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

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 TiO-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 TiO and then CoPi onto both Pt/TiO and TiO. In all reactions, Pt/TiO and CoPi/TiO were always more active and less active, respectively, than TiO. In comparison with Pt/TiO, CoPi/Pt/TiO was less active for phenol oxidation but more active for CP and DCP oxidation. CoPi/Pt/TiO was also more active than Pt/TiO for the photocatalytic reduction of O into HO. For DCP oxidation in a phosphate-free suspension at pH 7, however, CoPi/Pt/TiO was much less active than either Pt/TiO or TiO, which is ascribed to the dissolution of Co ions that act as recombination centers. It is proposed that the Co species, formed by the hole oxidation of Co in CoPi, are surface-bound and short-lived. They can react with a nearby adsorbed substrate (CP, DCP, and HO) but deactivate in the absence of either Pt (O 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 O reduction and organic oxidation.