Insights into Rh size-dependent reactivity of CO₂ methanation over Rh–Al₂O₃ catalysts

The hydrogenation of CO₂ to methane at atmospheric pressure is a significant chemical approach to achieve carbon neutrality and gain renewable energy. However, developing catalysts with high selectivity and high methane yield remains challenging. In this study, a series of Rh-Al₂O₃ catalysts with varying Rh particle sizes were prepared by modulating the Rh loading amounts. Rh nanoparticles (Rh NPs) were found to exhibit superior performance compared to Rh single atoms (Rh SAs) under identical reaction conditions. The sharp decrease in CH₄ selectivity at high temperature is dominantly attributed to the side reaction of dry reforming of methane instead of the limitation of reaction thermodynamics. It was found that there was the coexistence of formate and CO pathways in CO₂ methanation on Rh-Al₂O₃ catalysts regardless of Rh loadings and formate pathway is dominate for CO₂ methanation when the temperature high than 400 °C. Turnover frequency (TOF) calculations indicated that the theoretical CH₄ generation frequency of Rh NP was three times higher than that of Rh SA. Kinetic experiments and DFT calculations revealed that the dissociation and activation of H₂ is the key factor affecting the performance of Rh-Al₂O₃ catalyst. This study facilitates our understanding of Rh size-dependent chemistry for CO₂ methanation reaction.