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 (In2O3–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 In2O3–x(OH)y and may therefore impede sustained catalytic activity by blocking surface sites.