CO<sub>2</sub> Desorption\nDynamics Reveal Structure\nInsensitivity in Formate Decomposition on Copper Surfaces

The formation of formate (HCOO<sub>a</sub>) adsorbate\nfrom CO<sub>2</sub> and H<sub>2</sub> on Cu catalysts, one of the\nmost important\nelementary steps in methanol synthesis, is known to exhibit structure-insensitive\nkinetics and thermally nonequilibrium dynamics. In contrast, the kinetics\nof formate decomposition, the reverse reaction of formate formation,\ndiffers significantly across various Cu surfaces in terms of activation\nenergy. The decomposition mechanism and its structure sensitivity\nremain unclear. In this study, we investigated the structure-dependent\ndynamics of formate decomposition by analyzing the angular distributions\nof CO<sub>2</sub> and H<sub>2</sub> desorbed from Cu(111), Cu(110),\nand Cu(100) surfaces under ultrahigh vacuum conditions. All CO<sub>2</sub> angular distributions, measured between 400 and 500 K, were\nsharply collimated along the surface normal, following an identical\nfunction of cos<sup>8</sup> θ, regardless of surface\nstructure, formate coverage, or the presence of adsorbed oxygen. This\nlack of structure sensitivity can be attributed to the nearly identical\nlocal structure of the transition state (TS) on all Cu surfaces and\nthe presence of monodentate, rather than bidentate, formate just before\nthe TS. The results of this study align with previous molecular beam\nexperiments in which structure-insensitive dynamics were observed\nduring formate formation. In those experiments, hot CO<sub>2</sub> gas molecules directly reacted with H adatoms via an Eley–Rideal-type\nmechanism, leading to the formation of monodentate formate. Our findings\nunderscore the critical role of monodentate formate in CO<sub>2</sub> hydrogenation over Cu-based catalysts, particularly in methanol\nsynthesis.