pd alloy NPs/ZrO2 is a reusable photocatalysts for reductive

coupling of nitrobenzene.

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Figure 10: The photocatalytic stability of 3 wt% Ag−Pd alloy NPs/ZrO2 in

five cycles at 60 °C.

Conclusions

In summary, light can efficiently drive the reductive coupling of nitrobenzene

reactions with the photocatalysts of Ag-Pd alloy NPs supported by ZrO2 at ambient

conditions and within a shorter period (3 hours). Moreover, Ag-Pd alloy NPs on

ZrO2 shows superior photocatalytic activity compared to NPs made from the pure

component metals, with the optimum activity observed for alloy NPs with a Ag:Pd

weight ratio of 1:1. The combination of the light absorbing properties of the metallic

NPs and the electronic properties of the alloys results in a superior catalytic

performance regardless the support. Nevertheless, similar trends were observed for

Ag-Pd alloy NPs on Al2O3 as well. The knowledge developed in this study may

inspire further studies in novel photocatalysts of Ag and other transition metals on

different supports for an extensive range of organic synthesis driven by sunlight, an

inexhaustible and green energy source.

Acknowledgements

We gratefully acknowledge financial support from the Australian Research

Council (ARC DP150102110). The authors are thankful to Pengfei Han for

providing ɣ-Al2O3 for the experiments. The electron microscopy work was

95

performed through a user project supported by the Central Analytical Research

Facility (CARF), Queensland University of Technology.

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Table of Contents

99

Supporting Information

Silver and Palladium Alloy Nanoparticles Catalysts:

Reductive coupling of Nitrobenzene through Light

Irradiation Sunari Peiris,a Sarina Sarina,a* Chenhui Han,a Qi Xiao,b and Huai-Yong Zhua

a. School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and Technology, Queensland University of Technology, Brisbane, QLD 4001, Australia. b. CSIRO Manufacturing, Bayview Avenue, Clayton, VIC 3168, Australia. *Corresponding author email: s.sarina@qut.edu.au

100

(a)

(b)

Figure S1. TEM image of (a) Ag-Pd(1:1)/ZrO2 catalyst; (b) Ag-Pd(1:1)/Al2O3 catalyst.

101

(a)

102

(b)

Figure S2. (a) SEM image of Ag-Pd(1:1) /ZrO2 sample and the corresponding mapping of Zr, Ag and Pd elements.; (b) EDX spectrum of of Ag-Pd(1:1) /ZrO2 sample.

103

Table S1: The calculated corresponding Au-Pd molar in photocatalysts- ZrO2 and Al2O3.

Entry Catalyst Ag (Wt%)

Pd (Wt%)

Ag:Pd ratio Calculated

Wt. Ratio Experimental

Wt. Ratio Calculated

molar Ratio

1 Ag:Pd(2:1) 2 1 2:1 1.89:1 1.97:1 2 Ag:Pd(1:1) 1.5 1.5 1:1 1:1.01 1:1.01 3 Ag:Pd(1:2) 1 2 1:2 1:1.78 1:2.03 4 Ag 3 0 1:0 1:0 1:0 5 Pd 0 3 0:1 0:1 0:1

The alloy photocatalysts with different Ag and Pd contents, pure Ag and pure Pd catalyst supported by ZrO2/ Al2O3 were also prepared in the impregnation-reduction method for reference. The corresponding Ag-Pd molar ratios were calculated and calculated Ag-Pd weight ratios were compared with an experimental weight ratio obtains via SEM- EDX spectrum.

104

Characterization of products

The products were identified using an Agilent 6980 gas chromatography (GC) coupling with an Agilent HP5973 mass spectrometer equipped with a HP-5 column. Reference mass spectra from Scifinder are provided for comparison. Nevertheless spectra may reflect different instrument/ ionization methods:

a) 4-Methoxybenzenamine- m/z for C7H9NO is 123.15

Reference spectrum of 4-Methoxybenzenamine found from SciFinder:

105

b) Azobenzene, 4,4'-dimethoxy - m/z for C14H14N2O2 is 242.2

Reference spectrum of Azobenzene, 4,4'-dimethoxy found from SciFinder:

106

c) 4-Bromobenzenamine - m/z for C6H6BrN is 170.9

Reference spectrum of 4-Bromobenzenamine found from SciFinder:

107

d) 4-Methylbenzenamine - m/z for C7H9N is 107.0

Reference spectrum of 4-Methylbenzenamine found from SciFinder:

108

e) 4-Chlorobenzenamine- m/z for C6H6ClN is 127.0

Reference spectrum of 4-Chlorobenzenamine found from SciFinder:

109

f) Azobenzene, 4,4'-dichloro- m/z for C12H8Cl2N2 is 251.1

Reference spectrum of Azobenzene, 4,4'-dichloro- found from SciFinder:

110

g) 4-Iodobenzenamine - m/z for C6H6IN is 218.9

Reference spectrum of 4-Iodobenzenamine found from SciFinder:

111

h) Aniline - m/z for C6H7N is 93.0

Reference spectrum of Aniline found from SciFinder:

112

i) Azobenzene - m/z for C12H20N2 is 182.2

Reference spectrum of Azobenzene found from SciFinder:

113

j) 4-Aminobenzonitrile - m/z for C7H6N2 is 118.1

Reference spectrum of 4-Aminobenzonitrile found from SciFinder:

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Chapter 4: Supported gold based alloy

nanoparticle photocatalysts for

organic synthesis under visible

light irradiation

4.1 INTRODUCTORY REMARKS

This chapter includes one article ready to submit to RSC Advances.

In this Article, we focused on formation of carbon- nitrogen (C-N) bonds using

ZrO2 supported Au-Pd alloy nanoparticles under visible light irradiation at lower

reaction temperatures. Preparation of amines under mild and waste free conditions,

using inexpensive and readily available reactants is still a challenging goal. Herein,

we were able to synthesis N-substituted amines from nitroaromatics and alcohols. In

this article, plasmonic metal gold was alloyed with a non-plasmonic metal,

palladium; the photocatalytic activity increased remarkably compared to both pure

Au NPs and Pd NPs. The Au-Pd alloy nanoparticle absorb visible light and the light

excited energetic electrons on the metal alloy NP surface and activates the reactant

molecules. These heterogeneous catalysts can be conveniently recycled, which is an

important factor in the development of practical and cost-effective catalytic

processes. This study provides a general guideline for the applicability of the alloy

NP photocatalysts, which prepared by alloying transition metals with gold.

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116

Reductive N-alkylation of nitrobenzene with benzyl alcohol

by Au-Pd alloy nanoparticles under light irradiation

Sunari Peiris, Sarina Sarina*, and Huai-Yong Zhu

Abstract

Gold-palladium (Au-Pd) alloy nanoparticles on ZrO2 strongly absorb visible

light and exhibit significantly high photocatalytic activity for the formation of

carbon- nitrogen (C-N) bonds by reductive N-alkylation of nitrobenzene with benzyl

alcohol. Under optimized conditions, the catalyst with 1:1.86 molar ratios Au-

Pd/ZrO2 achieved the highest photocatalytic activity and selectivity. The

photocatalytic activity of the alloy nanoparticles depends on the reaction

temperature, intensity of the incident light and metal ratio. The finding of this study

may inspire further studies on Au alloy photocatalysts and the number of organic

syntheses using Au-Pd nanoparticles catalyst driven under visible light irradiation.

Introduction

The formation of carbon- nitrogen (C-N) bonds is one of the most significant

transformations in organic synthesis chemistry, because nitrogen containing

compounds are versatile building blocks for the synthesis of polymers, dyes,

pharmaceuticals and bio-active natural compounds.1-5

For the synthesis of C-N

bonds, the most traditional procedure is the alkylation of amines with organic

halides.6-8

However, these traditional processes often proceed under high pressure

and with the use of stoichiometric acids, which make negative consequence on

environment.7, 9-11

Therefore, the development of an easily recoverable

heterogeneous photocatalyst, which can resolve the problem of the homogeneous

systems is desirable. The direct use of readily available and inexpensive nitroarenes

and alcohols as starting materials are greatly attractive for the synthesis of secondary

amines.12-16

Recently, various nitroarenes have been used as a nitrogen source in the

formation of carbon-nitrogen(C-N) bonds through the borrowing-hydrogen process

using a large excess of benzyl alcohol derivatives, in moderate to good yields.17-22

The transfer of hydrogen from alcohols to nitro compounds allows generating

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primary amines and aldehydes. The aldehyde reacts with amine to form imine and

was reduced to obtain the final product.13, 20, 22-26

However, this method using a large

excess of alcohol to provide the hydrogen, which essential for nitrobenzene

reduction.17, 27

Here, we present the one-pot synthesis of secondary amines using an

equal molar ratio of nitrobenzene and alcohol as starting materials and the hydrogen

gas (1 atm pressure) without adding any base and within 6h. Therefore, the one-pot

synthesis of N-substituted amine from nitroaromatics and alcohols is more

economical and environmentally friendly.

Experimental Section

Catalysts Preparation:

Au-Pd/ZrO2 Catalyst: Au-Pd alloy photocatalysts with different Au/Pd ratios on

ZrO2 were prepared by impregnation-reduction method. For example, For example,

1.5 wt %Au−1.5 wt % Pd/ZrO2 (3%Au−Pd(1:1)/ ZrO2) was prepared by the

following procedure: 1.0 g ZrO2 powder was dispersed in 7.6 mL of 0.01 M HAuCl4

aqueous solution and 14 mL of 0.01 M PdCl2 aqueous solution (dissolved in 0.04M

NaCl) were added while magnetically stirring. 16 mL of 0.53 M lysine was then

added into the mixture with vigorous stirring for 30 min. To this suspension, 3 mL of

0.35 M NaBH4 solution was added drop wise over 20 min. The mixture was aged for

24 h and then the solid was separated, washed with water and ethanol, and dried at

60 °C. The dried solid was used directly as a catalyst. Catalysts with other Au/Pd

ratios and pure Au and pure Pd were prepared by a similar method, but using

different quantities of HAuCl4 aqueous solution or PdCl2 aqueous solution.

Au-Pd/LDH-P Catalyst: Au-Pd alloy photocatalysts with different Au/Pd ratios on

LDH-P were prepared by impregnation-reduction method. For example, For

example, 1.5 wt %Au−1.5 wt % Pd/ LDH-P (3%Au−Pd(1:1)/ LDH-P) was prepared

by the following procedure: 1.0 g LDH-P powder was dispersed in 7.6 mL of 0.01 M

HAuCl4 aqueous solution and 14 mL of 0.01 M PdCl2 aqueous solution (dissolved in

0.04M NaCl) were added while magnetically stirring. 16 mL of 0.53 M lysine was

then added into the mixture with vigorous stirring for 30 min. To this suspension, 3

mL of 0.35 M NaBH4 solution was added drop wise over 20 min. The mixture was

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aged for 24 h and then the solid was separated, washed with water and ethanol, and

dried at 60 °C. The dried solid was used directly as a catalyst. Catalysts with other

Au/Pd ratios and pure Au and pure Pd were prepared by a similar method, but using

different quantities of HAuCl4 aqueous solution or PdCl2 aqueous solution.

LDH-P support: The LDH with a Mg: Al molar ratio of 3:1 was produced

using a sol-gel process. 28

Mg(NO3)2•6H2O (115.4 g, 0.45 mol) and Al(NO3)3•9H2O

(56.3 g, 0.15 mol) were dissolved in 600 mL of deionized water to form an acidic

aqueous solution. The alkaline solution was made by dissolving NaOH (60.0 g, 1.5

mol) and Na2CO3 (26.5 g, 0.25 mol) in 1000 mL of deionized water. Acidic and

alkaline solutions were added drop wise simultaneously into 400 mL of deionized

water at 75 °C to obtain the precipitation. The pH value was controlled to be 10. The

suspension was aged for 3 h at 85 °C under stirring. The gel suspension was filtered

and kept at 80 °C for 16 h in an autoclave. The hydrothermally treated gel was

washed with deionised water until the washings reached a pH of 7. The resulting

precipitate was dried in oven overnight at 80 °C and grounded. The calcined (at 450

°C in a flow of 100 mL min-1

dry air for 8 h) LDH (2.0 g) was dispersed in 50 mL

Na3PO4 aqueous solution (0.02 mmol/L). The suspension was stirred at room

temperature for 12 h and finally, the solid (LDH-P) was washed and dried at 110 °C

for 10 h, the resultant solid was grounded and used as the support material.

Characterization of Catalysts

Transmission electron microscopy (TEM) images were acquired on a JEOL

JEM-2100 transmission electron microscope employing an accelerating voltage of

200 kV. The element line scanning was conducted on a Bruker EDX scanner

attached to the TEM. The composition (Au and Pd contents) of samples was

determined by using the energy-dispersive X-ray spectroscopy (EDS) attachment of

an FEI Quanta 200 scanning electron microscope (SEM). X-ray diffraction (XRD)

patterns of the catalyst samples were collected using a Philips PANalytical X’pert

Pro diffractometer. CuKα radiation (λ=1.5418 Å) and a fixed power source (40 kV

and 40 mA) were used. The diffuse reflectance UV-Visible spectra (DR−UV−vis) of

the samples were examined by a Cary 5000 spectrometer. The X-ray photoelectron

119

spectroscopy (XPS) data were acquired using a Kratos Axis ULTRA X-ray

Photoelectron Spectrometer.

Photocatalytic Reactions

Reductive N-alkylation of benzyl alcohol with nitrobenzene was conducted in a

hydrogen atmosphere and solution mixture bubbled with hydrogen for 1-2 min: 0.1

m mol benzyl alcohol, 0.1 m mol nitrobenzene, 2 mL of toluene and 30 mg of the

catalyst were added in a chamber in which a 500 W Halogen lamp (from Nelson,

wavelength in the range 400−750 nm) was used as a light source and the light

intensity was usually 0.8 W/cm2

(except for the experiments investigating the impact

of the intensity). The solution mixture was stirred with a magnetic stirrer during the

reaction and illuminated with incandescent light. The temperature of the reaction

system was vigilantly regulated with an air conditioner, which attached to the

reaction chamber. The reaction setups under dark condition were maintained at the

same temperature as the corresponding reactions under light irradiation by using oil

bath placed above a magnetic stirrer to the comparison. The details of the reaction

systems are given briefly as footnotes in Table for each reaction. At given irradiation

time intervals, 1 mL aliquots were collected and then filtered through a Millipore

filter (pore size 0.45 μm) to remove the catalyst particulates. The filtrates were

analysed by gas chromatography (HP6890 Agilent Technologies) with a HP-5

column to measure the concentration change of reactants and products.

Results and Discussion

In the present study, we prepared a series of Au-Pd alloy NP catalysts

supported on zirconia (ZrO2) with various different Au: Pd ratios by the

impregnation-reduction method.29-32

The as-prepared catalysts were characterized by

several techniques to confirm the composition and morphology. TEM image (Figure

1) shows that Au-Pd alloy nanoparticles (Au-Pd NPs) are well dispersed on ZrO2

support and the mean sizes of the particles are 6 nm. The metal composition and

homogeneous distribution of the catalyst was determined by using the energy-

dispersive X-ray (EDX) spectroscopy attachment of the scanning electron

120

microscope (SEM) (Figure S1). The X-ray diffraction (XRD) characterization was

performed on catalyst samples and no reflection peaks corresponding to either

metallic Au or Pd were observed owing to the low metal content (Figure 2). This

result suggests that the detection of alloy NP signals in XRD patterns is also closely

related to the ZrO2.

Figure 1: Catalyst characterization. (a) TEM images of the Au‐Pd(1:1)/ZrO2

catalysts; (b) Particle size distribution.

Figure 2: The XRD pattern of photocatalysts.

The X-ray photoelectron spectroscopy (XPS) of the samples shown in Figure 3

confirms that gold and palladium exist in the metallic state on ZrO2 support. The

binding energies of Au 4f7/2 and Au 4f5/2 electrons are 84.1 and 87.9 eV, respectively.

Moreover, the binding energies of Pd 3d3/2 and Pd 3d5/2 electrons are 340.1 and 334.9

eV, respectively. The significant feature of the Au−Pd alloy NPs, which makes them

121

useful as a photocatalysis, is that they intensely absorb visible light mainly through

the localized surface plasmon resonance (LSPR) effect of AuNPs. UV–Vis spectra

of these samples in Figure 4 indicate that the supported Au-Pd alloy NPs strongly

absorb visible light irradiation. The absorption peak in the visible light range (at 520

nm) observed for the Au NPs on ZrO2 supports is attributed to the LSPR absorption

of Au NPs.33, 34

Evidently, the visible light absorption by Au NPs is a prerequisite for

the photocatalytic activity. In contrast, zirconia exhibits a weak absorption band from

250 to 400 nm owing to the large band gap.35

The ZrO2 enables the uniform

distribution of Au-Pd alloy NPs on the support surface, and the readily recycling of

the catalysts after reaction.

Figure 3: X‐ray photoelectron spectra (XPS) binding energy of (a) Au 4f7/2 and Ag

4f5/2 for Au‐Pd NPs on ZrO2; (b) Pd 3d5/2 and Pd 3d3/2 for Au‐Pd NPs on ZrO2.

Figure 4: UV‐Visible diffuse reflectance spectra of the supported Ag‐Pd (1:1)

alloy NPs catalysts and their comparison with pure AuNPs/ZrO2 and PdNPs/ZrO2.

122

In literature, it has been reported that amine compounds could be synthesized

from the corresponding nitroaromatic compounds and benzylalcohol through various

supported metal catalysts comprising Au, Pd, Cr and Ag at higher temperatures,

pressures, strong base and/or longer reaction time (Table 1).15, 16, 27, 36

In the present

study, Au-Pd alloy NPs on ZrO2 could be used as an efficient photocatalyst for the

formation of C-N bonds from reductive N-alkylation of nitrobenzene with benzyl

alcohol (Entry 5, Table 1)

Table 1: Comparison of the reaction conditions and catalytic activity of various

heterogeneous catalysts reported in the literatures for the reductive N-alkylation of

nitrobenzene with benzyl alcohol.

OH

NO2 HN

+

Entry Catalyst Reaction Conditions Yield (%) Ref

1 Au/Ag–Mo nano-

rods

150 ◦C, 1 atm Ar, glycerol, 24h,

K2CO3

91 27

2 Au/Fe2O3 160 ◦C, 1 atm Ar, 8h, K2CO3 87

15

3 Ag/Al2O3 155 ◦C, 2 atm H2, 24h, K2CO3 93

16

4 Cu–Cr/ɣ-Al2O3 200 ◦C, 30 atm H2, 24h, K2CO3 90

36

5 Au-Pd/ZrO2

Present Study

80 ◦C, 1 atm H2, 6h, visible light 83

We prepared a series of supported Au-Pd alloy NPs catalysts with various

metal weight ratios. The 3% Au:Pd (1:1 Wt= 1:1.86 mol) alloy NPs catalysts was

found to be the most effective for the reductive N-alkylation reaction with excellent

yield and selectivity under mild reaction conditions (Table 2). Based on the current

experimental conditions, it is clear that the 3% Au-Pd(1:1) alloy NPs on ZrO2

photocatalyst shows decent conversion as well as product yield. The possible

explanation for these observations is that the charge heterogeneity at the alloy NPs’

surface results in the improved catalytic activity of the alloy structure. The alloying

affects the surface electronic properties of the NPs and the catalytic activity of the

alloy NP is significantly improved compared to mono metals. In terms of the

selectivity, all dark reactions were found to show a very poor amine yield during the

given reaction time. For example, the reductive N-alkylation of nitrobenzene with

benzyl alcohol could be proceeded efficiently at 80 ◦C in atmospheric H2, giving

>80 % yield (Entry 4, Table 2). Compared with those reported process for the

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catalytic N-alkylation of nitrobenzene with benzyl alcohol usually conducted under

harsh reaction conditions, this exhibits apparent advantage in the viewpoint from

green chemical synthesis. Blank experiments under otherwise identical reaction

conditions, but without metal alloy NPs and without any catalyst were also

conducted and were catalytically inactive in this transformation (Entry 8 & 9, Table

2).

Table 2: Photocatalytic activity test with different Au:Pd weight ratios the reductive

N-alkylation.

OH

NO2 HN

O

NH2

(c) (d) (e)

+ + +

(a) (b)

Entry Catalyst Light Reaction (Dark)

(Weight ratio) Conversion (a) (%) Yield (%)

(c) (d) (e)

1 Au/ZrO2 26 (20) / 64 (39) 36 (61)

2 3Au-Pd/ZrO2 62 (48) 6 (3) 52 (15) 42 (82)

3 2Au-Pd/ZrO2 77 (54) 65 (0) 24 (0) 11 (100)

4 Au-Pd/ZrO2 100 (64) 83(55) 8 (8) 9 (37)

5 Au-2Pd/ZrO2 85 (55) 67 (56) 20 (4) 13 (36)

6 Au-3Pd/ZrO2 68 (46) 56 (36) 25 (11) 19 (53)

7 Pd/ZrO2 49 (42) 32 (14) 35 (15) 33 (71)

8 ZrO2 6 (0) / 100 (0) /

9 No catalyst 6 (0) / 100 (0) /

Reaction conditions: photocatalyst 30 mg, Nitrobenzene 0.1 mmol,

Benzylalcohol 0.1 mmol, solvent-toluene 2 mL, 1 atm H2, reaction temperature 80

◦C, light intensity- 0.7 W/cm

2, reaction time 6 h. The conversions and yield were

calculated from the product formed and the reactant converted based on the

benzylalcohol conversion measured by gas chromatography.

The direct synthesis of amine from benzylalcohol and nitrobenzene in the

presence of different catalyst under irradiation with light was used for the

optimization of reaction conditions (Table 3). The influence of several critical

reaction conditions, such as solvents and reaction atmosphere, have been tested with

124

Au-Pd alloy NPs on ZrO2 and HT-PO43-

(LDH-P) catalysts. The Au-Pd supported on

ZrO2 has the highest activity and selectivity towards coupling reaction than on LDH-

P. The two most commonly used solvents were examined for the reaction with other

conditions were maintained unchanged. It is obvious that the Au-Pd alloy/ZrO2

catalyst exhibited the best performance when the toluene is solvent, under light

irradiation (entry 5). We carried out the reaction in argon, oxygen, hydrogen and air

atmospheres and found that hydrogen atmosphere promotes the reaction. In the

oxygen and air atmosphere, the prominent product was benzaldehyde (Entry 3, 6, 9

& 12 ). Notably, the reaction did show excellent activity, even without base additive,

which increases the green synthesis aspects.

Table 3: Optimization of reaction conditions for amine synthesis.

OH

NO2 HN

O

NH2

(c) (d) (e)

+ + +

(a) (b)

Entry Catalyst Solvent Atmosphere Conv.

[%] (a)

Yield. [%]

Au-Pd alloy/ZrO2

(c) (d) (e)

1 BTF Ar 100 20 80 0 2 BTF H2 52 9 / 42

3 BTF O2 100 / 100 / 4 Tol Ar 92 14 78 / 5 Tol H2 100 83 8 9

6 Tol O2 100 / 100 /

7

Au-Pd alloy/LDH-P

BTF Ar 94 10 84 / 8 BTF H2 78 47 12 41

9 BTF O2 100 / 100 / 10 Tol Ar 100 13 87 / 11 Tol H2 100 51 16 33

12 Tol O2 100 / 100 /

Reaction conditions: photocatalyst 30 mg, Nitrobenzene 0.1 mmol, Benzylalcohol

0.1 mmol, solvent 2 mL (Tol-toluene, BTF-Benzotrifluoride), 1 atm atmosphere,

reaction temperature 80 ◦C, light intensity- 0.7 W/cm

2, reaction time 6 h. The

conversions and yield were calculated from the product formed and the reactant

converted based on the benzylalcohol conversion measured by gas chromatography.

The influence of the irradiation light intensity on the reductive N-alkylation

was investigated (Figure 5). We have experimentally verified that higher light

125

intensities exhibit more efficient performance. Moreover, the results clearly show a

linear dependence, which further confirms the photocatalytic activity of Au-Pd alloy

metal NPs. When the irradiance was increased from 0.34 to 0.80 W.cm−2

with other

conditions unchanged, the conversion rate of benzyl alcohol increased from 70% to

100%, respectively.

Figure 5: Intensity influences on the reductive N-alkylation of nitrobenzene

and benzyl alcohol on Au:Pd(1:1)/ZrO2 ‐6 hours

We studied the evolution of the product during the time course of the reductive

N-alkylation of nitrobenzene and benzyl alcohol using Au-Pd/ZrO2 catalyst

(Figure 6). It can be seen that the conversion of the reaction increased progressively

over time, and the amine is the main product during the reaction. The selectivity and

conversion both reached to maximum from the six hours. Then, keeping other

reaction conditions identical, the longer reaction times (24h) were investigated.

However, the product yield doesn’t increase with the reaction times as expected.

126

Figure 6: Time-conversion plot for reductive N-alkylation using Au-Pd alloy/ZrO2.

The photocatalytic activities of the Au-Pd alloy NPs/ZrO2 catalysts was

tested with different reaction temperatures and found that the activity increases with

increasing reaction temperature. The experiment conducted under seven different

reaction temperatures, while maintaining the other reaction conditions constant. As

shown in Figure 7, the catalysts exhibit excellent conversion with the increasing

temperature. The product selectivity could be controlled by the reaction temperature

and dark/light conditions. Secondary amine was obtained as the main product under

light irradiation for the all the temperatures. Further, the selectivity for the desired

amine reaches to a maximum at 80 ◦C and then decreases. Based on the current

experimental conditions, it is clear that the Au-Pd alloy NPs on ZrO2 photocatalyst

shows decent conversion as well as product yield. In terms of the selectivity, all dark

reactions were found to show a poor amine yield during the given reaction time.

127

Figure 7. Conversion of reductive N-alkylation and selectivity of secondary amine,

aniline and benzaldehyde. (a) Under light irradiation (b) Under dark condition using

3% Au:Pd(1:1)/ZrO2 at different temperatures.

We provide a tentative mechanism for amine synthesis method starting from

benzylalcohol and nitrobenzene on ZrO2 supported Au-Pd alloy NPs photocatalysts.

Figure 8 and S2 were on the basis of our experimental observation and the literature.

26, 29, 30 The Au-Pd/ZrO2 catalyst was shown efficient towards production of amine

without any addition of base. This process involves mainly three steps, which

comprises the reduction of the nitrobenzene to aniline (Figure S2 (a)), oxidation of

the alcohol to aldehyde(Figure S2 (b)), and condensation of the aldehyde and the

aniline to form the corresponding amine by reducing imine(Figure S2 (c)).22, 26

Sarina et al. found that visible light irradiation of Au-Pd alloy NPs could

enhance the catalytic activity for oxidant-free dehydrogenation of aromatic alcohols

to the corresponding aldehydes at even ambient temperatures.29, 30

Firstly, the

abstraction of α-H atoms from the alcohol molecules take place from the −CH2−

group and alloy-H species is formed. Then the dehydrogenated species, undergo a

C−H bond cleavage, yielding aldehyde as the product (Figure 8-I).37, 38

In contrast,

128

the reduction of the nitrobenzene to aniline is mainly facilitated by palladium.22

Generally, Pd known as an classic hydrogen storage metal and able to store hydrogen

under mild conditions.39, 40

The most widely accepted mechanism proposes two

possible reaction pathways (Figure S2). One is the direct reduction of nitrobenzene,

which involves a sequential hydrogenation/dehydration process via nitroso and

hydroxy intermediates.41, 42

The second route involves a condensation reaction

between the nitroso and hydroxy products leading to the azoxy compound, which is

then hydrogenated/dehydrated to afford the azo compound. 43

The latter compound is

subsequently hydrogenated to the hydrazo, which finally generates the amine (Figure

8-II). Moreover, under harsh reaction conditions, such as high reaction temperatures

azobenzene rapidly reduced to aniline according to known Haber’s mechanism.41

None of the azobenzene or azoxy compounds were detected during the reaction.

Finally, benzaldehyde, condenses with the amine produced, generating an

imine and tends to follow the condensation route (Figure 8-III & S2-(c)).22

In this

sense, we speculated that the imine further reduced by abstracting H to produce

corresponding amine (Figure 8-IV) and ZrO2 supported Au-Pd alloy NPs catalyst

promotes the reductive N-alkylation of nitrobenzene and benzyl alcohol under light

irradiation.

Figure 8: Proposed reaction pathway of reductive N-alkylation of nitrobenzene

and benzyl alcohol on Au:Pd (1:1)/ZrO2.

129

For practical applications of heterogeneous catalyst, the recyclability of the

catalyst is a crucial factor.44

3% Au-Pd/ZrO2 catalyst was used for five consecutive

runs for reductive N-alkylation of nitrobenzene with benzyl alcohol to investigate the

reusability of the photocatalyst (Figure 9). The catalyst was separated by

centrifugation and washed thoroughly with ethanol three times. The dried catalyst

was directly used for subsequent reactions, while keeping the other reaction

conditions identical. The results confirmed the reusability of the Au-Pd alloy

NPs/ZrO2 catalyst without significant activity loss.

Figure 9: The photocatalytic stability of 3 wt% Au−Pd alloy NPs/ZrO2 in five cycles

at 80 °C.

Conclusions

In conclusion, we demonstrated that supported Au-Pd alloy NPs can

efficiently drive reductive N-alkylation of nitrobenzene with benzyl alcohol,

achieving excellent activity and yields under mild reaction conditions. Moreover,

decent yields of N-alkyl amines were achieved by 1:1 molar ratio of benzyl alcohol

and nitrobenzene. Further, Au-Pd alloy NPs/ ZrO2 photocatalyst can be efficiently

recycled after consecutive five reaction rounds without significantly losing activity.

The knowledge learnt in this study may inspire further studies on a wide range of

organic syntheses using supported alloy NP photocatalysts.

130

Acknowledgements

We gratefully acknowledge financial support from the Australian Research Council

(ARC DP150102110). The electron microscopy work was performed through a user

project supported by the Central Analytical Research Facility (CARF), Queensland

University of Technology.

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133

Supporting Information

Reductive N-alkylation of nitrobenzene with benzyl

alcohol by Au-Pd alloy nanoparticles under light

irradiation

Sunari Peiris,

a Sarina Sarina,

a* and Huai-Yong Zhu

a

a. School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and

Technology, Queensland University of Technology, Brisbane, QLD 4001, Australia.

*Corresponding author email: s.sarina@qut.edu.au

134

Figure S1. (a) SEM image of Au-Pd(1:1)/ZrO2 sample and the corresponding

mapping of Zr, O, Au and Pd elements.; (b) EDX spectrum of of Au-Pd(1:1) /ZrO2

sample.

135

Figure S2. Proposed reaction mechanism for reductive N-alkylation of nitrobenzene

with benzyl alcohol. a) two pathways in the reduction of nitrobenzene; b) alcohol

oxidation, and c) amine/aldehyde condensation product. 1

Reference

1. S. Sabater, J. A. Mata and E. Peris, Chem. - Asian J., 2012, 18, 6380.

136

Characterization of products

The products were identified using an Agilent 6980 gas chromatography (GC)

coupling with an Agilent HP5973 mass spectrometer equipped with a HP-5 column.

Reference mass spectra from Scifinder are provided for comparison. Nevertheless

spectra may reflect different instrument/ ionization methods:

a) Benzenemethanamine, N-phenyl- - m/z for C13H13N is 183

Reference spectrum of Benzenemethanamine, N-phenyl- - found from SciFinder:

137

b) Benzaldehyde - m/z for C7H6O is 106

Reference spectrum of Benzaldehyde- found from SciFinder:

138

c) Aniline- m/z for C6H7N is 93

Reference spectrum of Aniline- found from SciFinder:

139

Chapter 5: Conclusions & Future work

Conclusions

This work has contributed to the knowledge of novel metal NP photocatalysts

(non-plasmonic NPs and their alloy NPs) on various organic synthesis reactions

under visible light irradiation. Three types of meal NP photocatalysts have been

developed using Pd metal and used for coupling reactions under visible light

irradiation. This thesis includes the photocatalytic enhancement of the alloy NPs and

non-plasmonic Pd NPs. From the results of this study, the following conclusions can

be drawn:

In chapter 2, it was revealed that irradiation with light can significantly

enhance the intrinsic catalytic performance of non-plasmonic Pd transition metal NPs

at ambient temperatures and atmospheric pressure for several types of cross-coupling

and homo-coupling reactions. Pd metal NPs functions as together a visible light

harvester and a provider of catalytic sites. Generally, Pd metal NPs absorb the visible

light via interband electronic transitions. The energetic electrons excited by light

irradiation drive the photocatalytic reaction. The rate of the catalysed reaction

depends on the concentration and energy of the excited electrons and it can be

improved by tuning the incident light intensity and wavelength. The highest yield

was obtained (80-98%) for the 3% PdNPs on ZrO2 for all the four reactions. This

study provides insight into catalyst design for the activation of C-X bond and shown that

plasmonic excitation is not the merely mechanism involved in metal NPs under light

irradiation. This study broadens the application of non-plasmonic metals as

photocatalysts under visible light irradiation and the possibility of green approach for

the fine organic chemical synthesis.

In Chapter 3, we found an effective approach to expand the application of

AgNP as a photocatalysts by incorporating transition metals such as Pd. Generally,

palladium is well known to be catalytically active for many important organic

reactions. Therefore, the coupling of light absorption of Ag NPs (via LSPR) and

catalytic property of Pd in alloy structures can drive various chemical reactions.

Herein, the reductive coupling of nitrobenzene with Ag‐Pd alloy NPs/ZrO2

140

photocatalyst under visible light at ambient conditions was studied. Ag-Pd alloy NPs

on ZrO2 support is favourable for reduction of nitroaromatics (azobenzene yield of

~80 % in 3 hours) than Ag-Pd alloy NPs/Al2O3. Further, study reveals that the

highest yield of target products achieved when the alloys NPs have the Ag:Pd molar

ratio of 1:1.01. Alloy NPs with other Ag: Pd molar ratios exhibited much lesser

activity, either under light irradiation or in the dark. The catalytic activity of the

photocatalysts can be tuned through metal ratio, irradiation intensity & wavelength

and reaction temperature. The prospect of visible light irradiation driving chemical

synthesis has potential to deliver greener, controlled industrial processes especially

for temperature sensitive synthesis.

In Chapter 4, the photocatalytic application of Au-Pd NPs was extended to a

novel reaction. We have successfully fulfilled reductive N-alkylation of nitrobenzene

with benzyl alcohol by Au-Pd alloy nanoparticles under light irradiation at ambient

reaction conditions with Au-Pd alloy NPs/ ZrO2. These alloy NPs exhibit superior

catalytic performance when compared to pure non-plasmonic and plasmonic metal

NP photocatalysts when exposed to visible light under moderate reaction conditions.

Under optimized conditions, the catalyst with 1:1.86 molar ratios Au-Pd/ZrO2

achieved the highest photocatalytic activity and selectivity (>80 % yield) for

reductive N-alkylation of nitrobenzene with benzyl alcohol. The reaction rate

depends on the number of light-excited electrons and the number of reactant

molecules on the catalyst surface. The strong affinity of palladium towards organic

molecules increases the reactant molecules on the metal NPs and facilitates the light-

excited electron transfer. Moreover, the light absorption of Au-Pd alloy plays an

important role and by tuning light intensity and reaction temperature we can obtain

optimized the reaction activity. In addition, the reaction did show excellent activity,

even without base additive, which increases the green synthesis aspects. The

knowledge learnt in this study may inspire further studies on a wide range of organic

synthesis using supported Au-Pd alloy NP photocatalysts.

In general, work presented in this thesis will help guide successful design

photocatalysts using number of supported metal NPs for fine organic chemical synthesis.

141

Future Work

The chemical industry, which turns raw materials such as petroleum by-

products, minerals and farm products into valuable chemicals that are the ingredients

of life's essential objects, plays a vital role in our everyday life. According to US

Energy information Administration (EIA), the largest consumer of delivered energy

is the basic chemicals industry, which in 2012 accounted for about 14-19% of total

industrial energy consumption and expected rises in 2040. Therefore, it is practical to

discover a clean, renewable energy source to synthetic chemicals and sunlight stands

out as the most promising choice.

Direct metal photocatalysis (plasmonic, non-plasmonic and alloy) is a rapid

emerging research field and recently it achieved significant advancements.

Nevertheless, there is more space for further improvement on photocatalytic

performance and mechanism. Based on the outcomes of this thesis as well as

reported work, Future work can be proposed from the following aspects:

1. In chapter 2, we discussed supported non-plasmonic Pd metal NP photocatalyst

for coupling reactions under light irradiation. Based on current research, it is

beneficial to conduct the further studies on different other potential non-

plasmonic/ transition metal NPs, such as Ni, Ir, Ru and Co for fine chemical

synthesis under light irradiation. Many experiments on metal photocatalytic

reactions have examined the plasmonic properties of gold, silver and their alloy

combinations. If we could use supported transition metal NPs as photocatalysts,

such a system could attract manufacturing industries and lead to controlled,

simplified, and greener chemical synthesis. Furthermore, it is useful to study

and understand the underlying light absorption properties, mechanisms and

chemical stabilities for photocatalytic reactions. Additionally, practical

implementation of non-plasmonic/ transition metal NPs enhanced chemical

reactions will require the use of inexpensive compared to noble metals.

2. The findings obtained from chapter 3 provide useful guidelines for designing

efficient photocatalysts form Ag metal NPs. Alloying transition metals,

improves the utilisation of Ag for photocatalytic chemical synthesis in a cost-

effective way. The study of Ag-Pd alloy NPs photocatalysts could extend to

further research fields from two aspects: The first is to extend the application of

Ag-Pd alloy NPs to other chemical synthesis, such as cross-coupling reactions,

142

esterification reactions, etc. This will widen the use of Ag-Pd alloy NPs to

more applications in organic synthesis. The second is to develop new alloy NPs

photocatalysts using Ag plasmonic metal with a new photocatalyst structure for

chemical synthesis.

3. The photocatalysts made from plasmonic, non-plasmonic metals and their

alloys were illustrated a promising improvement under visible light irradiation.

There is a potential to extend the support scope of the catalyst by introducing

other materials such as conducting metal nitrides (TiN and ZrN), since they

exhibit metallic properties at visible frequencies. Furthermore, metal nitrides

stabilize oxidisable metals NPs (Eg: Cu NPs) and this widens the range of

metals that can be used in catalyst preparations. I have used ZrN as a support

for different metal NPs such as Pd and Ag-Pd alloy NPs and it exhibited decent

activity in the cross-coupling and nitrobenzene coupling reactions under visible

light irradiation. This work is underway and expected to be published in future.

4. The localised surface plasmon resonance (LSPR) wavelength of a plasmonic

nanostructure can be tuned by changing the particle geometry such as

composition, size, shape, etc. Metal NPs that can absorb the entire solar

spectrum could be designed by systematically regulating those NP’s

parameters. There have been a number of reports on the particle composition

tuning to activate various reactant molecules; however, there is limited

knowledge about the particle shape effect on the photocatalytic synthesis of

fine chemicals. Therefore, it is important to study the metal NPs shape effects

in relation to the shift of their plasmon excitation band and the ability of

activating the reactant molecules. The findings might lead to develop a

connection between reactant electronic structure and photocatalysts structure,

which allows regulating the product selectivity.