Operando spectroscopic studies on redox mechanism for CO2 hydrogenation to CO on In2O3 catalysts

The catalytic activities of In₂O₃ catalysts with different surface area for both cyclic unsteady-state (transient) and steady-state reverse water–gas shift (RWGS) reactions were systemically investigated. The initial CO formation rates during CO₂-oxidation of the H₂-reduced In₂O₃ catalysts were close to the CO formation rates in steady-state RWGS conditions at 325 °C, suggesting a redox mechanism for the steady-state RWGS reaction over the In₂O₃ catalysts. Transient kinetics for the In³⁺/In⁺ redox and products (H₂O, CO) formation during the cyclic H₂-reduction and CO₂-oxidation of In₂O₃ under periodic feeding of H₂ ↔ CO₂ were studied by time-resolved operando UV–vis and In K-edge X-ray absorption experiments at 325 °C. During the H₂-reduction, the surface In³⁺-O species was reduced to produce H₂O and In⁺-□ (□: oxygen vacancy). Subsequent re-oxidation of the In⁺-□ by CO₂ gave CO and the In³⁺-O species. The transient CO formation rates were close to the consumption rates of In⁺-□ under CO₂, providing a quantitative evidence on the redox mechanism for unsteady-state RWGS reaction over In₂O₃. These results indicate that the unsteady-state and steady-state RWGS reactions are primary driven by the In³⁺/In⁺ redox mechanism. Kinetic results show that the reoxidation step is the rate-limiting step in the steady-state RWGS reaction.