PHOTO CATALYTIC FIXATION OF DINITROGEN

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PHOTO CATALYTIC FIXATION OF DINITROGEN

Ph.D. Seminar I

G. Magesh

9-5-06

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Contents

Importance of fixation of dinitrogen

Properties of dinitrogen

Various methods for fixation of dinitrogen

Shortcomings in available methods

Merits of photo catalytic fixation of dinitrogen

Fundamentals of photo catalysis

Challenges in photo catalytic route

Ways of overcoming them

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Nitrogen - necessary for functioning of biomolecules and plant growth

Important component of fertilizers and medicines

Present in dyes, explosives and resins

Ammonia - starting material for nitrogen containing chemicals

Importance of fixation of dinitrogen

Usage of ammonia in various industries

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Nitrogen cycle

Various processes involved in nitrogen cycleEncyclopaedia Britannica, Encyclopaedia Britannica (1998)

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Haber process

Fixed nitrogen by bacteria and algae

Chile salt petre (Sodium nitrate)

Destructive distillation of decayed vegetable and animal matter

Reduction of nitrous acid and nitrites with nascent hydrogen

Decomposition of ammonium salts by alkaline hydroxides or quicklime

Mg3N2 + 6 H2O 3 Mg(OH)2 + 2 NH3

Sources of fixed nitrogen

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Fixed nitrogen before and after Haber process

Fixed Nitrogen Production (1000 tons)1913 1934

World 843.5 1972.0Chile 476.7 (56.5%) 141.8 (7.2%)

Germany 131.6 462.5Great Britain 99.5 175.0United States 39.5 256.7

Norway 22.0 65.5France 18.9 187.6Canada 12.7 41.1Belgium 11.0 109.8

Italy 6.3 98.6Japan 3.9 208.0Russia 3.2 45.0

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Properties of dinitrogen which makes it inert

Dinitrogen - two N atoms connected by triple bond

Breaking the NN bond is difficult - high dissociation energy of 942 kJ mol-1

Breaking first bond requires 540 kJ mol-1

Very weak base – no interaction with even strong acids

Non-polar

Initial hydrogenation is highly endothermic for N2

N2 + H2 N2H2 H = 213.5 kJ mol-1

2 C + H2 C2H2 H = -175.8 kJ mol-1

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Gas Property

Nitrogen Carbon Oxygen Argon

Ionization potential (eV) 14.3 11.256 13.614 15.755

Electron affinity (eV) 0.073 1.595 1.461 0

Solubility in water (mole/cm3) 0.083 Insoluble 0.153 0.140

Other important properties

High ionization potential and low electron affinity - difficult to reduce and oxidize

Solubility very less - reactions in solution phase - difficult

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LUMO

HOMO

22.9 eV

Activation of dinitrogen

Very difficult to activate dinitrogen using light, heat and potential

HOMO very low w.r.to e- acceptors

LUMO very high w.r.to e- donors

Molecular orbitals diagram of N2 molecule

T.A.Bazhenova and A.E.Shilov, Coord. Chem. Rev. 144 (1995) 69-145

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Redox potential dependence on the number of electrons transferred

Initial two electron transfer requires higher potential

NH3 formation - six electron process - less probable

Stepwise redox potentials

Chatt J, Camara L M P, Richards R L, New Trends in the Chemistry of Nitrogen Fixation, Academic Press, (1980)

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Thermodynamics of fixation of N2 to ammonia

N2 + 3H2 2NH3 H= -36 kJ mol-1

Change in entropy, S = - ve

II law of thermodynamics - Natural processes tend to increase the entropy

Formation of ammonia by this route cannot be a natural process

Spontaneous reaction G = – ve

G negative at very low temperatures

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Available methods of fixing dinitrogenHaber process

N2 + 3H2 2NH3400°C, 200 atm

Fe based catalyst

Various steps in Haber process

Water gas shift reaction

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Limitations with the Haber process

Forward reaction - reduction in number of molecules

Le Chatelier principle - high pressure – forward reaction

Not desired in industries - accidents and increased cost

Forward reaction - exothermic

Temperature must be minimum - Le Chatelier principle

To achieve high rates in industries - temperature at 400°C

Conversion of 15%

Hydrogen Obtained from fossil fuels – a limited resource

Production requires major part of plant and cost

Releases green house gases like CO2 and CO

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Biological fixation of dinitrogen

Enzyme nitrogenase

Present in soil bacteria, root nodules and algae

Two decades of research - mechanism not established

Enzyme contains Mo and Fe

Proposed mechanism - complexation of N2 to metal ions

Reduces bond strength - breaking 1st bond easier

Limitations with biological route: Nitrogenase - sensitive to O2 – requires O2 free environment

Sensitive to environmental conditions - temperature, pH

Cannot be used for large scale N2 fixation

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Fixation of dinitrogen by metal-nitrogen complexes

Fe, Ti, Zr, Mo - high affinity for N2

Electron rich ligands – TMS, phosphine

Perturbing N2 – donates e- to LUMO of N2

Structure of

[(TMS2N)2Ti]2-(N2)2-

complex

Compound N-N bond length(Å

)N2 gas 1.0975

H2NNH2 1.460

[(TMS2N)2Ti]2-(N2)2- 1.379

Limitation: N2 evolution during reduction

Fryzeuk M D, Johnson S A, Coord. Chem. Rev., 200 (2000) 379

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Haber process Dissociative adsorption of N2 – High temperature and pressure

Metal complex based reduction

Binding N2 – Perturb e- acceptor orbital (wave function)

e- donation LUMO of N2

Limited success

Look for

Perturb orbital (wave function) of e- donor and acceptor

e- donation to LUMO – N2 activation

Very strong N2 adsorption

Hydrogen addition – without interruption

Alternatives

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Merits of photo catalytic fixation of dinitrogen

Utilizes light and efforts are on to use sunlight - a renewable source

H2 for reduction obtained from water - a widely available source

No pollution associated with the process

Process of photo catalysis is well understood

Carried out at atmospheric pressure and room temperatures

Methods to perturb catalyst orbitals – transfer e- to LUMO

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Photo catalysis - reaction assisted by photons in the presence of a catalyst

In photo catalysis - simultaneous oxidation and reduction

Light excites electrons from valence to conduction band - electrons and

holes

Photo catalysis

Light induced excitation processes in a photo catalyst

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Metal• No band gap

• Only reduction or oxidation – band position

Semiconductor • Optimum band gap

• UV or Visible light

Insulator• High band gap

• Requires light - higher energy than UV light

Choice of materials as photo catalystChoices – Metals, semiconductors, insulators

Catalyst - absorb light in UV or visible region - easily available

Energy

Band gap of available materials

CBCB

CB

VB

VB

VB

Metal

Semiconductor

Insulator

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For reduction

Conduction band potential - more negative than potential of reduction reaction

For oxidation

Valence band potential - more positive than potential of oxidation reaction

OR Type – Oxidation and Reduction

R Type – Reduction

O Type – Oxidation

X type - None

Energy

-ve

+ve

Band positions of various types of semiconductors

Potential

Reduction (A / A-•)

Oxidation (D/D+•)

Types of semiconductors

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N2/NH3 = + 0.059 eV

H+/H2 = 0.000 eV

Conduction band potential - more negative than above potentials

H2O/O2 = 1.229 eV

Valence band potential - more positive than above potential

Very strong N2 adsorption

No photocorrosion

Good light absorption

Chemically inert

Requirements of photo catalyst for fixation of N2

Band positions of semiconductors w.r.to reactions

eVN2/NH3

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Photocorrosion

Oxidation potentials of catalysts w.r.to band positions

CdS, ZnS, ZnO undergo

photocorrosion

Activity decrease as the time increases

Catalyst gets oxidised

Oxidation potential of catalyst – More

-ve than desired oxidation reaction

potential

“S” deposition on catalyst - reduce

light absorption

h+ = hole

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Doping cations and anions – altering band positions

Increase in ionic character of M-X bond - band gap decreases and vice versa

% Ionic Character = ( 1 - exp [- (XM - XX)2 / 4] ) x 100 X- electronegativity

Semiconductor M-X Percentage ionic

characterTiO2

SrTiO3

Fe2O3

ZnOWO3

ZnS CdS

CdSe

Ti-OTi-O-Sr

Fe-OZn-OW-OZn-S Cd-S

Cd-Se

59.568.547.355.557.518.0 17.616.5

Selection criterion for dopant ions in semiconductor

Viswanathan B, Bull. Catal. Soc. India, 2 (2003) 71

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Photo catalytic fixation of dinitrogen

Schrauzer G N and Guth T D, J. Am. Chem. Soc., 99 (1977) 7189

First reported - Schrauzer and Guth in 1977 with moist TiO2 using UV light

Transfer of e- from CB to N2 directly or indirectly

Potential requirement - N2 reduction and photo-splitting of water - similar

Activation barrier in N2 reduction is high

Reduction of one mole of N2

N2 + 6H+ + 6e- 2 NH3

3H2O + 6h+ 3/2 O2 + 6H+ (requires 6 electrons)

Photo-splitting of water

2H+ + 2e- H2

H2O + 2h+ 2H+ + 1/2 O2 (requires 2 electrons)

h+ = hole

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Problems associated with photo catalytic fixation of N2

Oxidation of NH3 formed to nitrites and nitrates

Recombination of excited electrons

Simultaneous H2 evolution leading to its lesser availability

Less –ve conduction band potential of available catalysts

Oxidation reactions by the holes

Lesser adsorption of N2 on catalyst surface

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Fixation of N2 by iron based catalysts

Fixation of N2 by iron –TiO2 based catalysts - reported in 1977

Compound responsible - not established

Fe2Ti2O7 responsible

Has a bandgap of 2 eV

Fe2Ti2O7 Conduction band at –0.4 eV – compared to TiO2 (–0.2 eV) – high reduction potential

Valence band at +1.6 eV

CB (Fe2Ti2O7)

VB (Fe2Ti2O7)

N2/NH3

Band positions of Fe2Ti2O7

eV

1.6

Rusina O et al, Chem. Eur. J., 9 (2) (2003) 561

CB (TiO2)

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Fe2Ti2O7 exhibits more activity - presence of ethanol

Exhibits photocurrent doubling in presence of ethanol

Following mechanism explains above two observations

Mechanism

SC + h SC (h+, e-)

SC (h+, e-) + H2O SC (h+) + Had + OH-

SC (h+) + CH3CH2OH SC + CH3HC•OH + H+

SC + CH3HC•OH SC (e-) + CH3CHO + H+

SC (e-) + H2O SC + Had + OH-

N2 + Had N2H2 or NH3

Photocurrent doubling

h+ = hole

SC = Semiconductor

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Effect of noble metal dispersion

Recombination of electrons and holes - reduces efficiency

Solution - dispersing noble metals on TiO2 surface

Noble metals - high electron affinity - traps excited electrons immediately

Ranjit K T et al, J. Photochem. Photobiol. A: Chem., 96 (1996) 181

Metal Electron affinity

(eV)Ru 1.050Rh 1.136Pd 0.557Pt 2.127Fe 0.163Ti 0.079

Trapping of electrons by noble metals

2H+ + 2e- 2Hade-

N2 + 6Had 2NH3

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Effect of noble metal dispersion

Another advantage - reduces H2 evolution

Reduced H+ should be as Had – not evolved as H2

High H2 evolution – Low N2 reduction

Noble metals - promote adsorption of hydrogen on surface

Reduction order: Ru > Rh > Pd > Pt

H2 evolution overpotential and M-H bond strength follows same order

Higher loading of metal - lesser activity than TiO2 - hindrance to light absorption

Yie

ld o

f am

mon

ia (µ

mol

h-1)

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Fixation of N2 on TiOx- poly-3-methyl thiophene(P3MeT) composite

Drawback - Oxidation of ammonia to nitrites and nitrates

Convert to its salts immediately

A TiOx-conducting polymer doped with ClO4- used

NH3 formed reacts with ClO4- to form NH4ClO4 crystals

N2 reduction and conversion to NH4ClO4

SEM image of NH4ClO4 crystals on polymer surface

Hoshino K, Chem. Eur. J., 7 (13) (2001) 2727

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Less negative conduction band (CB) potential – Lower rate of reduction

At TiOx-polymer interface - alteration of bandposition - CB at –1.1 eV

CB TiO2 (-0.2 eV)

Increases reduction rate at interface

Polyfuran and polycarbazole - active

Reactivity order:

Carbazole > Furan > ThiopheneBand position change at TiO2-polymer

interface

More negative band position

Tomohisa O et al, J. Photopolym. Sci. Technol., 17 (1) (2004) 143

eV

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Role of hole scavengers in photo catalytic reduction

Sucrose, acetic acid, salicylic acid, formic acid, methanol and ethanol

- investigated with TiO2

No improvement for sucrose, acetic acid and salicylic acid

Improvement order: formic acid > methanol > ethanol

Tan T et al, J. Photochem. Photobiol. A: Chem., 159 (2003) 273

Holes in valence band:

Increases recombination

Involve in oxidation of NH3

Necessary to quench the holes formed

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• Supply electrons to conduction band

• Capable of reducing reactant by themselves

Redox potentials of reaction species

Reduction potential of the radical species

Formic acid, methanol and ethanol form

reducing radicals

HCOO- + h+ •COO- + H+

RCH2OH + h+ R•CHOH + H+

R•CHOH + SC RCHO + SC (e-) + H+ N2/NH3

Potential (eV) vs NHE

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Solvent effects on photo catalytic reduction

Effect of various alcohols as solvents on photo catalytic reduction

Activity order

Methanol > Ethanol > 1-propanol > 2-propanol > 1-butanol > (iso-butanol) 2-methyl-propan-1-ol

Brezolva V et al, J. Photochem. Photobiol. A: Chem., 107 (1997) 233

Properties of solvents which play a role:

Viscosity

Refractive index

Polarity

Stability of radicals

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High polarity:

More stabilization of the charge carriers

Property

Solvent

Viscosity (g cm-1 s-1)

Refractive index

Polarity

Methanol 0.544 1.326 0.60Ethanol 1.074 1.359 0.541-Propanol 1.945 1.383 0.522-Propanol 2.038 1.375 0.481-Butanol 2.544 1.397 0.47iso-Butanol 4.312 1.394 0.40

Properties of the various solvents

High viscosity:

Low diffusion coefficient

High refractive index:

Less penetration of light

Stability:

2-methyl-propan-1-ol(iso-butanol) > 1-butanol > 2-propanol > 1-propanol > Ethanol > Methanol

Stability of radicals - reverse order of activity

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High N2 bond strength - cleavage difficult

Dinitrogen complexation - weakens N-N triple bond - reductively

cleaved by various means

Conventionally reduced using LiAlH4, NaBH4, Al metal

Photoexcited electrons used for the reduction

Fixation of N2 on a CdS/Pt – [RuII(H-EDTA)(N2)]- system

Nageswara Rao N, J. Mol. Catal., 93 (1994) 23

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N2 fixation on a CdS/Pt/RuO2 – [Ru(H-EDTA)(N2)]- system

EDTA - sacrificial agent – enhances rate

Mechanism

Taqui Khan M M and Nageswara Rao N, J. Mol. Catal., 52 (1989) L5

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Influence of Ti3+ sites on fixation of N2

Adsorption of N2 - essential for e- transfer leading to reduction

Ti3+ defect sites: Increase N2 adsorption

Responsible for n-type semiconductivity

Directly gives electrons to N2

6 Ti4+-OH 6 Ti3+-OH

6 Ti3+-OH 6 Ti3+ + 3 H2O + 3/2 O2

6 Ti3+ + N2 + 6 H2O 6 Ti4+-OH + 2 NH3

Catalyst with more Ti3+ sites - more active for N2 reduction

Doping TiO2 - favorable preparation methodsRanjit K T and Viswanathan B, Ind. J. Chem., 35A (1996) 443

h

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Reasons

CB of photo catalyst – Not matching LUMO of N2

N2 adsorption – Not strong to perturb orbitals

Yields of ammonia – Not sufficient

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The activation of dinitrogen appears to be still intriguing. Even though, various methods of activation of dinitrogen have been attempted, the perturbations of the frontier wave functions of dinitrogen with respect to energy and symmetry have been considered to be the key.

However, in photocatalytic routes the frontier wave functions of the reacting species (photo catalysts) are perturbed so as to be able to interact with ground state wave functions of dinitrogen. It essentially means that the emphasis is shifted from the reacting species (i.e. dinitrogen) to the species with which the reacting species interacts.

However, even this shift in the emphasis does not seem to have provided the answer.

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Ammonia reactants

Steam reformingCH4(g) + H2O(g) CO(g) + 3 H2(g)

15-40% NiO/low SiO2/Al2O3 catalyst (760-816C)products often called synthesis gas or syngas

Water gas shiftCO(g) + H2O(g) CO2(g) + H2(g)

Cr2O3 and Fe2O3 as catalyst

carbon dioxide removed by passing through sodium hydroxide. CO2(g) + 2 OH-(aq) CO3

2-(aq) + H2O(l)

Biological N-FixationMost nitrogen is fixed by micro-organisms in the soil which include bacteria and cyanobacteria.

Some plants like legumes and alder trees have special adaptations on their roots to fix nitrogen which are called nodules.

This is an example of a symbiotic relationship between the plant and N-fixing bacteria.

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NH4Cl + Ba(OH)2 = NH3 + H2O + BaClDestructive distillation:

The decomposition of wood by heating out of contact with air, producing primarily charcoal

Magnesium nitride:

Fomed by interaction of magnesium with nitrogen in atmosphere

Reaction with quick lime:

2NH4Cl + CaO --> 2NH3 + CaCl2 + H2O

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N2

Structure of RuEDTAN2 complex

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According to Stoke’s –Einstein equation, Diffusion coefficient, D = kT/6 r Where r - radius of species

- viscosity of solvent

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N2 N2H5+ E = - 0.23 V

N2H5+ NH4

+ E = + 1.275 V

N2 NH4+ E = + 0.275 V

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Structures of polymers

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Li Be B C

Na Mg D Al Si

K Ca (Sc) Ti (V) Cr Mn Fe Co Ni Cu Zn Ga Ge

Rb Sr Y Zr Nb Mo (Tc) (Ru) Rh Pd Ag Cd In Sn

Cs Ba La (Hf) Ta W Re (Os) Ir Pt Au Hg Tl Pb

C A B E

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Structural basis of biological nitrogen fixationPhilosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences Volume 363, Number 1829 / April 15, 2005, 971 - 984 Biological nitrogen fixation is mediated by the nitrogenase enzyme system that catalyses the ATP dependent reduction of atmospheric dinitrogen to ammonia. Nitrogenase consists of two component metalloproteins, the MoFe-protein with the FeMo-cofactor that provides the active site for substrate reduction, and the Fe-protein that couples ATP hydrolysis to electron transfer. An overview of the nitrogenase system is presented that emphasizes the structural organization of the proteins and associated metalloclusters that have the remarkable ability to catalyse nitrogen fixation under ambient conditions. Although the mechanism of ammonia formation by nitrogenase remains enigmatic, mechanistic inferences motivated by recent developments in the areas of nitrogenase biochemistry, spectroscopy, model chemistry and computational studies are discussed within this structural framework.

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Composition in activated form(%)Fe2O3 1.1 - 1.7FeO 14.3 - 14.6Fe 79.7 - 81.6CaO 0.1 – 0.2SiO2 0.1 – 0.7MgO 0.3 - 0.6Al2O3 1.5 – 2.1K2O 0.2 – 0.5

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Free-living (asymbiotic)• Cyanobacteria• Azotobacter

Associative• Rhizosphere–Azospirillum• Lichens–cyanobacteria• Leaf nodules

Symbiotic• Legume-rhizobia• Actinorhizal-Frankia

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Structural Basis of Biological Nitrogen FixationJames B. Howard, Douglas C. ReesChem. Rev. 1996, 96, 29652982

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As Temperature increases, should drive the reaction to the left.

But, dissociation is only significant at high temperatures

- Very inefficient reaction (low reaction probability)

N2 + 3H2 2NH3 H=-36 kJ/mole Exothermic

1 N2(g) + * N2*

2 N2* + * 2N*

3 N* + H* NH* + *

4 NH* + H* NH2* + *

5NH2* + H*

NH3* + *

6 NH3* NH3(g) + *

7 H2(g) + 2* 2H*

This is the slow step

Physisorption

Why the reduction process is difficult

PHOTO CATALYTIC FIXATION OF DINITROGEN

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