KINETICS AND MECHANISM OF THE HETEROGENEOUS OXIDATION OF METHYL RADICALS ON SAMARIUM(III) OXIDE - IMPLICATIONS FOR THE OXIDATIVE COUPLING OF METHANE

The purely heterogeneous oxidation of CH3(g) on Sm2O3, in the presence and absence of O-2(g), was investigated in a very low pressure flow r eactor by molecular beam mass spectrometry, between 1000 and 1200 K. I n the presence of O-2, CH3 is quantitatively oxidized to H2O and COx a t steady state rates proportional to [CH3]Gamma 5-([O-2]) = [CH3]k(5)K (5)(1/2)[O-2](1/2)/(1 + K-5(1/2)[O-2](1/2)). Alternate or simultaneous measurement of oxidation rates for CH3 and CH4, the latter proportion al to [CH4]Gamma(4)([O-2]), on the same Sm2O3 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) - (648 0 +/- 130)/T(3). Equations 1-3 imply that (1) methyl radicals are oxid ized faster than methane on Sm2O3 below 1500 K and (2) each reaction o ccurs on distinguishable active sites generated by endothermic, but hi ghly exentropic, O-2 chemisorptive processes involving cooperative par ticipation of the solid. Transient experiments provide evidence on the relative timing of O-2 chemisorption, CH3 oxidation, and CO2 release. Sm2O3 is almost inert under anoxic conditions. Present results impose an irreducible floor to COx yields in the steady state oxidation of m ethane on Sm2O3 at low pressures, but open up the possibility of disen gaging CH3 and CH4 oxidations under other conditions.