Variation in the In₂O₃ Crystal Phase Alters Catalytic Performance toward the Reverse Water Gas Shift Reaction

Understanding the structure-catalytic activity relationship is crucial for developing new catalysts with desired performance. In this contribution, we report the performance of In₂O₃ with different crystal phases in the reverse water gas shift (RWGS) reaction, where we observe changing activity induced by a phase transition under reaction conditions. Cubic In₂O₃ (c-In₂O₃) exhibits a higher RWGS rate than the hexagonal phase (h-In₂O₃) at temperatures below 350 °C because of its (1) enhanced dissociative adsorption of H₂, (2) facile formation of the oxygen vacancies, and (3) enhanced ability to adsorb and activate CO₂ on the oxygen vacancies, as suggested both experimentally and computationally. Density functional theory results indicate that the surface oxygen arrangement on the cubic polymorph is key to rapid H₂ adsorption, which facilitates oxygen vacancy formation and subsequent CO₂ adsorption to yield high RWGS reactivity. At 450 °C and above, the activity of h-In₂O₃ increases gradually with time on stream, which is caused by a phase transition from h-In₂O₃ to c-In₂O₃. In situ X-ray diffraction experiments show that h-In₂O₃ is first reduced by H₂ and subsequently oxidized by CO₂ to c-In₂O₃. These findings highlight the importance of the crystal phase in the catalytic RWGS reaction and provide a new dimension for understanding/designing RWGS catalysts.