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