节点文献
锂基和钾基稀燃氮氧化物储存还原催化剂结构与性能研究
Study on the Structures and Performance of Li and K Based Lean-Burn NO_x Storage and Reduction Catalysts
【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NOx,因此有必要开发新的NOx消除技术。NOx储存-还原(NSR)技术是消除稀燃NOx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-MOx(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-MOx (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NOx的储存适宜温度为350 400 oC,在350 oC,NOx在催化剂Pt/Li/TiO2-MOx (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NOx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NOx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,NOx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NOx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NOx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NOx。
【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 (NOx) can not be effectively removed by traditional three-way catalysts (TWCs). So, it is necessary to explore new catalytic technique for lean-burn NOx abatement. The NOx storage-reduction (NSR) technique is a promising solution to lean-burn NOx 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-MOx (M=Al, Zr, Si, Sn) was investigated carefully. Based on NOx storage ability and SOx-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 NOx storage mechanisms were proposed.A series of NSR catalysts Pt/Li/TiO2-MOx (M=Al, Zr, Si, Sn) were prepared by sequential impregnation using the supports TiO2-MOx synthesized by co-precipitation. The NOx 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-MOx strongly depends on the total acidity of the supports, including Br?nsted acid or Lewis acid. The oxidation ability of Pt/Li/TiO2-MOx is largely determined by crystallite size of Pt. Larger Pt crystallite corresponds to stronger oxidation ability. In-situ DRIFT results show that the NOx is mainly stored as nitrate at 350 oC. At this temperature, NOx is mainly stored as ionic nitrates over Pt/Li/TiO2-MOx (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 NOx 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 NOx storage capacity. Taking both the NOx 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 NOx is mainly stored via bidentate nitrate intermediate at the temperature of 500 oC, while on Pt/Li/TiO2, NOx is mainly stored as ionic nitrates. The–OLi groups are regarded as the main NOx storage sites for Pt/Li/Al2O3 and Pt/Li/TiO2–Al2O3 catalysts, while lithium carbonate may be the prevailing NOx 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 NOx 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 NOx 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 NOx 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 NOx 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 NOx 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.
【Key words】 NOx storage; sulfur-resistance; Li-based catalysts; K-based catalysts; TiO2-Al2O3-La2O3;