Methane combustion over La2O3-based catalysts and γ-Al2O3 Original Research Article

The kinetics of methane combustion were examined over La2O3, Sr-promoted La2O3, 40% La2O3/γ-Al2O3 and γ-Al2O3 between 773 and 973 K, 0.4–5 Torr CH4 and 3–23 Torr O2. Rates (mol/(s g)) were highest for supported La2O3, but the rate on pure Al2O3 was one-third of this value; however, the specific activity (mol/(s m2)) on La2O3 was 130-fold greater than that on γ-Al2O3. The apparent activation energies were 25±2 kcal/mol for the La2O3 catalysts and 20 kcal/mol for γ-Al2O3. Respective reaction orders on CH4 and O2 were 0.69–0.75 and 0.12–0.18 for the La2O3-based catalysts compared to respective orders near 0.6 and 0.3 for γ-Al2O3. 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.rCH4=kPCH4PO21/2(1+KCH4PCH4+KO21/2PO21/2)2that fit the data well and indicated that CO, CO2 and H2O have no significant effect on this reaction under the reaction conditions employed here. An analysis of the enthalpies and entropies of adsorption for CH4 and O2 showed that these parameters were thermodynamically consistent, with indicated heats of adsorption for methane of around 20 kcal/mol CH4 on all four catalysts while for oxygen this value was around 30 kcal/mol O2 on the La2O3-based catalysts and near 20 kcal/mol O2 on γ-Al2O3. The activation energy calculated for the elementary step to form a methyl group on the surface was around 40 kcal/mol for all catalysts.