Abstract
An industry-defined evaluation protocol was used to evaluate the hydrocarbon trapping (HCT) and passive NOx adsorption (PNA) potential for BEA, ZSM-5, and SSZ-13 zeolites with ion-exchanged Pd or Ag. All materials underwent 700 °C degreening prior to exposure to an industry-derived protocol gas stream, which included NOx, ethylene, toluene, and decane as measured trapping species as well as common exhaust gasses CO, H2O, O2, CO2, and H2. Evaluation showed that BEA and ZSM-5 zeolites were effective at trapping hydrocarbons (HCs), as saturation was not achieved after 30 min of exposure. SSZ-13 also stored HCs but was only able to adsorb 20–25% compared to BEA and ZSM-5. The presence of Ag or Pd did not impact the overall HC uptake, particularly in the first three minutes. Pd/zeolites had significantly lower THC release temperature, and it aided in the conversion of the released HCs; Ag only had a moderate effect in both areas. With respect to NOx adsorption, the level of uptake was much lower than HCs on all samples, and Ag or Pd was necessary with Pd being notably more effective. Additionally, only Pd/ZSM-5 and Pd/SSZ-13 continue to store a portion of the NOx above 200 °C, which is critical for downstream selective catalytic NOx reduction (SCR). Hydrothermal aging (800 °C for 50 h) of a subset of the samples were performed: BEA, Pd/BEA, ZSM-5, Pd/ZSM-5, and Pd/SSZ-13. There was a minimal effect on the HC storage, ~10% reduction in capacity with no effect on release temperature; however, only Pd/SSZ-13 showed significant NOx storage after aging.
Highlights
Since the 1970s automotive emissions control has been a continual challenge due to increasing understanding of the hazards of pollutants and subsequently increasing regulatory efforts to lower harmful emissions
The SSZ-13 samples reach saturation well before 30 min and only uptake a total of 0.9–1.0 mmols C1 /gcat after 30 min. These results clearly indicate that the larger pore structures associated with BEA (0.7–0.8 nm) and ZSM-5 (0.5–0.6 nm) allow more uptake of the HCs studied here compared to SSZ-13 (~0.4 nm), which has smaller pores [49,50]
The pores of SSZ-13 are too small to store significant quantities of HCs, and BEA and ZSM-5 are preferred for hydrocarbon trap (HCT)
Summary
Since the 1970s automotive emissions control has been a continual challenge due to increasing understanding of the hazards of pollutants and subsequently increasing regulatory efforts to lower harmful emissions. NOx reduction catalysts (SCR), in addition to the DOC This emissions control system requires a minimum exhaust temperature to become fully effective, and a significant percentage of the overall emissions occur in the first. We present an evaluation of nine zeolite-based trapping materials utilizing a flow reactor protocol developed by a collaboration of industry, national laborutilizing a flow reactor protocol developed by a collaboration of industry, national laboratoatories, and academic researchers [38,39] to mimic realistic lean-burn automotive exhaust.
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