Effect of a Co-Based Oxygen-Evolving Catalyst on TiO₂-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 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 Co^IV species, formed by the hole oxidation of Co^II/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.