【作者】 卫婷；

【导师】 何洪；

【作者基本信息】 北京工业大学 ， 物理化学， 2010， 硕士

【摘要】 三效催化剂(Three Way Catalysts,简称TWCs),对汽车尾气中CO、HC和NOx等主要污染物可以同时具有很高的催化转化效率。而贵金属资源有限,新的机动车尾气排放法规日益严格,因此需要TWCs具有相对低的贵金属含量且具有较高的催化活性,需要在传统的TWCs制备方法上做出改进,找到新的突破。本文研究了超声膜扩散法(ultrasonic-assisted membrane reduction,UAMR)制备Pd纳米粒子,并从Pd纳米溶胶出发,制备了系列负载型纳米Pd催化剂,对其进行表征,考察其在三效催化模型反应中的反应性能。首先,采用正交试验设计探究UAMR法制备Pd纳米粒子的制备条件,并采用SEM,TEM和激光粒度仪对Pd纳米粒子进行了表征,确定UAMR法的制备最佳条件。结果表明,UAMR法制备的Pd纳米粒子形状为球形,单分散性较好。在平流泵流速10 mL·min-1,输液泵流速取100 r·min-1,金属溶液浓度取4×10-4 mol·L-1,还原剂摩尔比例取5:1,保护剂摩尔比取20:1,超声波频率取0 Hz,膜管尺寸为40 nm的条件下制备得到的Pd纳米粒子粒径最小,分散性好。然后从制备Pd纳米溶胶出发,选取γ-Al2O3、TiO2、SiO2、CeO2和Ce0.5Zr0.5O2五种不同载体,制备负载型纳米Pd催化剂,采用N2吸-脱附,XRD,TPR和ICP对催化剂进行了表征,并考察比较了各催化剂对CO+O2的催化活性差别。结果表明,催化剂Pd/Ce0.5Zr0.5O2具有较高的活性,其金属负载量最低,仅为0.27 wt%,为提高催化剂活性的同时降低贵金属用量提供了可能。最后研究催化剂Pd/Ce0.5Zr0.5O2和Pd/Al2O3,负载量为1 wt%为采用不同的方法制备了系列负载型Pd催化剂,深入研究催化剂的结构、形貌以及活性组分的结构组成、价态,金属分散度对催化活性的影响。结果表明,催化剂的活性与催化剂表面活性组分的氧化态和表面相对含量有关。Pd的高氧化态有利于CO催化氧化反应,催化剂表面的Pd物种对催化活性起到重要作用。更多还原

【Abstract】 Three-way catalyst can convert CO, NOx and hydrocarbons in the automotive exhaust gas to nonhazardous substances simultaneously with high efficiency. However, due to the shortage of noble metal resources and the increasingly strict vehicle emission control regulations. Therefore, the TWCs should preserve lower content of precious metals with higher catalytic activity than the TWCs prepared by traditional method. The traditional method of preparing TWCs should be improved. Hereon, we investigated the synthesis of Pd colloids by UAMR (ultrasonic-assisted membrane reduction , UAMR) method, the preparation of supported Pd nanocatalysts from a typical nano-sized Pd colloid, then characterized, and investigated its catalytic performance in the three way model reactions.Firstly, the orthogonal test method was used to explore the conditions for preparing Pd nanoparticles by the UAMR method, and the obtained Pd nanoparticles were characterized by SEM, TEM and laser particle size analyzer, to obtain the best conditions of the UAMR method to prepare Pd nanoparticles. The results showed that Pd nanoparticles with a nearly spherical shape and narrow particle size distribution can be achieved by UAMR method. The nano-sized and narrowly dispersed Pd nanoparticles could be synthesized under such conditions: the flow rate of the constant flow pump is about 10 mL·min-1, the rotating speed of the peristaltic pump is about 100 r·min-1, the solution concentration is 4×10-4 mol·L-1, the molar ratio of NaBH4/Pd is about 5:1, the molar ratio of PVP/Pd is 20:1, the ultrasonic frequency is 0 Hz and the micropore size of membrane is 40 nm.Secondly, the fabrication of supported Pd catalyst from a typical nano-sized Pd colloid with 1 wt% Pd loading was studied; several carriers such as Al2O3, TiO2, SiO2, CeO2, and Ce0.5Zr0.5O2 were selected. The catalysts were characterized by a combination of N2 adsorption/desorption, X-ray diffraction (XRD), Temperature-Programmed Reduction by H2 (H2-TPR) and Inductively Coupled Plasma-atom Emission Spectrometer (ICP-AES), and the catalytic behavior of the samples in the CO oxidation reaction was investigated. The results showed that the catalyst of Pd/Ce0.5Zr0.5O2 exhibited the lowest metal loading, the best catalytic activity for CO oxidation among the prepared supported Pd catalysts. This method of preparing catalyst provides a chance to reduce the usage of precious metal and to improve the catalytic activity of supported Pd catalysts. Finally, the investigation on several Pd/Al2O3 and Pd/Ce0.5Zr0.5O2 catalysts prepared by different processes was carried out. The effect of the catalysts structure, morphology, active components composition, chemical state (oxidized or reduced), and metal dispersion on the activity of supported catalysts was investigated. The results showed that the catalytic activity was effected by the oxidation state and the surface relative content of the surface active component. The high oxidation state of Pd was conducive to high CO oxidation reaction on the catalyst surface, the Pd species on the surface of the catalyst play an important role in catalytic activity.更多还原