Surface ZnO$_{x}$ on zirconia is highly active for high temperature methanol synthesis

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 ZrO$_2$ provided a beneficial effect while SiO$_2$ exerted a detrimental effect compared to bulk ZnO. Preparing co-precipitated ZnO-ZrO$_2$ 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 ZrO$_2$. Operando XAS coupled with modulation excitation spectroscopy unravelled that the surface ZnO was partly reduced under operating conditions, as surface ZnO$_x$, with x approximately equal to 0.98. In-situ DRIFTS further uncovered that the surface ZnOx activated CO$_2$ and formed methanol via carbonate, formate and methoxide species. ... mehrXPS finally showed that ZrO$_2$ withdrew electrons from ZnO, facilitating oxygen abstraction to form the partly reduced ZnO$_x$, which in turn facilitated the activation of CO$_2$.