NOx-Propylene Co-Adsorption Enables Low-Temperature NOx Desorption via Organic Nitrite/Nitro Species on Pd-Loaded Zeolites.

NOx and hydrocarbons coexist in the exhaust of diesel engines under conditions relevant to the passive NOx adsorber (PNA). However, the influence of hydrocarbons on NOx adsorption and desorption on Pd-zeolites as a PNA is not clearly understood and appears to depend strongly on the properties of the Pd-zeolites. In this study, the effect of propylene (C3H6), a representative light hydrocarbon, on the NOx adsorption-desorption behavior of Pd-loaded zeolites with different framework structures was systematically investigated to elucidate the role of hydrocarbons. NOx adsorption and desorption test results revealed that the presence of C3H6 increased the total amount of adsorbed NOx while lowering the desorption temperature. In addition, Pd/MFI and Pd/BEA exhibited a new low-temperature NOx desorption peak below 300 °C, contrasting with the previously reported high-temperature shift induced by C3H6. Various NOx-TPD and FT-IR analyses demonstrated that the coadsorption of NO and C3H6 generated organic nitrite (R-ONO) and nitro (R-NO2) species, which decompose below 300 °C to release NOx. These results provide new mechanistic insights into the C3H6-induced low-temperature NOx desorption on Pd-loaded zeolites, offering a basis for optimizing PNA materials and operating strategies suitable for low exhaust temperatures typical of diesel engines.