KINETIC-STUDIES OF THE CATALYTIC-OXIDATION OF METHANE .2. METHYL RADICAL RECOMBINATION AND ETHANE FORMATION OVER 1-PERCENT SR/LA2O3

The early stages of the catalytic oxidative conversion of methane to ethane over 1%Sr/La2O3 have been studied at 1086 K, using a heatable tubular flow reactor (containing the catalyst) coupled to a photoionization mass spectrometer. Temporally resolved concentrations of CH3, C2H6, and C2H4 were recorded along the catalyst bed. Conditions used resulted in negligible loss of the reactants (CH4 and O2, < 0.5%) and stable products (C2H6 and C2H4, < 2%). CH3 concentrations were observed first to grow and then to relax to steady-state concentrations during which time C2H6 and C2H4 accumulated. The observed temporal behavior of CH3 and C2H6 was quantitatively accounted for by a simple mechanism involving heterogeneous production of CH3 (whose rate law was fully characterized in part 1) and homogeneous loss of CH3 by recombination, CH3 + CH3 --> C2H6. The small amount of C2H4 formed (5 +/- 3% of the C2H6) can largely be accounted for by known gas-phase processes. Conversion of CH3 to C2H6 was stoichiometric, > 75%. No indication was found of any heterogeneous "deep oxidation" of methyl radicals or of any heterogeneous conversion of methane to C2 compounds (C2H6 or C2H4). Rate constants for the CH3 + CH3 reaction under the conditions of these experiments (which were used in the kinetic modeling of the experimental results) were obtained in separate experiments using the same reactor but with the catalyst removed, experiments in which the CH3 radicals were produced by laser photolysis of acetone and the decay of CH3 was monitored in time-resolved experiments.