SO2 deactivation in NO reduction by NH3: IV. Auger studies of deactivated catalysts in selective NO reduction

Abstract Deactivation of supported metal catalysts by SO 2 in the selective reduction of NO by NH 3 was studied in a differential, packed-bed flow reactor as a function of the gas-phase O 2 concentration, and foils of the same metal as the catalyst were placed in the reactor with the catalyst. Auger electron spectroscopy (AES) was used to analyze the metal foils subsequent to reaction. With no SO 2 in the feed gas, the rate of NO reduction over Pt Al 2 O 3 , Ru Al 2 O 3 , and Ni A1 2 O 3 was enhanced by the presence of 0.5 mole% O 2 , whereas that for Pd Al 2 O 3 was reduced. AES studies confirm that the deactivation of Pd in the sulfur-free system is due to oxidation of the metal. Although SO 2 severely poisons the catalytic activity in the NONH 3 SO 2 system, addition of 0.5 mole% O 2 to the feed gas largely restores the activity of all catalysts. This elimination of severe SO 2 deactivation correlates well with the incorporation of oxygen to high concentrations into the subsurface layers of the metal; oxygen appears to counteract the presence of sulfur incorporation. Only metal oxides and metal sulfides were formed; there was no evidence of sulfate formation.