(691g) Impact of Field Aging on the Redox Half Cycles of NH3- Selective Catalytic Reduction of NOx over Commercial Cu-SSZ-13 Monolith Catalysts

Cu-SSZ-13 zeolites are commercial catalysts for NH₃-selective catalytic reduction (SCR) of NOx present in diesel vehicle exhaust. Despite the commercial success, the associated reaction chemistry and mechanism are still not fully understood, which has made it difficult to assess the kinetic impact of field aging on the SCR reaction. A 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. Most of the reported experimental and theoretical studies have been performed only in the NH₃-solvation region (150°C-300°C). In the current work, we employ spatially resolved capillary inlet mass-spectrometry (SpaciMS) to investigate the SCR redox cycles at various spatial locations of 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 region and high temperature region. A 10-step 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. The SCR reaction was found to be OHC-limited in the NH₃-solvation region, and RHC-limited in the high temperature region. While field aging slowed down both RHC and OHC, its impact on OHC was greater, which made the SCR reaction OHC-limited even in the NH₃-solvated region. Understanding the impact of field aging on the SCR redox chemistry in both low and high temperature range will help in formulating SCR catalysts that will meet the rapidly increasing catalyst warranty and useful-life demand.