Study two kind different catalytic behaviors for K4H1.2[Co0.6(H2O)0.6SiW11.4O39.4]-cocatalyzed visible light driven water oxidation in pH 1–7 media

Abstract

Efficient water oxidation remains a particular challenge in water splitting that calls for developing robust and economic water oxidation catalysts (WOCs), preferably based on earth-abundant transition metals. Herein, we report a robust and noble metal-free catalytical system using BiVO4 as an effective light-harvesting material, a cobalt-containing polyoxometalate K4H1.2[Co0.6(H2O)0.6SiW11.4O39.4] (1) as molecular cocatalyst, and NaIO3 as a sacrificial electron acceptor for visible-light-driven water oxidation in a wide pH range 1–7. The combination of 1 + BiVO4 samples gives better oxygen evolution activities than that of pristine BiVO4. A turnover number (TON) of 326.3, apparent quantum efficiency (AQE) of 10.0%, and an O2 yield of 21.8% are obtained under pH 4. This work investigates two kind different catalytic behaviors for 1-cocatalyzed visible light driven water oxidation at pH 1–7 media, which exhibits a pH-induced dynamic change on the catalytic active species. Catalytic behavior, XPS, SEM and TEM collectively testify that 1 remains intact during photocatalytic water oxidation under pH 4–5. Conversely, under pH 1–3 and 6–7, the heterogeneous CoOx originated from 1 is formed on the BiVO4 surface, which is responsible for the enhancement of BiVO4 photocatalytic activity. Our work uncovers the borderline between homogeneous and heterogeneous WOCs using a cobalt-based polyoxometalate at different pH media, which helps to the study of stability for polyoxometalates when semiconducting material is used as the light-harvesting material.