Copper-exchanged SSZ-13 zeolites are commercially used for Selective Catalytic Reduction (SCR) of NOx emitted by lean-burn engines. The Standard SCR reaction, 4NO + 4NH3 + O2 → 4N2 + 6H2O, is a redox process consisting of a reduction half cycle (RHC) and an oxidation half cycle (OHC) that cycle the active Cu sites between the Cu(II) and Cu(I) states. Despite extensive efforts to elucidate the SCR mechanism and impacts of hydrothermal aging and sulfur poisoning, the impact of real-world field aging on the SCR activity of Cu-SSZ-13 has not been reported. 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 a commercial Cu-SSZ-13 monolith catalysts in their degreened, 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 transient response Cu-redox experimental protocol was developed to measure RHC, OHC and SCR transients on the Cu-SSZ-13 samples. Using kinetic models, these transients were then analyzed to understand the impact of field aging on SCR half-cycle kinetics. Moreover, NH3 TPD, SO2 TPD, DRIFTS, ICP-OES and NMR experiments were performed to investigate the changes in catalyst chemical property caused by field aging. The Cu speciation in the catalyst changed from being Z1CuOH-dominated in the degreened catalyst to Z2Cu-dominated in the field aged catalyst, while also incurring loss of Bronsted acid sites. Approximately 30% of isolated Cu sites in the field aged catalyst were found to be poisoned by bisulfites, bisulfates and sulfates originated from the sulfur impurity present in diesel fuel. The SCR activity of the catalyst was degraded by 50% at 200 °C at the front locations of the catalyst. Such an activity loss was not solely due to the decrease in Cu sites, but also due to changes in the RHC and OHC kinetics. Field aging was found to selectively slow OHC more than RHC. This appears mainly due to lower mobility of the Cu ions coordinated to sulfur-related species leading to lower formation of Cu-dimers necessary for OHC. Understanding the impact of field aging on Cu-SSZ-13 catalyst and SCR redox half cycles will help in formulating more durable catalysts capable of meeting the increasing useful-life regulations.

Field aging impact on the NH3 selective catalytic NOx reduction redox half cycles over a commercial Cu-SSZ-13 catalyst

ACS Spring 2022

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