Coupled NO and C₃H₆ Trapping, Release and Conversion on Pd/BEA: Evaluation of the Lean Hydrocarbon NO_<i>x</i> Trap

Abstract

Automotive emission control catalysts are not effective in eliminating CO, hydrocarbons (HCs), and NOx during engine cold start (below 200 °C). In this study, the effectiveness of coupled low-temperature NOx and C3H6 trapping and conversion on the model lean HC NOx trap (LHCNT) consisting of a washcoated Pd/BEA monolith was examined. Transient uptake and release experiments were conducted to obtain insight regarding competitive adsorption, release temperature, conversion activity, and water impact. NO and C3H6 uptake were found to be mutually inhibitive because of competition for adsorption on Brønsted acid sites. During desorption, the existence of multiple NO sorption sites was established. Although C3H6 inhibited NO adsorption on some of these sites, it delayed NOx desorption to higher temperatures. Furthermore, above 200 °C, Pd/BEA was able to catalyze the oxidation of stored HCs and exhibited NOx reduction activity. Addition of water inhibited both NOx and C3H6 uptake, but the inhibition was lessened through a high-temperature thermal pretreatment. Overall, Pd/BEA demonstrates a combination of inhibiting and promoting coupling effects during uptake and release and exhibits activity for both NOx reduction and C3H6 oxidation. The findings provide guidance for developing improved LHCNT materials.