(A) study on the role of copper and metal oxides for $CO_2$ hydrogenation to methanol

Abstract

Formate on copper metal or ZnO has been suggested as an intermediate in the hydrogenation of carbon oxides to methanol. The formates of copper and zinc were prepared by the addition of corresponding metal carbonates into formic acid solutions. Temperature-programmed decomposition (TPD) and temperature-programmed hydrogenation (TPH) experiments were performed with copper formate and zinc formate. The maximum decomposition temperature of copper formate was 450 K. On the other hand, that of zinc formate was 525 K. Formic acid was found to be the only organic product from the TPD and TPH reaction of the copper formate, while methanol and methyl formate were major organic products from the TPD and TPH reaction of the zinc formate. Methyl formate was a major product in TPH reaction under atmospheric pressure, whereas methanol became a major one at 27.2 atm. When ZnO was mixed into copper formate, methanol and methyl formate were formed from TPH reaction and the maximum decomposition temperature $(T_{max})$ of copper formate was shifted to 478 and 505 K from 450 K. The presence of ZnO was essential for the methanol formation in the formate reduction. In order to understand the formate hydrogenation capability of other oxides, copper formate mixed with metal oxide were hydrogenated at the atmospheric hydrogen pressure. Those results showed that all oxides took part in TPH reaction of copper formate, although there are some differences. The temperature-programmed IR (TPIR) spectroscopic investigations were performed with copper formate and ZnO mixture in helium atmosphere. Two absorption peaks at 1352.1 and 1552.1 $cm^{-1}$ began to decrease in intensity and two new peaks at 1382.6 and 1598.8 $cm^{-1}$ start to appear at 353 K. At the end of TPIR, the final spectrum was almost the same as the one of zinc formate. The interaction between copper and ZnO during CO2 hydrogenation was investigated. The conversion of carbon dioxide and the selectivity to methanol on Cu/Zn...