The Role of Zn–Hf Site Proximity and Oxygen Vacancies for Methanol Formation Over ZnHfO _x Catalysts Under CO ₂

ABSTRACT

Mixed metal oxides, such as ZnZrO _x , have attracted considerable interest as CO ₂ 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 ZnHfO _x catalyst that exceeds the intrinsic methanol formation rate of the reference ZnZrO _x at a Zn content of 20 mol% ( r _{MeOH,20ZnZrOx} = 0.59 mol _MeOH (mol _cat h) ⁻¹ , r _{MeOH,20ZnHfOx} = 0.68 mol _MeOH (mol _cat h) ⁻¹ ). Remarkably and in contrast to ZnZrO _x , the ZnHfO _x ‐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–V _O –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 ZnHfO _x catalysts (≤ 35 mol% Zn), but also in the form of isolated HfO _x 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.