In Situ Spectroscopy of Water Oxidation at Ir Oxide Nanocluster Driven by Visible TiOCr Charge-transfer Chromophore in Mesoporous Silica

An all-inorganic photocatalytic unit consisting of a binuclear TiOCr charge-transfer chromophore coupled to an Ir oxide nanocluster has been assembled on the pore surface of mesoporous silica AlMCM-41. When exciting the Ti(IV)OCr(III) --> Ti(III)OCr(IV) metal-to-metal charge-transfer chromophore of an aqueous suspension of IrxOy-TiCr-AlMCM-41 powder with visible light, oxygen evolution with a quantum efficiency of at least 13% was detected by Clark electrode measurements. In situ Fourier transform Raman and X-band electron paramagnetic resonance spectroscopy revealed the formation of superoxide species. Use of (H2O)-O-18 confirmed that the superoxide species originates from oxidation of water. Photolysis in the absence of persulfate acceptor led to accumulation of Ti(III) instead. The results indicate efficient photocatalytic oxidation of water at Ir oxide nanoclusters followed by trapping of the evolving O-2 by transient Ti(III) centers to yield superoxide. Given the flexibility of the synthetic method for selecting donor metals with appropriate redox potential, photocatalytic units consisting of a binuclear charge-transfer chromophore coupled to a water oxidation catalyst shown here constitute a step toward thermodynamically efficient visible-light water oxidation units.