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.