Performance of Passive NO_x Adsorbent Pd/ZSM-5

The existing NO_x post-treatment techniques of diesel vehicle exhaust have a poor performance during the cold start, passive NO_x adsorbent (PNA) is thus proposed. In this paper, Pd/ZSM-5 PNA was obtained by loading 1% Pd onto ZSM-5 zeolite in Si/Al ratio of 11.5 via incipient wetness impregnation and ion exchange, respectively. The effect of reaction atmosphere (with or without O₂ and H₂O) on the adsorbed-released NO_x performance of PNA with two Pd loading methods was investigated. The mechanism of the factors affecting the structure properties, Pd species state and acid properties of Pd/ZSM-5 was analyzed by X-ray diffraction, N₂ adsorption-desorption isotherm, transmission tlectron microscopy, in-situ diffuse reflectance infrared Fourier transform spectroscopy and NH₃-temperature programmed desorption, respectively. The O₂ effect in the pretreatment atmosphere was further evaluated. The results show that there are more NO active adsorption sites with O₂ in pretreatment and reaction atmosphere during incipient wetness impregnation, thus improving the NO adsorption capacity of Pd/ZSM-5. In addition, O₂ promotes the release of NO_x at a lower temperature (~200 ℃), and an incipient wetness impregnation method reduces the release temperature to 430 ℃, which is conducive to the material regeneration. H₂O inhibits NO adsorption, but promotes NO_x release capacity at a higher temperature. This work can provide a reference for the design of high-performance Pd/zeolites. High adsorption-release capacity is the important prerequisite for the development of high-performance PNA.