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/gamma-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 deNO(x) feed did not regenerate the catalytic activity fully whereas reduction with H-2 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 N-2 over a wide temperature window. The decrease in the SCR activity observed over the Ag/gamma-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/gamma-Al2O3 catalyst (plain or sulphated) mostly occurred on the alumina (clean or sulphated). (C) 2000 Elsevier Science B.V. All rights reserved.