Kinetics and mechanism of the heterogeneous oxidation of methyl radicals on samarium(III) oxide. Implications for the oxidative coupling of methane

Article abstract of DOI:10.1021/ja00118a022

The purely heterogeneous oxidation of CH₃(g) on Sm₂O₃, in the presence and absence of O₂(g), was investigated in a very low pressure flow reactor by molecular beam mass spectrometry, between 1000 and 1200 K. In the presence of O₂, CH₃ is quantitatively oxidized to H₂O and CO_x at steady state rates proportional to [CH₃]Γ₅-([O₂]) = [CH₃]k₅K₅^1/2[O₂] ^1/2/(1 + K₅^1/2[O₂]^1/2). Alternate or simultaneous measurement of oxidation rates for CH₃ and CH₄, the latter proportional to [CH₄]Γ₄([O₂]), on the same Sm₂O₃ sample as a function of [O₂] and temperature, led to the following expressions: log(k₅/k₄) = -(2.18 ± 0.35) + (3210 ± 301)/T (1), log(K₄/10⁹ M) = (1.89 ± 0.25) - (4170 ± 260)/T (2), log(K₅/10⁹ M) = (5.65 ± 0.11) - (6480 ± 130)/T (3). Equations 1-3 imply that (1) methyl radicals are oxidized faster than methane on Sm₂O₃ below 1500 K and (2) each reaction occurs on distinguishable active sites generated by endothermic, but highly exentropic, O₂ chemisorptive processes involving cooperative participation of the solid. Transient experiments provide evidence on the relative timing of O₂ chemisorption, CH₃ oxidation, and CO₂ release. Sm₂O₃ is almost inert under anoxic conditions. Present results impose an irreducible floor to CO_x yields in the steady state oxidation of methane on Sm₂O₃ at low pressures, but open up the possibility of disengaging CH₃ and CH₄ oxidations under other conditions.