Effect of a Co-Based Oxygen-Evolving Catalyst on TiO2-Photocatalyzed Organic Oxidation.

Abstract

Cobalt phosphate (CoPi) is a promising cocatalyst for the (photo)electrochemical oxidation of water over semiconductor electrodes in phosphate solution, but the effect of CoPi on organic oxidation reactions has been little studied. Herein, we report a compound-sensitive effect of CoPi on the TiO2-photocatalyzed oxidation of phenol, 4-chlorophenol (CP), and 2,4-dichlorophenol (DCP) in a phosphate-containing suspension at pH 7.0. A photochemical method was used to deposit Pt onto TiO2 and then CoPi onto both Pt/TiO2 and TiO2. In all reactions, Pt/TiO2 and CoPi/TiO2 were always more active and less active, respectively, than TiO2. In comparison with Pt/TiO2, CoPi/Pt/TiO2 was less active for phenol oxidation but more active for CP and DCP oxidation. CoPi/Pt/TiO2 was also more active than Pt/TiO2 for the photocatalytic reduction of O2 into H2O2. For DCP oxidation in a phosphate-free suspension at pH 7, however, CoPi/Pt/TiO2 was much less active than either Pt/TiO2 or TiO2, which is ascribed to the dissolution of Co2+ ions that act as recombination centers. It is proposed that the CoIV species, formed by the hole oxidation of CoII/III in CoPi, are surface-bound and short-lived. They can react with a nearby adsorbed substrate (CP, DCP, and H2O2) but deactivate in the absence of either Pt (O2 reduction catalyst) or phosphate (CoPi repairer). Moreover, there is a synergism between the CoPi-mediated hole transfer and the Pt-mediated electron transfer, that improves the efficiency of the charge separation and, consequently, increases the rates of O2 reduction and organic oxidation.