【作者】 刘咏；

【导师】 孟明；

【作者基本信息】 天津大学 ， 工业催化， 2008， 博士

【摘要】 NOx储存-还原(NSR)法是消除稀燃NOx的一种有效方法。目前广泛研究的Pt/Ba/Al2O3催化剂体系抗硫性能差。本文以碱金属K代替碱土金属Ba作为储存剂、以TiO2-ZrO2复合氧化物代替Al2O3,制备了Pt/K/TiO2-ZrO2催化剂。详细考察了载体焙烧温度对催化剂储存、抗硫和再生性能的影响,同时考察了K负载量、储存剂种类和载体效应对催化性能的影响,并对储存机制进行了深入研究。NOx储存实验结果表明,随载体焙烧温度升高,催化剂对NOx的储存量呈先增大后减小的趋势,800oC焙烧时,催化剂具有最大的NOx储存量。N2吸附脱附、XRD、TPD和in-situ DRIFTS等结果表明,影响储存能力的直接因素是载体的表面性能和K物种形态,与比表面积大小无明显关系。随焙烧温度升高,载体由无定形结构转变为ZrTiO4晶体结构,B酸位转变为L酸位,表面酸量明显下降,同时,K物种由-OK键和氧化物逐渐转变为K2CO3。由不同K物种形成的硝酸盐的稳定性次序为:K2CO3>氧化物>-OK键。随载体焙烧温度升高,硫酸盐脱附温度提高,催化剂抗硫性能逐渐减弱。H2-TPR结果表明,载体于500oC焙烧的样品中硫酸盐从200oC左右开始分解,在500oC之前基本完全脱附,还原温度比传统催化剂Pt/Ba/Al2O3降低200oC以上;载体于650oC焙烧时完全脱附温度升至约610oC;800oC焙烧时,脱附峰温升至625oC。K物种的存在形式影响硫酸盐的稳定性,K2CO3的分解降低了硫酸盐脱附温度。载体于650oC焙烧的样品硫化再还原后,储存能力最强;但载体于500oC焙烧的样品再生性能最好,储存能力达到新鲜催化剂的90%。载体于500或650oC焙烧,Pt/K/TiO2-ZrO2催化剂能同时较好地兼顾储存和抗硫性能。K2CO3负载量(5~30wt%)与催化剂储存能力呈顺变关系,但负载量大,硫酸盐脱附温度高,抗硫性能下降,K2CO3负载量不宜超过15wt%。储存组分Li、K、Ba、Mg和Sr对催化性能有不同影响,K和Li是储存和抗硫性能兼顾的适宜选择。载体效应的研究表明,TiO2-ZrO2与储存组分的作用最强,抗硫性能最好;Al2O3载体上储存组分易形成较大颗粒且难以脱除硫酸盐,抗硫性能最差。DRIFTS结果表明,载体于500oC焙烧时,反应路径主要是NO直接氧化成硝酸盐以及歧化反应生成硝酸盐;于650oC焙烧时,反应路径主要是NO直接氧化成硝酸盐以及亚硝酸盐;进一步提高焙烧温度至800oC,反应路径主要为NO直接氧化成硝酸盐,并以三维方式向体相扩散。随载体焙烧温度升高,NSR催化剂的最佳储存温区向高温方向移动。更多还原

【Abstract】 The NOx storage and reduction (NSR) technique proposed by Toyota provides a feasible approach to the abatement of lean-burn NOx. Up to now, the most widely studied catalyst system is Pt/Ba/Al2O3, which shows poor SO2-resisting ability. Thus, new NSR catalysts tolerant to sulfur poisoning are desiderated to be developed. In the present study, a series of NSR catalysts Pt/K/TiO2-ZrO2 with good sulfur-tolerant performance are prepared. The effect of calcination temperature of the support on storage capacity, sulfur durability and the regeneration property of the NSR catalysts are investigated in detail; meanwhile, the influences of the K loading, the kinds of storage components and the supports on the catalytic performance are also studied; in addition, the reaction routes and storage mechanisms are revealed or discussed.The results of NOx storage capacity (NSC) show that as the calcination temperature increases, the NOx storage capacities of the catalysts show volcano-type tendency, with the maximum appearing at 800oC. The results of N2 adsorption/desorption, XRD, TPD and in-situ DRIFTS show that the storage capacity is tightly related to the structures and chemical properties of the supports and the state of K speices, regardless of the specific surface areas. With the calcination temperature increasing, the structures of the supports are transformed from amorphous state at 500oC to ZrTiO4 crystalline at 650oC or above, while the total amounts of surface acidity decline evidently, accompanying with the transformation of Br?nsted acidic sites to Lewis acidic sites. The formation of -OK group arising from the interaction between the surface hydroxyl of support and the K-containing phases is not favorable to NOx storage, while the highly dispersed K2CO3 phase facilitates the NOx storage as nitrates. The sequence for the stability of nitrates formed from different K species is K2CO3> potassium oxide >-OK bond.The reduction temperature of sulfates formed on the catalysts shifts to high temperature as the calcination temperature of support increases. The results of H2-TPR reveal that the reduction of the sulfates formed on Pt/K/TiO2-ZrO2 catalyst with the support calcined at 500oC started from about 200oC and completely finished before 500oC, which is about 200oC lower than that of traditional Pt/Ba/Al2O3. The corresponding temperature for the catalyst with the support calcined at 650oC is elevated to 610oC, and at higher calcination temperature of 800oC, the major reduction peak further shifts to 625oC. The stability of sulfates is also influenced by the state of K species, the decomposition of K2CO3 has decreased the reduction temperature of the sulfates. After reduced in H2-containing atmosphere, the regenerated sample with the support calcined at 650oC shows the biggest NOx storage capacity, being about 60% of that for the fresh catalyst; and the regenerated one with the support calcined at 500oC possesses the best regeneration ability, whose storage capacity achieves 90% of that for the fresh sample; while the storage capacity for the regenerated sample with the support calcined at 800oC only reaches 20% of that for the fresh one. The catalysts Pt/K/TiO2-ZrO2 with the calcination temperature of support at 500 and 650oC possess not only high storage capacity but also novel sulfur-resisting ability.The results of NSCs show that the K loadings (5~30 wt%) are proportional to the storage capacities of the catalysts, but with the increase of K loading, the sulfates reduction shifts to higher temperature, decreasing the sulfur-resisting ability, so, the optimal loading for K2CO3 should not exceed 15 wt%. Among the different storage components of Li, K, Ba, Mg and Sr, K and Li are the best selection from the view of both the storage capacity and the sulfur resistance. The comparative studies of the support effect show that the interaction of TiO2-ZrO2 with the storage component is stronger than other supports, leading to the best SO2-resisting performance. The sulfate particles are easily agglomerated on the surface of Al2O3, which is hard to be removed, and therefore showing the worst performance for sulfur resistance.The DRIFTS results of NOx adsorption over the catalyst Pt/K/TiO2-ZrO2 indicate that the different storage mechanisms are followed for the samples with the support calcined at different temperatures. For the catalyst with the support calcined at 500oC, the reaction pathway consists of direct oxidation of NO to nitrate and the disproportionation reaction of NO2 with the formation of nitrate and NO; for the sample with the support calcined at 650oC, the main reaction routes may be the oxidation of NO to form nitrite and nitrate species; at higher calcination temperature of 800oC, the main reaction pathway is the formation of bulk nitrate species via the direct oxidation of NO and the three-dimensional transferring from surface to bulk. As the calcination temperature of support increases, the optimal temperature region for NOx storage as nitrate shifts to higher temperature direction.更多还原