Theoretical Study on CO₂ Hydrogenation on In₂O₃(111) Supported Single-Atom Catalysts: Horiuti-Polanyi versus Non-Horiuti-Polanyi Mechanism

In the field of catalytic hydrogenation, two primary mechanistic pathways, namely the Horiuti-Polanyi (HP) mechanism and the non-HP mechanism, have been extensively investigated. Current understandings suggested that the non-HP mechanism preferred to occur on the coinage metal surfaces, such as copper, silver, and gold, which exhibited low activity towards H₂ dissociation. Herein, we offered a detailed theoretical investigation into the mechanisms of CO₂ hydrogenation to formic acid on M₁-In₂O₃(111) surfaces, using density functional theory calculations. Our calculations provided novel insights into the preference of the non-HP mechanism on reduced single-atom noble metal catalysts, such as r-Rh₁-In₂O₃(111) and r-Ir₁-In₂O₃(111). In these cases, molecularly adsorbed H₂ would be polarized into H^δ−–H^δ+, thus facilitating the electrophilic attack to the O in CO₂. Conversely, the H^δ+ species, derived from heterolytically dissociated H₂, exhibited a strong affinity on the adjacent oxygen site at the M-O-In interface. This strong adsorption resulted in a higher energy barrier for CO₂ hydrogenation, thereby rendering the HP mechanism less viable than the non-HP one. Our results were anticipated to provide a deeper understanding of hydrogenation reactions on oxide-supported noble single-atom catalysts and theoretical guidance for the development of novel high-performance catalysts for catalytic hydrogenation reactions.