Direct Observation of Dioxymethylene and Formaldehyde as Hydrogenation Products of Formate Species on the Hydrogen-Adsorbed Cu(997) Surface

To elucidate the hydrogenation process of formate species on Cu-based model catalysts, we investigated the thermal process of formic acid on the clean Cu(997) surface and the H-adsorbed Cu(997) surface using temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). In the TPD spectra of HCOOH/H/Cu(997), desorption of formaldehyde was observed at 260–410 K. By comparing the results for the clean Cu(997) surface with those for the H-adsorbed Cu(997) surface, we have found that hydrogenated species are newly formed by the presence of hydrogen on Cu(997) after heating to 300 K using HREELS; the intermediate species is assigned to dioxymethylene. These results indicate that some of formate species are hydrogenated to dioxymethylene below 300 K and the dioxymethylene is decomposed and desorbed as formaldehyde between 260 and 410 K. In addition, the bidentate formate species at step sites are predominantly involved in the hydrogenation reaction. On the other hand, most of the bidentate formate species on the terrace are decomposed and desorbed as CO₂ and H₂; these formate species may appear as spectator species. We conclude that the Cu step site plays an important role in the hydrogenation of formate species to dioxymethylene and formaldehyde.