Light Induced Carbon Dioxide Reduction by Water at Binuclear ZrOCoII Unit Coupled to Ir Oxide Nanocluster Catalyst

An all-inorganic polynuclear unit consisting of an oxo-bridged binuclear ZrOCoII group coupled to an iridium oxide nanocluster (IrOx) was assembled on an SBA-15 silica mesopore surface. A photodeposition method was developed that affords coupling of the IrOx water oxidation catalyst with the Co donor center. The approach consists of excitation of the ZrOCoII metal-to-metal charge-transfer (MMCT) chromophore with visible light in the presence of [Ir(acac)3] (acac: acetylacetonate) precursor followed by calcination under mild conditions, with each step monitored by optical and infrared spectroscopy. Illumination of the MMCT chromophore of the resulting ZrOCoII-IrOx units in the SBA-15 pores loaded with a mixture of (CO2)-C-13 and H2O vapor resulted in the formation of (CO)-C-13 and O-2 monitored by FT-IR and mass spectroscopy, respectively. Use of O-18 labeled water resulted in the formation of O-18(2) product. This is the first example of a closed photosynthetic cycle of carbon dioxide reduction by water using an all-inorganic polynuclear cluster featuring a molecularly defined light absorber. The observed activity implies successful competition of electron transfer between the IrOx catalyst cluster and the transient oxidized Co donor center with back electron transfer of the ZrOCo light absorber, and is further aided by the instant desorption of the CO and O-2 product from the silica pores.