The Barrier for CO₂ Functionalization to Formate on Hydrogenated Pt

Abstract

Understanding heterogeneous catalysis is based on knowing the energetic stability of adsorbed reactants, intermediates, and products as well as the energetic barriers separating them. We report an experimental determination of the barrier to CO₂ functionalization to form bidentate formate on a hydrogenated Pt surface and the corresponding reaction energy. This determination was possible using velocity resolved kinetics, which simultaneously provides information about both the dynamics and rates of surface chemical reactions. In these experiments, a pulse of isotopically labeled formic acid (DCOOH) doses the Pt surface rapidly forming bidentate formate (DCO*O*). We then record the (much slower) rate of decomposition of DCO*O* to form adsorbed D* and gas phase CO₂. We establish the reaction mechanism by dosing with O₂ to form adsorbed O*, which efficiently converts H* or D* to gas phase water. H₂O is formed immediately reflecting rapid loss of the acidic proton associated with formation of formate, while D₂O formation proceeds more slowly and on the same time scale as the CO₂ production. The temperature dependence of the reaction rate yields an activation energy that reflects the energy of the transition state with respect to DCO*O*. The derived heat of formation for DCO*O* on Pt(111) agrees well with results of microcalorimetry. The maximum release of translational energy of the formed CO₂ provides a measure of the energy of the transition state with respect to the products and the barrier to the reverse process, functionalization of CO₂. The comparison between the results on Pt(111) and Pt(332) shows that the barrier for CO₂ functionalization is reduced by the presence of steps. The approach taken here could provide a method to optimize catalysts for CO₂ functionalization.