Kinetics and Mechanism of the Heterogeneous Oxidation of Methyl Radicals on Samarium(III) Oxide. Implications for the Oxidative Coupling of Methane

Abstract

The purely heterogeneous oxidation of CH_3(g) on Sm_2O_3, in the presence and absence of O_2(g), was investigated in a very low pressure flow reactor by molecular beam mass spectrometry, between 1000 and 1200 K. In the presence of O_2, CH_3 is quantitatively oxidized to H_2O and CO_x at steady state rates proportional to [CH3]Γ_5-([O_2]) = [CH_3]k_5K_^(s1/2)[O_2]^(1/2)/(1 + K_5^(1/2)[O_2]^(1/2)). Alternate or simultaneous measurement of oxidation rates for CH_3 and CH_4, the latter proportional to [CH_4]Γ_4([O_2]), on the same Sm_2O_3 sample as a function of [O_2] and temperature, led to the following expressions: log(k_5/k_4) = -(2.18 ± 0.35) + (3210 ± 301)/T (1), log(K_4/10^9 M) = (1.89 ± 0.25) - (4170 ± 260)/T (2), log(K_5/10^9 M) = (5.65 ± 0.11) - (6480 ± 130)/T (3). Equations 1-3 imply that (1) methyl radicals are oxidized faster than methane on Sm_2O_3 below 1500 K and (2) each reaction occurs on distinguishable active sites generated by endothermic, but highly exentropic, O_2 chemisorptive processes involving cooperative participation of the solid. Transient experiments provide evidence on the relative timing of O_2 chemisorption, CH_3 oxidation, and CO_2 release. Sm_2O_3 is almost inert under anoxic conditions. Present results impose an irreducible floor to CO_x yields in the steady state oxidation of methane on Sm_2O_3 at low pressures, but open up the possibility of disengaging CH_3 and CH_4 oxidations under other conditions.