Effect of ex situ treatments with SO2 on the activity of a low loading silver–alumina catalyst for the selective reduction of NO and NO2 by propene

A 1.2 wt.% Ag/γ-Al2O3 catalyst for the selective catalytic reduction of NO with propene (C3H6-SCR) was rapidly and permanently deactivated following the addition of 100 ppm of SO2 to the gas stream. DRIFTS studies performed on the deactivated silver–alumina material showed the formation of two different types of surface sulphate species. One of these surface species was a surface aluminium sulphate whereas the other corresponded to a sulphate associated with the silver phase. Treatment at higher temperatures in the deNOx feed did not regenerate the catalytic activity fully whereas reduction with H2 was able to do so by removing only the sulphate species associated with the silver phase. When NO2 was used as reactant, there was no need for a promoter for either the alumina or the sulphated alumina, both of those giving high conversions of the NO2 to N2 over a wide temperature window. The decrease in the SCR activity observed over the Ag/γ-Al2O3 upon sulphation is thought to be associated with a decrease of the activity for the oxidation of NO to ad-NOx species (x > 1) promoted by the silver. It is suggested that the reduction of ad-NOx species and NO2 by propene over the 1.2 wt.% Ag/γ-Al2O3 catalyst (plain or sulphated) mostly occurred on the alumina (clean or sulphated).