Methane combustion over La 2O 3-based catalysts and γ-Al 2O 3

Abstract

The kinetics of methane combustion were examined over La 2O 3, Sr-promoted La 2O 3, 40% La 2O 3/γ-Al 2O 3 and γ-Al 2O 3 between 773 and 973 K, 0.4–5 Torr CH 4 and 3–23 Torr O 2. Rates (mol/(s g)) were highest for supported La 2O 3, but the rate on pure Al 2O 3 was one-third of this value; however, the specific activity (mol/(s m 2)) on La 2O 3 was 130-fold greater than that on γ-Al 2O 3. The apparent activation energies were 25±2 kcal/mol for the La 2O 3 catalysts and 20 kcal/mol for γ-Al 2O 3. Respective reaction orders on CH 4 and O 2 were 0.69–0.75 and 0.12–0.18 for the La 2O 3-based catalysts compared to respective orders near 0.6 and 0.3 for γ-Al 2O 3. A reaction mechanism invoking quasi-equilibrated adsorption of all reactants and products along with methyl radical formation as a slow step produced a derived rate expression, i.e. r CH 4 = kP CH 4 P O 2 1/2 (1+K CH 4 P CH 4 +K O 2 1/2P O 2 1/2) 2 that fit the data well and indicated that CO, CO 2 and H 2O have no significant effect on this reaction under the reaction conditions employed here. An analysis of the enthalpies and entropies of adsorption for CH 4 and O 2 showed that these parameters were thermodynamically consistent, with indicated heats of adsorption for methane of around 20 kcal/mol CH 4 on all four catalysts while for oxygen this value was around 30 kcal/mol O 2 on the La 2O 3-based catalysts and near 20 kcal/mol O 2 on γ-Al 2O 3. The activation energy calculated for the elementary step to form a methyl group on the surface was around 40 kcal/mol for all catalysts.