TAP study of the mechanism and kinetics of the adsorption and combustion of methane on Ni/Al2O3 and NiO/Al2O3

The activation of methane on an industrial Ni/Al2O3 catalyst was studied in a TAP reactor between 723 and 823 K. Methane adsorbs on the reduced Ni catalyst with an activation energy of 54 kJ mol(-1). Oxidation of the Ni on the surface of the support up to 13% results in a strong decrease of the methane adsorption rate which is more than proportional to the decrease in the number of reduced Ni sites on the surface. Three parallel pathways are responsible for the adsorption of methane on the oxidized Ni catalyst. Two pathways consist of hydrogen abstraction of methane by oxygen, reversibly chemisorbed on two different active sites. The lifetime of oxygen chemisorbed on the two active sites is limited due to desorption and amounts to respectively 5 and 148 s at 823 K. The rate of adsorption of methane via the third pathway decreases on a time scale of 1000 s. The type of site that is responsible for this pathway cannot be regenerated by oxygen. The combustion of methane on the oxidized catalyst was studied by means of step experiments with methane and isotopically labeled oxygen. Chemisorbed oxygen abstracts hydrogen from gasphase methane and from adsorbed CHx species. Lattice oxygen is responsible for the carbon-oxygen bond formation. The structure of the Ni/Al2O3 catalyst was studied by means of TPR, XPS, and XRD. (C) 1999 Academic Press.