Surface species in CO 2 methanation over amorphous palladium/zirconia catalysts

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The mechanism of methane formation over a palladium/zirconia catalyst prepared by wet impregnation of amorphous ZrO 2 has been investigated by diffuse reflectance FTIR spectroscopy. The production rate of surface formate, which represents the pivotal surface intermediate, is high when starting from gaseous CO 2 and H 2; a considerably lower rate is observed upon hydrogen exposure of a carbonate-covered catalyst surface. Reference compounds have been adsorbed to determine the CO stretching frequency of a potential formyl (HCO) surface intermediate, which apparently gives rise to a broad absorption around 1700 cm −1. This band is not observed under CO 2 hydrogenation conditions. Instead, a characteristic absorption is detected at 1760 cm −1, which is assigned to CO triply bound to the palladium particles. The latter species is correlated with surface formate, in an equilibrium that involves surface hydroxyl groups on the zirconia. No further surface species are detected under CO 2 hydrogenation conditions. A mechanism for the hydrogenation Of CO 2 to methane is proposed that accounts for these observations; the importance of surface hydroxyl groups and anion vacancies on the zirconia surface is emphasized.