甲烷重整镍基催化剂制备及催化性能研究

【作者】 潘秉荣；

【导师】 刘昭铁；

【作者基本信息】 陕西师范大学 ， 高分子化学与物理， 2011， 硕士

【摘要】 天然气转化和利用已经成为一种新的能源途径,对于保障我国能源安全及解决环境污染问题具有重大的经济和现实意义。近年来,人们致力于将甲烷(天然气的主要成分)转化为高附加值的碳氢化合物,尤其是同时利用甲烷和二氧化碳两种“温室气体”催化重整制合成气是近几年来颇受关注的课题。甲烷、二氧化碳重整可以制备低H2/CO比的合成气(≤1),而低H2/CO比的合成气更适合生产甲醛、聚碳酸脂和乙醇等化工原料。虽然甲烷、二氧化碳重整具有环境和经济上的诸多优势,但工业上基于此重整反应的生产并不多。从生产成本和催化活性角度出发,镍基催化剂是最常用的催化剂,但甲烷、二氧化碳重整反应工业化的主要障碍之一是镍基催化剂的积炭问题。本论文以调控铈锆复合氧化物的结构和性质为目标,采用络合分解法制备了一系列CexZr1-xO2固溶体,选用等体积浸渍法制备了NiO/CexZr1-xO2型催化剂,通过BET、XRD、Raman、TG和TPR等手段分析了载体和催化剂的结构及性质。系统地考察了其催化CH4-CO2重整反应性能,并在最优催化剂上考察了甲烷三重整反应的催化性能。结果表明,以水杨酸作络合剂所制得Ce0.6Zr0.4O2负载12.5 wt.% Ni催化剂的活性和稳定性较好,并与催化剂的结构相关联。具体研究内容及主要结论如下：(1)通过对载体的TG、XRD、Raman和BET等表征数据分析,确定600℃作为金属配合物的焙烧温度。在600℃焙烧4 h,金属配合物已基本分解完全,形成了晶型规整的铈锆固溶体；在该温度下形成的铈锆固溶体具有最小的晶粒尺寸,作为催化剂的载体,具有适中的比表面积。(2)在较低反应温度、加压和高空速条件下通过CH4-CO2重整反应筛选催化剂,考察了反应空速和反应压力对CO2重整甲烷催化活性的影响。结果表明,反应压力和空速对反应的转化率和选择性都有不同程度的影响,其中压力影响更大。(3)考察了Ni引入方式、Ce/Zr比及Ni负载量对CO2重整甲烷催化活性的影响,确定了最佳催化剂。由水杨酸(S)制备的载体与NiO形成的催化剂(Ni/CZ-S)表现出较高的反应活性和稳定性；不同Ni引入方式和Ce/Zr比主要影响载体的氧化还原性能,进而影响NiO的还原行为；随着Ni含量提高,活性中心数量增多,这样即使在反应过程中积炭覆盖或氧化流失部分活性中心,仍能保持相对较高的反应活性和稳定性。(4)将最优催化剂应用于甲烷三重整反应,考察了空速和压力对甲烷三重整催化活性的影响。由于甲烷催化部分氧化反应(CPOM)的存在,空速对CH4转化率、H2收率和H2/CO影响较小,但是对CO2转化率影响相对较大；压力对三重整反应的转化率、H2收率和H2/CO影响显著,但当压力高于5 atm时,其影响相对减缓。(5)考察了原料气组成对甲烷三重整催化活性的影响。通过调变原料气比,在一定范围内,可以控制三重整反应生成的合成气H2/CO比(1.5～3.0之间)；在加压条件下,三重整反应中产生积炭仍制约着催化剂的使用寿命,其中CO歧化可能在该实验条件下占主导作用。更多还原

【Abstract】 The conversion and utilization of natural gas are important options for a new energy source, which has great economic and practical significance to protect China’s energy security and to solve the environmental pollution problem caused from the contaminants in fossil fuel. Recently, some work has been done on the conversion of methane(a main component of natural gas) into high valued hydrocarbon products. Especially, the catalytic carbon dioxide reforming of methane to syngas (CO+H2) using the two kinds of "greenhouse gas" at the same time became a popular issue in last few years. The syngas with lower H2/CO molar ratio is more suitable for production such as formaldehyde, polycarbonate, ethanol, and other chemicals. Consequently, considerable efforts have been devoted to the development of high-performance catalysts.Although the process of CH4-CO2 reforming has environment benefits and economic advantages, there are only a few commercial processes based on the CH4-CO2 reforming reaction. From the view of production cost and catalytic activity, Ni-based catalysts are more widely used for reforming of methane, but the major obstacle for commercialization of the CH4-CO2 reforming over Ni-based catalyst is still the rapid deactivation of catalysts caused by carbon deposition.Herein, for the purpose to vary the structures and properties of CexZr1-xO2 composite oxide, a series of cerium zirconium solid solutions were obtained via complex-decomposition method. The structural and textural properties of the CexZr1-xO2 were characterized by TG, XRD, and N2 adsorption-desorption techniques. The Ni/CexZr1-xO2 catalysts were prepared by impregnation method, and the catalytic performances of the nickel loaded samples were investigated for the CO2 reforming of methane. Then, catalytic performance of the optimum catalysts was investigated for the tri-reforming of methane. Reforming results indicate that 12.5wt.% Ni loaded on the Ce0.6Zr0.4O2 catalyst prepared by using salicylic acid as a complex agent shows the best performance among the catalysts investigated, which is consistent to the characterization results of the catalysts. Detail results and main conclusions are given follows:(1) Through analysing the data of TG, XRD, Raman and BET, calcinations temperature (600℃) of metal complexes was selected. After calcinated at 600℃for 4 h, metal complexes have completely decomposed, and formed cerium zirconium solid solutions with regular crystalline phase; Formed at 600℃for 4 h. cerium zirconium solid solutions, with least grain size and moderate specific surface area, is suitable for the support of catalyst.(2) The effects of GHSV (gas hourly space velocity) and pressure on catalytic performance for CH4-CO2 reforming reaction under lower temperature, high GHSV, and pressurized condition were developed. Reforming results show that conversion and selectivity of reaction are significantly influenced by GHSV and pressure, and pressure has greater effect.(3) The optimum catalyst was confirmed by investigating the factors of nickel introducing mode, Ce/Zr molar ratio, and content of nickel. Due to different interaction forces between NiO and support prepared by using salicylic acid (S) as a complex agent, Ni/CZ-S shows the best performance among the catalysts investigated. Reduction behavior of support and NiO particle are direcitly affected by nickel introducing mode and Ce/Zr molar ratio. With an increase of nickel content, active site increases. Although active site reduces with the increase of time on stream, catalyst still can maintain higher activity and stability.(4) The effects of GHSV and pressure on catalytic performance for tri-reforming of CH4 were investigated. Owing the CPOM existence, CH4 conversion, H2 yield and H2/CO molar ratio are not significantly influenced by GHSV, but significantly influenced by pressure. With a further increase of pressure (≥5 atm), the influence of pressure became weaker relatively.(5) The influences of raw materials ratio on catalytic performance for tri-reforming of CH4 were investigated. H2/CO of syngas can be regulated via adjusting the ratio of raw materials in a certain range (1.5～3.0). Under pressurized conditions, the life-time of catalyst is still restricted by carbon deposition formed during tri-reforming of methane. Noteworthy, carbon deposition comes from CO disproportionation reaction, being a major factor for catalyst deactivation.更多还原