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Wencka, Magdalena; Kovac, Janez; Dasireddy, Venkata D. B. C.; Likozar, Blaz; Jelen, Andreja; Vrtnik, Stanislav; Gille, Peter; Kim, Hae Jin; Dolinsek, Janez (2018): The effect of surface oxidation on the catalytic properties of Ga3Ni2 intermetallic compound for carbon dioxide reduction. In: Journal of Analytical Science and Technology, Vol. 9
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Background: In 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 Ga3Ni2 noble metal-free intermetallic compound, a new catalyst for the CO2 reduction to CO, CH4 and methanol. Methods: The instability of the oxidized Ga3Ni2 surface 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 H-2 and CO on the Ga3Ni2 catalyst surface was determined by H-2 and CO temperature-programmed desorption. CO2 conversion experiments were performed by using the catalyst material reduced in hydrogen at temperatures of 300 and 600 degrees C. Results: XPS study of the Ga3Ni2 surface 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 degrees 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 degrees C was ineffective to change the surface composition. Catalytic tests were in agreement with the XPS results, where an increased CO2 conversion for the catalyst reduced with hydrogen at a higher temperature followed an increased Ni/Ga surface concentration ratio. Conclusions: The 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. Ga3Ni2 and other Ga-Ni intermetallic compounds are good examples of binary intermetallic catalysts, whose catalytic performance is strongly affected by exposure to the air.