The selective catalytic reduction of nitric oxide with methane over nonzeolitic catalysts

This article deals with recent progress in the development of nonzeolitic catalysts for the selective catalytic reduction (SCR) of nitrogen oxides (NO_x) with methane. Although metal-exchanged zeolitic catalysts exhibit high activities for this reaction, most of these metastable structures suffer from deactivation problems when exposed to wet exhaust streams at elevated temperatures. Nonzeolitic oxide catalysts have the potential for improved durability because many of these are thermodynamically stable structures. The NO_x reduction activity and poisoning resistance of lanthanide oxides, Group IIA oxides, Group IIIB oxides, as well as gallium-, tin-, and palladium-based catalysts are reviewed. Current opinions on mechanistic aspects of the SCR reaction are discussed in relation to the development of more active catalytic systems. A detailed experimental study of the SCR of NO_x over rare earth oxides has also been performed. The role of absorbed oxygen in the activation of CH₄ and consumption of CH₃ radicals is discussed and related to the catalytic properties of the rare earth oxides. The presence of highly basic centers is shown to promote catalytic activity, and the presence of weakly absorbed oxygen species is found to be detrimental to catalytic selectivity. Oxides with lower electronic conductivities are found to be more selective than catalysts with higher conductivities. The selectivities of rare earth oxides for the SCR of NO_x with methane and the oxidative coupling of methane are also compared and contrasted.