【作者】 戴宇；

【导师】 王幸宜；

【作者基本信息】 华东理工大学 ， 工业催化， 2012， 博士

【摘要】 随着科技的飞速发展以及人类活动领域的不断扩大,人类对大自然造成的危害越来越大,环境污染问题越来越严重。含氯挥发性有机化合物(CVOCs)作为大气污染物的主要来源之一,已经引起了各国的广泛关注。由于CVOCs在工业及人类生活的各个领域都有非常广泛的应用,因此,如何处理各种排放的CVOCs是环境保护的重要课题。常用的CVOCs处理方法有催化加氢脱氯、催化水蒸气重整、光催化氧化法以及催化燃烧方法。在这些方法中,催化燃烧方法作为一种能耗小,处理温度低,净化效率高的方法,被得到广泛的应用。催化燃烧方法中常用的催化剂一般有贵金属负载型催化剂、钙钛矿型非贵金属催化剂以及过渡金属氧化物催化剂。本文选用价格便宜且对环境污染较小的过渡金属MnOx催化剂,结合我国含量丰富的稀土资源,制备掺杂稀土元素Ce和La的MnCeLaOx催化剂,选用氯苯作为CVOCs的模型分子,考察氯苯在MnCeLaOx催化剂上的催化燃烧反应性能,并对其机理进行相应研究,取得了一些有意义的结果。以硝酸锰、硝酸铈及硝酸镧为前驱体,通过柠檬酸络合法制备MnCeOx、 MnCeLaOx以及单组分氧化物催化剂。在1000ppm氯苯,15000ml·h-1空速以及10%02浓度条件下对不同催化剂上氯苯催化燃烧反应进行考察。结果表明,MnOx, CeO2, La2O3活性较低,90%氯苯转化率所需的反应温度(T90)高于440℃C。在MnOx催化剂中引入少量Ce后,MnCeOx催化剂活性大幅度提升,T90降到246℃C。通过XRD, XPS, TPR和Raman等表征显示,Ce的引入使得部分Mn物种进入Ce02的立方萤石结构的晶格中,形成具有良好氧流动性的锰铈固溶体。锰铈固溶体结构热稳定性差,在高温下Mn物种易析出,使固溶体分解,从而导致催化剂活性的降低。引入La元素,通过形成Mn-O-Ce-O-La的结构,能够有效地提高锰铈固溶体的热稳定性,维持MnCeLaOx催化剂的高催化活性。MnCeLaOx催化剂对氯苯的催化燃烧活性随着Mn含量的升高而提高,如Mn(0.86)CeLaOx催化剂上氯苯完全转化温度仅为225℃C,反应主要产物为CO2. H2O以及HCl；在较高的反应温度下,有少量C12生成。MnCeLaOx催化剂的活性与Mn含量的关系归结于在固溶体与富Mn相间存在的临界面构成的高活性中心。随着Mn含量的降低,MnCeLaOx催化剂的结构呈现出MnCeOx固溶体向CeLaOx固溶体的转变,催化剂的氧化还原能力逐渐减弱,催化剂活性降低。采用微反-色谱-质谱技术对MnCeLaOx催化剂进行活性稳定性测试。结果表明,当反应温度低于300℃C时,各种Mn/Mn+Ce+La比例的催化剂都发生失活,结合反应前后催化剂表面分析,表明氯苯的解离发生在固溶体结构与富Mn相的临界面上,生成的氯物种与该临界面上的Ce物种发生强相互作用,阻塞了催化剂的活性位。当反应温度高于350℃C时,各催化剂上氯苯转化率可达稳定值。在350℃C下用空气中对失活Mn(0.86)CeLaOx催化剂进行处理,低温活性得到部分恢复。对反应尾气进行检测,发现了大量Cl2及HCl的生成,表明Ce物种周围的Mn物种利用固溶体中的活性氧通过Deacon反应将氯物种转化成Cl2离开催化剂表面。通过对反应动力学的研究得知,反应速控步骤为吸附在催化剂表面的氯物种从表面的离去。氢源如H2O和庚烷的加入不能阻止催化剂的失活,但可以抑制Cl2的生成。使用柠檬酸络合法,将KMnO4作为MnCeLaOx催化剂中Mn物种的前驱体之一对MnCeLaOx催化剂进行进一步改性,KMnO4引入量为0.5%-40%。氯苯催化燃烧反应活性的测试结果表明,少量地引入KMnO4使得催化剂的氧化还原能力略有提升,而过多KMnO4的引入,会因为K含量的增加使得MnCeLaOx催化剂中的固溶体被破坏,同时由于K元素的强碱性,在催化剂表面产生大量的吸附氧,阻碍了氯苯在催化剂表面的吸附,反而降低了催化剂对氯苯催化燃烧的活性。作为对氯苯催化燃烧应用方面的拓展,利用钙钛矿催化剂高热稳定性的优点,研究钙钛矿型催化剂上氯苯催化燃烧的反应,使用溶胶凝胶法制备ABO3(A=La; B=Co,Mn,Fe)钙钛矿型催化剂,并使用Ce, Sr, Pr, Sm以及Cu对催化剂进行改性,以提高催化剂的催化燃烧活性,引入的比例为10%-50%,最后在钙钛矿催化剂上通过浸渍法负载1%Ru,试图解决催化剂存在的氯中毒。研究结果表明,杂元素的引入,一定程度地改变了钙钛矿型催化剂的催化燃烧活性,对于LaMn03型钙钛矿催化剂活性变化最为明显,并且LaMnO3及Lao.9Ceo., Co03催化剂有着较高的催化燃烧的转化率,T90分别为332℃C以及345℃C。更多还原

【Abstract】 With the rapid development of science and technology and the expansion of human activities, awareness concerning natural environment is seriously necessary for sustainability. Chlorinated volatile organic compounds (CVOCs), which are widely used in industries, may become a source of atmospheric pollutants and the solution of the contamination is coming up on the agenda.Technologies dealing with this problem include catalytic hydrodechlorination, catalytic steam reforming, photocatalytic oxidation and catalytic combustion. Among these, catalytic combustion, for low energy consumption, low processing temperature and high efficiency, has been widely used and is considered to be a very promising environmental technology. The common catalysts used in the CVOCs catalytic combustion are mainly the supported noble metal catalysts, transition metal oxide catalysts and solid acid catalysts. In this paper the MnOx catalyst which is cheap and environmental friendly is employed. Through modification by adding Ce and La into MnOx, catalysts of MnCeLaOx were prepared for the catalytic combustion of chlorobenzene(CB) as a model of COVCs. The catalytic behaviors and mechanism of the catalytic combustion of CVOCs over MnCeLaOx were investigated.A series of catalysts of MnCeOx, MnCeLaOx and single metal oxide catalysts, which were prepared by complexation method, were tested in the catalytic combustion of chlorobenzene under the condition of1000ppm CB,10%O2, with the balance of N2at GHSV=15,000h"1. The activities of pure MnOx, CeO2and La2O3are not good enough with the T90of CB combustion above440℃. But the activity is much improved with the addition of Ce into the MnOx, with the result of T90of CB combustion decreasing to246℃. Characterization by XRD, XPS, TPR and Raman shows that the addition of Ce makes a large portion of Mn species enter CeO2fluorite lattice, leading to the formation of MnCeOx solid solution which owns high redox ability. Decomposition of MnCeOx solid solution at the high calcination temperature causes the problem of thermal stability. The addition of La, which leads to form the structure of Mn-O-Ce-O-La, can promote the thermal stability of MnCeOx solid solution even calcined up to750℃, and accordingly the catalyst presents high activity. The catalytic activities in the catalytic combustion of CB over the series of Mn(y)CeLaOx catalysts with various ratios of Mn/(Mn+Ce+La) are investigated. The catalytic activities are improved with the increase in the ratios of Mn/(Mn+Ce+La), and Mn(0.86)CeLaOx exihibits the highest catalytic activity with the temperature of complete combustion for CB as low as225℃. The high activity of Mn(0.86)CeLaOx ascribes to the high mobility of oxygen on the interface between MnCeOx and MnOx. The products from the catalytic combustion of CB were detected, and all catalysts under study have more than99.5%selectivity to carbon oxides. The mainly products are CO2, H2O and HC1, and trace of polychlorobenzene was formed at high temperature for the catalysts with low content of Mn.The stable activities of Mn(y)CeLaOx catalysts were tested by GC-MS. The deactivation ocurred at low temperature during the reaction on all of the catalysts when the temperature below300℃. Combined with the characterizations of the fresh and used catalysts, the possible reason is that the dissociation of CB occurs on the interface between MnCeOx and MnOx and the produced Cl species absorbed strongly on the Ce species around the interface causing deactivation. But activities keep stable above350℃. The treatment in air can recover partly the activity of the used MnCeLaOx catalysts at350℃, and a large amount of Cland HC1were detected in the reaction effluent. This indicates that the Deacon reaction becomes possible on the Mn species around the interface and Ce species, which removes the produced Cl species in the formation of CI2. The addition of H2O or heptane can improve the removal of Cl species from the surface of catalysts. Based on the investigation of kinetics of CB combustion, the rate controlling step of CB combustion over the MnCeLaOx catalysts is the removal of Cl species from the surface of catalysts.KMnCv was employed as one of the precursors for Mn species to the modified theMnCeLaOx catalysts with the content of KMnCo4from0.5%to40%. The characterizations and activity tests show that addition of low content of KMnCu improves the redox ability of MnCeLaOx catalysts. With the increase of the content of KMno4, the activities of CB combustion decreased, due to the destruction of MnCeOx solid solution. XPS result shows that the greatly increased amount of surface absorbed oxygen ascribes to the strong basicity of K element, which retards the absorption of CB on the surface of catalysts.The perovskite-like catalysts with high thermal stability such as LaCoO3, LaMnO3and LaFeCO3were also prepared for investigation. And further these catalysts were modified by the added species of Ce, Sr, Pr, Sm and Cu. The activity results show that the doped species affect the activities of perovskite-like catalysts, and Lao.5Sro.5MnO3and LaogCeoiCoO3catalysts present good activities for CB catalytic combustion with T90of CB combustion about332℃and345℃respectively. Finally,1%of Ru was loaded on the perovskite catalysts to solve the problem of poisoning by Cl species, but the results are not good enough owing to the different catalytic mechanism between perovskite and Ru in the catalyst.更多还原