Hydrogenation of CO2 to Methanol Catalyzed by In2O3: Oxygen Vacancies, Surface Hydroxyl Groups, and Temperature Dependence for Reactivity Revealed by 17O NMR.

Catalytic hydrogenation of CO2 over In2O3 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 17O labeling strategy combined with high-field solid-state NMR (ssNMR) up to 18.8 T, we achieve exceptionally high spectral resolution of 17O NMR, enabling explicit separation of distinct surface oxygen species. This approach allows tracing the thermal evolution of these species and probing their reactivities toward CO2 activation and hydrogenation. Notably, surface OIn3 sites are identified as the OV precursors, with a formation barrier of approximately 200°C. Unexpectedly, room-temperature oxygen exchange between In2O3 and CO2 is observed, enabled by the high sensitivity of the new 17O NMR method. Further probing with 13CO2 and/or H2, at variable temperatures, we are capable of unraveling the roles between OVs, InOHs and surface lattice oxygen species in CO2/H2 activation and their stepwise conversion into formate intermediates and ultimately methanol, which may prompt rational designs to lower the reaction temperature for In2O3.