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The formation of formate (HCOOa) adsorbate from CO2and H2on 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 CO2and H2desorbed from Cu(111), Cu(110), and Cu(100) surfaces under ultrahigh vacuum conditions. All CO2angular distributions, measured between 400 and 500 K, were sharply collimated along the surface normal, following an identical function of cos8θ, 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 CO2gas 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 CO2hydrogenation over Cu-based catalysts, particularly in methanol synthesis.

We employ time-slice and velocity map ion imaging methods to explore the quantum-state resolved dynamics in thermal N2O decomposition on Pd(110). We observe two reaction channels: a thermal channel that is ascribed to N2products initially trapped at surface defects and a hyperthermal channel involving a direct release of N2to the gas phase from N2O adsorbed on bridge sites oriented along the [001] azimuth. The hyperthermal N2is highly rotationally excited up to J= 52 (v″ = 0) with a large average translational energy of 0.62 eV. Between 35 and 79% of the estimated barrier energy (1.5 eV) released upon dissociation of the transition state (TS) is taken up by the desorbed hyperthermal N2. The observed attributes of the hyperthermal channel are interpreted by post-transition-state classical trajectories on a density functional theory-based high-dimensional potential energy surface. The energy disposal pattern is rationalized by the sudden vector projection model, which attributes to unique features of the TS. Applying detailed balance, we predict that in the reverse Eley–Rideal reaction, both N2translational and rotational excitation promote N2O formation.

Understanding gas–surface reaction dynamics, such as the rupture and formation of bonds in vibrationally and translationally excited (‘hot’) molecules, is important to provide mechanistic insight into heterogeneous catalytic processes. Although it has been established that such excitation can affect the reactions occurring via dissociative mechanisms, for associative mechanisms—in which the gas-phase reactant collides directly with a surface-adsorbed species—only translational excitation has been observed to affect reactivity. Here we report a bond-formation reaction that is driven by the vibrational energy of reactant molecules and occurs via an (associative) Eley–Rideal-type mechanism, in which the reaction takes place in a single collision. Hot CO2in a molecular beam is found to react with pre-adsorbed hydrogen atoms directly on cold Cu(111) and Cu(100) surfaces to form formate adspecies. The vibrational energy of CO2is more effective at promoting the reaction than translational energy, the reaction rate is independent of the surface temperature and the experimental results are consistent with density functional theory calculations. Vibrational and translational energies have previously been observed to promote reactions at surfaces occurring via dissociative mechanisms. Now, it has been shown that the reaction of CO2with surface-adsorbed atomic hydrogen—which occurs via an associative (Eley–Rideal-type) mechanism—can be driven by vibrationally exciting CO2.