Surface ZnOx on zirconia is highly active for high temperature methanol synthesis

Abstract

Zinc containing mixed metal oxides and supported zinc oxide are stable and selective methanol synthesis catalysts at temperatures where a subsequent methanol to hydrocarbons reaction can occur directly. This work provides fundamental insights into ZnO-based high temperature methanol synthesis catalysts. A pronounced support effect was observed, where ZrO2 provided a beneficial effect while SiO2 exerted a detrimental effect compared to bulk ZnO. Preparing co-precipitated ZnO-ZrO2 catalysts showed that the initial activity correlated with the amount of amorphous ZnO on the surface of the support and that the catalytic activity increased with time on stream as zinc oxide migrated out of a solid solution with ZrO2 and onto the support surface. Hence the active phase appeared to be ZnO surface species and not zinc oxide in a solid solution with ZrO2. Operando XAS coupled with modulation excitation spectroscopy unravelled that the surface ZnO was partly reduced under operating conditions, as surface ZnOx, with x approximately equal to 0.98. In-situ DRIFTS further uncovered that the surface ZnOx activated CO2 and formed methanol via carbonate, formate and methoxide species. XPS finally showed that ZrO2 withdrew electrons from ZnO, facilitating oxygen abstraction to form the partly reduced ZnOx, which in turn facilitated the activation of CO2.