被动式NOx吸附剂Pd/ZSM-5的性能影响因素

Performance of Passive NOx Adsorbent Pd/ZSM-5

柴油车尾气NO后处理技术对冷启动阶段NO净化效果较差,被动NO吸附剂(PNA)应运而生。分别采用等体积浸渍法和离子交换法将1%Pd负载于m(Si)/m(Al)为11.5的ZSM-5分子筛,获得Pd/ZSM-5 PNA材料。考察反应气氛(有无O_(2)和H_(2)O)对2种不同Pd负载方式PNA吸附-释放NO性能的影响。采用X射线衍射、N吸附-脱附曲线测定、透射电子显微镜、原位漫反射Fourier变换红外光谱和NH程序升温脱附手段,从Pd/ZSM-5结构性质,Pd物种形态和酸性质分析以上因素的影响作用机制,并进一步评价预处理气氛中O_(2)作用。结果表明:反应气和预处理气氛存在O_(2)及等体积浸渍法中存在更多NO_(x)活性吸附位点从而提高Pd/ZSM-5的NO_(x)吸附能力。此外,O_(2)促进NO低温(~200℃)释放,等体积浸渍法使释放温度降至430℃,有利于材料再生。H_(2)O抑制了NO_(x)吸附,但可提高NO高温释放能力。为设计高性能Pd/分子筛提供建议,具备高吸附-释放能力是开发高性能PNA的重要前提。

The existing NOpost-treatment techniques of diesel vehicle exhaust have a poor performance during the cold start, passive NOadsorbent (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 Oand H_(2)O) on the adsorbed-released NOperformance 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, Nadsorption-desorption isotherm, transmission tlectron microscopy, in-situ diffuse reflectance infrared Fourier transform spectroscopy and NH-temperature programmed desorption, respectively. The Oeffect in the pretreatment atmosphere was further evaluated. The results show that there are more NO_(x) active adsorption sites with Oin pretreatment and reaction atmosphere during incipient wetness impregnation, thus improving the NO_(x) adsorption capacity of Pd/ZSM-5. In addition, Opromotes the release of NOat a lower temperature (~200 ℃), and an incipient wetness impregnation method reduces the release temperature to 430 ℃, which is conducive to the material regeneration. H_(2)O inhibits NO_(x) adsorption, but promotes NOrelease 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.