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 Co 3 O 4 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 Co 3 O 4 nanoclusters formed inside the mesoporous support using hexane/water as the combination. The resulting catalyst consists of Co 3 O 4 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 Ce 4+ /Ce 3+ (chemical) systems, and it was found that smaller Co 3 O 4 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 Co 3 O 4 nanoparticles together with two SiO 2 - and γ-Al 2 O 3 -supported ones were investigated. Photocatalytic studies show that both supported Co 3 O 4 nanoparticles exhibited significant enhancement (50–80%) in oxygen evolution activity, compared with bare Co 3 O 4 nanoparticles. However, switching from the SiO 2 to γ-Al 2 O 3 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 Co 3 O 4 -based water oxidation catalysts is to provide a medium to physically separate Co 3 O 4 nanoclusters from aggregation, leading to superior photocatalytic activities.