CO₂ Hydrogenation on Ru Single-Atom Catalyst Encapsulated in Silicalite: a DFT and Microkinetic Modeling Study

The critical levels of CO₂ emissions reached in the past decade have encouraged researchers into finding techniques to reduce the amount of anthropogenic CO₂ expelled to the atmosphere. One possibility is to capture the produced CO₂ from the source of emission or even from air (i.e., direct air capture) by porous materials (e.g., zeolites and MOFs). Among the different usages of captured CO₂, its conversion into light fuels such as methane, methanol, and formic acid is essential for ensuring the long-awaited circular economy. In the last years, single-atom catalysts encapsulated in zeolites have been considered to this purpose since they exhibit a high selectivity and activity with the minimum expression of catalytic species. In this study, a detailed mechanism composed by 47 elementary reactions, 42 of them in both forward and reverse directions and 5 of them that correspond to the desorption of gas products just forwardly studied), has been proposed for catalytic CO₂ hydrogenation over Ru SAC encapsulated in silicate (Ru₁@S-1). Periodic density functional theory (DFT) calculations along with microkinetic modeling simulations at different temperatures and pressures were performed to evaluate the evolution of species over time. The analysis of the results shows that carbon monoxide is the main gas produced, followed by formic acid and formaldehyde. The rate analysis shows that CO_(g) is formed mainly through direct dissociation of CO₂ (i.e., redox mechanism), whereas COOH formation is assisted by OH. Moreover, the Campbell's degree of rate control analysis suggests that the determining steps for the formation of CO_(g) and CH₂O_(g) gas species are their own desorption processes. The results obtained are in line with recent experimental and theoretical results showing that Ru₁ SACs are highly selective to CO_(g), whereas few atom clusters as Ru₄ increase selectivity toward methane formation.