Flux-matching conditions at TiO2 photoelectrodes: Is interfacial electron transfer to O2 rate-limiting in the TiO2-catalyzed photochemical degradation of organics?

Janet M. Kesselman, Gary A. Shreve, Michael R. Hoffmann, Nathan S Lewis

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Abstract

A flux-matching condition has been applied to determine whether O2 reduction is rate-limiting under photocatalytic conditions for the degradation of CHCl3 at rutile TiO2 single-crystal electrodes. In this approach, the potential dependence of the photooxidation current density is compared to the potential dependence of the current density for O2 reduction. The potential at which the oxidation and reduction fluxes are equal determines the operating potential and the steady-state flux that will flow through the crystal under no applied bias. If this flux-matching condition occurs when the cathodic flux equals the flux of photogenerated carriers, then the predicted quantum yield should approach unity; otherwise, recombination should be significant in the TiO2. Our measurements indicate that significant recombination will occur for the oxidation of typical organic molecules in H2O over a range of pH values. The data also indicate that Pt catalysis of O2 reduction should be beneficial for the oxidation of organic molecules, as would the use of alternate electron acceptors such as Fe(CN)6 3-. The O2 reduction data and rotating disk electrode data collected in this work allow a quantitative comparison to theoretical estimates of the electron transfer rate constant for O2 reduction at TiO2. We also present an elucidation of the previously published theoretical treatments of TiO2 charge transfer rate constants in view of the new data collected herein.

Original languageEnglish
Pages (from-to)13385-13395
Number of pages11
JournalJournal of Physical Chemistry
Volume98
Issue number50
Publication statusPublished - 1994

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ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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