Degradation Study of Cu-SSZ-13 Selective Catalytic Reduction Catalysts

NOx reduction in a diesel engine after-treatment is challenging. Unlike three-way catalysts in a gasoline engine that operate under stoichiometric conditions, where NOx can react with stoichiometric amounts of CO and hydrocarbons, in diesel exhaust there is an excess of O2 under which conditions three-way catalysts are ineffective at reducing NOx. The current solution for NOx reduction in diesel engine exhaust is selective catalytic reduction of NOx by NH3 (NH3-SCR). The current commercialized catalyst for the NH3-SCR reaction is Cu-SSZ-13.
Even though many advantages can be found with Cu-SSZ-13 catalysts compared to others, sulfur poisoning is still a problem. In this work, we first measured SCR and NH3, NO, CO, and SO2 oxidation performance after SO2 or SO2 + SO3 exposure on catalysts that had been exposed to mild hydrothermal aging conditions. We found there is a strong correlation between the extent of sulfur poisoning and the catalyst oxidation ability. The results suggest that with a great extent of SO2 oxidation, or more oxidation of SOx species on the catalyst surface, more extensive catalyst poisoning occurs.
Next, we studied which Cu species were more strongly affected by sulfur. Calculations performed by Keka Mandal, Asanka Wijerathne and Professor Paolucci suggest that Cu dimers strongly bind sulfur. A CO titration method and diffuse reflectance ultra-violet spectroscopy (DR UV-Vis) were used to evaluate a change in amount of static Cu dimers sites before and after sulfur exposure. We found out that Cu dimers were lost after SO2 or SO2 + SO3 exposure and there appear to be at least two different Cu/sulfur species based on different impacts of regeneration after SO2 or SO2 + SO3 exposure conditions.
Finally, we studied if different exhaust gas components might have different effects on mild hydrothermal aging. We found that mild hydrothermal aging with ethylene can lead to different oxidation conversion and slightly different high-temperature SCR performance compared to mild hydrothermal aging without any in the exhaust mixture. We believe this is due to different Cu site distributions during mild hydrothermal aging with and without ethylene. We further propose that during mild hydrothermal aging with ethylene, Z2Cu can be transformed back to ZCuOH.