Preparation Of Platinum Group Metal Catalysts And Their Electrocatalytic Performances

Proton exchange membrane fuel cell (PEMFC) has been received widely attentions due to its high specific energy, no pollution for the environment, the safety for storage and transportation of fuel, and simple structure, etc. However, some problems, such as low electrocatalytic activity of the anodic and cathodic catalysts restrict its practical application. Therefore, the preparation of the anodic and cathodic catalysts with high electrocatalytic performance is an important study subject in PEMFC application.The mutual relation of the preparation, construct and performance of platinum group metals are studied in the thesis. The research work contains two parts:(Ⅰ) the preparations of carbon supported Pt metal catalysis and their electrocatalytic performance for methanol oxidation; (Ⅱ) the preparations of Pd and Pd base metal catalysts and their electrocatalytic performance for formic oxidation and oxygen reduction. The main results obtained are as follows:1. Pt/C catalysts with the different Pt particles sizes are prepared with using SnCl2 as the reductant and methanol as the organic solvent. The effect of Pt particles sizes on the methanol oxidation, ethanol oxidation and oxygen reduction are investigated. The improved preparation method is very simple and inexpensive comparing with Bonnemann method. It is found that the average size of Pt particles can be simply controlled by adjusting the evaporating temperature of the solvent. X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements show that the evaporating temperatures of the solvent affect the average sizes of the Pt particles. When the evaporating temperatures of the solvent are 65,60,50,40, and 30℃, the average sizes of the Pt particles in the Pt/C catalysts prepared are:2.2,3.2,3.8, 4.3 and 4.8 nm, respectively. X-ray photoelectron spectroscopic (XPS) results demonstrate that the small Pt particles are easily oxidized. The electrochemical studied show the optimum Pt particles size for methanol oxidation, ethanol oxidation and oxygen reduction is 3.8,3.2 and 3.2 nm, respectively.2. Single-wall carbon nano-tubes (SWNTs), multi-wall carbon nano-tubes (MWNTs) and Vulcan XC-72 carbon (XC-72) are used as supporting carbon materials to prepare Pt/XC-72, Pt/SWNTs and Pt/MWNTs catalysts in tetrahydrofuran/water/ethanol mixture solution. TEM and XPS measurements demonstrate that the type of supporting carbon material affects significantly the morphology and the electronic structure of supported Pt nano-particles (NPs). Electrochemical measurements indicate that the Pt/SWNTs catalyst exhibited the highest current density, the lowest onset oxidation potential and the best stability for methanol electro-oxidation among the three samples, indicating SWNTs are an ideal anode catalyst supporting material for the practical application of direct methanol fuel cells.3. The carbon supported Pd (Pd/C) catalyst is prepared with the improved complex-reduction method. Herein, sodium ethylenediamine tetracetate (EDTA) is used as stabilizer and complexing agent. The average size of the Pd particles in the as-prepared Pd/C catalyst is about 2.1 nm, and the Pd particles in the Pd/C catalyst possess the excellent uniformity. The obtained Pd/C catalyst shows high electrocatalytic activity and stability for the formic acid oxidation.4. The carbon-supported Pd-Fe catalyst (Pd-Fe/C) is prepared in the H2O/tetrahydrofuran (THF) mixture solvent under the low temperature. The homemade Pd-Fe/C catalyst contains two forms of iron species, alloying and non-alloying Fe. The alloying Fe species is hardly dissolved in 0.5 M H2SO4 solution, while the non-alloying Fe species is easily dissolved in 0.5 M H2SO4 solution. The electrochemical measurements show the electrocatalytic activity of the Pd-Fe/C catalyst with the acid treatment for the oxygen reduction is higher than that of the Pd-Fe/C catalyst without acid treatment, illustrating that the non-alloying Fe species suppresses the electrocatalytic activity of the Pd-Fe/C catalyst. In contrast, the alloying Fe species promotes the electro catalytic activity of the Pd-Fe/C catalyst for the oxygen reduction, which is likely attributed to the change of the electron structure of Pd atom and/or bond length of Pd-Pd in the Pd-Fe/C catalyst.5. The carbon supported Pd-P (Pd-P/C) anodic catalyst in direct formic acid fuel cell (DFAFC) is prepared with using ethanol as solvent and white phosphorus as reductant. Pd and P atomic ratio in Pd-P/C catalyst is 100:23, and the P0 content is about one third. P element can enter into the lattice of Pd metal, and form the Pd-P alloy. Because alloying P could decrease the adsorption affinity of CO and H on Pd, and the 3d electron density of Pd, alloying P element can promote the formic acid (FA) oxidation through the direct pathway. Therefore, the electrocatalytic performance of the Pd-P/C catalyst for the FA oxidation is much better than that of the Pd/C catalyst.6. The carbon-supported Pd-Au catalysts (Pd-Au/C) with different alloying degree are prepared in the aqueous solution with and without tetrahydrofuran (THF) by a chemical reduction method. XRD and high resolution transmission electron microscopy (HRTEM) measurements show that the alloying degree of the Pd-Au/C catalyst prepared in the aqueous solution is much lower than that of Pd-Au/C catalyst prepared in the H2O/THF mixture solution, indicating the presence of THF can obviously enhance the alloying degree of Pd-Au nano-particles during the preparation of Pd-Au/C catalyst. The electrochemical measurements illustrate the electrocatalytic activity of Pt-Au/C catalyst for the formic acid electrooxidation is strongly dependent on alloying degree of Pd-Au nanoparticles. The Pd-Au/C catalyst with high alloying degree shows a higher electro catalytic activity and stability for the formic acid electrooxidation compared to the Pd-Au/C catalyst with low alloying degree, which can be ascribed to enhancement of CO tolerance and possible suppression of dehydration pathway in the course of formic acid electrooxidation.6. PdCo bimetallic hollow nano-spheres anodic catalyst has been synthesized through a simple simultaneous reduction reaction using polyglycol as template. TEM images show that the average diameter of the PdCo hollow nano-spheres is about 80 nm and the shell thickness is about 9.0 nm. The electrochemical measurements indicate that the electrocatalytic activity and stability of PdCo hollow nano-spheres catalyst for formic acid oxidation is much better than that of the PdCo solid nano-spheres and Pd solid nano-spheres, which is. ascribed to the unusual chemical and physical properties of the PdCo hollow nano-spheres, such as the interior micro/nano structure, the particular electronic property, and the promotion of the direct dehydrogenation path of formic acid oxidation.