Adsorption and Dissociation of H₂O on Monolayered MoS₂ Edges: Energetics and Mechanism from <i>ab Initio</i> Simulations

The dissociation of water on 2D monolayer molybdenum disulfide (MoS₂) edges was studied with density functional theory. The catalytically active sites for H₂O, H, and OH adsorption on MoS₂ edges with 0% (Mo-edge), 50% (S50-edge), and 100% (S100-edge) sulfur coverage were determined, and the Mo-edge was found to be the most favorable for adsorption of all species. The water dissociation reaction was then simulated on all edges using the climbing image nudged elastic band (CI-NEB) technique. The reaction was found to be endothermic on the S100-edge and exothermic for the S50- and Mo-edges, with the Mo-edge having the lowest activation energy barrier. Water dissociation was then explored on the Mo-edge using metadynamics biased <i>ab initio</i> molecular dynamics (AIMD) methods to explore the reaction mechanism at finite temperature. These simulations revealed that water dissociation can proceed by two mechanisms: the first by splitting into adsorbed OH and H species produced a particularly small activation free energy barrier of 0.06 eV (5.89 kJ/mol), and the second by formation of desorbed H₂ and adsorbed O atom had a higher activation barrier of 0.36 eV (34.74 kJ/mol) which was nevertheless relatively small. These activation barrier results, along with reaction rate calculations, suggest that water dissociation will occur spontaneously at room temperature on the Mo-edge

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–<i>x</i>(OH)_<i>y</i>) 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–<i>x</i>(OH)_<i>y</i> and may therefore impede sustained catalytic activity by blocking surface sites