Reaction mechanism of CO2 methanation over Rh/TiO2 catalyst

Rh/TiO2 has been regarded as a very promising catalyst for the low-temperature CO2 methanation. However, the atomic-level reaction mechanism that dictates the reactivity and selectivity of CO2 reduction over Rh/TiO2 catalyst remains elusive. The reaction mechanism governed by a delicate interplay of surface reaction chemistry and thermodynamics was systematically investigated using density functional theory calculations. Theoretical results indicate that significant charges accumulate at the perimeter of the interface between support TiO2 and Rh nanoparticle. Metal-support interface is identified as the most active site for CO2 adsorption and activation over Rh/TiO2 catalyst. Compared with the direct C-O bond cleavage pathway and formate pathway, the reverse water-gas shift (RWGS) reaction followed by CO hydrogenation is much more thermodynamically and kinetically favorable for CO2 methanation over Rh/TiO2 catalyst. The RWGS + CO hydrogenation pathway via H2COH* dissociation dominates CO2 methanation due to the relatively lower energy barrier. CO2 methanation via the RWGS + CO hydrogenation pathway prefers to proceed through the channel: CO2* -> COOH* -> CO* -> COH* -> HCOH* -> H2COH* -> CH3* -> CH4*. H-assisted COOH* dissociation is identified as the rate-determining step of CO2 methanation over Rh/TiO2 catalyst. Finally, a reaction network is established to understand the atomic-level reaction mechanism of CO2 methanation over Rh/TiO2 catalyst. These mechanistic insights can guide the rational design of catalyst active centers to boost the activity and selectivity of CO2 reduction.