Pd-promoted reduction and restructuring of an In₂O₃-based catalyst for CO₂ hydrogenation at room temperature

An unconventional reaction mechanism in an In₂O₃/Pd(1 1 1) inverse model catalyst for the CO₂ hydrogenation reaction has been uncovered: In₂O₃ is partially reduced at room temperature in a reaction atmosphere as a result of its direct contact with Pd(1 1 1), which is an efficient H₂ splitter. The reduction induces changes in surface free energy, leading to a dynamical restructuring at the In₂O₃/Pd(1 1 1) interface via formation of InO_x and outward diffusion of Pd, as revealed by ambient pressure X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and density functional theory simulations. This dynamical restructuring eventually promotes the growth of 2D InPd_yO_x nanodomains as the catalytically active phase and the exclusive formation of methanol upon hydrogenation of CO₂ at room temperature. A comparable high selectivity toward CH₃OH was found in more realistic bulk catalytic systems (2 wt% Pd/In₂O₃ catalyst and commercial CZA catalyst). Scanning tunneling microscopy under ultrahigh vacuum and ambient pressure reaction atmospheres further reveals the structural dynamics at the InO_x/Pd(1 1 1) interface, where we follow in situ the evolution of the InO_x particles on Pd(1 1 1) and the mobility of the InPd_yO_x nanodomains in a CO₂ + H₂ environment. The present findings of the formation of a mixed oxide phase in a dynamically restructuring metal/reducible-oxide interface indicate further implications for other heterogeneous catalytic systems beyond the present CO₂ hydrogenation example and highlight the importance of in situ investigations.