How Cycling between Sulfur Exposure and High-temperature Desulfation Affects Deactivation of a Cu-SSZ-13 Selective Catalytic Reduction Catalyst

Reducing NO to N2 in diesel engine exhaust is typically accomplished using a Cu/CHA catalyst. Although commercially available, Cu/CHA catalysts are still susceptible to degradation in performance via sulfur poisoning. In this study, we used different approaches to sulfur poisoning. We compared the results from these approaches to a fresh catalyst, one that had only undergone hydrothermal aging (HTA), i.e. no exposure to S, and an engine aged sample. For S poisoning, the catalysts were first exposed to SO2 at 400 degrees C then a desulfation phase at 575 degrees C. We tested samples after 1 cycle, 5 cycles and 100 cycles. Multiple cycles of SO2 exposure and desulfation led to an accumulation of S, and loss in multinuclear Cu species as well as reducible Cu. These factors led to a decrease in NOx conversion as well. Correlations were examined between the different Cu sites and NOx reduction, with the best being related to dynamic dimers. Most of the results suggest residual sulfate species (ZCuxSOy). The results demonstrate that a single sulfur exposure and regeneration process did not represent an engine-aged catalyst, in extent of SCR performance and in changes to Cu speciation.