Preparation And Characterization Of Supported Nano-Metal Catalysts And Their Properties

The preparation, characterization and catalytic properties of supported metal nanoparticle catalysts have attracted considerable scientific and commercial attention due to the unique catalytic properties of metal nanoparticles and the important role of supported metal catalyst in heterogeneous catalysis. The catalytic performance of supported metal nanoprticle catalyst is dependent upon the size of metal particles and surface properties of support, which influence the electron state of metal particle. Modern high-performance catalysts are now designed to take into account the significant contribution of the support and to engineer the size of metal particles carefully. In addition, in order to study the effect of particle size or metal-support interaction on catalytic performance, producing a narrow particle size distribution of small metal particles and size-controlled nanoparticles are necessary. However, in spite of the intensive research in this field in last decades, the preparation of supported metal nanoparticles with a desired size and properties still remains a challenge.Direct conversion of methane into useful products is one of the most challenging subjects to be studied in heterogeneous catalysis. Amariglio and van Santen have demonstrated the possibility of higher hydrocarbon formation from methane by a low-temperature two-step reaction sequence on supported noble metal catalysts. However, the effect of metal particle size and support nature on the formation and praduct distribution in methane conversion is not well known.Based on above case, the main goal of the present work is to develop supported metal nanoparticles catalyst with uniform size for difference catalytic purposes. Three kinds of catalysts are prepared and discussed: (i) Pt and Ru metal nanoparticles stabilized by polymer PVP and Beta zeolite, (ii) Pt nanoparticles supported Y or Beta zeolite, and (iii) Pt nanoparticles located in pore of MCM-41, then used as the catalysts for methane two-step conversion to investigate the size effect of metal nanoparticles and support effect on catalytic activity and selectivity over supported metal nanoparticles. Some contribution and progress made are following aspects:Preparation and characterization of platinum nanoparticles stabilized by polymer PVP and Beta zeoliteThe method of preparing supported platinum nanoparticles was developed in methanol-water mixture by the reduction of PtCl62" by methanol in the presence of poly(N-vinyl-2-pyrrolidone)(PVP) and Beta nano-zeolite. The resulting supported Pt nanoparticles catalysts of Pt-PVP/Beta are charactered by multianalytical approach including TEM, EXAFS, DRS, CO-FTIR and XPS with respect to the dispersion state, electron state and stability of Pt particle. The result show that the metal particle deposited on Beta zeolite are well-distributed, high dispersed, size uniform and stable in air, their size can be adjusted by changing the ratio of PVP monomer to Pt and metal loading and upon increasing the concentration of PVP the size of metal particle decrease. Moreover, XPS and CO-FTIR of displayed that the electron properties of Pt nanoparticle is hardly affected by the surface properties of zeolite support, this means that the metal-zeolite interaction is greatly weakened due to the presence of PVP, but instead, PVP rather loosely convers the most of surface for metal particles. A lack of support effect and the uniform, size-controlled particle in Pt-PVP/Beta catalysts could be expected to provide a favorable system for the chacterization of the surface structure of metal particles as well as the investigation for the relationship between the catalytic behavior and size of metal particles.Characterization and catalytic application of Ru nanoparticles stabilized by PVP and Beta zeoliteAs Pt-PVP/Beta catalysts, ruthenium nanoparticle catalysts supported on Beta zeolite were prepared by alcohol reduction of RuCl3 solution in the presence of PVP. It is found based on the results of characterization of TEM and CO-FTIR, that mono-size, high dispersed and well separated Ru nanoparticles can be formed and their size can be controlled by changing the amount of PVP at constant loading. The catalytic activity of this kind catalyst, as a heterogeneous catalyst, was tested in liquid phase hydrogenation of benzene at 333K and 3.0MPa of hydrogen pressure for the characterization of the accessibility of the ruthenium particles and its catalytic performance. The reaction result exhibits that Ru-PVP/Beta is active for benzene hydrogenation reaction, this means the metal atoms exposed to surface are able to coordinated benzene and activate hydrogen. It is interesting to observe that when water is added to the reaction system, the rate of benzene hydrogenation reaction increase significantly, this result can be owed to the swellability of catalysts in water, originating from the hydrophilicity of PVP, to increase the accessibility of ruthenium particles to reactant, so as to increase the rate of benzene. Moreover, theRu-PVP/Beta catalysts have good stability and can be easily recovered, giving the advantage of a truly heterogeneous catalysis system.The activation and conversion of methane over Pt-PVP/Beta catalystFirstly, the interaction of methane with Pt particles in Pt-PVP/Beta catalyst was observed by FTIR. In spite of no infrared C-H bands arrributable to CHX species were observed, it is undoubtedly that methane can be chemisorbed on Pt-PVP/Beta catalyst based on that the intensity of CO-FTIR decreased with time after CH4 admission. Following, two-step conversion of methane over Pt-PVP/Beta catalysts was investigated, and the C2-C6 alkanes were obtained during hydrogenation after methane chemisorption. Unlike Pt/support (HY, NaY, HBeta, NaBeta, SiO2) catalysts prepared by the general impregnation method, the catalytic activity and selectivity of methane conversion on Pt-PVP/support catalysts depend hardly on the nature of support at same metal loading and the ratio of PVP to platinum. This phenomenon can be explained by XPS result over Pt-PVP/support, that the blinding energy of Pt on difference support is close to. It is again proved that support effect is greatly weakened in this kind of catalyst, due to the presence of PVP. The size effect of Pt particles on catalytic conversion of methane was studied over Pt-PVP(l:x)/Beta ( x=2, 5, 10, 15 ).The catalytic results show that the activity of catalysts increases for methane conversion with decrease of particle size, and the distributions of C2+ products vary in favor of the heavier hydrocarbons as the particle size increase. The effect of size originates from the structure-sensitive of methane dissociation, resulting in different distribution of CHX on particle with different size.The effect of zeolite acidity on methane two-step conversion over Pt/Y and Pt/Beta catalystsZeolites with nanosize pore (or cage) have been particularly attractive as supports of metal nanoparticles in practical application. Because of small size and easily modifiable environmentof metal nanoparticle supported on zeolite, they are well-suited for the study of the effect of metal-support interaction on catalytic performance. Here, two series of catalysts of Pt/NaY, Pt/HNaY, Pt/HY as well as Pt/NaBeta , Pt/HBeta with identical metal dispersion, but different acidity of support were prepared respectively, and used as the study of the effect of zeolite acidity on methane two-step conversion. The finding that catalytic activity rely very closely on support acidity was discussed in relation with the different electron state of the Pt nanoparticles. The results showed that the activity ofcatalyst increased with increasing support acidity and the selectivity of C2-C6 products varied with support acidity. Moreover, the distribution and reactivity of surface carbonaceous species chemisorbed on catalyst, generating from the methane decomposition at first step were characterized by temperature programmed hydrogenating (TPH). The formation of Cot species, which is known to give C2+ hydrocarbon on hydrogenation, and reactivity of surface carbon are enhanced on acidic support compared to neutral support. The changes of catalytic properties are due to electron deficient nature of platinum particles on acidic support from the interaction of metal and support.The preparation of Pt nanoparticles supported on MCM-41 and the support effect on methane two-step conversionMCM-41 mesoporous materials possess the advantage of uniform size and shape of pore, large surface area, as well as a tunable pore size and surface properties. Thus, there is a great potential for the development and application of metal nanoparticles supported on MCM-41 with tailored physical and chemical properties in catalysis. In the present paper, a series of MCM-41 mesoporous materials with different Si/Al ratio( 00 ,15 and 10 ) extracted by and that with Si/Al ratio of 10 extracted by different media (C2H5OH , HO Ac/ C2H5OH, NH4NO3/C2H5OH) were prepared and used as supports for Pt-supported catalysts. These catalysts show the obvious different catalytic behaviors for methane conversion to higher alkanes through two-step procedure, and both activity and products distribution depend on the ji/Al ratio, sodium content and extraction medium of supports. N? adsorption results of supports and catalysts revealed the pore structure and surface area of MCM-41 are influenced by Si/Al ratio and hence resulting in different dispersion states of metal over them. When extracting with solution of an acid or salt in ethanol, ion exchange is achieved simultaneously, and resulting in the changes of electronic properties of Pt particles supported on them. The Pt catalyst based on the pure silica MCM-41 presented the most CHx species arising from decomposition of CH4 on the surface of catalyst and a superior activity for C2+ formation, the high Pt dispersion, originating from large surface area of support, and suitable chemical composition of pure silica MCM-41 can be responsible for its high activity. The high metal dispersion and the formation of electron-deficient Pt particles on Al-MCM-41 extracted by acidic media may account for their higher catalytic activity.