X-ray photoelectron spectroscopy and scanning tunnel microscope studies of formate species synthesized on Cu(111) surfaces

The synthesis of formate species on Cu(111) surfaces by hydrogenation of CO₂ at atmospheric pressure was studied by x-ray photoelectron spectroscopy and scanning tunnel microscope (STM). The initial formation rate of formate was measured to be 2.1×10^-4 molecules site ^-1 s^-1 at P_H2 = PCO₂= 380 Torr and 333 K. The saturation coverage of formate species was estimated to be ⊖_HCOO= 0.24, where ⊖=1 corresponded to the number of Cu surface atoms. No promotional effect of Zn on the formation of formate species over a Cu(111) surface was observed, indicating that the promotional effect of Zn previously observed for methanol synthesis on Cu(111), is not ascribed to promotion in the formate formation, but is ascribed to promotion in a hydrogenation process of formate species. STM images were obtained for individual formate species synthesized on a clean Cu(111) surface by hydrogenation of CO₂ at P_H2 = P_co2 = 380 Torr and 333 K for 20 min. Formate species arranged in a hexagonal structure with a distance of 6.6±0.2 Å were attributed to a (√7×√ 7) R 19.1° periodicity. A different domain with ac(2×8) structure and the boundary with the (√7×√7) R19.1° domain were observed. It was found that formate species tended to be arranged with their molecular planes parallel to each other.