Hydrogen dissociation sites on indium-based ZrO2-supported catalysts for hydrogenation of CO2 to methanol

Abstract

The formation and nature of surface indium species in zirconia-supported catalysts for the hydrogenation of CO2 to methanol has been investigated by infrared (IR) spectroscopy. We studied the dissociation of hydrogen on In2O3/m-ZrO2, In2O3/t-ZrO2, In2O3/am-ZrO2 and m-ZrO2:In catalysts (m-, t- and am- refers to monoclinic, tetragonal and amorphous, respectively and m-ZrO2:In is a solid solution material), with and without a redox pretreatment. Indium hydride species and hydroxyl groups form at room temperature on the surface of all redox-treated catalysts upon their exposure to hydrogen. The activity and concentration of surface indium sites capable of heterolytic activation of H2 is the highest in In2O3/m-ZrO2(redox). The sites for the dissociation of hydrogen also exist, although in lower concentration, on the surface of calcined In2O3/m-ZrO2 and m-ZrO2:In catalysts (evacuated at 400 °C), i.e. the catalysts featuring the highest activity in the hydrogenation of CO2 to methanol. Noteworthy, the room temperature reaction between CO2 and InH species of redox-treated catalysts gave surface formate species, i.e. intermediates of the methanol synthesis pathway, only for In2O3/m-ZrO2(redox) and m-ZrO2:In(redox), highlighting more favourable reactivity of InH species and carbonates on the m-ZrO2 support. In situ X-ray absorption spectroscopy (XAS) at the In K-edge demonstrates the transformation of In2O3/m-ZrO2, during reduction in H2 at 400 °C, into highly dispersed In sites with an average oxidation state between In2+ and In0. Subsequent oxidation recovers the In3+ oxidation state (in the in situ XAS experiment) and forms a m-ZrO2:In solid solution. Thus, H2 dissociation in the most active m-ZrO2:In catalyst proceeds on In3+–O–Zr4+ sites dispersed in m-ZrO2, forming In–H and Zr–OH sites.