NO x -Propylene Co-Adsorption Enables Low-Temperature NO x Desorption via Organic Nitrite/Nitro Species on Pd-Loaded Zeolites

High Resolution Image Download MS PowerPoint Slide NO x and hydrocarbons coexist in the exhaust of diesel engines under conditions relevant to the passive NO x adsorber (PNA). However, the influence of hydrocarbons on NO x 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 (C 3 H 6 ), a representative light hydrocarbon, on the NO x adsorption–desorption behavior of Pd-loaded zeolites with different framework structures was systematically investigated to elucidate the role of hydrocarbons. NO x adsorption and desorption test results revealed that the presence of C 3 H 6 increased the total amount of adsorbed NO x while lowering the desorption temperature. In addition, Pd/MFI and Pd/BEA exhibited a new low-temperature NO x desorption peak below 300 °C, contrasting with the previously reported high-temperature shift induced by C 3 H 6 . Various NO x -TPD and FT-IR analyses demonstrated that the coadsorption of NO and C 3 H 6 generated organic nitrite (R–ONO) and nitro (R–NO 2 ) species, which decompose below 300 °C to release NO x . These results provide new mechanistic insights into the C 3 H 6 -induced low-temperature NO x desorption on Pd-loaded zeolites, offering a basis for optimizing PNA materials and operating strategies suitable for low exhaust temperatures typical of diesel engines.