Hydrogenation of Formate Species Using Atomic Hydrogen on a Cu(111) Model Catalyst

The reaction mechanism of the CH₃OH synthesis by the hydrogenation of CO₂ on Cu catalysts is unclear because of the challenge in experimentally detecting reaction intermediates formed by the hydrogenation of adsorbed formate (HCOO_a). Thus, the objective of this study is to clarify the reaction mechanism of the CH₃OH synthesis by establishing the kinetic natures of intermediates formed by the hydrogenation of adsorbed HCOO_a on Cu(111). We exposed HCOO_a on Cu(111) to atomic hydrogen at low temperatures of 200-250 K and observed the species using infrared reflection absorption (IRA) spectroscopy and temperature-programmed desorption (TPD) studies. In the IRA spectra, a new peak was observed upon the exposure of HCOO_a on Cu(111) to atomic hydrogen at 200 K and was assigned to the adsorbed dioxymethylene (H₂COO_a) species. The intensity of the new peak gradually decreased with heating from 200 to 290 K, whereas the IR peaks representing HCOO_a species increased correspondingly. In addition, small amounts of formaldehyde (HCHO), which were formed by the exposure of HCOO_a species to atomic hydrogen, were detected in the TPD studies. Therefore, H₂COO_a is formed via hydrogenation by atomic hydrogen, which thermally decomposes at ∼250 K on Cu(111). We propose a potential diagram of the CH₃OH synthesis via H₂COO_a from CO₂ on Cu surfaces, with the aid of density functional theory calculations and literature data, in which the hydrogenation of bidentate HCOO_a to H₂COO_a is potentially the rate-determining step and accounts for the apparent activation energy of the methanol synthesis from CO₂ on Cu surfaces.