Photocatalytic reduction of CO2
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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.
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Photocatalytic reduction of CO2
- 1. Photocatalytic reduction of CO2 Apratim K, Karthick M, Manohar K.H.
- 2. 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.
- 3. 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
- 4. Pictorial representation of Artificial photosynthesis Courtesy of Toshisba Corporate Research & Development Center Fig 1: Mechanism of Artificial photosynthesis
- 5. 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.
- 6. 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
- 7. 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
- 8. TiO2 as Photocatalyst High efficiency in UV irradiation. Wide availability Lack of Toxicity Durability and Stability Easy 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
- 9. 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
- 10. 1. Doping Metallic 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.
- 11. 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.
- 12. 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
- 13. 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.
- 14. 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
- 15. 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
- 16. Fig 5: Schematic of experimental setup Nanoscale, 2014, 6, 14305; DOI: 10.1039/c4nr05371k CO2 photoreduction experiment
- 17. 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
- 18. 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
Editor's Notes
- One of the reasons for this low efficiency associated with the use of TiO2 could be the low reduction potential of electrons in its conduction band (-0.1 V) that is much lower than the theoretical thermodynamics requirements for one electron reduction of CO2 Band gap engineering has to be done
- Surface heterojunction: concept is proposed on the basis of the DFT calculations to explain the difference in the photocatalytic activity of anatase TiO2 with coexposed {001} and {101} facets. Bi-crystalline TiO2 : Brookite and Anatase show improved photocatalytic activity.
- >Some metal NPs, such as gold, silver and copper, when of appropriate size and morphology, exhibit absorption bands in the visible region as a consequence of the collective oscillation of electrons on the surface of the NPs (“surface plasmon band”). In particular, the surface plasmon band absorption of Au NPs appears as a very broad band at λ max around 560 nm expanding from 400 to 700 nm >To reduce CO2 into carbon monoxide or hydrocarbons, electrons in the semiconductor are required to have more negative chemical potential and for water oxidation, holes need to have more positive potential
- Glycerol (6% ethylene glycol, 5% NH4F) + 15% DI water N atoms were doped into TiO2NTs by a facile hydrothermal method. The hydrothermal process was subsequently conducted at 120 °C for 12 h and then the autoclave was cooled to room temperature naturally An appropriate amount of H2PtCl6·6H2O solution based on Pt content in the sample (0.05, 0.1 and 0.2 wt.%) was mixed with TO-NP in distilled flask. The resulting slurry was stirred in the rotary dryer at 333 K for 2 h and maintained at 363 K for 30 min to evaporate the water completely. The solid residue is further dried at 383 K for 12 h and calcined in air at 773 K for 2 h.
- Anodic Reaction: Ti + 2H2O TiO2 + 4H+ + 4e- Chemical dissolution: TiO2 + 4H+ + 6F- TiF62- + 2H2O Before anodization, a nano scale TiO2 passivation layer is on the Ti surface. When constant voltage is applied, a pit is formed on the TiO2 layer. As anodization time increases, the pit grows longer and larger, and then it becomes a nanopore. Nanopores and small pits undergo continuous barrier layer formation. (e) After specific anodization time, completely developed nanotubes are formed on the Ti surface. In general, the mechanism of TiO2 nanotube formation in fluorine-ion based electrolytes is said to occur as a result of three simultaneous processes: the field assisted oxidation of Ti metal to form titanium dioxide, the field assisted dissolution of Ti metal ions in the electrolyte, and the chemical dissolution of Ti and TiO2 due to etching by fluoride ions, which is enhanced by the presence of H+ions (Shankar et al. 2007). TiO2 nanotubes are not formed on the pure Ti surface but on the thin TiO2oxide layer naturally present on the Ti surface. Therefore, the mechanism of TiO2 nanotubes formation is related to oxidation and dissolution kinetics. Schematic diagram of the formation of TiO2nanotubes by anodization process is shown in Figure 1. For a description of the process displayed inFigure 1, the anodization mechanism for creating the nanotube structure is as follows:
- The in situ monitoring of the surface reaction intermediates, the investigation of the rate limiting step and the process dynamics of adsorption and desorption of these species finally should lead to the synthesis of new photocatalysts with higher efficiency, selectivity, long-term stability and economic competitiveness more effort should be put into the design of co-catalysts that are able to enhance the chemisorption of CO2 from the mixture of CO2 and H2O, to activate CO2 molecules preferentially and to catalyze the product formation with controllable selectivity. The lack of a single measure of efficiency which would allow for an unequivocal comparison of heterogeneous photocatalytic systems has to be considered as a serious impediment to advancing the field. The situation could be partially ameliorated by standardizing the experimental conditions as well as by publishing at least two of the popular measures, one related to the amount of the catalyst (e.g. formation rate) and one related to the strength of the illumination (e.g. photonic efficiency). An alternative would be to devise a new measure that encompasses both relationships. Both approaches would be valuable steps forward for a consistent assessment of the efficiency of any system with respect to others and thereby enable identification of the best performing ones and paths for further improvement.