Deactivation of accelerated engine-aged and field-aged Fe–zeolite SCR catalysts

A single-cylinder diesel engine with an emissions control system – diesel oxidation catalyst (DOC), Fe–zeolite selective catalytic reduction (SCR) catalyst, and diesel particulate filter (DPF) – was used to perform accelerated thermal aging of the SCR catalyst. Cyclic aging is performed at SCR inlet temperatures of 650, 750 and 850 °C for up to 50 aging cycles. To assess the validity of the implemented accelerated thermal aging protocol, a field-aged SCR catalyst of similar formulation was also evaluated. The monoliths were cut into sections and evaluated for NOx performance in a bench-flow reactor. While the rear section of both the field-aged and the accelerated engine-aged SCR catalysts maintained high NOx conversion, 75–80% at 400 °C, the front section exhibited a drastic decrease to only 20–35% at 400 °C. This two-tiered deactivation was also observed for field-aged samples that were analyzed in this study. To understand the observed performance changes, thorough materials characterization was performed which revealed two primary degradation mechanisms. The first mechanism is a general Fe–zeolite deterioration which led to surface area losses, dealumination of the zeolite, and Fe2O3 crystal growth. This degradation accelerated above 750 °C, and the effects were generally more severe in the front of the catalyst. The second deactivation mechanism is linked to trace levels of Pt that are suspected to be volatizing from the DOC and depositing on the front section of the SCR catalyst. Chemical evidence of this can be seen in the high levels of NH3 oxidation (80% conversion at 400 °C), which coincides with the decrease in performance.