Hydrogenation of CO ₂ to Methanol Catalyzed by In ₂ O ₃ : Oxygen Vacancies, Surface Hydroxyl Groups, and Temperature Dependence for Reactivity Revealed by ¹⁷ O NMR

Abstract

ABSTRACT Catalytic hydrogenation of CO 2 over In 2 O 3 has attracted intense interest due to its exceptionally high methanol selectivity and broad tunability through structural modification. However, the precise formation mechanism of oxygen vacancies (OVs) and exact roles of hydroxyl groups (InOHs), the two most important surface structures, remain poorly understood due to the limited resolution of currently available characterization methods. Here, by employing an advanced 17 O labeling strategy combined with high‐field solid‐state NMR (ssNMR) up to 18.8 T, we achieve exceptionally high spectral resolution of 17 O NMR, enabling explicit separation of distinct surface oxygen species. This approach allows tracing the thermal evolution of these species and probing their reactivities toward CO 2 activation and hydrogenation. Notably, surface OIn 3 sites are identified as the OV precursors, with a formation barrier of approximately 200°C. Unexpectedly, room‐temperature oxygen exchange between In 2 O 3 and CO 2 is observed, enabled by the high sensitivity of the new 17 O NMR method. Further probing with 13 CO 2 and/or H 2 , at variable temperatures, we are capable of unraveling the roles between OVs, InOHs and surface lattice oxygen species in CO 2 /H 2 activation and their stepwise conversion into formate intermediates and ultimately methanol, which may prompt rational designs to lower the reaction temperature for In 2 O 3 .