Effect of the Support on the Photocatalytic Water Oxidation Activity of Cobalt Oxide Nanoclusters

Oxygen evolution from water driven by visible light is one of the key reactions for solar fuel production. In this paper, we investigated the effect of the support on photocatalytic water oxidation under visible light using cobalt oxide as a multielectron catalyst. A range of supported Co₃O₄ nanoclusters have been successfully synthesized via wet impregnation and bisolvent methods. Compared with the wet impregnation approach, the bisolvent method allowed us to obtain a high quality catalyst with all the Co₃O₄ nanoclusters formed inside the mesoporous support using hexane/water as the combination. The resulting catalyst consists of Co₃O₄ nanoclusters with a very small particle size (∼25 nm) and narrow size distribution. Catalytic water oxidation experiments were performed in [Ru­(bpy)₃]²⁺-persulfate (photochemical) and Ce⁴⁺/Ce³⁺ (chemical) systems, and it was found that smaller Co₃O₄ cluster sizes resulted in higher water oxidation activity. In addition, KIT-6 was found to be a better support than SBA-15, which is likely due to the fact that the 3D porous structure of KIT-6 offers more accessible pores than the 1D channels in SBA-15. To further elucidate the role of support in the photocatalytic oxygen evolution, bare Co₃O₄ nanoparticles together with two SiO₂- and γ-Al₂O₃-supported ones were investigated. Photocatalytic studies show that both supported Co₃O₄ nanoparticles exhibited significant enhancement (50–80%) in oxygen evolution activity, compared with bare Co₃O₄ nanoparticles. However, switching from the SiO₂ to γ-Al₂O₃ support does not significantly change the activities, indicating composition and surface properties of the support do not participate in rate-limiting steps in oxygen evolution. It can be concluded that the major role of catalyst supports in Co₃O₄-based water oxidation catalysts is to provide a medium to physically separate Co₃O₄ nanoclusters from aggregation, leading to superior photocatalytic activities