Preparation And Electrochemical Performance Of Carbon Nanotubes Supported Pt-au Nanoparticles

Since their discovery, carbon nanotubes (CNTs) have become one of the popular materials in many research areas, owing to their large surface area, significant mechanical strength, excellent electron transportation and high chemical stability. Especially, they are of great promising as the catalyst carrier. Meanwhile, metal nanoparticles are highly catalytic activity and have become great new potential catalysts in various fields, as its small size, large specific surface area, large number of surface atoms and many unsaturated atoms resulting in a large number o f surface dangling bonds and unsaturated bonds. In recent years, carbon nanotubes supported metal nanoparticles composites have been the focus of intensive study in the fields of fuel cells and electrochemical sensors. The nanocomposites greatly enhance the electrochemical performance due to their special geometric effects and electronic effects. In addition, the composition, size, dispersion, morphology and structure of nanoparticles are the key factors affecting their catalytic activity greatly. Therefore, in this thesis, studies on the preparation of CNTs supported metal nanocomposites and their application for fuel cells and sensors were carried out, and some valuable results were obtained. The main points of this thesis are summarized as follows:(1) First, MWCNTs was functionalized by soaking in a mixed acid of sulfuric acid and nitric acid. Then a series of catalysts (named PtAu/MWCNTs) with different Pt/Au molar ratio were prepared by reduction of H₂PtCl₆·6H₂O and HAuCl₄·3H₂O with ethylene glycol as a reducing reagent and sodium citrate as a stabilizing reagent. By changing the deposition order of metals, Au@Pt/MWCNTs (weight percent of total PtAu = 20%, and Pt/Au molar ratio was 4:1) nanocomposites were prepared by successive reduction of HAuCl₄·3H₂O and H₂PtCl₆·6H₂O. Transmission electron microscopy and X-ray diffraction were used to characterize the nanocomposites. The result indicates that the PtAu nanoparticles with an average diameter of about 4.0 nm are supported on MWCNTs. The results of electrocatalytic oxidation of methanol show that the PtAu/MWCNTs with Pt/Au molar ratio of 4:1 displays best catalysis. It proved that incorporation of an appropriate amount of Au nanoparticles can greatly enhance the activity of the catalyst. Meanwhile, it was observed that the activity of PtAu(4:1)/MWCNTs is higher than that of the Au@Pt(1:4)/MWCNTs, which may illustrate that the structure of the catalysts has a great influence on their catalytic activity.(2) PtAu-alloy nanoparticles supported on multiwalled carbon nanotubes (MWCNTs) were successfully prepared by simultaneous reduction of H₂PtCl₆·6H₂O and HAuCl₄·3H₂O with sodium borohydride as a reducing reagent and sodium citrate as a stabilizing reagent. The morphology and composition of the composite catalyst were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The results show that the PtAu alloy nanoparticles with an average diameter of about 3.5 nm and narrow size distribution are supported on MWCNTs. Electrocatalytic oxidation of formic acid at the PtAu/MWCNTs nanocomposite electrode was investigated in a solution containing 0.50 mol/L H₂SO₄ as supporting electrolyte by cyclic voltammogram and chronoamperometry. The results demonstrate that the PtAu/MWCNTs catalyst exhibits higher activity and stability for eletro-oxidation of formic acid than the commercial Pt/C catalyst, reflecting by its lower onset potential (-0.05 V), oxidation mainly occurring in a low potential range of -0.05 V +0.65 V, higher peak current density of 3.12 mA/cm2, good stability and repeatability. Furthermore, CO stripping voltammetry was carried out to explore the role of Au in the composite materials on the electrocatalytic oxidation of formic acid.(3) Nanoparticles containing platinum and gold supported on MWCNTs (Pt@Au/MWCNTs) were successfully prepared by successive reduction of H₂PtCl₆·6H₂O and HAuCl₄·3H₂O with ethylene glycol as a reducing reagent and sodium citrate as a stabilizing reagent. The morphology and composition of the composite catalyst were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. The results showed that Pt@Au nanoparticles with the average size of about 4.0 nm, a narrow distribution range, well dispersed and almost no agglomeration, were successfully deposited on MWCNTs. A novel sensor for determination of DA was fabricated with Pt@Au/MWCNTs nanocomposite-modified glassy carbon electrode coating with Nafion. The electrode exhibits excellent performance towards the oxidation and detection of DA in the presence of ascorbic acid. A high sensitivity of 1.16 mA cm^-2 (mmol/L)^-1 with a wide linear concentration range of dopamine up to 120μmol/L and a low detection limit of 8.0×10^-8 mol/L were obtained in neutral solution. The interference of ascorbic acid can be eliminated effectively. Moreover, the modified electrode presented high stability and reproducibility. The present work provides a simple and practical approach to selective detection of DA in the presence of ascorbic acid. Therefore it is of great promising to detect DA in the field of biological samples.