Impact of Field Aging on the Redox Half Cycles of NH3-Selective Catalytic NOx Reduction over a Commercial Cu-SSZ-13 Monolith Catalyst

Dhruba Jyoti  Deka, Oak Ridge National Laboratory

Copper-exchanged SSZ-13 zeolites are commercial catalysts for NH₃-selective catalytic reduction (SCR) of NO_x emitted by diesel and other lean-burn vehicles. Despite their commercial success, the associated reaction chemistry and mechanisms are still not fully understood, which has made it difficult to assess the kinetic impacts of field aging on the SCR reaction. The Standard SCR reaction, 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O, consists of two redox halves: the reduction half cycle (RHC) and the oxidation half cycle (OHC) that cycle the Cu-inventory between 2+ and 1+ oxidation states. Experimental and theoretical studies performed on model Cu-SSZ-13 catalysts in the NH₃-solvation region (150°C-300°C) have shown a rather complicated relationship between the redox cycles and Cu-species. In the current work, we employ an intra-catalyst measurement technique called spatially resolved capillary inlet mass-spectrometry (SpaciMS) to investigate the SCR redox cycles at various spatial locations within the commercial Cu-SSZ-13 monolith catalysts in their de-greened, hydrothermally aged, and field aged forms, in the temperature range of 200°C-450°C, thereby encompassing both the NH₃-solvation and high-temperature regions. A 10-step transient response experimental protocol was developed to measure RHC, OHC and SCR transients on the Cu-SSZ-13 samples with various initial Cu-oxidation state partitioning. With the help of kinetic modeling, these transients were then analyzed to understand the half-cycle kinetics, role of Cu-species and Bronsted acid sites. Moreover, NH₃ temperature-programmed desorption experiments were performed to further elucidate the roles of Lewis-bound NH₃ and Bronsted-bound NH₃ in RHC and OHC.

In general, the SCR reaction was found to be OHC rate-limited in the NH₃-solvation region, and RHC rate-limited in the high-temperature region. While the majority of the present literature shows that RHC and OHC have first and second order dependency on Cu-sites, respectively, our results show that the dependency on Cu-sites can be second order for both half cycles. Field aging slowed down both RHC and OHC; however, its impact on OHC was greater, which made the SCR reaction OHC rate-limited even in the NH₃-solvated region. Field aging also impacted the dependency of RHC and OHC on Cu-sites. The automotive industry is required to meet not only the permissible emissions level, but also the rapidly rising catalyst-warranty and useful-life demands. Understanding the impact of field aging on the SCR redox chemistry in both the low- and high-temperature regions will help in formulating more durable SCR catalysts.