(Photo)electrochemical behavior of selected organic compounds on TiO2 electrodes. Overall relevance to heterogeneous photocatalysis

Oxidation potentials of resorcinol, 5-chlororesorcinol 4,6-dichlororesorcin ol, catechol, 3,4-dihydroxybenzoic acid and 1,2,4-trihydroxybenzene were me asured on particulate TiO2 (Degussa P-25) thin films, immobilized on optica lly transparent SnO2 conducting glass electrodes, by cyclic voltammetry in 0.5M KCl aqueous electrolyte solutions. The effect of adsorption on oxidati on potentials was examined with the compounds adsorbed on the TiO2 particle surface. Scan rate dependencies of oxidation peak currents indicate that a dsorbed species are consistently characterized by less positive oxidation p otentials compared to those attributed to solution free species; the differ ence ranges from about 0.2 to 0.8 V. Results show that depending on the nat ure of the working electrode, associating a single oxidation potential to s uch compounds does not adequately describe their electrochemical behavior. Such observations have relevance in heterogeneous photocatalysis in that pr edictions of whether a substance will be photooxidized or photoreduced cann ot be based on Fermi levels (redox potentials) of the redox couples in homo geneous solutions. (C) 2000 Elsevier Science S.A. All rights reserved.