Performance of Passive NOx Adsorbent Pd/ZSM-5
)Abstract
The existing NOx post-treatment techniques of diesel vehicle exhaust have a poor performance during the cold start, passive NOx 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 NOx 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 NOx 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 NOx 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.
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