【作者】 郑建东；

【导师】 任晓光；

【作者基本信息】 北京化工大学 ， 化学工程， 2008， 博士

【摘要】 甲烷催化燃烧是抑制NO_x排放最有效的方法,近些年来已得到广泛的研究。选择具有高活性和高温稳定性的催化剂是甲烷催化燃烧技术中期待解决的关键问题。贵金属催化剂具有高的催化活性,但价格昂贵且易高温烧结;钙钛矿催化剂面临同样的问题。目前,六铝酸盐系列催化剂被认为是用于甲烷催化燃烧最合适的催化剂。因此,制备热稳定性好及活性高的六铝酸盐催化剂是本论文研究的主要目标。本文分别采用共沉淀、溶胶凝胶和反相微乳液三种方法制备催化剂,通过不同离子掺杂对六铝酸盐LaAl₁₂O_19-δ进行修饰,以甲烷催化燃烧为目标反应,借助于BET、XPS、XRD、SEM、TG-DTA、TPR等实验技术,考察了制备方法和不同离子掺杂对催化剂性能的影响。采用共沉淀法制备LaMnAl₁₁O_19-δ催化剂,首先研究了焙烧温度和焙烧时间对催化剂的影响,结果表明,焙烧温度在1200℃,焙烧时间为4h可以形成完整的六铝酸盐晶型。其次考察了沉淀剂的种类和沉淀温度以及沉淀剂加入量对所制备催化剂的性能影响。实验结果表明,以碳酸铵为沉淀剂、沉淀温度为90℃、n（碳酸铵）/n(M²⁺)为1.5时制备的催化剂LaMnAl₁₁O_19-δ活性较高,T_10%为474℃,T_90%为662℃。在溶胶凝胶法中,采用金属螯合凝胶法和金属醇盐水解法两种技术制备催化剂。分别考察不同螯合剂、不同溶剂以及醇盐水解时水的加入量对催化剂的影响。实验发现以蔗糖为螯合剂所得催化剂的活性较好。金属醇盐水解法用异丙醇做溶剂所得催化剂的催化活性要高于正丁醇和乙醇,并保持了较大比表面积。醇盐水解时,水和金属醇盐的摩尔比为1.5,所制备催化剂的活性较好。对于反相微乳液法制备催化剂,主要针对反相微乳液体系的选择及反应条件进行了考察。结果发现TX-100/正己醇/环己烷/水四者配比在1:1:2:1,反应温度为25℃,溶液浓度为0.75mol/L的条件下可以形成稳定的反相微乳液体系,并进一步用该体系制备催化剂。首次提出了以K作为镜面阳离子,不但可以形成完整的六铝酸盐,而且所制备催化剂具有较高的催化活性。用微乳醇盐水解法制备K₂Mn_xAl_12-xO_19-δ系列催化剂,当Mn的掺杂量为1时所制备的催化剂K₂MnAl₁₁O_19-δ具有较高的催化活性,其起燃温度T_10%为458℃,完全转化温度T_90%为676℃,Mn的掺杂量进一步增多导致在晶体结构中出现钙钛矿杂质。通过上述几种方法制备催化剂,分别采用不同金属离子对样品LaAl₁₂O_19-δ催化剂进行了单一掺杂和复合掺杂。实验表明,Mn²⁺和Fe³⁺容易进入六铝酸盐骨架并促进六铝酸盐晶体的形成,其他的金属离子（镍、钻、铜、锌、锆等）很难进入六铝酸盐骨架,不利于六铝酸盐晶体的形成。可能是因为Mn²⁺和Fe³⁺的半径与Al³⁺的半径比较接近。Mn离子有利于提高催化剂的低温活性,Fe离子对提高催化剂的高温活性有重要作用。进行金属复合掺杂铝离子时,发现金属Mn和Fe之间存在着较好的协同作用,二者共同掺杂所得催化剂LaFeMnAl₁₀O_19-δ具有更高的起燃活性和高温稳定性。过量的Mn、Fe离子会导致杂质相出现。100h稳定性试验表明催化剂LaFeMnAl₁₀O_19-δ具有很好的高温稳定性和水热稳定性。与共沉淀法和溶胶凝胶法相比,反相微乳液法得到的催化剂前驱体混合更为均匀,样品具有较高的比表面积和催化活性。XPS和TPR的分析结果说明,在六铝酸盐催化剂中Mn是以Mn³⁺和Mn²⁺两种价态形式存在,而Fe是以Fe³⁺的形式存在。更多还原

【Abstract】 Catalytic combustion is an effective method to suppress the emission of nitrogen oxide from combustors. This potential application in energy generation industry has attracted many research interests in recent decade. The effect of catalytic combustion technology on industry application is to find suitable catalysts, with excellent activity and high heat-resistant ability. Generally, noble metal catalysts have high catalytic activity. However, their high cost and poor high temperature stabilities are obvious disadvantages. Perovskite catalysts are also not ideal for catalytic combustion at high temperature. Hexaaluminates are considered to be one of the most suitable materials for high-temperature catalytic combustion of methane because of their unique crystal structure. The aim of this paper is to find advanced hexaaluminate type catalysts, with high catalytic activity and high thermal stability. The co-precipitation, sol-gel and reverse microemulsion methods were employed to prepare catalysts. Based on LaAl₁₂O_19-δ. catalytic combustion catalyst was synthesized by substitution of different metal ions. The effects of preparation methods and various ions composite substitution on the samples crystal structure and performance of the catalysts were investigated by means of BET, XPS, XRD, SEM, TG-DTA , TPR and miniature fixed bed with methane combustion as model reaction.The co-precipitation method was employed to prepare LaMnAl₁₁O_19-δ catalyst sample for methane combustion. Calcination temperature and duration time was considered in the synthesis experiment. The XRD results showed that the crystal phase of hexaaluminates was formed when sample catalysts were calcined at 1200℃for 4 hours. The precipitator and precipitation temperature were also factors affected on the sample performances. Investigation showed salvolatile was better than the others in the process of co-precipitation. Under the precipitation temperature of 90℃and molar ratio (salvolatile/ M²⁺)of 1.5, the catalyst sample had higher activity , which light-off T₁₀% and total conversion T₉₀% were 474℃and 662℃, respectively.The metal chelated-gel and hydrolysis of metal alkoxide methods were employed to synthesize catalyst samples. During the process of the metal chelated-gel method, the catalyst, which the sucrose was used as chelating agent, exhibited higher catalytic activity in methane combustion. In the process of the hydrolysis of metal alkoxide method, the catalyst had the highest activity when the molar ratio of H₂O/ A1（OC₃H₇）₃ is equal to 1.5.How to obtain steady system of reverse microemulsion is a key factor to prepare catalyst sample with reverse microemulsion method. Experiment results indicated that the optimum ratio of Triton X-100 / n-hexanol / cyclohexane / water was 1:1:2:1. when the temperature was 25℃and the concentration of precursors was 0.75mol/L, a steady reverse microemulsion was formed under above ratio. This system can be used as the reaction media to prepare hexaaluminate catalysts. K ions were used as mirror plane cations to prepare substituted hexxaluminate catalysts. Experimental results showed that the catalyst samples had good crystal structure and lower temperatures of initial and complete conversion for methane combustion. A series of K₂Mn_xAl_12-xO_19-δ samples were prepared by reverse microemulsion-hydrolysis of metal alkoxide method. The K₂MnAl₁₁O_1-δ catalyst had higher activity with T₁₀%as 458℃and T₉₀% as 676℃. Increased of Mn ions in precursor would be result in the perovskite phase in the samples.The effects of various ions substitution on performance of the catalysts were investigated in above three methods. The results showed that the radius of Mn²⁺ and Fe³⁺ were similar to Al³⁺, so Mn²⁺ and Fe³⁺ easily accessed hexaaluminate skeleton and promoted the formation of pure single heaaluminate phase. While the other ions（ Ni, Co, Cu, Zn, Zr, et al.）, which had larger ionic radius , were difficult to enter hexaaluminate skeleton. The introduction of single Mn could improve the catalytic activity at low temperature and make methane light-off temperature shift to lower value. Single Fe could make methane completely convert at lower temperature. The LaFeMnAl₁₀O_19-δ catalyst which were prepared by any methods possessed the lowest temperatures of initial and complete conversion of methane, probably due to the more redox capacity of active sites. After 100h on-stream, no obvious decrease of catalytic activity was observed for LaFeMnAl₁₀O_19-δ sample. It can be concluded that LaFeMnAl₁₀O_19-δ catalyst had excellent catalytic performance and thermal stability for methane combustion.The mixture produced in reverse microemulsion can be easily converted to hexaaluminate catalysts with higher surface area and activity compared to that produced by co-precipitation or sol-gel methods. The results of TPR and XPS showed that Mn ion was existed in Mn²⁺ and Mn³⁺ in the catalyst, while the Fe ion was existed in Fe³⁺.更多还原