CO2 HYDROGENATION OVER NICKEL ZIRCONIA CATALYSTS FROM AMORPHOUS PRECURSORS - ON THE MECHANISM OF METHANE FORMATION

An amorphous Ni64Zr36 alloy is used as a precursor for a carbon dioxide hydrogenation catalyst. Upon exposure to CO2 hydrogenation conditions, the glassy metal is partly transformed into crystalline metallic nickel particles and less well-ordered zirconium dioxide. A conventional Ni/ZrO2 catalyst, which was synthesized by coprecipitation and calcination of the amorphous precipitate, served as a reference. The as-prepared catalyst exhibits a very similar catalytic behavior to the alloy-derived catalyst. The structural and chemical changes are characterized by gas adsorption, X-ray diffraction, and thermal analysis. In CO2 hydrogenations over these catalysts, methane is almost exclusively produced, and traces of ethane are formed besides, as evidenced by gas chromatography. To study the origins of this selectivity behavior, in situ diffuse reflectance FTIR spectroscopy has been applied. Surface species in carbon dioxide as well as carbon monoxide hydrogenation reactions are correlated with the formation of gas-phase products. CO2/H2 mixtures rapidly yield surface formate as the immediate precursor of the methane product on the coprecipitated catalyst. Doubly and singly bound adsorbed CO is detected besides; doubly bound CO is observed to originate from surface formate. In CO/H2 reactions, higher saturated hydrocarbons are produced besides methane. The formation of these products proceeds from singly bound adsorbed CO via a dissociative mechanism. The respective hydrogenation steps are discussed in detail in a corresponding reaction scheme.