Preparation and Characterization of Ni5Ga3 for Methanol Formation via CO2 Hydrogenation

A nickel–gallium catalyst (Ni5Ga3) with superior catalytic performance for the hydrogenation of CO2 into methanol was prepared by co-precipitation, and calcined for 7 h at 600 °C in a flowing stream of air containing hydrogen. On the basis of measurements of X-ray photoelectron spectra (XPS), the fraction of integrated area of Ga2O3 over the Ni5Ga3 surface decreased significantly from 75.24 to 53.24%. Spectra showing X-ray absorption near-edge structure (XANES) indicate that the oxidation state of gallium atoms shifted from Ga(0) to Ga(III) indicating that Ga is a reactive component. Extended X-ray absorption fine structure (EXAFS) showed that the first shells of gallium and its adjacent atoms in fresh and used Ni5Ga3 were Ga–Ga and Ga–O, respectively. The coordination number, 5.56, of Ga atoms in used Ni5Ga3 was larger than that, 5.31, in the fresh catalyst; the bond Ga–O of length 1.94 Å is shorter than the bond Ga–Ga of length 1.98 Å. These results of XPS and XANES/EXAFS analyses indicate that the reactive gallium in Ni5Ga3 is first oxidized to Ga2O3 and then carbonated to GaCO3 over the catalyst surface during the formation of methanol. That formation was demonstrated through the appearance of specific peaks at 3681, 2982, 1345, and 1053 cm−1 for the absorption of methanol in infrared spectra recorded in situ. At 250 °C and 50 bar, the greatest conversion of CO2 was 95.7% and the greatest yield of methanol was 72.2%. In terms of the formation of dimethyl ether, the optimal reaction temperature with greatest selectivity (47.6%) and yield (27.8%) was 350 °C. Most importantly, the conversions of CO2 at 150, 250 and 350 °C were much larger than the equilibrium conversions, 47.6, 47.8 and 47.2%, of CO2, which indicates that Ni5Ga3 prevented the formation of water so that methanol could be produced continuously. For the formation of methanol with Ni5Ga3, the optimal Gibbs energy was 4.23 kJ mol−1; the equilibrium constant was 0.378 h−1.