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 NOx emissions from diesel engines. These catalysts can achieve high NOx 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 (8 vol.%) to quantify progressive effects of ammonium sulfate formation. BET surface area is used to measure the impact of ammonium sulfate on the Cu/zeolite catalysts’ physical properties. 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.