Indium oxide (In₂O₃) based catalysts are active for the selective hydrogenation of CO₂ to methanol that likely proceeds over oxygen vacancy sites (V_O^∙∙ sites) of an active In₂O_3−x phase formed in situ.[1–3] However, an excessive generation of V_O^∙∙ sites leads to their precipitation to In⁰ (deactivation due to over-reduction of In₂O₃).[3] An atomic-scale understanding of how to modulate the reactivity of V_O^∙∙ sites in In₂O₃ is currently incomplete.[4,5] We hypothesized that the abundance and stability of V_O^∙∙ sites in In₂O₃ is tunable by changing its defective structure through doping, eventually aiming to improve the overall catalyst performance. Using Sn- or Zr-doped In₂O₃, we observed diverging dopant-induced catalytic performances under CO₂ hydrogenation conditions, i.e., a rapid deactivation in Sn-doped In₂O₃ and a stable, high activity in Zr-doped In₂O₃. Relying on various operando techniques, we uncovered that the V_O^∙∙ sites in Sn-doped In₂O₃ were unreactive towards CO₂, agglomerated, and precipitated to In⁰ and Sn⁰ that were molten under reaction conditions. At the same time, V_O^∙∙ sites in Zr-doped In₂O₃ were highly reactive towards their replenishment by CO₂, resulting in high catalyst activity and stability of the c-In₂O₃ phase.

Fresh Sn- and Zr-doped In₂O₃ (In_1.9Sn_0.1O_3.05 and In_1.9Zr_0.1O_3.05) were phase pure (c-In₂O₃ structure) based on XRD measurements. The analysis of the local atomic structure probed by PDF (Figure a, top) revealed dopant-induced local strain in both In_1.9Sn_0.1O_3.05 and In_1.9Zr_0.1O_3.05 relative to the reference undoped nano-In₂O₃. The strain due to the dopant insertion is evident from the peak in the difference PDF for the doped materials, while no such peak was observed for nano-In₂O₃ (Figure a, bottom). Despite their structural similarity, In_1.9Sn_0.1O_3.05 and In_1.9Zr_0.1O_3.05 exhibited contrasting catalytic performances for the hydrogenation of CO₂ to methanol (Figure b). In_1.9Sn_0.1O_3.05 showed a low initial activity and underwent rapid deactivation, while In_1.9Zr_0.1O_3.05 displayed a high methanol yield with no deactivation. Operando PDF data of the catalysis after 4 h time on stream (TOS) revealed that the structure of In_1.9Zr_0.1O_3.05 remained in the c-In₂O₃ type, while In_1.9Sn_0.1O_3.05 (and nano-In₂O₃) displayed a strongly altered PDF after 4 h TOS (Figure c), originating from molten metallic indium (and tin).

Our study demonstrates the structural dynamics of In₂O₃-based catalysts during CO₂ hydrogenation to methanol. Altering the defective structure of c-In₂O₃ through doping is an effective approach to tune the structural dynamics towards the desired improvement of the catalytic performance.

[1] J. Ye, C. Liu, D. Mei, Q. Ge, ACS Catalysis, 2016, 3, 1296–1306.
[2] O. Martin, A. J. Martín, C. Mondelli, S. Mitchell, T. F. Segawa, R. Hauert, C. Drouilly, D. Curulla-Ferré, J. Pérez-Ramírez, Angewandte Chemie International Edition, 2016, 55, 6261–6265.
[3] A. Tsoukalou, P.M. Abdala, D. Stoian, X. Huang, M.-G. Willinger, A. Fedorov, C. R. Müller, Journal of the American Chemical Society, 2019, 141, 13497–13505.
[4] A. Tsoukalou, P. M. Abdala, A. Armutlulu, E. Willinger, A. Fedorov, C. R. Müller, ACS Catalysis, 2020, 10, 10060–10067.
[5] T. P. Araújo, S. Mitchell, J. Pérez-Ramírez, Advanced Materials, 2024, 2409322, 1–22.