Metadynamics-Biased ab Initio Molecular Dynamics Study of Heterogeneous CO₂ Reduction via Surface Frustrated Lewis Pairs

The recent discovery of frustrated Lewis pairs (FLPs) capable of heterolytically splitting hydrogen gas at the surface of hydroxylated indium oxide (In₂O_3–x(OH)_y) nanoparticles has led to interesting implications for heterogeneous catalytic reduction of CO₂. Although the role of surface FLPs in the reverse water-gas shift (RWGS) reaction (CO₂ + H₂ → CO + H₂O) has been experimentally and theoretically demonstrated, the interplay between surface FLPs and temperature and their consequences for the reaction mechanism have yet to be understood. Here we use well-tempered metadynamics-biased ab initio molecular dynamics to obtain the free energy landscape of the multistep RWGS reaction at finite temperatures. The reaction is simulated at 20 and 180 °C, and the minimum energy reaction pathways and energy barriers corresponding to H₂ dissociation and CO₂ reduction are obtained. The reduction of CO₂ at the surface FLP catalytically active site, where H₂ is heterolytically dissociated and bound, is found to be the rate-limiting step and is mostly unaffected by increased temperature conditions; however, at 180 °C the energetic barriers associated with the splitting of H₂ and the subsequent adsorption of CO₂ are reduced by 0.15 and 0.19 eV, respectively. It is suggested that increased thermal conditions may enhance reactivity by enabling the surface FLP to become further spatially separated. Product H₂O is found to favor dissociative adsorption over direct desorption from the surface of In₂O_3–x(OH)_y and may therefore impede sustained catalytic activity by blocking surface sites.