Formation and reactions of methyl radicals on metal oxides

Abstract

Over La2O3 and the transition metal oxide catalysts LiNiO2 and NaMnO4/MgO it was demonstrated that the coupling of the surface-generated gas-phase Ch4 radicals constitutes a major pathway for the formation of C2+ hydrocarbons (mainly C2H6 and C2H4) in the oxidative dimerization of methane. The kinetic isotope effect (KIE) observed in the generation of Ch3 radicals exhibited a dependence on the residence time in the post-reaction zone. Metal oxides differ greatly with respect to their reactivities with Ch3 radicals. Oxides in which the cations exhibit accessible multivalent oxidation states are highly reactive. The reactions of CH3 radicals with these metal oxides are believed to take place via a mechanism in which cations are reduced as a result of the electron transfer from CH3 radicals. The reactions are characterized by small activation energies and sticking coefficients that result from either electron-tunneling or a direct electron transfer which is limited by the weak adsorption of CH3 radicals. The methoxide ions that are produced ultimately form CO2 and CO via formate ions. The addition of Na2CO3 to La2O3, CeO2 and Tb2O3 has marked effect on the catalytic properties of these materials. The effect is most dramatic for CeO2, which, upon addition on Na2CO3, is transformed from a total oxidation catalyst to a reasonably selective catalyst for the formation of C2+ hydrocarbons. A sodium carbonate/oxide phase largely covers the lanthanide oxide surface. Accordingly, the Na/Ln[x]O[y] catalysts are similar to pure Na2CO3 with respect to their catalytic properties. The sodium compound inhibit reactions between CH3 radicals and the underlying metal oxide and provide a new active species, Na2O2. The active and selective NaMnO4/MgO catalyst also was partially covered with a sodium carbonate/oxide phase. Two phases may be responsible for the good performance of the catalyst: a crystalline phase of the structure of Mg6MnO8 and a sodium phase on the surface. The formation of the former is promoted by the presence of Na+ ions. The role of the sodium phase is the same as in Na/Ln[x]O[y] catalysts.