CAS OpenIR
基于反应-扩散的银颗粒形貌的可控制备及其催化性能的研究
林强
Subtype博士
Thesis Advisor韩永生
2018
Degree Grantor中国科学院大学
Degree Discipline化学工程
Keyword银催化剂,反应-扩散,形貌调控,选择性
Abstract

银具有优异的物理化学特性,广泛用于电器、光学、医学以及化学等领域,特别是在催化领域中,银是乙烯氧化制环氧乙烷(EO)的工业化催化剂,其选择性对催化剂的性能具有重要影响。银催化剂的选择性每提高1 %,生产1万吨EO就可以节省乙烯成本约81万元人民币,2017年,我国EO产能已经超过790万吨,因此高选择性银催化剂的开发具有重要的经济价值。目前,银催化剂年均需求量已经超过1500 m3,全世界银催化剂的年均需求量大约7000 m3。银颗粒的形貌对催化剂性能具有重要影响,合理调控银颗粒的形貌,对于制备高效银催化剂具有重要的意义。基于此,本论文的工作由中国科学院过程工程研究所和中国石油化工股份有限公司北京化工研究院燕山分院合作开展,通过调控反应和扩散速率合成了不同形貌的银颗粒,并将银颗粒负载于氧化铝载体表面,研究了银颗粒的形貌对银催化剂性能的影响。本文的主要发现如下:(1) 通过黏度调控组装单元的扩散速率,在溶液(胶)内合成出具有菱形结构的银颗粒。实验发现,在含银溶液(胶)中,反应初期可以形成大量微小的纳米银五孪晶。由于晶格缺陷,五孪晶更倾向于互相聚集以降低表面自由能。采用LAMMPS方法的模拟结果表明,随着溶液(胶)黏度的增加,五孪晶沿着(100)晶面连接和聚集的几率增加,因而更倾向于沿着(100)晶面二维生长,并最终形成菱形结构。通过调控溶胶的黏度,进而调控溶胶中五孪晶的扩散速率,可以调控五孪晶的聚集模式:在无溶胶接近水的黏度条件下,易于形成无规则结构、形状各异的银颗粒聚集体;溶胶浓度较低,易于聚集形成椭圆型分层的盘状银颗粒;溶胶浓度较高,易于形成较为规则的菱形结构的银颗粒。而在黏度相近的不同溶胶体系中,可以产生形貌相近的银颗粒聚集体。上述结果表明,通过调节黏度来调控溶液中作为组装单元微粒的扩散速率,可以得到复杂的介晶结构,对分析和研究复杂体系中介晶生长和演化的机理具有重要指导意义。(2) 通过简化工艺,利用浸渍法将硝酸银负载于高纯α-氧化铝载体表面,研究了银盐在氧化铝表面的反应、扩散过程。研究发现,该过程可以分为低温(硝酸银熔化)、中温(硝酸银分解形成银颗粒)和高温(银颗粒的迁移和聚集)三个阶段。TEM及XRD的表征结果表明,负载于氧化铝表面的硝酸银可在较低温度(低于纯硝酸银分解温度)下发生分解反应并产生银单质颗粒。原位SEM的观察结果表明,热处理过程中,硝酸银会在载体表面熔融,银颗粒逐步从熔融液体中析出,并随着熔融原料的流动在载体的表面重新分布,因此液相的生成对于负载型催化剂表面活性颗粒的分布具有重要的影响。上述结果表明,采用原位技术实时观测和研究银在载体表面反应、迁移及聚集的过程,可以明确负载型催化剂中活性组分的生长和演化的规律,为负载型催化剂的研究提供新的表征方法和研究思路。(3) 通过调控反应速率的方法制备了不同形貌的银颗粒,并将银颗粒负载于Al2O3载体表面制成了银催化剂,研究了银颗粒的形貌对银催化剂性能的影响。研究发现,(111)晶面择优的含枝状银颗粒的催化剂的选择性最高。XRD、SEM及TEM的表征结果表明,随着银的还原反应速率的降低,银颗粒的多层级结构逐渐退化,形貌从枝状变为花状最后变为球状,其表面暴露的Ag(111)晶面也相应逐步减少,说明银颗粒表面暴露更多的Ag(111)晶面及丰富的多层级结构,更有利于乙烯环氧化反应。因而,控制乙烯环氧化过程中银的晶面结构的变化是提高银催化剂效能的关键,该发现对工业高选择性银催化剂的开发和应用具有很好的指导作用。;Silver (Ag) is widely used in electronics, optics, medicine and chemistry for its excellent properties. Especially in catalysis, silver is the unique industrial catalyst for the ethylene epoxidation. The selectivity to ethylene oxide (EO) is the important issue of silver catalysts. The increase of 1% to the selectivity of silver catalysts saves the cost of ethylene about 810,000 RMB for synthesizing 10,000 tons of EO. In 2017, the EO capacity in China has reached more than 7.9 million tons and the annual demand for silver catalysts has exceeded 1500 m3 in China and reached 7000 m3 around the world, which show that silver catalysts have great economic benefits.The morphology of silver particles are key factors to affect the performance of catalysts. It is of great significance to prepare high efficient silver catalysts by regulating the morphology of silver particles. So, in this dissertation, collaborated by Institute of Process Engineering, Chinese Academy of Sciences and Yanshan Branch, Beijing Research Institute of Chemical Industry, Sinopec, silver particles with diverse morphologies are synthesized by regulating the reaction and diffusion rates. Then, the catalytic performances of silver catalysts are studied by loading different structured silver particles onto the surface of Al2O3 carriers. The main findings in this work are shown as follows:(1) Silver particles with rhomboid structures are synthesized by controlling the viscosity of the solutions. In the experiments, a large number of tiny 5-fold twinned silver nanocrystals are formed in the silver solution (glue) during the initial stage of reaction. These five twins tend to aggregate to reduce the total surface energy due to the lattice defects. Discovered by molecular simulation via the LAMMPS method, the probability of aggregation of the five twins along the (100) facets increases with the increase of solvent viscosity, which leads to a two dimensional orientation growth mode along (100) facets, and forming the rhombic structured particles. Thus, regulating the diffusion rate of the 5-fold twinned crystals in the solution (glue) via controlling the viscosity of the solution (glue) can change the aggregation mode of the 5-fold twinned crystals: In the pure water, irregular aggregates are formed; in the low concentration gel solution, round multi-layered plates are formed; and in the high concentration gel solution, regular rhombic silver particles are formed. Moreover, silver aggregates with similar morphologies are generated in the different gel solutions at the similar viscosity. The results above suggest that the diffusion of particles in the solution plays an important role in shaping complex mesocrystal structures, which promises an effective solution for the controllable synthesis of complex mesocrystal system.(2) The silver nitrate is loaded onto the surface of a high purity alpha-alumina carrier by a simplified impregnation method. The reaction and diffusion process of silver salts on the surface of alumina is also studied by using in situ SEM, TEM and et al. The results indicate that this process can be divided into three steps: low temperature step (silver nitrate melts), moderate temperature (silver nitrate decomposes into silver particles) step and high temperature step (silver particles migrate and aggregate). The TEM characterizations and spectra of XRD show that silver nitrate on the surface of alumina decomposes to form metallic silver particles at the lower temperature than the decomposition temperature of pure silver nitrate. The in situ SEM characterizations prove that silver particles gradually precipitate from the molten silver nitrate during heating. The flux of molten salt drives silver particles disperse and redistribute on the surface of the alumina. Therefore, the formation of molten salt has an important influence on the distribution of catalyst particles. The above results indicate that using in-situ technology to observe the reaction, migration and aggregation of silver provides a new method to study the process of the evolvement of active components in supported catalysts.(3) Silver catalysts are prepared by loading silver particles synthesized by reaction kinetics control onto the industrial Al2O3 supports. Then, the performance of the silver catalysts are evaluated. The results show that the catalyst loading with silver dendrites which largely exposes Ag(111) facets has the highest selectivity. In the synthesis of silver particles, with the decrease of the reduction rates, the silver dendrites, flowers and spheres are formed sequentially. The variation of silver particles from dendritic to flower-like then to spherical leads to the decrease of the exposed Ag (111) facets, which results in the descrease of the selectivity of ethylene epoxidation. This study indicates that regulating the silver morphologies provides a valuable approach to develop high selectivity silver catalyst. 

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/40729
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
林强. 基于反应-扩散的银颗粒形貌的可控制备及其催化性能的研究[D]. 中国科学院大学,2018.
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