【作者】 何俊俊；

【导师】 孟明；

【作者基本信息】 天津大学 ， 工业催化， 2010， 博士

【摘要】 稀薄燃烧技术不仅能减少燃油消耗,而且能减少CO₂和碳氢化合物的排放。然而在稀燃条件下,传统的三效催化剂不能有效去除稀燃尾气中的NO_x,因此有必要开发新的NO_x消除技术。NO_x储存-还原（NSR）技术是消除稀燃NO_x的一种有效方法。目前广泛研究的Pt/Ba/Al₂O₃体系因抗硫性差,难以推广使用。本文以碱金属（Li和K）代替碱土金属Ba作为储存剂,以TiO₂基复合氧化物TiO₂-MO_x（M=Al, Zr, Si, Sn）为载体,制备了相应的NSR催化剂,并筛选出合适的载体TiO₂-Al₂O₃,对其组份配比进行了优化,研究了负载不同Li含量的催化剂性能并与不同Ba含量的催化剂进行了比较。同时考察了掺杂La₂O₃的影响,对La₂O₃的含量进行了优化,并对载体焙烧温度,储存剂K的前驱体的影响进行了系统考察。在此基础上,提出了氮氧化物的储存机理。采用共沉淀法合成了一系列TiO₂基复合氧化物TiO₂-MO_x （M=Al, Zr, Si, Sn）,并分步浸渍贵金属Pt和Li制成NSR催化剂,结果表明:NSR催化剂Pt/Li/TiO₂经Al₂O₃或ZrO₂改性后,其储存能力大幅增加。原因是催化剂的比表面积的提高和贵金属Pt分散度的降低。催化剂的抗硫能力与载体的总酸量密切相关,总酸量越大,抗硫性能越好。而催化剂的氧化能力由贵金属的粒子大小（分散度）决定,分散度越低,粒子越大,其氧化性能越强。以碱金属Li作为储存剂的NSR催化剂对NO_x的储存适宜温度为350 400 oC,在350 oC,NO_x在催化剂Pt/Li/TiO₂-MO_x （M=Al,Zr,Si,Sn）上只是以单一的离子硝酸盐形式吸附。催化剂硫中毒主要因为形成了体相硫酸盐。对于TiO₂-Al₂O₃负载的NSR催化剂体系,优化了载体TiO₂-Al₂O₃中的原子比例,并进一步研究了Li基催化剂与Ba基催化剂之间的差异。结果表明:TiO₂的掺杂可以大幅提高催化剂Pt/Li/Al₂O₃的抗硫性。与纯TiO₂作为载体相比,Pt与Li在TiO₂-Al₂O₃复合载体上的分散性更好,催化剂的NO_x储存量也较大。从催化剂的储存能力和抗硫性能考虑,适宜的TiO₂添加量为:质量比TiO₂/（TiO₂ + Al₂O₃）=0.40,摩尔比Ti/（Ti + Al）=0.30。In-situ DRIFT表征结果表明:500 oC下,NO在Pt/Li/Al₂O₃和Pt/Li/TA （40）上主要以双齿硝酸盐形式储存,主要储存活性位为-OLi。而在Pt/Li/TiO₂则是以离子态硝酸盐形式吸附,主要吸附中心为Li2CO3。以碱金属Li和碱土金属Ba作为储存剂时,当催化剂含有相同摩尔量的碱性组分时,Li基与Ba基催化剂的NO_x储存量大致相当,但Li基催化剂具有更好的抗硫性。为了改进Pt/K/TiO₂-Al₂O₃催化剂的热稳定性和抗硫性能,对载体进行了改性,发现载体中掺杂少量La₂O₃可以显著催化剂Pt/K/TiO₂-Al₂O₃的热稳定性,储存及抗硫性能。La₂O₃的最佳掺杂量为:质量比La₂O₃/（TiO₂ + Al₂O₃ + La₂O₃）为3%。当载体在较低温度焙烧时（500 oC）,主要以无定形形式存在,载体酸性较强,K在载体上主要以-OK形式存在,NO_x在催化剂上以单齿或双齿硝酸盐的形式吸附,此时的硝酸盐热稳定性差,催化剂的储存能力不大。当载体在较高温度焙烧时（750 oC）,载体表面酸性较弱,K在载体上主要以K2CO3的形式存在,K2CO3相对于-OK更容易与NO_x作用,生成自由硝酸根,其热稳定性较高,因此高温焙烧的载体负载的NSR催化剂对NO_x的储存能力更大。然而,K2CO3也容易与SO₂反应生成难以脱除的硫酸盐。不同K盐的前驱体能显著影响NSR催化剂Pt/K/TiO₂-Al₂O₃-La₂O₃的储存性能。采用KNO3为前驱体时,新鲜样品的催化剂储存能力最强,但抗硫性最差。采用KCl为前驱体时,新鲜样品的储存能力最弱,但抗硫性最强。In-situ DRIFT表征结果表明,采用不同前驱体制得的样品在350 oC富氧气氛下主要以离子态硝酸盐形式储存NO_x。更多还原

【Abstract】 Lean-burn combustion is a promising technique to increase fuel efficiency and decrease hydrocarbons （HCs） and carbon dioxide （CO₂） emission. However, under lean-burn condition nitrogen oxides （NO_x） can not be effectively removed by traditional three-way catalysts （TWCs）. So, it is necessary to explore new catalytic technique for lean-burn NO_x abatement. The NO_x storage-reduction （NSR） technique is a promising solution to lean-burn NO_x pollution. At present, the most widely studied catalyst system is Pt/Ba/Al₂O₃, which can be hardly applied extensively due to its poor sulfur-resistance. In this work, alkali metal Li and K was used as storage medium to replace Ba. The performance of Li-based catalysts supported on TiO₂ and TiO₂-MO_x （M=Al, Zr, Si, Sn） was investigated carefully. Based on NO_x storage ability and SO_x-resisting performance, the TiO₂-Al₂O₃ is selected as the support. The weight ratio of TiO₂/（TiO₂ + Al₂O₃） was optimized, meanwhile, the difference between Li-based and Ba-based catalysts was systematically investigated. In addition, the effect of La₂O₃ doping on the performance of Pt/K/TiO₂-Al₂O₃ was also studied. The optimized weight ratio La₂O₃/（TiO₂ + Al₂O₃ + La₂O₃） in the support is 3%. Besides, the effects of calcination temperature of the support and the different precursor of K were also investigated. Based on this, potential NO_x storage mechanisms were proposed.A series of NSR catalysts Pt/Li/TiO₂-MO_x （M=Al, Zr, Si, Sn） were prepared by sequential impregnation using the supports TiO₂-MO_x synthesized by co-precipitation. The NO_x storage capacity （NSC） of fresh Pt/Li/TiO₂ is greatly improved after doped with Al₂O₃ or ZrO₂ due to the increase of specific surface area and decrease of Pt dispersion. The regeneration of sulfated Pt/Li/TiO₂-MO_x strongly depends on the total acidity of the supports, including Br?nsted acid or Lewis acid. The oxidation ability of Pt/Li/TiO₂-MO_x is largely determined by crystallite size of Pt. Larger Pt crystallite corresponds to stronger oxidation ability. In-situ DRIFT results show that the NO_x is mainly stored as nitrate at 350 oC. At this temperature, NO_x is mainly stored as ionic nitrates over Pt/Li/TiO₂-MO_x （M=Al, Zr, Si, Sn）. Sulfur poisoning of the catalysts is mainly resulted from the formation of bulk sulfates.As for the NSR catalysts supported on TiO₂-Al₂O₃, the weight ratio of TiO₂ to TiO₂ + Al₂O₃ was optimized, and the difference between Li-based and Ba-based catalysts for NO_x storage and sulfur-resistance was investigated. The doping of TiO₂ into the Al₂O₃ could significantly improve the sulfur-resistance performance of the catalyst Pt/Li/Al₂O₃. Compared with those on pure TiO₂, the Pt and lithium species are more highly dispersed on TiO₂–Al₂O₃ mixed oxides, giving higher NO_x storage capacity. Taking both the NO_x storage capacity and the sulfur-resistance performance into account, the optimal weight ratio of TiO₂/（TiO₂ + Al₂O₃） in the catalysts is 40%. In-situ DRIFT results show that on Pt/Li/Al₂O₃ and Pt/Li/TiO₂–Al₂O₃ NO_x is mainly stored via bidentate nitrate intermediate at the temperature of 500 oC, while on Pt/Li/TiO₂, NO_x is mainly stored as ionic nitrates. The–OLi groups are regarded as the main NO_x storage sites for Pt/Li/Al₂O₃ and Pt/Li/TiO₂–Al₂O₃ catalysts, while lithium carbonate may be the prevailing NO_x storage phase for Pt/Li/TiO₂. When Pt/Li/TA（40） and Pt/Ba/TA（40） possess equal molar amounts of storage medium, they show almost the same NO_x storage ability. However, the Pt/Li/TA（40） exhibits much better sulfur-resistance performance than the Ba-based NSR catalyst.To improve the thermal stability and sulfur-resistance of Pt/K/TiO₂-Al₂O₃, the support TiO₂-Al₂O₃ is further modified by La₂O₃. The La₂O₃ doping can obviously improve both the NO_x storage capacity and the sulfur-resisting performance of Pt/K/TiO₂-Al₂O₃. The most suitable weight ratio of La₂O₃/（TiO₂ + Al₂O₃ + La₂O₃） is 3%. When the support was calcined at 500 oC, it exists in amorphous state and possesses large amount of acidity, with the K existing mainly as–OK groups. On the corresponding catalyst, the main NO_x storage species are monodenate or bidenate nitrates. As the support was calcined at 750 oC, the surface hydroxyl groups greatly decrease and even disappear. In this case, K2CO3 is the dominating storage medium which is more efficient for NO_x storage than–OK groups. In a summary, the catalyst with its support calcined at higher temperature possesses higher NSC, however, K2CO3 can react with SO₂ more easily to form sulfates, decreasing its sulfur-resistance. The performance of Pt/K/TiO₂-Al₂O₃-La₂O₃ is also influenced by different K precursors. When KNO3 is used, the fresh catalyst possesses the best NO_x storage ability but the worst sulfur-resistance performance. On the contrary, when KCl is used, the fresh catalyst possesses the highest sulfur-resistance performance and the lowest NSC value. In-situ DRIFT results indicate that the storage mechanism is varied with different calcination temperature of the support and the different K precursors.更多还原