The kinetics and mechanism of methanol synthesis by hydrogenation of CO2 over a Zn-deposited Cu(111) surface

The hydrogenation of CO2 over a Zn-deposited Cu(111) surface has been studied using an X-ray photoelectron spectroscopy (XPS) apparatus combined with a high-pressure flow reactor. It was shown that the turnover frequency (TOF) for methanol formation linearly increased with Zn coverage below ϑZn=0.19 and decreased above ϑZn=0.20. The optimum TOF obtained at ϑZn=0.19 was thirteen-fold larger than that of the Zn-free Cu(111) surface. On the other hand, the TOF for CO formation started to decrease at ϑZn=0.10 and approached zero at ϑZn=0.5. No promotional effect of Zn was thus observed for the reverse water-gas shift (RWGS) reaction on Cu(111). Post-reaction surface analysis by XPS showed the formation of formate species (HCOOa) on the Cu(111) surfaces. The formate coverage linearly increased with the Zn coverage below ϑZn=0.15, suggesting that the formation of the formate species was stabilized by the Zn species. The relation between ϑHCOO and ϑZn is similar to that between TOF and ϑZn; thus, the formate species is considered to be the reaction intermediates during methanol formation, and the amount of the formate species should determine the rate of the reaction. It was found that the surface chemistry of the Zn-deposited Cu surface drastically changed at ϑZn=0.15. At higher Zn coverages (ϑZn>0.15), Zn on Cu(111) was readily oxidized to ZnO during the CO2 hydrogenation reaction. On the other hand, at low Zn coverages below ϑZn=0.15, Zn was partially oxidized in the absence of oxygen in ZnO or Oa on the Cu surface under the reaction conditions. It was suggested that the Zn on Cu(111) was directly bound to the oxygen in the surface formate species as the role of the active sites.