Photoelectrochemical oxidation
Photoelectrochemical oxidation (PECO) is the process by which light enables a semiconductor to promote a catalytic oxidation reaction. While a photoelectrochemical cell typically involves both a semiconductor (electrode) and a metal (counter-electrode), at sufficiently small scales, pure semiconductor particles can behave as microscopic photoelectrochemical cells. [clarification needed] PECO has applications in the detoxification of air and water, hydrogen production, and other applications.
Reaction Mechanism
The process by which a photon initiates a chemical reaction directly is known as photolysis; if this process is aided by a catalyst, it is called photocatalysis.[1] If a photon has more energy than a material's characteristic band gap, it can free an electron upon absorption by the material. The remaining, positively charged hole and the free electron may recombine, generating heat, or they can take part in photoreactions with nearby species. If the photoreactions with these species result in regeneration of the electron-donating material—i.e., if the material acts as a catalyst for the reactions—then the reactions are deemed photocatalytic. PECO represents a type of photocatalysis whereby semiconductor-based electrochemistry catalyzes an oxidation reaction—for example, the oxidative degradation of an airborne contaminant in air purification systems.
The principal objective of photoelectrocatalysis is to provide low-energy activation pathways for the passage of electronic charge carriers through the electrode electrolyte interface and, in particular, for the photoelectrochemical generation of chemical products.[2] With regard to photoelectrochemical oxidation, we may consider, for example, the following system of reactions, which constitute TiO2-catalyzed oxidation.[3]
- TiO2 (hv) → TiO2 (e− + h+)
- TiO2(h+) + RX → TiO2 + RX.+
- TiO2(h+) + H2O → TiO2 + HO. + H+
- TiO2(h+) + OH− → TiO2 + HO.
- TiO2(e−) + O2 → TiO2 + O2.−
This system shows a number of pathways for the production of oxidative species that facilitate the oxidation of the species, RX, in addition to its direct oxidation by the excited TiO2 itself. PECO concerns such a process where the electronic charge carriers are able to readily move through the reaction medium, thereby to some extent mitigating recombination reactions that would limit the oxidative process. The “photoelectrochemical cell” in this case could be as simple as a very small particle of the semiconductor catalyst. Here, on the “light” side a species is oxidized, while on the “dark” side a separate species is reduced.[4]
Photochemical oxidation (PCO) versus PECO
The classical macroscopic photoelectrochemical system consists of a semiconductor in electric contact with a counter-electrode. For N-type semiconductor particles of sufficiently small dimension, the particles polarize into anodic and cathodic regions, effectively forming microscopic photoelectrochemical cells.[2] The illuminated surface of a particle catalyzes a photooxidation reaction, while the “dark” side of the particle facilitates a concomitant reduction.[5]
Photoelectrochemical oxidation may be thought of as a special case of photochemical oxidation (PCO). Photochemical oxidation entails the generation of radical species that enable oxidation reactions, with or without the electrochemical interactions involved in semiconductor-catalyzed systems, which occur in photoelectrochemical oxidation.[clarification needed]
Applications
PECO may be useful in treating both air and water.[6][7]
History
In 1938, Goodeve and Kitchener demonstrated the “photosensitization” of TiO2—e.g., as evidenced by the fading of paints incorporating it as a pigment.[8] In 1969, Kinney and Ivanuski suggested that a variety of metal oxides, including TiO2, may catalyze the oxidation of dissolved organic materials (phenol, benzoic acid, acetic acid, sodium stearate, and sucrose) under illumination by sunlamps.[6] Additional work by Carey et al. suggested that TiO2 may be useful for the photodechlorination of PCBs.[9]
See also
- Photocatalysis
- Photolysis
- Photochemistry
- Photocatalytic water splitting
- Photoelectrochemical cell
- List of photochemists
Further reading
- I. U. I. A. Gurevich, I. U. V. Pleskov, and Z. A. Rotenberg, Photoelectrochemistry. New York: Consultants Bureau, 1980.
- M. Schiavello, Photoelectrochemistry, photocatalysis, and photoreactors: Fundamentals and developments. Dordrecht: Reidel, 1985.
- A. J. Bard, M. Stratmann, and S. Licht, Encyclopedia of Electrochemistry, Volume 6, Semiconductor Electrodes and Photoelectrochemistry: Wiley, 2002.
References
- ↑ D. Y. Goswami, Principles of solar engineering, 3rd ed. Boca Raton: Taylor & Francis, 2015.
- ↑ 2.0 2.1 H. Tributsch, "Photoelectrocatalysis," in Photocatalysis: Fundamentals and Applications, N. Serpone and E. Pelizzetti, Eds., ed New York: Wiley-Interscience, 1989, pp. 339-383.
- ↑ O. Legrini, E. Oliveros, and A. Braun, "Photochemical processes for water treatment," Chemical Reviews, vol. 93, pp. 671-698, 1993.
- ↑ D. Y. Goswami, "Photoelectrochemical air disinfection " US Patent 7,063,820 B2, 2006.
- ↑ A. J. Bard, "Photoelectrochemistry and heterogeneous photo-catalysis at semiconductors," Journal of Photochemistry, vol. 10, pp. 59-75, 1979.
- ↑ 6.0 6.1 L. C. Kinney and V. R. Ivanuski, "Photolysis mechanisms for pollution abatement," 1969.
- ↑ D. Y. Goswami, J. Klausner, G. Mathur, A. Martin, K. Schanze, P. Wyness, et al., "Solar photocatalytic treatment of groundwater at Tyndall AFB: field test results," in Proceedings of the... Annual Conference, American Solar Energy Society, Inc, 1993.
- ↑ C. Goodeve and J. Kitchener, "Photosensitisation by titanium dioxide," Transactions of the Faraday Society, vol. 34, pp. 570–579, 1938.
- ↑ J. H. Carey, J. Lawrence, and H. M. Tosine, "Photodechlorination of PCB's in the presence of titanium dioxide in aqueous suspensions," Bulletin of Environmental Contamination and Toxicology, vol. 16, pp. 697–701, 1976.
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