The Role of Zn–Hf Site Proximity and Oxygen Vacancies for Methanol Formation Over ZnHfO <i> _x </i> Catalysts Under CO ₂ Hydrogenation Conditions

Abstract

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 (&gt; 70%) and catalytic stability at elevated reaction temperatures (&gt; 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 (&gt; 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 &gt; 35 mol%), explaining the high selectivity (and activity) over a wide range of Zn contents.