【作者】 邹志强；

【导师】 孟明；

【作者基本信息】 天津大学 ， 工业催化， 2010， 博士

【摘要】 催化净化是消除汽车尾气污染同时提高空气质量的最有效方法。随着各国汽车尾气排放法规的日益严格,传统的三效催化剂已经不能解决汽车尾气净化所面临的两大新的难题,即冷启动污染和稀燃氮氧化物（NOx）污染,因为传统三效催化剂对机动车冷启动时排放的CO和烃类,以及稀燃发动机排放的NOx的净化效率很低。虽然丰田公司早在上世纪90年代中期就提出了NOx储存还原的概念（NSR）,并开发了Pt/Ba/Al₂O₃体系的NSR催化剂,但是这类NSR催化剂因抗硫性能差,很难推广使用。本论文针对冷启动和稀燃氮氧化物污染问题,开展相关应用基础研究,研发高性能的低温氧化型催化剂和同时具有优良储存性能和抗硫性能的NSR催化剂具有重要意义。本文选取氧化性能优越的Co基氧化物催化剂为研究对象,从催化剂的组成、结构、制备方法、组分间的相互作用以及反应机理等方面开展了较系统的研究;对于稀燃NSR催化剂,则以酸性复合载体负载的K基NSR催化剂为研究对象,考察了助剂Co和Ce对NSR催化剂Pt/K/TiO₂-ZrO₂结构和性能的影响,包括储存和抗硫性能,同时还考察了载体中掺杂Al₂O₃的影响,对Al₂O₃的含量进行了优化,并对储存介质K物种的存在形式进行了深入研究和讨论,提出了催化剂的构效关系及K物种的分布模型。采用双表面活性剂辅助合成法制备了系列Co基混合氧化物La-Co-Zr-O、Co-Ce-Zr-O和La-Ce-Co-Zr-O等,BET和孔径分布测试结果表明,该系列催化剂具有较高的比表面积和均一的介孔孔径,较传统共沉淀法有明显优势。程序升温还原结果（H2-TPR）表明,催化剂中Co-O键的活动度与其氧化性能密切相关。助剂Ce的加入能够显著提高Co-O键的活动度,从而增强活性。当La和Ce以合适的比例共存于混合氧化物中,催化剂的活性和热稳定性进一步增强,同时该催化剂La-Ce-Co-Zr-O还具有良好的抗硫性能。在Co-Ce-Zr-O体系中引入微量贵金属Pd（0.044%）后,催化剂对CO的氧化活性大大提高,但对C₃H₈氧化却没有促进作用。程序升温氧化（TPO）结果显示,贵金属促进作用的本质主要为氧溢流,即Pd解离吸附气相氧为反应提供活性氧物种。对于CO氧化,氧气的吸附活化是关键步骤;而对于C₃H₈氧化,反应速率可能更取决于C-H键的活化。为进一步提高Co基混合氧化物的热稳定性,采用非离子表面活性剂三嵌段共聚物P123辅助合成了系列Co-Ce-M（M=Cu、Fe、Ni和La）混合氧化物催化剂,高分辨电镜（HR-TEM）和BET结果表明,该系列催化剂具有蠕虫状孔道结构和高比表面积。采用原位红外（DRIFTS）对含不同助剂的样品上CO的氧化机理进行了研究。发现Cu掺杂的催化剂形成了对CO氧化的双活性位,CuO和Co₃O₄都参与了反应,在该催化剂上不但有碳酸盐中间物种,还有羰基Cu物种。H2-TPR和O₂-TPD结果表明,这些催化剂的表面晶格氧的活动度与其CO氧化活性密切相关。Cu的掺杂使得催化剂在500℃和650℃焙烧后,活性大大提高,而La的掺杂在高温焙烧时能有效抑制催化剂的烧结,维持催化剂的氧化活性。Fe的掺杂降低了催化剂表面晶格氧的活动度从而降低了催化剂的活性。Ni的掺杂改变了催化剂上CO氧化反应的路径,此时表面吸附氧取代表面晶格氧成为反应的主要活性物种,该催化剂的活性较不掺杂的催化剂明显降低。对于NSR催化剂Pt/K/TiO₂-ZrO₂,考察了助剂Co或Ce的加入对其储存和抗硫性能的影响。结果表明,Co或Ce的引入能够大大增强催化剂的氧化能力从而显著提高了催化剂对NOx的储存能力。但是催化剂经过硫化和再生后,采用助剂改性的催化剂的储存量比未改性的Pt/K/TiO₂-ZrO₂催化剂反而略有下降,循环TPR测试结果表明,在改性的催化剂上形成了难以脱除的硫酸盐,尤其是含Co的催化剂最为明显,该催化剂具有最强的氧化能力从而促进了硫酸盐的生成。助剂Ce对催化剂Pt/K/TiO₂-ZrO₂性能的影响更取决于Ce的添加方法。采用机械混合法添加Ce的催化剂具有较高的氧化能力和表面K/Ti比,硫化再生后的储存能力仍高达142μmol/g。随着未来燃油中硫含量不断减少,助剂Ce改性的NSR催化剂Pt/K/TiO₂-ZrO₂具有一定的应用前景。为了改进Pt/K/TiO₂-ZrO₂催化剂的抗硫性能,对载体进行了改性,发现载体中添加Al₂O₃可以显著提高催化剂Pt/K/Al₂O₃-TiO₂-ZrO₂的储存和抗硫能力,Al₂O₃的最佳掺杂量为Al/（Ti+Zr）原子比为3/1。当载体在较低温度焙烧时,K物种的主要存在形式是-OK基,NOx储存后,主要以单齿或双齿硝酸盐形式存在;当载体在高温焙烧时表面羟基大大减少直至基本消失,此时储存组分主要以K₂CO₃形式存在,这种K₂CO₃较-OK基能更有效地与NOx反应,生成自由硝酸钾,因此催化剂的储存能力显著提高,然而K₂CO₃物种同样易与SO₂作用,生成相当稳定的硫酸盐物种,从而降低了催化剂的抗硫与再生性能。催化剂中K负载量的提高,使K₂CO₃逐渐取代-OK基成为NOx储存的主要物种,虽然储存量迅速增加,但抗硫性能也显著下降。结合原位红外和载体的表面酸性表征结果（NH3-TPD和吡啶吸附）,提出了不同K负载量和不同焙烧温度时催化剂表面K物种的分布模型。更多还原

【Abstract】 Catalytic purification is the most effective method to eliminate pollutants in vehicle exhaust. The conventional TWC is not efficient enough to remove CO and hydrocarbons released during cold-start period and NOx from lean-burn engines. For NOx reduction, the NOx storage-reduction （NSR） catalyst Pt/Ba/Al₂O₃ was proposed by Toyota, but the low sulfur-resisting ability limits its application. In order to solve these problems, Co-based mixed oxides are selected as the catalysts for low-temperature oxidation of CO and hydrocarbons. The compositions, structures, preparation methods of the catalysts, and reaction mechanisms are investigated. The acidic mixed oxides supported K-based NSR catalysts were prepared. The effect of additive Co or Ce on the catalyst Pt/K/TiO₂-ZrO₂ is studied. In addition, the support TiO₂-ZrO₂ was modified by Al₂O₃ doping. The existing state of the storage medium K was investigated and the distribution modes of K are proposed.A series of Co-based mixed oxides with high specific surface area were synthesized by dual surfactants-assisted method. The activities for CO and C₃H₈ oxidation are related to the mobility of Co-O bond. The dopant CeO₂ can promote the mobility of Co-O bond and then improve the activity. When La and Ce coexist in the catalyst, the activity and thermal stability can be further enhanced. The ultra-low content of noble metal Pd can promote the CO oxidation, but can hardly influence C₃H₈ oxidation. This promotion derives from oxygen spillover. The oxygen activation is the crucial step for CO oxidation while C₃H₈ oxidation depends on C-H bond activation.A series of Co-Ce-M （M=Cu, Fe, Ni or La） mixed oxides were prepared by surfactant-assisted method using co-polymer P123. The different CO oxidation mechanisms were revealed by DRIFTS. Dual active sites have formed on the Cu doped catalyst. The mobility of surface lattice oxygen is relevant with the CO oxidation activity. Cu doping improves CO oxidation activity of the catalyst calcined at 500oC or 650oC, while La doping effectively suppresses the sintering of the catalyst when it is calcined at a high temperature. Fe doping always decreases the mobility of surface lattice oxygen and then decreases the activity. Ni doping has altered CO oxidation mechanism and the weakly bound active oxygen replaces surface lattice oxygen to act as the main active oxygen species.The effect of additive Co or Ce on the storage and sulfur-resistance performance of the catalyst Pt/K/TiO₂-ZrO₂ was investigated. The results show that the Co or Ce addition can improve the storage performance. However, after sulfation and regeneration, the NOx storage capacity （NSC） of the modified catalyst is more or less lower than that of the unmodified one due to more stable sulfates are formed on the modified catalysts. The effect of Ce addition on Pt/K/TiO₂-ZrO₂ largely depends on the addition mode. The mechanically prepared Ce-promoted catalyst possesses considerable NOx storage capacity of 142μmol/g after sulfation and regeneration.The Al₂O₃ doping can obviously improve both the NOx storage capacity and sulfur-resistance performance of the catalyst Pt/K/Al₂O₃-TiO₂-ZrO₂. When the support is calcined at lower temperatures, K exists mainly in the form of–OK groups. After NOx adsorption, the main storage species are monodentate or bidentate nitrates. When the support is calcined at a higher temperature, K₂CO₃ is the dominating storage medium forming ionic free nitrates, which is more efficient for NOx storage than–OK groups. As a result, the catalyst with its support calcined at higher temperatures possesses higher NSC, however, K₂CO₃ also react with SO₂ more easily to form stable sulfates, determining its worse sulfur-resistance performance. With the increase of K loading, K₂CO₃ becomes the main storage medium with the NSC increasing, but with the sulfur-resistance decreasing.更多还原