Photocatalytic water oxidation by very small cobalt domains on a silica surface

Abstract

Small domains of cobalt on silica (CoSBA) were prepared by the reaction of Co[N(SiMe₃)₂]₂ and SBA-15, resulting in a range of surface structures as the cobalt loading varied from 0.27 to 5.11 wt%. X-ray absorption spectroscopy (XAS) was employed to characterize these surface structures, which range from single-site cobalt atoms to small clusters of Co₃O₄. The CoSBA materials exhibit photochemical water oxidation catalysis, revealing distinct catalytic activities associated with characteristic types of surface structures that are dominant in particular concentration regimes. The catalytic turnover frequency for water oxidation of an isolated single-site cobalt atom (0.0143 s⁻¹) is much greater than that observed for a surface atom of a small cluster Co₃O₄ on silica (0.0006 s⁻¹). The CoSBA catalysts were recyclable for more than seven catalytic cycles (>200 turnovers) with additional sacrificial oxidant, and no leaching of cobalt was observed. Post-catalytic analysis of CoSBA by XAS revealed that the cobalt atoms were partially oxidized to Co³⁺, without exhibiting significant surface migration and aggregation of cobalt atoms.