【作者】 罗金勇；

【导师】 孟明；

【作者基本信息】 天津大学 ， 工业催化， 2009， 博士

【摘要】 催化净化是消除汽车尾气污染,提高空气质量的最有效方法之一。传统的三效催化剂不能有效消除低温冷起动阶段排放的一氧化碳和烃类,以及稀燃发动机排放的氮氧化物,因此,开发高性能低成本的低温氧化催化剂和具有良好抗硫性能的氮氧化物储存还原催化剂具有重要意义。本文选取少量贵金属促进的铈基复合氧化物催化剂为研究对象,对催化剂的制备过程、组分之间的相互作用、以及CO和C₃H₈催化氧化的机理等进行了系统研究;同时,对稀燃氮氧化物储存还原催化剂Pt/Ba/Al₂O₃进行了深入研究,重点考察了助剂Fe的添加及制备方法等对催化剂微观结构的影响,并与催化剂的储存和抗硫性能进行关联。首先,我们利用表面活性剂辅助合成法制备了系列Co₃O₄-CeO₂复合氧化物。BET和孔径分布测试结果表明,该方法制得的催化剂具有较高比表面积和较均匀的介孔。对催化剂的结构研究发现,该催化剂具备包裹结构,仅少量Co离子暴露于表面并与CeO₂相互作用。这种结构保证了Co₃O₄-CeO₂在三维方向上具有最为强烈的相互作用,使得该催化剂表现出独特的氧化还原性能。活性测试结果发现,该催化剂表现出优良的氧化性能。结构和机理研究的结果显示:CO和丙烷氧化所需活性位不同,CO氧化主要发生在Co₃O₄-CeO₂界面,而丙烷氧化主要发生在与表面Co₃O₄微晶相邻的晶格氧上。同时,在催化剂中添加少量Pd后,样品对CO氧化活性急剧提高提高,即Pd-Co₃O₄间存在显著的催化协同效应,但对丙烷氧化活性没有影响,催化机理和速控步骤的不同决定了催化协同效应的有无,据此,我们针对CO氧化从分子水平上提出了加Pd前后截然不同的动力学反应路径。为进一步简化制备步骤并保证催化剂具有较大的比表面积,对表面活性剂辅助合成法进行了改进,采用一步合成法。同时为发现普适的反应规律和探索新的催化体系,我们对贵金属和过渡金属氧化物进行调变。结果发现,该方法制备的催化剂仅有微量的Pd暴露于表面,但这些Pd物种与过渡金属氧化物间存在着对CO氧化的协同效应。原位DRIFTS结果表明,协同效应本质为二者相互作用能产生活泼的氧物种,这些氧物种能与CO迅速反应生成反应中间产物-双齿碳酸盐物种（1587和1285 cm-1）。协同效应的大小取决于Pd与金属氧化物MOx作用的强弱程度。其中,FeOx和MnOx这两种金属氧化物与CeO₂间能形成固溶体,促进了这种相互作用,使得CO氧化起燃温度较不添加Pd的催化剂分别降低了70 oC和100 oC以上。尤其是Pd-MnOx-CeO₂催化剂在室温下对CO的转化率即可达到80%。但对于丙烷氧化,速控步骤是C-H键的断裂而非氧的活化。C-H键断裂的难易在很大程度上取决于3d过渡金属氧化物的d电子结构。随着d电子增加,3d过渡金属氧化物对丙烷氧化表现出双峰行为。论文还研究了助剂Fe的添加对Pt/Ba/Al₂O₃催化剂结构、NOx储存和脱硫性能的影响,结果表明,尽管Fe的添加能抑制BaSO₄颗粒的长大,但对NOx的储存和硫的脱除均是不利的。EXAFS、原位DRIFTS和循环TPR表征结果表明:Pt-Ba间的相互作用对NOx储存和硫的脱除是非常重要的。Pt-Ba间相互作用不仅促进了储存中的关键步骤-NOx溢流,同时也有助于BaSO₄选择性地还原为H2S,促进硫的脱除和催化剂的再生。在Pt/Ba/Al₂O₃中加入Fe,经历氧化还原循环后,Pt-Fe间形成合金,使得活性位Pt被Fe及其氧化物覆盖,抑制了Pt-Ba间的相互作用,导致储存能力和抗硫能力的下降。最后,利用嵌段共聚物P123作为模板,合成了介孔Pt/BaCO₃-Al₂O₃ NOx储存还原催化剂,并与传统浸渍法制备的催化剂进行比较,同时对催化剂的结构和性能进行了系统研究。结构表征表明:介孔Pt/BaCO₃-Al₂O₃具有较高的比表面积,均匀的孔径和较高的热稳定性。Ba物种三维高度分散,且与Al₂O₃强烈作用,所有的BaCO₃均以低温碳酸钡形式存在。同传统浸渍样品比较,介孔样品作为NSR催化剂具有明显的优势,比如较高的NOx储存能力、较低的硫的吸附能力以及较强的脱硫能力。同时在NOx和SOx吸附后没有体相物种生成。更多还原

【Abstract】 Catalytic purification of automotive exhaust is one of the most efficient methods to improve the air quality. Since the conventional three-way catalysts can not effectively reduce the emissions of CO and hydrocarbons during the cold star period, as well as NOx from the lean-burn engines, it is necessary to develop highly active and low-cost oxidation catalysts, and highly sulfur-resistant NOx storage-reduction catalysts. In this dissertation, the preparation process of the noble metal-promoted CeO₂-based mixed oxide catalysts, the component interaction, and the catalytic mechanism for CO and C₃H₈ oxidation were systematically investigated, aiming at developing new catalyst system for the abatement of hazardous emissions during cold start. Meanwhile, the effect of Fe additive and preparation method on the microstructure of the Pt/Ba/Al₂O₃ NSR catalyst were carefully probed, and the structures were well correlated with the catalytic performance for NOx storage and desulfation.Firstly, mesoporous Co₃O₄-CeO₂ mixed oxide catalysts with high surface area were successfully synthesized by a surfactant-assisted method. The Co₃O₄ crystallites in these catalysts are encapsulated by nanosized CeO₂ with only a small fraction of Co ions exposing on the surface and strongly interacting with CeO₂. Such structure maximizes the interaction between Co₃O₄ and CeO₂ in three dimensions, resulting in unique redox properties. The results of activity measurements indicated that these catalysts exhibit excellent oxidation performance. By correlating the structure with the activity, it was found that the active site requirements for CO and C₃H₈ oxidation are different. CO oxidation preferentially occurs at the interface between Co₃O₄ and CeO₂, whereas C₃H₈ oxidation takes place on the neighboring surface lattice oxygen sites in Co₃O₄ crystallites. It was also found that the introduction of a small amount of Pd to Co₃O₄-CeO₂ can remarkably promote the CO oxidation, but can hardly alter the C₃H₈ oxidation. The different behaviors for CO and C₃H₈ oxidation are determined by their different reaction mechanisms and different rate-determining steps, on this basis, two totally different kinetic reaction pathways on molecular level for CO oxidation were proposed for Co₃O₄-CeO₂ and Pd/Co₃O₄-CeO₂ catalysts.In order to simplify the preparative procedures and maintain high surface area of the catalyst at the same time, a series of Pd-prmoted MOx-CeO₂ （M=Mn, Fe, Co, Ni, Cu） mixed oxide catalysts were synthesized by the surfactant-assisted method in one step. The aim of the study was to investigate the reaction mechanism from a broad point of view, and design new catalyst system. It was found that only trace amounts of Pd species are exposed on the surface, but there is a synergistic effect between these species and 3d transition metal oxides for CO oxidation. According to the in-situ DRIFTS results, the synergistic essential originates from the interaction-assisted generation of active oxygen species between Pd and MOx, which react readily with CO, forming bedentate carbonate （1587 and 1285 cm-1） as intermediates. The extent of synergism depends on the strength of interaction. Since a solid solution is formed between CeO₂ and MnOx or FeOx, very strong interaction between Pd and MOx is generated, resulting in the greatly enhanced CO oxidation activity. The light-off temperatures for Pd-doped Mn and Fe-containing catalysts, as compared with the Pd-free catalysts, are decreased by more than 70 and 100 oC, respectively. In particular, a CO conversion as high as 80% can be achieved at room temperature over the Pd-MnOx-CeO₂ catalyst. Whereas for C₃H₈ oxidation, the C-H bond activation, but not the oxygen activation, consists of the rate-determining step. The C-H bond activation ability is largely determined by the d-electron configurations of M cations, and a double-peak phenomenon can be derived with the 3d-transition metal oxides.The influence of the introduction of Fe on the structures, NOx storage and sulfur removal performance of the Pt/Ba/Al₂O₃ catalyst was studied. The results showed that although the introduction of Fe can greatly inhabit the growth of BaSO₄ particles, it is detrimental to the NOx storage and sulfur removal. Based upon the results of EXAFS, in-situ DRIFTS and repeated H2-TPR, it was found that the interaction between Pt and Ba species is of great importance for the NOx storage and sulfur removal. The Pt-Ba interaction not only accelerates the NOx spillover which is a key step during storage, but also facilitates the selective reduction of BaSO₄ into H2S, favorable to sulfur removal and catalyst regeneration. The introduction of Fe to the Pt/Ba/Al₂O₃ catalyst decreases the Pt-Ba interaction by encapsulation of Pt in the matrix of Fe/FeOx after repeated redox cycles, leading to the decrease of NOx storage capacity and sulfur removal ability.A mesoporous NSR catalyst Pt/BaCO₃-Al₂O₃ was synthesized by using tri-block copolymer P123 as template. Systematic comparative studies were performed on the structural and catalytic performance between this catalyst and the conventional impregnated one. The results of structural characterization show that the mesoporous catalyst exhibits high specific surface area, uniform pore size and high thermal stability. The Ba-containing species are highly dispersed in three-dimensions and strongly interacted with Al₂O₃, and all the BaCO₃ presents as LT-BaCO₃ （BaCO₃ with low thermal stability）. Compared with the impregnated catalyst, the mesoporous sample possesses great advantages in serving as NSR catalysts, such as enhanced NOx trapping ability, lower sulfation degree and higher desulfation extent. In addition, after NOx and SOx sorption, no bulk phase of barium nitrates and sulfates are formed in the mesopotous catalyst.更多还原