Secondary reactions of methyl radicals with lanthanide oxides: their role in the selective oxidation of methane

Abstract

Reactions between methyl radicals and certain members of the lanthanide oxide series have been studied by using a matrix isolation electron spin resonance technique. Methyl radicals react extensively with the lanthanide-metal oxides CeO{sub 2}, Pr{sub 6}O{sub 11}, and Tb{sub 4}O{sub 7}, all of which have multiple cationic oxidation states. By contrast, the oxides La{sub 2}O{sub 3}, Nd{sub 2}O{sub 3}, Sm{sub 2}O{sub 3}, Eu{sub 2}O{sub 3}, and Yb{sub 2}O{sub 3} react with CH{sub 3}{sup {sm bullet}} radicals to only a small extent. Consistent with this observation, the former group of oxides is ineffective in the generation of CH{sub 3}{sup {sm bullet}} radicals which emanate into the gas phase. The reaction of CeO{sub 2} with CH{sub 3}{sup {sm bullet}} radicals is strongly inhibited by the addition of Na{sub 2}CO{sub 3}, and consequently radical production is enhanced. This effect is manifested in the catalytic properties of CeO{sub 2}, which is a complete oxidation catalyst in its pure form but becomes a good catalyst for the oxidative coupling of methane following the addition of Na{sub 2}CO{sub 3}.