【作者】 李丽；

【导师】 阎子峰；

【作者基本信息】 中国石油大学 ， 化学工程与技术， 2007， 博士

【摘要】 随着能源危机的全球化和环境污染的加剧,新型高效、环境友好的能源开发是我们面临的迫切问题。燃料电池因具有效率高、质量轻、污染小等优点,在可移动发电机、汽车、小型电动摩托车、便携式电子产品以及军事、太空等许多领域有潜在的应用前景。然而,燃料电池的商业化进程面临着氢的生产、存储、运输和成本等问题。解决这些问题的最有效的方法是选择合适的具有高能量密度的液体燃料催化转化在线产氢。氨作为一种富氢化合物,不含碳元素、能量密度高、价格低廉、携带方便、制造技术成熟,作为燃料电池燃料特别是可移动车载燃料具有很大的发展潜力。然而,氨解反应的研究尚处于初级阶段,氨解催化剂的制氢效率不能满足车载燃料电池,因此,设计和合成高效稳定的氨解催化剂具有重要的意义。本文首先研究了负载型钌基催化剂的氨解反应活性,考察了以介孔碳分子筛和多孔碳黑为载体的钌基催化剂的氨解反应性能,并比较了不同碳载体负载的钌基催化剂的氨分解性能,借用各种表征手段分析了载体和钌催化剂的结构,详细的研究了载体性质(孔结构、表面性能和石墨化程度)对钌基催化剂的金属分布状态及氨分解催化性能的影响以及金属粒子大小对氨解催化活性的影响,初步讨论了纯氨分解反应机理。其次,在上述氨解反应研究的基础上,选用介孔氧化铬为催化剂考察其氨分解性能。采用固相热分解方法成功的合成了介孔氧化铬材料,考察了反应的温度、反应时间、前驱体和表面活性剂配比以及表面活性剂的种类等制备条件对纳米介孔氧化铬材料结构性能的影响。采用各种表征手段表征了纳米介孔氧化铬材料的物理化学性质和结构,并与催化活性关联,初步探讨了氧化铬的结构和氨解催化活性之间的相互关系。研究的结果表明,用硬模板法制备的CMK-3介孔碳分子筛载体的比表面、孔分布和表面性能对活性组分钌的分散度有重要影响。高比表面和合适孔径分布有利于活性组分在载体上的充分分散。酸碱表面修饰对介孔碳分子筛的孔结构、载体的表面性质有较大的影响,同时影响了活性组分在载体上的分散。对介孔碳分子筛负载的钌催化剂的氨解活性研究发现,Ru的聚集状态、颗粒大小、载体的酸碱性和表面性能是影响催化剂的氨解催化活性的重要因素。用化学气相沉积法合成了高比表面多孔结构碳黑材料,反应温度和催化剂的组成等制备条件对碳黑材料的结构都要较大的影响。以碳黑为载体的钌催化剂的氨分解活性明显高于CMK-3介孔碳分子筛负载的钌催化剂,不同的制备条件得到的碳黑载体的钌催化剂的氨解性能也有明显的差别,本文引入了Vnano参数来描述碳黑样品的晶粒大小,用来解释碳黑的结构对氧化反应活性和氨解催化性能影响并建立了较好的定量关系。在以上研究的基础上,进一步考察了不同碳载体负载钌催化剂的氨解性能发现,载体的孔结构影响活性组分的分散度和颗粒大小,对氨解活性的影响无规律;载体的电子效应对氨解活性有较大的影响,这主要是因为载体的导电性越强,越有利于电子在载体和活性组分间的迁移。控制合适的活性组分颗粒大小对于氨解反应至关重要,合适的钌颗粒大小为3.0-5.0nm。分别用柠檬酸、十六烷基三甲基溴化铵和P123表面活性剂为导向剂,采用固相热分解的方法成功地合成了纳米介孔氧化铬材料。通过控制反应的温度、时间和原料配比得到具有微孔-介孔复合结构的纳米氧化铬。不同的表面活性剂所得的纳米介孔氧化铬材料的结构和性能差异较大。将氧化铬材料用于氨分解反应,发现氧化铬具有一定的氨解活性,在氨分解过程中,催化剂的组成发生了变化,氧化物部分氮化,氨解性能并没有明显的变化,表明氮化铬是氨解反应的活性中心。更多还原

【Abstract】 In the context of globalization of energy crisis and severity of environmental pollution, the development of new efficient environment-friendly energy sources is of great importance and high emergency. Due to high efficiency, low weight and pollution, fuel cells have good potential application in many fields such as mobile generators, automobiles, mini electric motors, portable electronics, and military, aerospace, etc. Nevertheless, the biggest obstacles to commercial fuel cells are hydrogen production, storage, transportation and cost. The most effective way to solve these problems is on-board hydrogen generation by liquid fuels with high hydrogen density. Due to free COx, high energy density and hydrogen capacity, catalytic ammonia decomposition is promising as pure hydrogen carrier for fuel cells. However, the catalytic activity of ammonia decomposition is still low and could not meet the demand of on-board fuel cells. Therefore, the design and synthesis of highly efficient and stable ammonia catalyst is of great significance.In this paper, the activities of Ru catalysts supported on different carbons were studied. The relationship between the nature of carbon supports (including pore structure, surface property, and graphitization) and Ru dispersion on the supports was investigated. And the influence of properties of carbon supports as well as metal particle size on catalytic performance was studied in detail. The mechanism of ammonia decomposition was discussed primarily. On the basis of results above mentioned, mesoporous metal oxides were applied for catalytic decomposition of ammonia. A novel mesoporous chromium oxide was synthesized by solid thermal decomposition. The synthesis conditions of temperature, time, ratio of precursors and surfactants as well as surfactant species were studied for their influence on structure of the mesoporous chromium oxides. In addition, the catalytic activities of ammonia decomposition on Cr2O3 catalysts were discussed primarily.It is showed that the properties of CMK-3 mesoporous carbon molecular sieve synthesized by hard template method have great influence on Ru dispersion. Higher specific surface area and appropriate pore size distribution is advantageous for sufficient dispersion of Ru on the supports. The chemical treatment of carbon support produces significant changes in carbon surface chemistry and these in turn can have dramatic effects on both catalyst dispersion and catalytic activity. In conclusion, Ru state and particle size, as well as acid-base properties and surface properties of the supports are important factors for ammonia decomposition.Alternately, high surface area porous carbon blacks were synthesized by chemical vapor decomposition (CVD). The conditions such as reaction temperature and catalyst composition are crucial to the structure of the carbon blacks. Ru catalysts supported on carbon blacks showed distinctly higher activities of ammonia decomposition compared with the one supported on CMK-3. And different structure of carbon blacks as Ru supports showed obviously different catalytic activities for ammonia decomposition. And a parameter, Vnano, was introduced in this paper for characterization of the size of carbon crystallite, which explained quantitatively the influence of the structure of the carbon blacks on oxidation and ammonia decomposition activity.Further studies on the performance of Ru catalysts loaded on different carbons showed that pore structure of supports has influence on Ru dispersion and particle size, and has no evident influence on catalytic activities of ammonia decomposition. The electronic effect of supports and Ru particle size has notable influence on catalytic activity. The proper Ru particle size is 3.0-5.0nm.Mesoporous chromium oxides were successfully synthesized by solid thermal decomposition with citric acid, CTAB and P123 as structure directing agent. The pore structure and size can be tuned by the synthesis condition. The results of catalytic performance showed that chromium oxide has certain ammonia decomposition activity. The composition of catalyst changed to chromium nitride with ammonia decomposition, but the performance of catalyst showed no significant reduction, which means chromium nitride also is the active site of ammonia decomposition.更多还原