CO₂ Desorption Dynamics Reveal Structure Insensitivity in Formate Decomposition on Copper Surfaces

The formation of formate (HCOOa) adsorbate from CO2 and H2 on Cu catalysts, one of the most important elementary steps in methanol synthesis, is known to exhibit structure-insensitive kinetics and thermally nonequilibrium dynamics. In contrast, the kinetics of formate decomposition, the reverse reaction of formate formation, differs significantly across various Cu surfaces in terms of activation energy. The decomposition mechanism and its structure sensitivity remain unclear. In this study, we investigated the structure-dependent dynamics of formate decomposition by analyzing the angular distributions of CO2 and H2 desorbed from Cu(111), Cu(110), and Cu(100) surfaces under ultrahigh vacuum conditions. All CO2 angular distributions, measured between 400 and 500 K, were sharply collimated along the surface normal, following an identical function of cos8 θ, regardless of surface structure, formate coverage, or the presence of adsorbed oxygen. This lack of structure sensitivity can be attributed to the nearly identical local structure of the transition state (TS) on all Cu surfaces and the presence of monodentate, rather than bidentate, formate just before the TS. The results of this study align with previous molecular beam experiments in which structure-insensitive dynamics were observed during formate formation. In those experiments, hot CO2 gas molecules directly reacted with H adatoms via an Eley–Rideal-type mechanism, leading to the formation of monodentate formate. Our findings underscore the critical role of monodentate formate in CO2 hydrogenation over Cu-based catalysts, particularly in methanol synthesis.