Effect of Catalyst Preparation on Catalytic Activity: VI. Chemical Structures on Nickel/Alumina Catalysts: their Impact on the Rate-Determining Step in the Hydrogenation of Carbon Monoxide

Abstract

Titration of the carbon pool subsequent to steady-state reaction of hydrogen and carbon monoxide over Ni/Al 2O 3 catalysts in conjunction with temperature-programmed surface reaction studies has been performed to assess the impact of nickel speciation on the rate-determining step in the methanation reaction. Mass balances for the desorption products reveal significant differences that depend on the weight loading of the catalyst. Low-weight loading catalysts prepared by incipient wetness consist of NiAl 2O 4 reactive centers as revealed by electron spectroscopy for chemical analysis and carbon monoxide temperature-programmed reaction experiments. Data are presented which support the hypothesis the rate of methanation is controlled by the rate of surface-bound carbon monoxide dissociation over the catalysts. On the other hand, high-weight loading catalysts prepared by this method consist of “particle-like” nickel. Over these catalysts, the rate of methanation is found to be controlled by the rate of hydrogenation of CH x . It is proposed that variation in catalyst preparation procedures can account for the wide-spread controversy in the determination of rate-determining step for methanation over Ni/Al 2O 3 catalysts.