【作者】 李倩；

【导师】 孟明；

【作者基本信息】 天津大学 ， 工业催化， 2010， 博士

【摘要】 碳烟和NOx是柴油车尾气中的主要污染物。如何降低碳烟的燃烧温度同时提高NOx的还原效率是降低柴油车污染的关键。高效的氧化型催化剂可以显著降低碳烟催化燃烧的活化能,而具有还原活性位的催化剂可以提高碳烟还原NOx的效率。本文选取Co、Mn为主体元素的水滑石基复合氧化物作为催化剂,并用K对催化剂进行改性,优化了催化剂的组成、K的负载量、催化剂的焙烧温度等参数。另外,基于in situ DRIFTS表征结果,对NOx的储存物种进行了深入研究以揭示NOx的储存路径,并对反应机理进行了探讨。首先,用pH值共沉淀法成功制备了CoMgAlO水滑石基氧化物并负载了不同含量的K,考察了K负载量对催化剂的碳烟燃烧活性和NOx储存性能的影响。在200⁴00 oC的柴油车尾气温度范围内,所有催化剂都显示出了很高的催化活性。当K的负载量达到4.5 wt.%时,改进效果比较显著。K的添加增加了活性Co物种的数目,增强了体相晶格氧的移动性,从而提高了催化剂的碳烟燃烧活性。此外,K的添加使含氮储存物种多元化,因而提高了催化剂的NOx储存能力。电子给体K物种的存在有利于气相O₂分子和NO分子的吸附和活化。随着K含量的增加,含氮物种从螯合双齿硝酸根逐步转化为单齿硝酸根以及最终的自由态硝酸根离子,储存NOx的含K物种主要为K2O。选取K的负载量为4.5 wt.%的样品,进一步深入研究了K的作用的本质、焙烧温度对催化剂活性相的影响,以及碳烟催化燃烧和NOx储存/还原的机理。结果表明,对于碳烟燃烧,无K催化剂的活性随焙烧温度的升高而降低;而在含K催化剂上,即使800 oC焙烧,碳烟燃烧的高活性也可以基本维持,认为K和Co的强相互作用促使了新相K-Co-O中活性氧物种的生成,这种活性氧物种更容易与催化剂表面的碳烟进行反应。氧可以持续地从气相迁移至催化剂表面,从而维持了含K催化剂上碳烟燃烧的高活性。另外,K的添加有效提高了催化剂对NOx的消除活性。Co₃O₄、CoAl₂O₄或类似CoAl₂O₄的尖晶石都是碳烟还原NOx反应中的活性相。其中以600 oC焙烧的样品NOx消除活性最高（³2%）,将其归因于高的表面K/Co比导致的K与Co之间的强相互作用。当催化剂表面上的NO被K-Co-O中的活性晶格氧物种氧化为NO₂时,NO₂可以迅速地与K物种进行反应形成硝酸盐物种,继而被碳烟还原。另外,还考察了MnMgAlO水滑石基复合氧化物催化剂对碳烟催化燃烧、NOx储存以及碳烟-NOx共消除的性能,着重探讨了不同Mn含量对催化剂结构和性能的影响。结果表明,对碳烟燃烧,Mn_1.5Mg_1.5AlO和Mn_1.0Mg_2.0AlO催化剂的活性最佳。结构表征结果显示,随Mn含量的增加,Mn物种从MnAl₂O₄和Mg₂MnO₄逐渐转变成Mn₃O₄和Mn₂O₃。认为Mg₂MnO₄中高度可还原性的Mn⁴⁺物种为碳烟燃烧过程中最主要活性物种。对NOx消除反应,Mn_1.0Mg_2.0AlO的活性最高,基于in situ DRIFTS的结果,发现随着Mn含量的增加,体相存储的NOx物种从线性亚硝酸盐转变成了离子态硝酸盐,最后以螯合双齿硝酸盐的形式存在。认为Mg₂MnO₄中Mn⁴⁺的存在可以促进具有高氧化性能的硝酸盐的形成;MnAl₂O₄中的Mn²⁺物种可能是碳烟还原NOx的活性位。适当含量的Mn²⁺和Mn⁴⁺的共存对整个NOx消除过程是有利的。最后,用K对Mn_1.5Mg_1.5AlO水滑石基复合氧化物催化剂进行了改性,并考察了K的负载量对催化剂性能的影响。当K负载量低于10 wt.%时,随着K的增多,碳烟的氧化速率加快,但碳烟的最高转化速率对应的温度基本不变,认为主要遵循溢流机理。但是当K负载量高于10 wt.%时,碳烟的特征燃烧温度有所降低,但是碳烟燃烧速率与无K的催化剂相当,认为主要遵循NOx辅助的气相反应机理。KNO₃分解产生的NO₂与碳烟反应,从而降低了碳烟的燃烧温度。加入K以后,NOx的还原效率先升高后降低,当K负载量为7.5 wt.%时达到峰值。认为适量K的掺杂有利于单齿硝酸盐的生成,与亚硝酸盐、螯合双齿硝酸盐和离子硝酸盐相比,它与碳烟的反应性更强,对应的NOx消除效率也更高。当K负载量大于15wt.%时,反应过程中KNO₃分解致使K相严重流失;而当K负载量低于10 wt.%时,K₂Mn₄O₈中K与Mn物种的强相互作用稳定了K相,从应用角度考虑,这些催化剂更具有应用前景。更多还原

【Abstract】 Soot and NOx are main pollutants in the emission of diesel engines. How to decrease the soot combustion temperature and increase the NOx reduction efficiency is important for the elimination of these pollutants. Highly effective oxidation catalysts can remarkably decrease the activation energy of soot combustion reaction, and the catalysts with reductive active sites can increase the efficiency of NOx reduction. In this dessertation, Co- and Mn-based hydrotalcite-derived catalysts, as well as the K-promoted ones, are selected. The composition of catalysts, the loading of K and the calcination temperature are optimized. Moreover, the stored NOx species were investigated carefully by in situ DRIFTS in order to reveal the reaction mechanisms.Firstly, hydrotalcite-based CoMgAlO mixed oxides were synthesized by pH-constant coprecipitation and then impregnated with K. All the catalysts display good catalytic activity in the range of 200⁴00 oC. When the weight loading of K reaches 4.5 %, prominent enhancement effect was observed. The addition of K increases the amount of active Co sites, leading to the high soot oxidation activity. Higher NOx trapping efficiency is also obtained over K-promoted catalysts. As dopant K mounts up, N-related species vary from chelating bidentate nitrates to monodentate nitrates and ionic nitrates gradually. The main storage phase is K2O.The loading of K is fixed at 4.5 wt.%. Further researches were carried out on the influence of calcination temperature. High catalytic performance could be maintained on K-promoted catalysts. The strong interaction between K and Co induces the formation of the active oxygen species in a new K-Co-O phase, which readily reacts with soot. Continuous transferring of oxygen from gaseous phase ensures the high soot combustion activity. The NOx reduction activity can also be improved. The catalyst calcined at 600 oC shows the highest NOx removal efficiency （³2%）, attributing to the strong interaction between K and Co. Therefore, NO molecules can be easily oxidized to NO2 by the referred active oxygen species and then stored as nitrates on K species, which can be reduced by soot.Moreover, the performance of hydrotalcite-derived MnMgAlO catalysts was evaluated. The influence of the Mn content on the activity of the catalysts is carefully studied. Mn1.5Mg1.5AlO and Mn1.0Mg2.0AlO show the best catalytic performance for soot combustion. As dopant Mn increases, major Mn-related species vary from MnAl₂O₄ and Mg₂MnO₄ to Mn₃O₄ and Mn₂O₃. The highly reducible Mn⁴⁺ in Mg₂MnO₄ is most active for soot combustion. The Mn_1.0Mg_2.0AlO exhibited the highest activity for NOx removal reaction. The existence of Mn⁴⁺ in Mg₂MnO₄ can enhance the formation of ionic nitrates; the Mn²⁺ in MnAl₂O₄ is proposed to be the active site for NOx reduction. The coexistence of appropriate amounts of Mn²⁺ and Mn⁴⁺ is beneficial to the whole reaction.Finally, Mn_1.5Mg_1.5AlO catalyst promoted by K was investigated and the effect of K was also studied. For soot combustion, when the dopant K was lower than 10 wt.%, the average soot oxidation rate is lowered. The spillover mechanism is proposed. While when the content of K is higher than 10 wt.%, the characteristic temperatures of soot combustion are decreased. NOx-aided gas-phase mechanism was proposed. As dopant K increases, the NOx reduction percentage reaches the maximum value at the point of 7.5 wt.% of K. The introduction of proper content of K can facilitate the formation of monodentate nitrates, which show high reactivity with soot, leading to the higher removal efficiencies. When the K content is lower than 10 wt. %, the strong interaction between K and Mn species in K₂Mn₄O₈ phase stabilizes the K species, making the catalysts possess higher thermal stability, in the view of application, these catalysts are more suitable for practical use.更多还原