Catalytic properties of Ag/Al₂O₃ catalysts for lean NO _x reduction processes and characterisation of active silver species

Abstract

A study of the lean NO _x reduction activity employing different reductants over Ag/Al₂O₃ samples prepared from reverse microemulsions or impregnation with EDTA-complexes is presented. A multitechnique approach is employed for characterisation of the samples and/or processes taking place in the course of the NO_x-SCR reaction with propene and propane. Results by in situ-DRIFTS reveal that, for the propene reductant, silver provides a new path for hydrocarbon activation involving generation of adsorbed acrylate species as a partially oxidised active intermediate, in line with previous proposals for other non-noble metal systems. It is shown, mainly on the basis of XAFS studies, that active silver species are related to well dispersed silver aluminate-like phases with tetrahedral local symmetry and a relatively high disorder in the oxygen first shell.