Flux-Matching Conditions at TiO₂ Photoelectrodes: Is Interfacial Electron Transfer to O₂ Rate-Limiting in the TiO₂-Catalyzed Photochemical Degradation of Organics?
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)₆³⁻. 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.
© 1994 American Chemical Society. Received: July 6, 1994; In Final Form: September 16, 1994. We gratefully acknowledge ARPA (NAV 5 HFMN N0001 149211901) and the National Science Foundation, Grant CHE-9221311, for support of this work, and J.M.K. acknowledges the National Science Foundation for a predoctoral fellowship. We thank Dr. Com for a preprint of ref 51.