Chelate stabilized metal oxides for visible light photocatalyzed water oxidations

Abstract

Visible light driven photocatalytic water oxidations were undertaken that compared lactate stabilized molecular and nanoparticle cobalt complexes and calcium manganese oxides as simple mimics of the PSII CaMn₄O₅ catalyst. Analysis showed chelated cobalt oxides formed as <5 nm particles whilst Ca–Mn and Mn oxides were composed of nanoparticles organized into ∼150 nm spherules. O₂ yield, turnover frequency and quantum yields were determined for the chelated materials and compared to sintered CaMn₃O₆ and Co₃O₄ counterparts. Results show two distinct stages of O₂ generation took place with the chelated calcium manganese oxides, a Ca : Mn molar ratio of 1 : 3 gave highest O₂ yield in the initial stage. Significantly, O₂ generation at extended reaction time re-occurred without addition of fresh reagents and was determined to be promoted by in situ coating of the lactate metal oxide particles with cobalt captured from the decomposed pentaminecobalt(III)chloride electron acceptor. Time course TEM, XPS and EDX analysis indicated the secondary catalyst accumulated cobalt mainly as Co₃O₄. These nano-micro particles are readily reused and produced a superior O₂ output of 85% maximum theoretical yield. Chelated cobalt catalysts resulted in TOF and O₂ yield superior to a laser ablated counterpart, whereas calcium manganese oxide lactates promoted generation of effective recyclable water oxidation catalysts which also minimized toxicity and waste for the photocatalytic system.