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 reactor by molecular beam mass spectrometry, between 1000 and 1200 K. In the presence of O-2, CH3 is quantitatively oxidized to H2O and COx at 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 proportional 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) - (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, O-2 chemisorptive processes involving cooperative participation 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 methane on Sm2O3 at low pressures, but open up the possibility of disengaging CH3 and CH4 oxidations under other conditions.