Coupled NO and C3H6 Trapping, Release and Conversion on Pd-BEA
Sotirios (Sam) Malamis, University of Houston
Automotive emission control technologies such as the three-way catalyst (TWC) and diesel oxidation catalyst (DOC) + selective catalytic reduction (SCR) are largely effective in eliminating CO, hydrocarbons (HCs), and NOx above 200°C. However, the slip of pollutants during the vehicle warm-up period or low temperature operation continues to be a challenge. To mitigate these cold-start emissions, NOx and hydrocarbons may be trapped on selective adsorbents at low temperature and released and converted at higher temperature. In this study the effectiveness of coupled NOx and propylene trapping and conversion on a model Passive NOx Adsorber (PNA) comprising a washcoated Pd-BEA monolith is examined. Uptake and release experiments are conducted over a wide range of operating conditions to assess the effectiveness of the material in terms of trapping (NO, NO2, C3H6) and conversion (NOx reduction, C3H6oxidation). The results enable a deeper understanding of the competitive adsorption, release temperature, oxidation and reduction activity, among other aspects. It is found that NO and C3H6uptake are mutually inhibitive due to competition for adsorption on Brønsted acid sites. During desorption, the existence of multiple NO sorption sites is established. While propylene inhibits NO adsorption on some of these sites, it promotes NO storage on Pd and delays desorption to higher temperatures. Water also inhibits NO uptake, but the inhibition can be lessened through a high temperature thermal pretreatment that appears to disperse Pd. There is also evidence for C3H6oligomerization and coupling, which increases uptake and leads to the release of a complex distribution of hydrocarbons. Upon catalyst warm-up, the PNA can catalyze the oxidization of stored HCs to CO2, providing additional functionality to the catalyst.