The reaction mechanism of CH3OH synthesis by hydrogenation of CO2 on Cu catalysts is unclear because of the challenge in experimentally detecting reaction intermediates formed by the hydrogenation of adsorbed formate (HCOOa). Since formate is thermally stable on Cu surfaces, it is thought that a large activation energy is required for subsequent formation of the intermediate, but the intermediate is difficult to detect due to its thermal instability. On the other hand, the ability of the Cu surface for the dissociation of H2 is low, so that a temperature of 400K or higher is required for the hydrogenation of formate. Such circumstances make it difficult to detect intermediates after formate. We thus exposed formate 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 exposure of HCOOa on Cu(111) to atomic hydrogen at 200 K and was assigned to the adsorbed dioxymethylene (H2COOa) species. The intensity of the new peak gradually decreased with heating from 200 to 290 K, whereas the IR peaks representing formate species increased correspondingly. In addition, small amounts of formaldehyde (HCHO), which were formed by exposure of formate species to atomic hydrogen, were detected in the TPD studies. Therefore, dioxymethylene is formed via hydrogenation by atomic hydrogen, which thermally decomposes at ~250 K on Cu(111). We propose a potential diagram of methanol synthesis via dioxymethylene from CO2 on Cu surfaces, with the aid of density functional theory calculations and literature data, in which the hydrogenation of bidentate formate to dioxymethylene is potentially the rate-determining step and accounts for the apparent activation energy of methanol synthesis from CO2 on Cu surfaces.

Reaction intermediates of methanol synthesis from CO2 on a Cu(111) model catalyst

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