The Role of Zn-Hf Site Proximity and Oxygen Vacancies for Methanol Formation Over ZnHfOx Catalysts Under CO2 Hydrogenation Conditions.

Mixed metal oxides, such as ZnZrOx, have attracted considerable interest as CO2 hydrogenation to methanol catalysts due to their high methanol selectivity (> 70%) and catalytic stability at elevated reaction temperatures (> 300°C). In this work, we introduce a novel ZnHfOx catalyst that exceeds the intrinsic methanol formation rate of the reference ZnZrOx at a Zn content of 20 mol% (rMeOH,20ZnZrOx = 0.59 molMeOH(molcath)-1, rMeOH,20ZnHfOx = 0.68 molMeOH(molcath)-1). Remarkably and in contrast to ZnZrOx, the ZnHfOx-based catalysts exhibit a high methanol selectivity (> 70%) up to Zn contents of 99.5 mol% despite the segregation of ZnO. Operando spectroscopy, in combination with computational analysis, identifies the Zn-VO-Hf motif as the active site for methanol formation that proceeds via the formate-methoxy pathway. Such active sites are not only present in solid solution-type ZnHfOx catalysts (≤ 35 mol% Zn), but also in the form of isolated HfOx clusters on segregated ZnO surfaces (for high Zn contents of > 35 mol%), explaining the high selectivity (and activity) over a wide range of Zn contents.