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 TiO 2 -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 2 and then CoPi onto both Pt/TiO 2 and TiO 2 . In all reactions, Pt/TiO 2 and CoPi/TiO 2 were always more active and less active, respectively, than TiO 2 . In comparison with Pt/TiO 2, CoPi/Pt/TiO 2 was less active for phenol oxidation but more active for CP and DCP oxidation. CoPi/Pt/TiO 2 was also more active than Pt/TiO 2 for the photocatalytic reduction of O 2 into H 2 O 2 . For DCP oxidation in a phosphate-free suspension at pH 7, however, CoPi/Pt/TiO 2 was much less active than either Pt/TiO 2 or TiO 2, which is ascribed to the dissolution of Co 2+ 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 H 2 O 2 ) but deactivate in the absence of either Pt (O 2 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 2 reduction and organic oxidation.