Controllable Synthesis And Catalysis Performance Of Silver Particles With Diverse Morphologies Regulated By Reaction And Diffusion

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.