MECHANISTIC AND KINETIC-STUDIES OF THE REACTIONS OF GAS-PHASE METHYL RADICALS WITH METAL-OXIDES

Reactions of CH3. radicals with metal oxide powders have been studied quantitatively for the first time in an attempt to understand the very large differences in selectivity that occur during the catalytic oxidation of methane. The sticking coefficients for CH3. radicals on ZnO, CeO2, and MgO were determined to be 1.8 X 10(-5), 2.1 x 10(-6), and 1.2 X 10(-7), respectively. The relative specific reaction rates of CH3. radicals with metal oxides were found to follow the series ZnO > MoO3 > NiO > CeO2 > MgO congruent-to Li+/MgO. From the absolute sticking coefficients and the relative reaction rates, first-order rate constants for the CH3. radical reactions with metal oxides were determined. Methyl radicals react with MoO3/SiO2 by electron transfer, which is apparent in the reduction of Mo6+ to Mo5+. An analogous reaction apparently occurs on CeO2 at 100-degrees-C, but the electron is subsequently transferred to molecular oxygen, forming 02- on tbc surface. As a result of this reduction reaction, methoxide ions are produced, and these yield formate ions. The methoxide and formate ions are believed to be intermediates in the nonselective oxidation of CH4 to CO and CO2. For the selective oxidative dimerization of CH4, a small sticking coefficient is required so that a CH3. radical may reflect off a catalyst surface many times before it collides and reacts with another CH3. radical. Thus, ZnO, MoO3, NiO, and CeO2 are nonselective oxidation catalysts, whereas Li+/MgO is a reasonably active and selective catalyst for the oxidative dimerization reaction.