Preparation And Properties Of New Composite Carbon Materials Supported Noble Metal Catalysts

Core-shell structure with low noble metal catalysts is a new type of catalysts in recent years.They enhance the catalytic activity of noble metal and reduce the cost of the catalyst due to the interactions between core and shell. They are widely used in electro catalysis, heterogeneous catalysis and biosensing, especially in potential applications of fuel cells. So they are considered to be the very promising catalysts for large-scale commercialization. But the core shell nano scale catalysts are very easy agglomeration in catalytic synthesis reaction and cyclic use due to the small particle size and higher coordination unsaturation surface atoms, which greatly reduces the activity area and selectivity. To solve this problem, the carrier is often used to as the particle supporter. So the structure and properties of the carrier is the key factor also affecting the activity of catalytic synthesis reaction. Therefore the development of a new catalyst support is one of the effective ways to improve the catalytic performance of noble metal catalysts.In this thesis, a series of new nanocomposites were synthesized using multi walled carbon nanotubes (MWCNTs) and graphene as carriers to support noble metal and core shell structure. The performance of noble metal catalyst was improved through the functional carrier and the synergistic effect of the core and shell.The results are as follows.1. Ni@Pd nanoparticles with core/shell structure uniformly dispersed on multi-walled carbon nanotubes (Ni@Pd/MWCNTs) is successfully prepared via a two-step strategy:impregnation-reduction methodand replacement method. The Ni@Pd/MWCNTs composite was characterized by transmission electronmicroscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis.It shows a uniform dispersion of Ni@Pd nanoparticles with core/shell structure on MWCNTs with the average particle size of3.4nm. XRD show that positive shift of the Pd peaks occurs obviously on the Ni@Pd/MWCNTs comparing with the Pd/MWCNTs, indicating the Ni atoms enter into the Pd crystals which caused the narrow transformation of the Pd crystal lattice distance. The single crystalline Ni@Pd particles are confirmed, the lattice planes with a interlayer distance of0.203nm in the core are indexed to Ni (111) crystal planes, the outer layer with the lattice space of0.224nm corresponds to Pd (111) crystal planes. The Ni@Pd/MWCNTs composite was used as electrocatalyst for alcohol oxidation in alkaline media for fuel cells. The electrocatalytic activity of ethanol oxidation on Ni@Pd/MWCNTs is2.3times higher than that of Pd/MWCNTs electrocatalyst at the same Pd loadings. The enhanced electrocatalytic properties could be attributed to not only the electric synergistic effect between Pd and Ni, but also the high use ratio of Pd for its shell structure.2. Well dispersed Ni@Pd bimetallic nanoparticles on multi-walled carbon nanotubes (Ni@Pd/MWCNTs) are prepared and used as catalysts for the oxidation of benzyl alcohol. Scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy analysis, and X-ray diffraction were performed to characterise the synthesized catalyst. The results show a uniform dispersion of Ni@Pd nanoparticles on MWCNTs with an average particle size of4.0nm. The as synthesised catalyst was applied to the oxidation of benzyl alcohol. A99%conversion of benzyl alcohol and a98%selectivity of benzaldehyde were achieved by using the Ni@Pd/MWCNTs (Pd:0.2mmol) catalyst with water as a solvent and H2O2as oxidant at80℃. The catalytic activity of Ni@Pd/MWCNTs towards benzyl alcohol is higher than that of a Pd/MWCNTs catalyst at the same Pd loadings. The catalyst can be easily separated due to its magnetic properties.3. The uniform dispersion of new highly active Ni@Pd core-shell nanoparticle catalysts supported on graphene (Ni@Pd/graphene) was prepared via a two-step procedure involving a microwave synthesis method and a replacement method. Several characterization tools, such as X-ray powder diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) were employed to study the phase structures, morphologies and properties of the Ni@Pd/graphene composite. The results indicated that a uniform dispersion of Ni@Pd core-shell structure nanoparticles on graphene have an average particle size of4nm. The Ni@Pd/graphene composite was used as an electrocatalyst for alcohol oxidation in alkaline media for fuel cells. The electrocatalytic activity of Ni@Pd/graphene for ethanol oxidation is3times higher than that of the Pd/grapheme electrocatalyst at the same Pd loading. The enhanced electrocatalytic properties could be attributed not only to the electric synergistic effect between Pd, Ni and graphene, but also the high use ratio of Pd due to its shell structure.4. The over oxidation of graphene nanosheets were produced by the modify Hummers method with strong oxidants. Abundant oxygen-containing functional groups were present in grapheme oxide, which enable the solubilization of oxidized graphene sheets and thus allow for the intercalation of molecules such as metal precursors into the interlayer space of GO(grapheme oxide). Unfortunately, when the metal ions and GO are mixed and co-reduced to form metal-graphene composite, the subsequent chemical reduction of GO (with metal ions) is required, which makes the abundant oxygen-containing functional groups lost and tends to form irreversible agglomerates or even restack to form graphite. The weak interaction between metal and graphite surfers results in a severe agglomeration of catalytic metal nanoparticles and leads to the loss of its advantage of an ultra-high surface area. To solve this problem, some other intermediates such as poly diallyldimethylammonium chloride (PDDA) and Dimethyldiallylammonium chloride (DMDAAC) have to be introduced.5. Dimethyldiallylammonium chloride modified reduced graphene oxide supported Pd nanoparticles (Pd/DMDAAC-RGO) were fabricated by polyol microwave heating method. The Pd/DMDAAC-RGOhybrid was characterized by transmission electromicroscopy (TEM), energy-dispersive X-ray spec-troscopy (EDS), X-ray diffraction (XRD) analysis and electrochemical tests. High Pd metal loadings, up to80wt.%with a mean size of1.8nm, were densely in situ decorated on DMDAAC-modified RGO surfaces.Compared with traditional carbon-based Pd catalysts, Pd/DMDAAC-RGO exhibits better activity and stability for ethanol oxidation in alkaline media with the same Pd content on the electrode. This improvedactivity indicates that DMDAAC plays a crucial role in the dispersion and stabilization of Pd nanoparti-cles on RGO sheets and DMDAAC-RGO are able to an alternative support for Pd immobilization in directethanol fuel cells and other catalytic devices.6. A simple one-pot micro wave-poly ol reduced method was used to anchor platinum nanoparticles on graphene with the aid of PDDA, forming a Pt/PDDA-G hybrid (Pt/PDDA-G). High Pt metal loadings, up to85wt.%with a mean size of1.4nm, were densely in situ decorated on PDDA-modified grapheme surfaces. The electrochemical tests showed that the activity and stability of Pt supported on PDDA-graphene hybrid substrates for methanol oxidation were better than that of Pt supported on graphene sheets, also better than the widely used Pt/carbon black electrocatalysts with the same Pt content on the electrode. This improved activity indicates that PDDA plays a crucial role in the highly dispersion and stabilization of Pt nanoparticles on graphene and PDDA-G are able to an alternative support for Pt immobilization in direct methanol fuel cells.