CONTROLLED PERIODIC ILLUMINATION IN HETEROGENEOUS PHOTOCATALYSIS


Overview

Heterogeneous photocatalysis is an advanced oxidation process for water and air purification. In water, dispersions of UV illuminated semiconductors, typically TiO2, catalyze the oxidation of dissolved organic pollutants through the photogeneration of strong oxidants such as hydroxyl radicals. The process begins when a long wave UV photon (l < 385 nm) excites a TiO2 particle creating an electron/hole pair. The electrons and holes react with adsorbed molecules and ions such as water, oxygen, hydroxyl groups, and in some cases directly with the organic compound, to form a variety of radical species. The primary oxidizing agent is the hydroxyl radical. The hydroxyl radicals react with the organic pollutant, completely mineralizing most compounds, leaving carbon dioxide, water, and mineral acids as products.

Hundreds of studies in photocatalysis have been recently reviewed and compiled in conference proceedings (1-3). These studies provide extensive insight into photocatalysis, but the field is far from being completely understood. There are still many questions about the reaction mechanisms that may never be answered. The micro-crystalline particles make it difficult to use surface analysis techniques that usually provide information about the reactive species. Also, mechanistic results inferred from large, well characterized single crystal surfaces do not account for the unique electronic properties of the small particles. Furthermore, as photocatalysis is commercially developed for water purification, more problems have arisen. Some of the problems include determining the best reactor design, overcoming low photoefficiencies, deciding whether to use a slurry or a fixed catalyst, and generating toxic intermediates and incomplete oxidation products in large scale systems. My research addresses some of those problems by investigating Controlled Periodic Illumination (CPI) to increase the photoefficiency and novel reactor designs that incorporate CPI.

My recent research showed that Controlled Periodic Illumination (CPI) of TiO2 increased the photoefficiency of aqueous formate decompositions by a factor of three (4,5). CPI is a cyclic process where the catalyst is briefly exposed to light, generating a relatively small number of holes and electrons. A dark period follows, during which all the physical and chemical processes (electron transfer reactions, radical reactions, diffusion, adsorption, desorption, etc.) associated with the process occur. During this dark period, the particle "relaxes" and returns to its original state, ready to be illuminated again. Only generating a small number of holes and electrons on the particle at a time may be the key to using those holes and electrons more efficiently. Limiting the number of holes and electrons may reduce the impact of direct and indirect recombinations since these are thought to be second order processes (6).

Current Research

The most promising fundamental investigation is examining the CPI effect in variety of compounds that have relatively well understood, but different photocatalytic reaction mechanisms. The research should include all types of photocatalytic hydrocarbon processing. Enhancing the photoefficiency of any of these photocatalytic systems would assist in making them feasible alternatives to conventional reaction processes.

The overall objective of the above work is to provide an adequate understanding of CPI effects. This understanding will logically lead to fundamental research on developing modified, or new photocatalytic materials. These materials would require minimal dark times and/or could accommodate longer illumination times. An optimal catalyst would not require a dark recovery time. It would be highly efficient during continuous illumination. Possible catalyst improvement methods include deposition of promoter metal co-catalysts such as platinum, pre-reduction and doping of the semiconductors, and development of new materials with physical, chemical, and electronic properties that reduce or eliminate the need for CPI.

References

  1. Fox, M. A., Dulay, M. T. Chem. Rev. 1993, 93, 341.
  2. Legrini, O. Oliveros, E., Braun, A. M. Chem. Rev. 1993, 93, 671.
  3. Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H.; Eds.; Elsevier: Amsterdam, 1993.
  4. Matthews, R. W., in Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H.; Eds.; Elsevier: Amsterdam, 1993; p. 121.
  5. Turchi, C. S., Mehos, M. S., Pacheco, J., in Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H.; Eds.; Elsevier: Amsterdam, 1993; p. 789.
  6. Sczechowski, J. G., Koval, C. A., Noble, R. D. J. Photochem. Photobio. A: Chem. 1993, 74, 273.