【作者】 孙红；

【导师】 全燮；

【作者基本信息】 大连理工大学 ， 环境工程， 2007， 博士

【摘要】 整体式金属载体由于具有比陶瓷载体更高的机械强度、导热率和更低的排气阻力，在现代汽车尾气治理领域受到越来越广泛的关注。但是，金属载体的比表面积很小，使用时需要在表面负载一层高比表面积的陶瓷涂层，而金属载体与陶瓷涂层材料的热膨胀系数相差较大，使得载体与涂层之间的结合强度较差，涂层易皲裂、剥落。这是限制整体式金属载体在实际中不能被广泛应用的一个原因。其次，由于金属载体加工程序复杂，相应增加了载体的制作成本。近年来，贫燃发动机因良好的燃油经济性和排放特性，成为车用发动机发展的一个重要方向。但是贫燃条件下，汽车尾气中含有大量的氧气使得传统的三效催化剂对NO_x的去除几乎没有效果。而利用尾气中未燃烧完全的烃类为还原剂，选择催化还原NO_x是一种具有潜力的净化贫燃发动机尾气的方法。目前烃类选择催化还原NO_x的催化反应一般反应温度较高，催化活性窗口较窄，以贵金属为催化活性组分时还存在选择性差，产物中有大量N₂O副产物等问题。针对金属载体以及贫燃条件下选择催化还原NO_x中存在的问题，本研究采用电沉积法在金属丝网表面制备氧化铝涂层，然后将负载有氧化铝涂层的金属丝网加工成蜂窝状金属丝网载体，并将其应用于贫燃条件下丙烯选择催化还原NO_x的研究。主要开展了以下几个方面的工作：（1）采用电沉积法，在金属不锈钢丝网表面负载氧化铝涂层。具体考察了影响涂层生长的因素，例如电沉积液、沉积电压、沉积时间、Zeta电位，聚丙烯酸浓度等。实验结果显示，以氧化铝的乙醇溶液为电沉积液，将溶液的pH值控制在8.8附近，聚丙烯酸浓度为2.035 mg·L^-1，超声分散40 min时，电沉积液能稳定存在，并且适宜电沉积过程的进行。此时，采用沉积电压为10 V，沉积时间为10 min可以在金属丝网基体表面制备适宜厚度的氧化铝涂层。进一步探索了提高涂层附着强度的方法，实验结果表明，采用HCl对金属丝网载体进行预处理不仅可以清洗其表面的污染物，而且可以提高涂层与基体的结合强度；在电沉积液中添加异丙醇铝可以提高涂层的抗热振性能，其浓度为0.27 g·L^-1时经过13个热振循环实验后，涂层的损失率为5.0 wt．％；通过加入铝粉、在800℃焙烧温度提高涂层与金属基体的结合力，增强了涂层的抗热振性能和抗机械振动性能，而且氧化铝涂层的表面形态以及性质没有因焙烧温度的升高而被损坏。SEM电镜显示涂层表面均匀没有皲裂，XRD表明氧化铝涂层仍是主要以γ-Al₂O₃晶形存在。将制备的涂层浸渍硝酸镧后，能明显的提高涂层的抗机械振动性能，超声振荡达到35 min时，涂层的损失率仅为44.0wt．％。（2）将负载氧化铝涂层的平板状和瓦楞状金属丝网交替堆积，加工成型为蜂窝状金属丝网载体。通过计算发现，与孔密度相近的堇青石载体（孔密度为50）相比较，蜂窝状金属丝网载体具有更高的前端开口程度和几何比表面积；与薄壁堇青石载体（孔密度为400）相比较，几何表面积仅降低25.0％左右；蜂窝状金属丝网载体没有因为比表面积的增加而增大载体的压力降。从成型工艺看，蜂窝状金属丝网载体成型简单，以金属丝网包裹，接头处不需要焊接，在一定程度上降低了制作成本。（3）制备了Pd／Al₂O₃、Pd／TiO₂／Al₂O₃、Pd／CeZr／TiO₂／Al₂O₃三种蜂窝状金属丝网催化剂，并采用丙烯选择催化还原NO_x的实验来评价其在贫燃条件下对NO_x的催化活性。结果表明Pd含量为0.23％时，Pd／CeZr／TiO₂／Al₂O₃蜂窝状金属丝网催化剂在低温条件下具有较高的催化活性，而且催化活性窗口较宽，出口产物没有检测到N₂O的生成。N₂和C₃H₆-O₂-N₂中进行的TPD实验结果说明，在Pd／CeZr／TiO₂／Al₂O₃蜂窝状金属丝网催化剂中存在两个不同的含氮中间产物的形成中心，一个是在TiO₂，另一个是在CeZr混合氧化物，而且它们也是NO_x催化反应的活性中心。进一步系统地考察了Pd的含量、氧气的含量和空速对Pd／CeZr／TiO₂／Al₂O₃蜂窝状金属丝网催化剂催化活性的影响以及催化剂的稳定性。（4）比较了颗粒状催化剂、50目和400目堇青石催化剂和蜂窝状金属丝网催化剂对丙烯选择催化还原NO_x反应的催化性能，结果显示蜂窝状金属丝网催化剂在低温下具有最高的NO_x转化率。非稳态实验表明，蜂窝状金属丝网催化剂比堇青石催化剂能更快对温度的变化做出响应。而且蜂窝状金属丝网催化剂对丙烯具有较低的起燃温度。总之，本研究采用电沉积法可以在金属丝网载体上制备附着牢固的氧化铝涂层；以Pd为催化活性组分的蜂窝状金属丝网催化剂，在低温下对NO_x具有较好的还原能力，并且显示出比整体式陶瓷载体更优越的催化性能和热响应性能。这为蜂窝状金属丝网催化剂实际应用于汽车尾气净化处理提供实验依据。更多还原

【Abstract】 Metallic monoliths offer plenty of advantages over ceramic monoliths, such as stronger mechanical strength, higher thermal conductibility and lower pressure drop, and now have become most attractive supports in automotive emissions controls. However, the metallic monoliths could not be applied in practice because of the low specific surface area. It is necessary to deposit the ceramic oxide washcoat with high surface area over metallic monoliths. Since thermal expansion coefficient of the metallic support is different from the ceramic oxide washcoat, the adhesion of washcoat is poor which leads to the washcoat chap and flake away. Otherwise, metallic monoliths are expensive due to complicated manufacturing and the high price of metal materials. Recently, lean-bum engines have become the main option for the vehicles due to lower consumption of fuel and lower emission. However, the exhaust from the lean-burn engines contains a large amount of oxygen, the three-way catalysts have been proved to be not efficient on removing NO_x. The selective catalytic reduction of NO_x by hydrocarbons （HC-SCR） has attracted considerable interest as a method to control emissions from engines operated under lean-burn conditions. Nevertheless, the rather high reaction temperature and narrow window are the main drawbacks of these catalysts to be used under lean-burn conditions. Specially, supported precious metal catalysts exhibit low selectivity of N₂, N₂O being formed in substantial amounts.In this study,γ-Al₂O₃ washcoat on wire mesh was prepared by electrophoretic deposition and selective catalytic reduction of NO_x by propene over the wire-mesh honeycomb catalysts was performed under lean burn conditions. The main works are as following:（1） Washcoat deposited on metallic wire mesh was prepared usingγ-Al₂O₃ powders by electrophoretic deposition. The preparation parameters of washcoat were investigated, such as the deposited solution, the deposited voltage, the deposited time, Zeta potential and the amount of polyscylic acid. The alumina powders were dispersed in ethanol and pH was about 8.8. Then, polyscylic acid was added as additives and its concentration was 2.035 mg·L^-1. Finally, the suspension was mixed in an ultrasonic bath for 40 min. The suspension was stable and suitable for the electrophoretic deposition experiment. Under this condition, a uniformly thickness alumina washcoat on metallic wire mesh was prepared under 10 V for 10 min.In order to improve the washcoat adhesion, wire meshes were dipped in HCl solution for wiping off the dirts and enhancing the cohesion between the wire mesh and washcoat. When the concentration of aluminum isopropoxide was 0.27 g·L^-1, the sample exhibited excellent adhesion onto the support in thermal shock, losing only 5.0 wt.% after 13 times of thermal shock. The ability of thermal resistant and vibration resistant was improved by adding aluminum powders and higher calcinations temperature, and the phase structure of washcoat doesn’t change due to increase temperature. SEM showed that alumina washcoat was even and no chap. XRD revealedγ-Al₂O₃ was main monocrystalline presented in the coating. In addition, after the samples were immersed in lanthanum nitrate solution, the loss of washcoat decreased and it was 44.0 wt.% for 35 min in ultrasonic test.（2） After depositing Al₂O₃ washcoat, the wrie-mesh honeycomb was manufactured by stacking alternatively corrugated and plain wire meshes. The calculation results showed that the wire-mesh honeycomb had more open frontal area （OFA） and geometric surface area （GSA） than cordierite monolith with 50 cpsi. The GSA of wire-mesh honeycomb decreased 25% relative to thin wall cordierite monolith with 400 cpsi. But the pressure drop of wire-mesh honeycomb didn’t increase due to the increasing of GSA. The manufacture process of wire-mesh honeycomb was simply. Metallic wire mesh was adopted to pack the honeycomb and it didn’t need to solder the joining, which could reduce the price of wire-mesh honeycomb.（3） The selective catalytic reduction of NO_x by C₃H₆ was investigated over Pd/Al₂O₃, Pd/TiO₂/Al₂O₃, Pd/CeZr/TiO₂/Al₂O₃ wire-mesh honeycomb prepared under lean-burn conditions. The results showed that 0.23%- Pd/CeZr/TiO₂/Al₂O₃ wire-mesh honeycomb catalyst exhibited high catalytic activity at low temperature over a broad temperature range and N₂O was not detected in measurable quantities in the outlet flow and other byproducts containing nitrogen were not detected. The results of temperature-programmed desorption experiments in N₂ and in C₃H₆-O₂-N₂ mixture suggested that there were two storage site of ad-NO_x on Pd/CeZr/TiO₂/Al₂O₃ wire-mesh honeycomb catalyst, one was TiO₂ and another was CeZr. Except of the storage site, TiO₂ and CeZr also participated in selective catalytic reduction reaction. The effect of Pd content, oxygen, space velocity and reaction time on the catalytic activity of Pd/CeZr/TiO₂/Al₂O₃ wire-mesh honeycomb catalyst was further investigated.（4） Comparing with pellet catalyst, cordieilte monolith catalysts with 50 and 400 cpsi, wire-mesh honeycomb catalyst exhibited highest activity of removal NO_x at low temperature. In unsteady-state experiment, conversion of NO_x over wire-mesh honeycomb catalyst was quickly stabilized in short time after temperature elevation, though that of ceramic monolith catalyst was slowly increased and it should be taken more time for reaching a steady-state conversion. Furthermore, the light-off temperature of propene was lowest.In conclusion, the firm alumina washcoat on wire mesh honeycomb was prepared by electrophoretic deposition. Pd supported on wire-mesh honeycomb showed better activity for selective catalytic reduction of NO_x by propene than ceramic monolithic catalysts under lean burn condition at low temperature, which provide the theoretical and experimental basis for the application of wire-mesh honeycomb in automotive exhaust treatment.更多还原