The kinetics and mechanism of methanol synthesis by hydrogenation of CO 2 over a Zn-deposited Cu(111) surface

Abstract

The hydrogenation of CO 2 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 ϑ Zn=0.19 and decreased above ϑ Zn=0.20. The optimum TOF obtained at ϑ Zn=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 ϑ Zn=0.10 and approached zero at ϑ Zn=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 (HCOO a) on the Cu(111) surfaces. The formate coverage linearly increased with the Zn coverage below ϑ Zn=0.15, suggesting that the formation of the formate species was stabilized by the Zn species. The relation between ϑ HCOO and ϑ Zn is similar to that between TOF and ϑ Zn; 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 ϑ Zn=0.15. At higher Zn coverages ( ϑ Zn>0.15), Zn on Cu(111) was readily oxidized to ZnO during the CO 2 hydrogenation reaction. On the other hand, at low Zn coverages below ϑ Zn=0.15, Zn was partially oxidized in the absence of oxygen in ZnO or O a 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.