Formation and Decomposition of Ammonium Sulfate Species over a Small Pore Cu/Zeolite Catalyst

Cu/zeolite selective catalytic reduction (SCR) catalysts are used globally to reduce NO_x emissions from diesel engines. These catalysts can achieve high NO_x conversion efficiency, and they are hydrothermally durable under real world diesel exhaust environments. However, Cu/zeolite catalysts are susceptible to sulfur poisoning and require some type of sulfur management even when used with ultra-low sulfur diesel (ULSD). In the present study, the authors seek to better illuminate the chemical processes responsible for ammonium sulfate formation and decomposition occurring in Cu/zeolite SCR catalysts. Reactor-based experiments are first conducted with a real-world concentration of SO₂ (0.5 ppmv) and a typical diesel exhaust water vapor concentration (7 vol.%) to quantify progressive effects of ammonium sulfate formation. A second group of experiments probe the chemical decomposition of ammonium sulfate via NO titration. The “movement” of sulfate species during this process is monitored with temperature programmed desorption experiments. Finally, the effect of NO₂ on ammonium sulfate is investigated via either co-feeding during ammonium sulfate formation or post-feeding following ammonium sulfate formation, since it is expected that NH₃, SO₂, and NO₂ will all be present at the catalyst surface during real-world operation. The authors believe the results presented herein can support the broad research effort to continue to improve low temperature NO_x conversion which is notably degraded by the formation of ammonium sulfate species.