Photocatalytic reduction of CO2

Apratim K, Karthick M, Manohar K.H.

Introduction

The threat of global warming is high due to the extensive use of fossil fuels.

Using non-renewable resources is a viable solution.

Sunlight can be converted in two ways - into electrical energy and into chemical energy

Water splitting and CO2 are two important methods which can be used in solar cells.

Artificial Photosynthesis

The basic process can be split into four steps:

Generation of charge carriers (electron–hole pairs) upon absorption of photons with suitable energy from light irradiation

Charge carrier separation and transportation

Adsorption of chemical species on the surface of the photocatalyst

Chemical reactions between adsorbed species and charge carriers

Pictorial representation of Artificial photosynthesis

Courtesy of Toshisba Corporate Research & Development Center

Fig 1: Mechanism of Artificial photosynthesis

Photocatalysis

A good photocatalytic material should be able to-

Separate electron - hole pairs generated and prevent recombination.

Transfer electrons to the surface for chemical reaction

Provide catalytic surface for the chemical reaction to take place.

Materials which generate catalytic activity when exposed to light are called photocatalysts.

Possible Redox reactions

Chem. Commun., 2016, 52, 35; DOI: 10.1039/c5cc07613g

Table 1: Some possible reactions related to photocatalytic conversion of CO2 with H2O

Drawbacks

Mismatch between the absorption ability of semiconductor and the solar spectrum

Charge recombination or poor charge carrier separation

Low solubility of CO2 in water (approximately 33 µmol in 1 ml of water at 100 KPa and room temperature)

Problem of back reactions during CO2 reduction

Water reduction to hydrogen is a competing reaction

TiO2 as Photocatalyst

High efficiency in UV irradiation.Wide availability Lack of ToxicityDurability and StabilityEasy to synthesize at nanoscale (TiO2 nanotubes)

The PROBLEM?

Lack of Photo-response under visible light irradiation

Avelino Corma , Hermenegildo Garcia; Photocatalytic reduction of CO2 for fuel production: Possibilities and Challenges; Journal of Catalysis 308 (2013) 168–175

TiO2 - Modifications

• Retarding fast charge recombination

• Reducing the band gap: improving photoresponse

Doping &

Creating Heterojunction

• Selectivity towards a single product• Increasing the recombination time:

Permanent effect• Solution to photocorrosion

Loading a Co-catalyst

Eg: Au/Ag NPs on TiO2

1. DopingMetallic Non-Metallic

Replacing Ti4+ by Fe3+, Pt4+ or Pd2+

Photocorrosion- Leaching and deactivation

Replacing Oxygen/Oxygen vacancies by C, N or S

More stable TiO2 photocatalysts

Reproducibility problems due to variation in dopant concentrations and its location.

1) A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515–582.2) C. Burda, Y.B. Lou, X.B. Chen, A.C.S. Samia, J. Stout, J.L. Gole, Nano Lett. 3 (2003) 1049–1051.

2. Forming a Heterojunction

Fig 2: Schematic Representation of ‘surface heterojunction’ effect in TiO2 , {001} and {101} facets

Improve light absorption and charge separation

Autonomous effect: due to the field generated

Semiconducting QDs can be used to create heterojunctions

Results in better visible light response

J. Yu, J. Low, W. Xiao, P. Zhou and M. Jaroniec, J. Am. Chem. Soc., 2014, 136, 8839–8842.

3. Loading a Co- Catalyst

Fig 3: Proposed mechanism for the photocatalytic hydrogen generation assisted by Au NPs on the TiO2 surface

Surface plasmon band characteristic: Absorption of visible light Supplying e- s to the CB of TiO2

State of charge separation

Reproducibility X Water oxidation X Redundant reactions Product Selectivity

Avelino Corma , Hermenegildo Garcia; Photocatalytic reduction of CO2 for fuel production: Possibilities and Challenges; Journal of Catalysis 308 (2013) 168–175

Desirable properties

Int. J. Mol. Sci. 2014, 15, 5246-5262; doi:10.3390/ijms15045246

How to accomplish the property Property Effect

Small particle size High surface area High adsorption

Crystalline material Single site structure Homogeneity

Engineering the band gap Light absorption Higher efficiency

Preferential migration along certain direction

Efficient charge separation Low recombination

Presence of co-catalysts Long lifetime of charge separation Possibility of chemical reactions

High crystallinity High mobility of charge carriers More efficient charge separation

Adequate co-catalysts Selectivity towards single product Efficient chemical process

Table 2: Compendium of all the desired properties and necessary modifications for efficient photocatalysis.

TiO2 nanotubes via anodization

Titanium foils - degreased using acetone, methanol, rinsed with DI water and blow dried

Ethylene glycol + Ammonium fluoride is used as electrolyte, Pt as counter electrode

Electrochemical anodization carried out at 50V for 3 hours

Annealed at around 450℃ for 2 hours for obtaining the crystalline phase

Free standing nanotubes can also be obtained by methanol evaporation

Chem. Mater. 2008, 20, 1257–1261

Biomaterials and Biotechnology Schemes Utilizing TiO2 Nanotube Arrays, By Karla S. Brammer, Seunghan Oh, Christine J. Frandsen and Sungho Jin ISBN 978-953-307-609-6, Published: September 15, 2011

Anodization mechanism

Fig 4: Schematic illustration of TiO2 nanotube formation

Fig 5: Schematic of experimental setup

Nanoscale, 2014, 6, 14305; DOI: 10.1039/c4nr05371k

CO2 photoreduction experiment

CO2 Photoreduction experiment Before the start of the experiment the steel chamber is heated to 80℃ to remove

the desorbed gases

Photocatalyst is kept inside a steel chamber along with water droplets covered with an optical grade Quartz window

The chamber is vacuumed and desired pressure of CO2 is filled

The solar simulator is switched on to irradiate the sample for a period of time

After the experiment is over, the output valve can be connected to Mass Spectrometer to analyse the reaction products

Nanoscale, 2014, 6, 14305; DOI: 10.1039/c4nr05371k

Conclusion and further work

The efficiency of artificial photocatalysis is generally lower than in natural photosynthesis

Alcohols/amines can be used in place of water to generate H+ ions

Need for optimal catalyst for CO2 reduction to be applied commercially

Lack of a single measure of efficiency which would allow for an unequivocal comparison of heterogeneous photocatalytic systems

In-situ spectroscopic techniques for understanding elementary steps

Developing efficient co-catalysts for activation and selective reduction of CO2