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Synthesis, Characterization And Catalytic Performance Evaluation Of Mesoporous Carbon Supported Gold Catalysts

Posted on:2014-12-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:H Z GuFull Text:PDF
GTID:1221330482468169Subject:Applied Chemistry
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The discovery of gold nanoparticles as catalysts is an important progress in catalysis in recent years. The particle size of the Au nanoparticles and the nature of the supports were the key factors in determining the catalytic performance of supported gold catalysts. Therefore, the preparation of homogeneously dispersed and stable gold nanoparticles plays a very important role in the study of gold catalyst. Among various of catalyst supports, the oxides were thoroughly studied compared with carbonaceous materials, but the later have their own advantages including the stable structure, chemical inertness, intrinsic hydrophobic nature, as well as easy recovery of precious metal by simple burning the carbonaceous supports away. Carbonaceous supports supported noble metal catalysts have been deeply studied and reported in literatures, even more, these serious catalyst have been widely used in the industrial scale. As a new group of carbonaceous materials, mesoporous carbon has shown potential applications in various fields of nano-materials and catalysis. It is an ideal support candidate for the preparation of high-dispersion catalysts, based on its high surface area, regular structure, and uniform controllable nanopore size distributions. With its unique structure the mesoporous carbon supported metal catalysts often show a superior catalytic performance. Researches on ordered mesoporous carbon supported noble metals like Pt、Pd and Ru are increasing dramatically in recent years. However, few literatures about ordered mesoporous carbon supported Au catalysts are reported. The reason is mainly due to the inert surface of mesoporous carbon, which fails to stabilize the gold precursor as well as the formed Au nanoparticles. Therefore, the exploration of new preparation method for gold/ordered mesoporous carbons catalysts preparation has great significance in fundamental research and practical applications.According to the existing problems of gold/ordered mesoporous carbons preparation, a novel method named as modified-hard template method was put forward in this thesis to prepare mesoporous carbon supported gold catalysts with homogeneously dispersed gold nanoparticles around 2.3 nm. The innovation point of this method was anchoring the metal precursor ions on the hard template by a surface modification scheme to prevent the gold nanoparticles from sintering. The AuPd-OMC bimetal catalysts were also prepared by same preparation method. All the catalysts were investigated using Fourier transform infrared spectrum, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and nitrogen adsorption/desorption methods. The structure and catalytic hydrogenation property of the Au-OMC catalysts were fully studied. It is the first time to report that the hydrogenation activity of the carbon supported Au catalyst is on a par with the one on oxide metal. Main points of this work are listed as follows:(1) The hard template SBA-15 was modified to anchoring the metal precursor ions; while the phenolic resin was used as the carbon source. The obtained carbon supported gold catalyst was defined as Au-OMC-P2, which has regular mesostructure and highly dispersed Au nanoparticles. The surface area is 1049 m2/g, and the particles size distribution is 2.3±0.5nm. By this preparation method, the reported detrimental effect of chloride could be avoided, and the interaction between Au nanoparticles and the functionalized amino groups could prevent the gold nanoparticles from sintering and aggregation. Au-OMC-P2 showed the greatest activity for the hydrogenation of substituted nitrobenzene compounds among the catalysts tested. In the hydrogenation of m-nitro toluene, Au-OMC-P2 gave a conversion of 99.4%, TOF of 1021 h-1, with a selectivity of 98.1%. In the liquid phase hydrogenation of cinnamaldehyde, Au-OMC-P2 catalyst showed an unusual property for highly selective hydrogenation at C=C bond of cinnamaldehyde. The Au-OMC-P2 catalyst also can be applied in the oxidation of benzyl alcohol to benzaldehyde with a high selectivity of 99.8%.(2) Bimetallic nanoparticles have attracted intensive attention in the field of catalysis. The synergistic effect between the two components in bimetallic nanoparticles introduces a mutual influence on the neighboring atoms, which leads to unique electronic and structural properties of the nanoparticles and helps ameliorate the catalytic performance of monometallic NPs. Therefore, preparation of highly dispersed bimetallic nanoparticles as catalysts has been an important topic in the field of catalysis. The bimetal AuPd-OMC catalysts were also prepared by the modified-hard template method. Separate Au and Pd nanoparticles were found to be dispersed on the OMC support. The small Au nanoparticles around 2.5 nm were attached to the pore walls of the OMC support, while the larger Pd nanoparticles between 10-20 nm stayed on the outer surface of the OMC support. By increasing the metal loading of the Au content, the dispersion of the metal nanoparticles was improved, and the AuPd alloy nanoparticles with a mean particles size of 6.0 nm were formed. The catalytic performance of AuPd-OMC catalysts containing separate Au and Pd nanoparticles for cinnamaldehyde hydrogenation was found superior than the others catalysts. The AuPd-OMC catalyst gave a high TOF of 120 h-1 with a selectivity of 96.2% under mild reaction conditions. Moreover, the selectivity of hydro cinnamaldehyde was maintained at about 88% until the end of the reaction. The monometallic Au-OMC catalysts was inactive under the given reaction conditions. The AuPd-OMC catalyst gave the highest TOF which is around 3 times higher than the monometallic Pd-OMC catalyst. Hydrogen spillover was proved to be the main reason for the activity enhancement. The Pd nanoparticles were the main active sites producing the active hydrogen species, while the Au nanoparticles served as active hydrogen acceptors and diluted the Pd active sites, thereby suppressing the deep hydrogenation process.
Keywords/Search Tags:Mesoporous carbon, Gold nanoparticles, Palladium nanoparticles, Highly dispersed, Bimetallic catalysts, Catalysis
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