Mass spectrometric studies of the low-pressure oxidation of methane on samarium sesquioxide

Abstract

The oxidation of methane over samarium sesquioxide, in the presence and in the absence of gaseous oxygen, has been investigated in a low-pressure flow reactor dynamically coupled to a mass spectrometer between 859 and 1100 K. In the presence of O_2(g) the reaction is catalytic. At very low pressures rates become first- and half-order with respect to methane and oxygen, respectively, revealing the reversible dissociative chemisorption of oxygen on the catalyst. The kinetic behavior observed over the entire range of pressures studied here is shown to be consistent with competitive Langmuir adsorption of both reactants followed by chemical reaction of CH_4(g) with chemisorbed oxygen. The rate constants for the latter step exhibit Arrhenius temperature dependence with E_3 = 115 kJ mol^(-1). In the absence of O_2(g) the rate of oxidation displays a slow induction period, whose duration depends on temperature and methane pressure, followed by sudden acceleration up to a maximum before decreasing to zero when the solid becomes covered with a carbon deposit. Product distribution changes along this process. This pattern is quantitatively reproduced after oxide regeneration. Finally, it was found that the oxide can only be reduced by H_2(g) up to the onset of the fast oxidation stage, Le., that after this treatment the solid remains able to rapidly oxidize methane, revealing the remarkable specificity of this process.