【作者】 王仲鹏；

【导师】 上官文峰；

【作者基本信息】 上海交通大学 ， 环境工程， 2007， 博士

【摘要】 柴油机以其低油耗、高功率、耐久性好的优势,在汽车、轮船以及各种动力装置上得到日益广泛的应用,但其尾气排放中的有害物质对环境和人类健康带来了严重的危害。世界各国都制定了日益严格的柴油机排放法规,柴油机排气污染控制是防止大气污染必须解决的重要环境问题,成为能源与环境领域的一个重大研究课题。柴油机排气控制主要针对碳颗粒物（PM）和氮氧化物（NOx）进行,由于PM和NOx的机内净化存在trade-off关系,发展柴油机尾气后处理技术成为同时降低PM和NOx排放的有效手段。本文模拟柴油机排气环境,采用程序升温反应技术,以类水滑石纳米复合材料为前体制备了系列具有介孔结构的混合氧化物催化剂,在同一催化床层上使干碳烟（soot）和NOx互为氧化还原剂实现污染物的同时去除,并运用多种现代分析技术,揭示了催化剂结构和性能之间的关系,在计算催化反应动力学基础上,分析了类水滑石材料同时催化去除NOx和soot的反应机理。具体工作如下:1.采用稳态共沉淀法和超声辅助法合成了镁铝水滑石（HT）纳米复合材料,两种方法合成的HT均为较规则的六边形片状颗粒,粒径大约在40～60 nm范围,后者可在超声处理一定时间内（10～20 min）得到粒径较小、结晶度较好晶相单一的HT。超声处理可以缩短HT合成时间,促进层间阴离子交换。以超声处理20 min合成的HT为前体经450℃焙烧可制得孔径均一的介孔混合氧化物LDO,其比表面积大,孔径分布窄,最大几率孔径分布约4 nm,SBET为168.8 m²·g^-1,孔容为0.37 cm³·g^-1。通过控制前体HT的合成条件,可有效控制LDO的孔径分布。2.以稳态共沉淀法合成的含过渡金属的类水滑石纳米复合材料M（П）Al-HT （M代表Ni、Co和Cu）为前体,制备了MAlO和K/MAlO两类具有相同介孔结构的混合金属氧化物催化剂。研究了过渡金属对物相组成、热稳定性、孔结构性质以及氧化还原性能的影响,MAlO和K/MAlO具有良好的soot催化氧化能力;与soot非催化燃烧相比,MAlO的起燃温度T_i下降200℃左右,大约为320℃,负载K₂CO₃后,K/MAlO的T_i和Tp都向低温方向移动;反应气氛中NO不但降低了soot的燃烧温度,而且提高了soot的氧化反应速率,这主要是反应中生成氧化能力更强的NO₂和在催化剂表面吸附生成的硝酸盐物种的促进作用。3.采用稳态共沉淀法在较大Co/Al比范围内（2～7）合成晶相单一的纳米类水滑石CA-HT,以此为前体制备的CAO-500和CAO-800混合氧化物呈Co尖晶石相,两类催化剂具有较大的表面积,80%以上的孔都位于中孔范围,平均孔径介于20～40 nm之间;XPS结果表明钴以Co²⁺和Co³⁺同时存在于尖晶石结构中,催化剂表面除存在晶格氧外,还有大量的吸附氧。两类催化剂具有良好的同时催化去除soot和NOx的能力,CAO-800的soot催化燃烧活性以及N₂的选择性S_N2/C都高于同一Co/Al比的CAO-500;CAO催化剂的介孔结构有利于气体分子的吸附、扩散和脱附,因而具有较高的N₂的选择性。4CAO-800是一个综合性能好的催化剂,T_i = 290℃,S_N2/C = 3.5%,S_N2O = 15.1%。通过XPS和H₂-TPR等方法推测,在CAO催化剂同时去除soot和NO反应中,soot的催化燃烧过程中溢流机理和氧化还原机理协同发生作用。4.以Cu取代型类水滑石（Cu-Mg/Al-HT）为前体制备了具有介孔结构的含Cu混合氧化物催化剂,其晶相以CuO相为主,同时伴生少量的尖晶石相,具有较大的比表面积(20～100 m²·g^-1),SBET随Cu取代量增加而降低,样品80%以上的孔位于中孔范围。含Cu混合氧化物具有良好的同时催化去除soot和NOx的能力,Cu取代量增加或焙烧温度升高,催化剂的soot燃烧活性和NO去除性能呈上升趋势;与纯CuO相比,类水滑石催化剂活性大大提高,这与其介孔结构和较大的比表面积有关。催化剂适宜的焙烧温度为800℃,Cu的最佳取代量为3.0,即活性较好的催化剂3.0Cu-800,其T_i = 260℃,S_N2/C=4.37%,S_N2O = 16.6%。5.Soot非催化燃烧时生成CO₂的表观活化能E_a = 158.4 kJ·mol^-1,T_i = 530℃,以具有介孔结构的含Cu混合氧化物为催化剂时E_a下降至60 kJ·mol^-1左右,T_i大幅下降至260～300℃,同时有大量的N₂生成,这表明通过催化燃烧可以降低生成CO₂的活化能,使soot可在较低的温度下得以催化去除;以CuO为主要晶相的催化剂同时去除soot和NOx的反应中,指前因子和表观活化能之间存在补偿效应。6.以3.0Cu-800为催化剂,研究了soot含量、NO浓度、O₂浓度、气体流速、催化剂与碳颗粒物的接触方式以及催化剂重复使用对同时催化去除soot和NOx的影响。研究发现,soot含量、气体流速和升温速率变化对T_i影响不大,而NO或O₂浓度的增加提高了起燃活性,但对NO最大去除率X_N2-max影响不大;良好的接触状况有利于soot和NOx的同时催化去除,而松接触条件下催化剂仍具有相当高的活性;催化剂经重复使用后活性下降,是由于在反应中生成的中间产物如碳酸盐等吸附于催化剂表面,覆盖了表面的活性位,从而抑制了催化剂的活性。7.宏观反应动力学研究表明在非稳态的程序升温反应中,当催化剂与PM混合物中的微量碳基本不变,动力学分析是可行的。在上述实验的基础上,利用催化反应表观速率常数r与反应温度T间的指数关系（Arrhenius方程）进行了NO-O₂-soot催化反应动力学的计算;在低温反应温度区间内,获得了同时去除PM和NOx反应中CO₂生成和N₂生成的经验速率方程。8.在分析NOx在类水滑石催化剂表面吸附、脱附过程以及NOx吸附物种的基础上,结合催化反应历程及前人对反应机理的探讨,推测了含Cu类水滑石材料同时催化去除soot和NO的反应机理。该机理充分考虑了NOx吸脱附过程对催化反应的影响,认为O₂的存在促进了反应的进行,NO/O₂在催化剂表面生成的NO₂和大量吸附物种提高了催化反应速率。总之,含过渡金属的类水滑石材料具有良好的同时催化去除soot和NOx的能力,而该技术则集“PM的捕集、PM的催化燃烧、NOx的催化还原”三功能于一体,有望成为一种非常有发展前途的柴油机排放后处理技术。更多还原

【Abstract】 The high efficiency, economy and durability of diesel engines have resulted in widely use in various power systems, trucks, buses, ship and nonroad vehicles in recent years. Meanwhile, the pollutants emitted by diesel engines have been causing severe environmental and human health problems. More and more stringent regulations have been established by many countries. Then the emission control on diesel engines has been an important research topic in energy and environmental area.Nitrogen oxides （NOx） and particulate matters （PM） are the main harmful substances. Since the reduction of NOx and PM cannot be accomplished by engine modifications alone, aftertreatment technology should be developed. A promising process to meet this demand is the simultaneous catalytic removal of NOx and soot. In this paper, mesoporous mixed oxides derived from hydrotalcites have been firstly used for NOx-soot removal under simulated diesel emission conditions. The detailed works are as follows:1. Mg-Al hydrotalcites （HT） nanometer materials have been prepared by two different methods: conventional co-precipitation （the CP method） and co-precipitation assisted by ultrasound （the US method）. Mixed oxides （LDO） was derived from LDH calcined at 450℃for 6 h. The results showed that well crystallized LDH of high purity could be obtained by the US method within acceptable times. Ultrasonic treatment could also accelerate anions exchange in the interlayer space. The mixed oxide, whose precursors were prepared by 20 minutes of ultrasonic treatment, showed mesoporous structures with monomodal pore size distribution and a very small amount of micropores. Its BJH desorption pore size distribution exhibited a narrow peak with maxima at 4 nm with SBET of 168.8 m²·g^-1 and pore volume of 0.37 cm³·g^-1. The amount of micropores decreased and the pore size distribution become broader when the ultrasonic radiation time exceeds 40 minutes. The pore size distribution of LDO can be controlled by ultrasonic treatment.2. MAlO (where M = Ni²⁺, Co²⁺ and Cu²⁺) mixed oxides derived from hydrotalcites and potassium-promoted MAlO catalysts （designated as K/MAlO） have been studied for soot oxidation. The catalysts were characterized by XRD, N₂ adsorption, TPR, FT-IR and TPO techniques. The hydrotalcites calcined at 800℃have large surface areas in the range 17-88 m²/g and uniform mesoporous features, which resulted in high activity for diesel soot oxidation under the conditions of tight contact between soot and catalyst powders. Potassium increased the activity due to the improvement of surface mobility. The presence of NO_x considerably enhanced the catalytic soot oxidation rate. The enhancement was attributed to the acceleration of soot oxidation due to NO₂ as a strong oxidizing agent and intermediates of nitrate and/or nitrite species formed on the catalyst surface.3. Co-Al mixed oxides （CAO） was prepared by co-precipitation method from hydrotalcites （HT） as precursors, and their catalytic activity was investigated for the simultaneously catalytic removal of NOx and diesel soot particulates by the Temperature-programmed Reaction （TPR） technique. All HT samples present well crystallized, layered structures, no excess phases were detected. A nonstoichiometric spinel phase was formed by calcining the CAO at 500℃and 800℃, irrespective of the Co/Al ratio. Both the activity of soot oxidation and the selectivity to N₂ formation of CAO catalysts calcined at 800℃were higher than that at 500℃. The observed difference in the catalytic performance was related to the redox properties of the catalysts and the crystallite size of HT precursors. The active species might come from Co₃O₄, which acted for redox-type mechanism for soot oxidation in the NO_x-soot reaction.4.Mesoporous mixed oxides catalysts were derived from Cu substituted Mg/Al hydrotalcites at the ratio of M²⁺/M³⁺ = 3. CuO was the predominant phase in the catalyst, while traces of spinels were also detected. Cu-containing mixed oxides had large surface areas in the range 20-100 m²/g and 80% of catalyst pores were mesopores. The catalysts showed high activity for the simultaneously catalytic removal of NO_x and diesel soot particulates. Both the activity of soot oxidation and the selectivity to N₂ formation increased with the increase of Cu content or calcined temperature. Compared with pure CuO, hydrotalcite derived catalysts had high activities, which maybe related to the mesoporous structure and large suface areas. The optimal calcined temperature was 800 ℃and the appropriate Cu/Mg molar ratio was 3.0. Then 3.0Cu-800 may be a good catalyst with high activity (T_i = 260℃, S_N2/C = 4.37%, S_N2O = 16.6%).5. The apparent activation energy （E_a） for CO₂ formation and T_i value under noncatalytic soot combustion was 158.4 kJ·mol^-1 and 530℃respectively. Both E_a and T_i value decreased when simultaneous NOx-soot removal reactions took place over Cu-containing mixed oxides catalysts. Large amounts of N₂ were formed at the same time. The compensation effect between the apparent activation energy and the pre-exponentail factor were observed over Cu-containing mixed oxides.6. Effects of reaction conditions including soot content, concentrations of inlet gas, total flow rate, heating rate, contact conditions and reuse of catalyst were investigated over 3.0Cu-800 samples. Soot content, total flow rate and heating rate hardly affect the ignition temperature of soot, while NO and O₂ concentration positive affect the catalytic acitivity. The contact between catalyst and soot was a very important factor for the catalytic performance. The relatively high activity over 3.0Cu-800 catalyst under loose contact correlated with the low melting point and high partial pressure of CuO phases. The catalytic performace decreased during catalyst reuses due to the coverage of active sites by the adsorption inermediates species in the reactions.7. The kinetics analysis of non-steady TPR results for the simultaneous NOx-soot removal has been revealed to be possible when a substantial amount of the charged soot remained in the soot/catalyst mixture. Based on the above experiments, the reaction orders of CO₂ and N₂ formation in NO-O₂-soot reactions at lower temperatures were calculated by Arrhenius-type plot. The power law expressions of reaction rates of CO₂ and N₂ formation were obtained.8. The reaction mechanisms of simultaneous NOx-soot removal over 3.0Cu-800 were discussed based on the NOx adsorption-desportion behavior, catalytic experiments and the results by other researchers. In the proposed mechanisms, the effect of NOx adsoption-desportion on the reaction was considered. The coexisting of O₂ promoted the NOx adsorption. NO₂ and the adsorption species formed on the catalysts surface increased the reation rate.In conclusion, transition metal containing hydrotalcites derived catalysts showed high activity for the simultaneously catalytic removal of nitrogen oxides and diesel soot particulates. This technology can be regarded as a combined process of PM trapping, soot oxidation and NOx reduction by soot, and, if realized, this should be the most desirable aftertreatment of desel exhausts because it is capable of simultaneously removing both harmful substances.更多还原