QUANTITATIVE STUDIES OF METHYL RADICALS REACTING WITH METAL-OXIDES

The reactive sticking coefficients of CH3. radicals, produced by the t hermal decomposition of azomethane, were determined over ZnO, CeO2, La 2O3, Sr/La2O3, and Li/MgO at 650 degrees C. The intrinsic sticking coe fficients ranged from 6 x 10(-8) for La2O3 activated at 900 degrees C to 3 x 10(-5) over ZnO. In the presence of even 0.1 mTorr of O-2 the s ticking coefficient for the La2O3 sample increased to 3 x 10(-6). Alth ough the other nonreducibie oxides exhibited similar behaviors, the ma gnitude of the effect was not as great, Methyl radicals are believed t o react via electron transfer at the surface. The electron transfer ma y be either to metal ions, in the case of reducible oxides, or to O-2, in the case of nonreducible oxides. Reactions of CH3. radicals with s everal high surface area, porous silicas and zeolites were also studie d. The sticking coefficient on a high-purity silica was found to be 1 x 10(-8) at 200 degrees C. An analysis of the stable products indicate d that the reaction of CH3. radicals with these surfaces exceeded the coupling reactions that might have occurred at the surfaces. That is, the surfaces did not function as an effective ''third body'' in the co upling reaction.