Zr(IV) surface sites determine CH3OH formation rate on Cu/ZrO2/SiO2 - CO2 hydrogenation catalysts

Abstract

Cu/ZrO 2 /SiO 2 are efficient catalysts for the selective hydrogenation of CO 2 to CH 3 OH. In order to understand the role of ZrO 2 in these mixed-oxides based catalysts, in situ X-ray absorption spectroscopy has been carried out on the Cu and Zr K-edge. Under reaction conditions, Cu remains metallic, while Zr is present in three types of coordination environment associated with 1) bulk ZrO 2 , 2) coordinatively saturated and 3) unsaturated Zr(IV) surface sites. The amount of coordinatively unsaturated Zr surface sites can be quantified by linear combination fit of reference X-Ray absorption near edge structure (XANES) spectra and its amount correlates with CH 3 OH formation rates, thus indicating the importance of Zr(IV) Lewis acid surface sites in driving the selectivity toward CH 3 OH. This finding is consistent with the proposed mechanism, where CO 2 is hydrogenated at the interface between the Cu nanoparticles that split H 2 and Zr(IV) surface sites that stabilizes reaction intermediates. Density and role of Lewis acidic Zr(IV) surface sites in Cu/SiO 2 /ZrO 2 based catalyst to promote CO 2 -to-CH 3 OH synthesis is elucidated by in-situ X-ray absorption spectroscopy.