摘要：AbstractBackgroundIn a routine handling of a catalyst material, exposure to air can usually not be avoided. For noble metal catalysts that are resistant to oxidation, this is not an issue, but becomes important for intermetallic catalysts composed of two or more non-noble chemical elements that possess much different standard enthalpies of the oxide formation. The element with higher affinity to oxygen concentrates on the surface in the oxide form, whereas the element with lower affinity sinks into the subsurface region. This changes the number of active sites and the catalytic performance of the catalyst. We have investigated the instability of the surface composition to oxidation of the Ga3Ni2noble metal-free intermetallic compound, a new catalyst for the CO2reduction to CO, CH4and methanol.MethodsThe instability of the oxidized Ga3Ni2surface composition to different heating–annealing conditions was studied by X-ray photoelectron spectroscopy (XPS), used to determine the elemental composition and the chemical bonding in the near-surface region. The dispersion of active sites available for the chemisorption of H2and CO on the Ga3Ni2catalyst surface was determined by H2and CO temperature-programmed desorption. CO2conversion experiments were performed by using the catalyst material reduced in hydrogen at temperatures of 300 and 600 °C.ResultsXPS study of the Ga3Ni2surface subjected to different heating–annealing conditions has revealed that the concentration of Ga at the oxidized surface is strongly enhanced and the concentration of Ni is strongly depleted with respect to the values in the bulk. By annealing the surface at 600 °C in ultra-high vacuum, the oxides have evaporated and thermal diffusion of atoms near the surface has partially reconstructed the surface composition towards the energetically more favorable bulk value, whereas annealing at a lower temperature of 300 °C was ineffective to change the surface composition. Catalytic tests were in agreement with the XPS results, where an increased CO2conversion for the catalyst reduced with hydrogen at a higher temperature followed an increased Ni/Ga surface concentration ratio.ConclusionsThe instability of the active surface chemical composition to oxidation in air must be taken into account when considering noble metal-free intermetallic catalysts as alternatives to the conventional catalysts based on noble metals. Ga3Ni2and other Ga–Ni intermetallic compounds are good examples of binary intermetallic catalysts, whose catalytic performance is strongly affected by exposure to the air.
关键词：Ga;3;Ni;2;intermetallic catalyst;Surface instability to oxidation;CO;2;catalytic conversion
