Chemical deactivation of Ag/Al2O3 by sulphur for the selective reduction of NOx using hydrocarbons

The hydrocarbon-SCR activity of Ag/Al2O3 catalysts is severely deactivated after low temperature (350 °C) sulphur ageing in the form of SO2 exposure. Catalysts aged with SO2, NO and hydrocarbon present accumulate a significantly larger amount of SO42− than those aged in the presence of only O2, H2O and SO2 when exposed to an equivalent amount of S. Following sulphation of the catalyst most of the sulphur can be removed by a high temperature (600 °C) treatment in the reaction gas. Regeneration in the absence of hydrocarbon is ineffective. The hydrocarbon-SCR activity of the sulphated catalyst using model hydrocarbons such as n-C8H18 can be restored after a high temperature pre-treatment in the reaction gases. However this desulphation process fails to regenerate the hydrocarbon-SCR activity when diesel fuel is used in the activity test. TPR studies show that a major fraction of the sulphur species present in the catalyst is removed by such pre-treatment, but the slight residual amount of sulphur is sufficient to inhibit the activation of the diesel fuel on the Ag catalyst. The nature of the hydrocarbon species present for the hydrocarbon-SCR reaction and during the regeneration strongly influences the activity. In general aromatics such as C7H8 are less effective for reducing NOx and regenerating the sulphated catalyst.