New Insights into the Active Surface Species of Silver Alumina Catalysts in the Selective Catalytic Reduction of NO

The performance of silver alumina catalysts and silver aluminate was studied in the selective catalytic reduction (SCR) of NO by propene. The use of boehmite during the impregnation step ensured a strong interaction between the silver species and the alumina surface in the final calcined catalyst. Thus, higher silver loadings (5−7 wt % silver) could be used without significant loss in selectivity to N₂ during SCR. The nature of the silver species and the formation of adsorbed surface species during SCR and hydrogen-assisted SCR (H₂-SCR) was studied by activity measurements and by combined in situ IR and X-absorption spectroscopic measurements. The combination of these techniques in the same reaction cell allowed simultaneous monitoring of the state of silver and the formation of surface species under realistic reaction conditions. The active silver species on alumina support were concluded to be 2-dimensional oxidic Ag_n^δ+ species. Silver aluminate was ruled out as a possibility for an active phase for the SCR reaction. The oxidic Ag_n^δ+ species were present under both SCR and H₂-SCR reaction conditions and even on a prereduced catalyst under the SCR reaction conditions. Hydrogen is proposed to enhance the formation of adsorbed surface species, especially nitrites, but not to change the nature of the active silver sites.