Sulfur Poisoning of Selective Catalytic Reduction (SCR) Automotive Emissions Catalysts – Why They Still Work
William S Epling
Chemical Engineering, University of Virginia
Sulfur dioxide is a common poison in automotive catalysis, and thus the catalyst design must take into account changes with time, or controls developed for catalyst regeneration to mitigate sulfur impacts. The sulfur compounds originate from oil and fuel, are combusted in the engine, which mainly leads to SO2 as the sulfur species. In this talk, sulfur poisoning of metal-exchanged small pore zeolite selective catalytic reduction (SCR) catalysts will be highlighted. Specifically, the SO2 poisoning effects on the SCR reaction over Cu-SAPO-34 and Cu-SSZ13 catalysts were investigated. Surface species formed during exposure of the catalysts to sulfur, NOx and NH3 were characterized with in-situ DRIFTS. Temperature programmed desorption (TPD) was also used to characterize the samples after exposure to sulfur. Results clearly demonstrate that ammonium sulfate forms and tends to be the key low-temperature degradation mode. Furthermore, Cu sulfate species that form can be recovered via exposure to NH3, as the NH3 will lead to ammonium sulfate, which decomposes at significantly lower temperatures that Cu sulfates. This aspect, the reversible nature of the sulfur poisoning, is what allows these catalysts to be used commercially.