汽车尾气净化低温起燃催化剂研究

【作者】 贺小昆；

【导师】 孙加林； 胡劲；

【作者基本信息】 昆明理工大学 ， 材料学， 2011， 博士

【摘要】 2008年我国排放法规从国Ⅲ阶段进入到国Ⅳ阶段,通常国Ⅲ催化剂在车辆启动100s内起燃,而国Ⅳ催化剂需要在70s内起燃才能满足国Ⅳ法规对HCs、NOx进一步削减50%的要求,因此本文将车辆冷启动阶段低温快速起燃催化剂技术作为研究重点。国内外报道的低温快速起燃催化剂技术的单因素研究较多,本文从汽车三效催化剂的载体几何物理性能、改性氧化铝物理化学性能、涂层材料物理化学性能、涂层材料与贵金属协同催化作用、贵金属分布5个方面开展了多项因素与催化剂低温活性的协同作用研究,并通过优化各项因素改善催化剂低温起燃特性,制得可满足国Ⅳ排放法规的三效催化剂。(1)载体实验：采用400cell/inch2.600cell/inch2.900cell/inch/2三种孔密度载体制备催化剂,发动机台架评价结果表明,载体孔密度与起燃活性密切相关。增加载体孔密度可减小孔壁厚、增大几何表面积和前端开口面积比,使载体热传递速度加快,同时孔道几何表面积增大为活性金属提供更大的反应面积,从而缩短催化剂起燃时间,减少贵金属用量,增强空燃比(A/F)调控能力。当孔密度一定时,增大载体体积和涂层涂敷率也可以提高活性金属反应面积从而降低起燃温度,当涂敷率超过45%后背压升高、热传递系数减小是导致催化剂起燃温度上升的主要原因。实验表明采用600cell/inch2载体制备国Ⅳ催化剂可有效降低起燃温度。(2)氧化铝制备及表征：优化氧化铝中La、Ba掺杂量和氧化铝稳定性,并进一步研究了改性氧化铝负载Pd、Rh的催化还原活性。BET、XRD、H2-TPR表征结果说明,Al2O3掺杂La、Ba后比表面积、孔容、孔径稳定性增强,由于La和Ba对氧化铝的稳定机制不同,XRD表明1050℃煅烧后La、Ba掺杂量较高的改性氧化铝中出现LaAlO3和BaAl2O4,导致比表面积快速衰减。2.5%La2.5%Ba/Al2O3-02(GAL)在1050℃煅烧后出现单一BaAl2O4物相,La起保护AlOH的作用,因此GAL具有最佳的比表面积和孔容稳定性,比表面积88.43m2/g,孔容0.31 cc/g,达到国内较好水平。GAL在550℃、950℃煅烧后负载Pd、Rh,H2-TPR结果表明,950℃煅烧后的GAL由于表面羟基氧化铝AlOH与Pd、Rh的锚定效应被改变,Pd在GAL表面的分散度增加了10%,Pd/GAL低温还原温度比Rh/GAL低30℃～40℃,具有更优的低温还原活性。(3)涂层材料制备及表征：对物理法制备的涂层材料GAL/CZA的组份配比进行优化,并研究涂层材料与Pd、Rh的协同催化作用。BET、H2-TPR结果表明,掺杂氧化物间表面积的差异以及同质程度,可以减少单组分颗粒间的接触和相互作用程度,与纯GAL和纯CZA相比,GAL和CZA掺杂后可明显降低催化材料的还原温度,并提高高温稳定性。GAL和CZA掺杂配比实验发现,GAL可增强催化材料比表面积稳定性、降低储氧能力(OSC)稳定性,CZA则相反,考虑到比表面积和储氧能力的稳定性,适宜配比的涂层材料为GAL0.4CZA0.6。从GAL/CZA载Pd、Rh的H2-TPR结果发现,低于200℃时CZA对增强还原活性起主要作用,300℃～400℃时Ba2+与PdO/Rh203发生协同作用提升催化材料还原活性,高于700℃时GAL对增强还原活性起主要作用。发动机台架测试结果表明,GAL0.4CZA0.6制备的催化剂性能优于纯CZA制备的国Ⅲ催化剂,新鲜催化剂CO、HCs、NOx起燃温度下降15℃～20℃,老化催化剂CO、HCs、NOx起燃温度下降20℃～30℃。(4)贵金属与涂层材料相互作用实验：选择GAL0.4CZA0.6担载不同Pd/Rh量、不同Pd/Rh配比催化材料样品,H2-TPR表征结果表明,贵金属量高使活性位增加提高了反应活性,但对贵金属分散度影响不明显,贵金属载量1%的样品1050℃老化前后的低温还原峰温度比0.35%样品低10℃～20℃；贵金属配比实验说明,Rh可促进Pd发生协同催化作用,提高贵金属分散度,降低起燃温度,Pd:Rh=3:1样品老化前后的低温还原峰温度比Pd:Rh=33:1样品低20℃～30℃。(5)催化剂制备实验：选择GAL0.4CZA0.6作为涂层材料负载Pd、Rh,分别采用浸渍法和化学沉积法制备催化剂样品,并在发动机台架快速老化100小时后进行性能测试,结果表明,浸渍法制备的新鲜催化剂起燃温度223℃～232℃,空燃比窗口0.48,OSC为432.77mg.L-1,100小时老化催化剂起燃温度384℃～386℃,空燃比窗口0.05,OSC为20.73,性能优于化学沉积法制备的催化剂,同时该催化剂性能优于国Ⅲ催化剂(新鲜催化剂起燃温度>260℃,老化催化剂起燃温度>400℃)。采用浸渍法制备贵金属区域担载的单级双载催化剂系列样品,1050℃/4h老化前后的样品测试结果表明,催化剂前端局部加浓的贵金属区域设计可通过增加贵金属活性位,促进低温时催化剂快速起燃。实验确定的贵金属总量为1.15g/L、贵金属组合为(前催1.6g/L@19：1+后催0.7@9：1)的单级双载催化剂方案,新型国Ⅳ催化剂起燃温度比国Ⅲ催化剂降低了30℃～40℃,样品整车排放测试结果达到国Ⅳ排放标准,整车实验冷启动起燃时间小于50s,优于国Ⅲ催化剂的70s～100s的水平,达到国Ⅳ催化剂性能的国际先进水平。贵金属用量比国外原装催化剂减少60%,比本企业提供给原车的早期国Ⅳ催化剂减少28%。更多还原

【Abstract】 The urban atmospheric pollution control has been listed as the important strategic task of China’s sustainable development in the 21st century. Automobile exhaust emissions are the main sources of air pollution in cities. China’s automobile exhaust emission regulations have rapidly developed from GBⅠto GBⅣsince 2000. More stringent emission regulations make higher demands on emission control technique. Three-way catalyst(TWC) is always the research emphasis as the core components of vehicle emission control system. Started from China’s GBⅢ/GBⅣ, the cold start emission level of vehicle were required to test. When starting a cold engine for 30s～60s,60%～80% uncombusted hydrocarbons (HCs) are exhausted because of its fuel-rich running. But the traditional underfloor TWC cannot meet its light-off temperature to effectively treat emission pollution in 30s～60s. Therefore, the research on rapid light-off catalyst technique during the cold start becomes the hot investigation on the more stringent emission regulations.A great deal of research results demonstrated that, the rapid light-off catalyst technique is not just the research of catalyst formula and catalytic activity. Catalyst light-off characteristic research is a system engineering which often depends on many complex factors, such as carrier performance, catalyst formula, preparation process and structure design. The research was supported by the National High Technology Research and Development Program (863) of China, Yunnan Province Reserve Talented Person of Academic and Technical Leader Program. In allusion to China automobile industry urgent needs to GBⅣcatalyst which has rapid light-off characteristics, this research was based on the rapid light-off catalyst technique to explore the relationship between carrier properties, washcoat material property and the precious metal property with light-off characteristics. The key influencing factors for rapid light-off characteristics of catalyst were systematically investigated. The low-cost GBIVcatalyst which contained precious metals of 30g/ft3～35g/ft3(or 1.06g/L～1.2g/L) have been developed successfully. Since the systematic research for rapid light-off catalyst is less in our country, this research contents and low-cost targets in this thesis have innovation and advancement. Main research contents include the five parts, such as analyzing the relationship between cell structure of honeycomb ceramic substrate with light-off characteristics of catalyst, relationship research between pore structure of alumina with light-off characteristics of catalyst, the research of impact of washcoat material on light-off characteristics of catalyst, the cooperative effect research of washcoat with the precious metals on light-off characteristics of catalyst, and catalyst preparation.1. Substrate researchThis paper calculated and verified the relationship between the cell density, volume, backpressure of three types of substrates with different cell densities (400cpsi,600cpsi,900cpsi) and light-off factor(LOF). The results tested on engine bench demonstrated that, (1) Higher cell density of substrate could improve the light-off performance and thermal transfer characteristics of catalysts. Especially, the higher cell density has superior compensation effect for degradation of the surface area and catalytic activity caused by high temperature aging. The light-off performance of catalyst mainly depended on the cooperation effect between the cell density of substrate and the precious metals loading. The higher cell density of substrate benefited to rapid light-off when the precious metals loading were less than 1.0g/L. For the normal cell density substrate of 400cpsi, the increase of the precious metals loading to improve light-off activity was most effective when the precious metals are more than 1.0g/L. (2) The effect of increasing cell density of substrate on air-fuel ratio characteristic of catalyst was stronger than increasing precious metals loading. It was effective measurement that higher cell density enlarged the operating window of catalyst. (3) For the same precious metals loading, the increase of substrate volume benefited to obtain lower light-off temperature than the increase of cell density of substrate. (4) The backpressure test results of catalyst with 400cpsi substrate showed that, Hs decreased when the coating rate was more than 45%. Therefore, the coating rate of 45% was optimal for rapid light-off of catalyst with 400cpsi substrate.2. Research on the pore structure of aluminaThree kinds ofγ-Al2O3 with middle pores were prepared to the series of modifying alumina of AlBaLa oxide. The ratio of modifying element was La/Ba=1. The effect of (La+Ba) contents on the pore structure of AlLaBa oxide was observed. The screened modifying alumina loaded Pd, Rh. The relationship among the modifying agent content, pore structure and the precious metals in the modifying alumina with light-off characteristics were observed. Al2O3-02 was the optimal alumina raw material by the characteristics of BET, XRD, H2-TPR.2.5%La and 2.5% Ba were added in Al2O3-02 to form 2.5%La2.5%Ba/Al2O3-02 (abbreviate:GAL), which have optimal surface area, pore volume and thermal stability. Pd and Rh were loaded on GAL which has calcined at different temperatures. When the calcine temperature was lower than 950℃, GAL loaded Rh3+ had optimal dispersity. When the calcine temperature was higher than 950℃, Pd/Rh/GAL has optimal reduction activity at lower temperature, and the low temperature reduction activity of Pd/GAL was better than Rh/GAL。3. Research on the washcoat materalsThe Ce-Zr-Al complex oxides (CZA) were physically mixed into GAL. The catalytic activity of CZA and CZA/GAL was examined. Pd and Rh were loaded on the different GAL/CZA hybrid. The effect of mixing ratio of GAL and CZA on the catalytic reduction behavior, oxygen storage capacity (OSC) and surface area of Pd/Rh catalyst were examined by H2-TPR, BET, etc. High temperature aging was used to study the light-off characteristic and thermal stability of catalyst. New washcoat material of GAL0.4CZA0.6 has optimal stability and cooperativity between surface area and OSC.The fast aging condition and OSC stability of the above Pd/Rh catalyst were investigated on engine bench. The fast aging condition of catalyst screening was obtained when the inlet temperature was 830℃, the light-off activity and OSC of catalyst decreased fastest during the aging time of 0h～20h. The air-to-fuel ratio (A/F) decreased fastest during the aging time of 0h～10h. The above condition could be used to quickly examine catalyst durability.4. Reciprocity research between the washcoat and the precious metalsFour contents and ratios of palladium nitrate and rhodium nitrate were loaded on GAL0.4CZA0.6.The catalytic reduction behavior and OSC of the samples were examined by H2-TPR, the precious metal dispersity characteristics. The thermal aging method was used to investigate the light-off activity and thermal stability. Further discussion of the reciprocity mechanism between the washcoat and the precious metals were carried out. The test results showed that, the fresh catalyst loaded higher Rh content had the better light-off performance. After high temperature aging, the light-off activity of the catalyst loaded higher Pd/Rh content was optimal. The catalytic material P03R (the ratio of Pd and Rh was 3:1) had optimal precious metal dispersity and catalytic activity. The precious metal dispersity of P03R was 35% higher than P33R (the ratio of Pd and Rh was 33:1). The catalyst SAM7 based on P03R had superior performance.5. Catalyst Preparation with lower temperature lightTo study the effect of the preparation process of Pd/Rh with GAL0.4CZA0.6 and the localized loading design of the precious metals on catalyst light-off activity. The effect of two kinds of the preparation processes of Pd/Rh with GAL0.4CZA0.6 on catalytic activity was compared. Furthermore, the catalytic performances of eight sets of single-stage with two-block catalysts were investigated. Through the engine bench and vehicle test results, the excellent performance of the catalyst prepared by the technique A (immersion method) was shown. The single-stage catalyst SAM 10 (the precious metal content:1.15g/L@9:1) prepared by Pd/Rh/GALo.4CZA0.6 and the sigle-stage with two block catalyst SAM11(the precious metal content in front block was 1.6g/L@19:1, and the precious metal content in rear block was 0.7@9:1) had met GBⅣ. SAM10 and SAM11 had the superior light-off performance. The emission results of CO, HCs, NOx were better than the old catalyst. One of the best emission control performances on vehicle was SAM11. The lower precious metal content met 1.15g/L@14:1. This indicated that the single-stage catalyst matched vehicle has got a good emission control effect through optimizing washcoat performance. The single-stage with two block catalyst has more superior light-off performance and emission control capacity after optimizing the process for the precious metal localized loading. The precious metal content of catalyst SAM11 was 60% less than the foreign original catalyst. The precious metal content of catalyst SAM11 was 28% less than our company’s old catalyst. Therefore, the new GBⅣcatalyst could offer the higher ratio of performance and price, and stronger competitive advantage.In allusion to the technical difficulty of cold-start emission control in GBIV implementation, the thesis was based on developing rapid light-off catalyst technique. It presented experiment method, experiment results, experiment conclusion and analytic view. The catalyst design idea was based on the combined numerical calculation, sample preparation, characterization, reaction mechanism analyses, simulation and verification. The GBIV catalyst product contained the precious metals of 1.15g/L which have high value of market popularization were successfully developed. Many rapid light-off techniques for effectively control exhaust gases during the cold-start were put forward. The catalyst light-off technique is a systemic engineering. Only combined technique of catalytic material, catalyst, system integration and vehicle matching can promote technique innovation of rapid light-off catalyst. It is very important to research the correlation between the above techniques. The systemic investigation idea has some innovation as follows:(1) The innovation in the aspect of substrate applied technology can offer reference and guidance of substrate selection for satisfying the stricter regulation; (2) The catalyst preparation technique of localized loading has advanced its property; (3) New washcoat material of alumina with Ce-Zr-Al oxides and preparation technique has novelty and advanced property; (4) The precious metal decreasing technique would make GBⅣcatalyst product have competitive advantage and market popularization value. This research has suggested a series of technical views, new material and new techniques to serve for catalyst researchers. The research achievements can offer the technical supports for China’s catalyst product technique progress and product marketization. It is of great significance to promote emission control level of China’s automobile industry and environmental protection development.更多还原