碳烟和NOx同时净化催化剂表面活性氧作用机理的探讨

【作者】 朱昌权；

【导师】 梁红；

【作者基本信息】 广州大学 ， 化学工艺， 2011， 硕士

【摘要】 随着经济的高速发展,机动车数量的持续增长,车辆排放造成的环境污染问题日趋严重。柴油车因其效率高、节油效果显著,得到了许多国家的重视。本文系统地阐述了柴油车排放污染物的组成、危害以及其控制技术,包括机前控制、机内净化控制和排气后处理技术。对柴油车排气净化催化剂进行了分类概括,以及对催化剂表面活性氧的研究进行了概述。研究的催化剂共分两个体系:1)镧钴钙钛矿型催化剂体系,主要包括Ce、Zr、Mn部分取代钙钛矿;2)铈锆固溶体催化剂体系,主要包括La、Co、Mn掺杂改性固溶体。通过程序升温氧化（TPO）、氢气程序升温还原（H2-TPR）、X-射线衍射仪（XRD）、比表面积（BET）、傅立叶变换红外光谱仪（FT-IR）、氧气程序升温脱附（O₂-TPD）、X-射线光电子能谱仪（XPS）等表征手段对样品进行了测试。考察了不同体系催化剂表面活性氧物种的性能,并通过原位漫反射红外光谱（in situ DRIFT）研究了在同时去除碳烟颗粒物（PM）和氮氧化物（NO_x）反应过程中,30%Mn/Ce_0.7Zr_0.3O₂催化剂表面活性氧物种的作用机理。经过研究得出了以下主要结论:1)对LaCoO₃催化剂制备方法考察发现,络合燃烧法制备的催化剂活性显著。采用Ce、Zr、Mn部分取代LaCoO₃制备的催化剂,少量过渡金属取代能够促进钙钛矿的催化活性,并且不影响钙钛矿的晶相结构。其中LaCo_0.95Mn_0.05O₃催化剂的活性最好,碳烟的起燃温度和最大燃烧温度分别降到了306℃和505℃,NO的转化率达到27.1%。2)络合燃烧法制备不同摩尔比的铈锆固溶体,通过结构表征和活性评价确定固溶体的铈锆比。采用La、Co、Mn掺杂改性Ce_0.7Zr_0.3O₂制备的催化剂,不同程度地形成了三元固溶体结构,适量过渡金属的掺杂提高了固溶体催化剂同时去除PM和NO_x的催化活性,其中30%Mn/Ce_0.7Zr_0.3O₂复合氧化物显示出了最高的活性,碳烟的起燃温度和最大燃烧温度分别降到了298℃和504℃,NO的转化率达到30.6%。3)Mn部分取代的LaCo_1-xMn_xO₃系列催化剂中,少量Mn进入钙钛矿结构更有利于产生缺陷,增加了Oˉ的浓度,提高钙钛矿的活性。4)掺杂不同含量Mn的x%Mn/Ce_0.7Zr_0.3O₂系列催化剂中,适量的Mn掺杂可以提高催化剂表面O₂-的浓度,提高固溶体同时去除PM和NO_x的活性。5)30%Mn/Ce_0.7Zr_0.3O₂催化剂同时去除PM和NO_x的活性与表面吸附的O₂-物种和NO₃^-物种有着直接的关系,中间物种-NCO的形成在该反应过程起到关键的作用。更多还原

【Abstract】 With the development of economy and increase in the number of vehicle, environmental pollution is becoming more and more severe. Diesel has focused much more attention from many countries around the world due to its high efficiency and low fuel consumption. In this dissertation, exhausts compositions, harm and its control technologies including fore-engine, internal-engine and after-treatment are summarized. The types of diesel exhaust purification catalysts are classified and introduced, and research advance of superficial active oxygen is also summed up.Catalysts investigated in our research are divided into two systems: 1) Lanthanum-Cobalt perovskite-type catalysts, including doped Ce, Zr, Mn. 2) Ceria-Zirconia solid solution catalysts, including doped La, Co, Mn.A series of technologies such as Temperature programmed oxidation （TPO）, BET, X-ray diffraction （XRD）, Fourier transformation infrared spectrometry （FT-IR）, Temperature programmed Reduction （H₂-TPR）, Temperature programmed desorption （O₂-TPD）and in situ diffuse reflection infared Fourier transformed （in situ DRIFT）, X-ray Photoelectron Spectroscopy （XPS） were used to determine the catalysts. The performance of surface active oxygen species on different sestem catalysts were investigated. On the basis of the experimental results of in situ DRIFT, a reaction mechanism was proposed, which can preferably explain the reaction process of simultaneous removal of PM and NO_x over 30%Mn/Ce_0.7Zr_0.3O₂.Results obtained in this dissertation are followed:1) It was revealed the activity of LaCoO₃ catalyst was significant with citric acid complex combustion. The catalysts were prepared by partly substituted LaCoO₃ with Ce, Zr, Mn. Substituted with small amount of transition metals can promote the catalytic activity of perovskite and does not affect the crystalline structure of perovskite. The activity of LaCo_0.95Mn_0.05O₃ catalyst was best, its Ti and Tm respectively downed to 306℃and 505℃, and the conversion of NO achieved to 27.1%.2) Ceria-Zirconia solid solution with different the ratio of ceria-zirconia were prepared by complex combustion method. It determined the best ratio of ceria-zirconia through structure characterization and activity evaluation. The catalysts which prepared by doped Ce_0.7Zr_0.3O₂ with La, Co, Mn formed the ternary solid solution. Doped with moderate transition metals can promote the catalytic activity of solid solution catalysts. The activity of 30%Mn/Ce_0.7Zr_0.3O₂ catalyst was best, its Ti and Tm respectively downed to 298℃and 504℃, and the conversion of NO achieved to 30.6%.3) During partly Mn-substituted LaCo_1-xMn_xO₃ catalysts, small amount of Mn-doped was good for the defects, increased the concentration of Oˉ, improved the activity of perovakite.4) Among x%Mn/Ce_0.7Zr_0.3O₂ with different doping of Mn, Moderate Mn-doped can increase the concentration of O₂- on the surface of catalysts, improved the activity of solid solutions for simultaneous removal of PM and NO_x.5) The catalytic performance of 30%Mn/Ce_0.7Zr_0.3O₂ was directly related to the superoxide species （ O₂- ） and nitrate species （ NO₃^- ） which adsorbed on the surface of 30%Mn/Ce_0.7Zr_0.3O₂. They were active species during the reaction of PM and NO_x, and further, the formation of the isocyanate species （ -NCO ） was clarified.更多还原