The Preparation Of Tungsten Trioxide Supported Platinum Catalysts And Their Electrocatalytic Activity In Methanol Oxidation

The tungsten trioxide has been a promising carrier for fuel cell catalysts due to the special chemical and physical properties and high stability in acid solution. The Pt/WO3 catalyst exhibits enhanced electrocatalytic activity in the process of methanol, formic acid and other small organic molecules oxidation. However, the low specific surface area and electrical conductivity of WO3 blocking its application in direct methanol fuel cells. The methods increasing specific surface area and electrical conductivity were studed to improve the catalytic activity and reduce the cost of catalyst in this paper. Three kinds of tungsten trioxide with diffrernt morphology and struture were obtained visa spay drying method, Calcination method and the phase transformation route. And then load them with Platinum particles by using NaBH4 reducing method. The structure and morphology of the catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electronic microscope, transmission electron microscopy as well as energy disperse spectroscopy, and electrochemical method was used to test the electrocatalytic property of catalysts toward the methanol oxidation. The results obtained are as follows:1. Hollow mesoporous tungsten trioxide microsphere(HMTTS) was synthesized by spay drying method. The surface area of HMTTS is about 28 m2·g-1. And Pt/HMTTS were prepared by using NaBH4 reducing method. The characterizations show that the platinum particles with an average size of 5 nm, which successfully distributed on the surface of HMTTS. Cyclic voltammetry indicates that the mass specific current of Pt/HMTTS is 230 mA·mg-1Pt, which is higher than that of Pt/C(160 mA·mg-1Pt) and Pt/WO3(80.7 mA·mg-1Pt). This result indicates that the electrocatalytic activity of platinum towards methanol oxidation has been improved by HMTTS; Line scan voltammetry indicate that The onset potential of Pt/HMTTS is lower than Pt/C and Pt/WO3 catalysts, which indicates that Pt/HMTTS catalyst requires less energy to carry out the methanol oxidation reaction; Chronoamperometry of Pt/HMTTS catalyst for 6000 s shows the highest initial mass specific current all the time, indicating excellent catalytic activity and stability.2. Hollow mesoporous tungsten trioxide dendritic(b-WO3) was synthesized by combining the hydrothermal method with sacrifice template method. Pt/b-WO3 catalyst was prepared through the improved liquid phase reduction method. The results of the characterization indicated that b-WO3 had hollow microsphere dendritic structure with a length of 6 micrometers and a width of 2 micrometers and the Pt nanoparticles with 7.2 nm were highly dispersed on the surface of b-WO3. The isotherms are identified as type Ⅳ, which is representative of the mesoporous materials. The pore-size distribution showed a large number of pores of around 20-120 nm. The BET surface area of b-WO3 is about 24 m2·g-1. The electrocatalytic activity and stability of the catalysts towards methanol electro-oxidation were investigated using cyclic voltammetry and chronoamperometry. The as-prepared Pt/b-WO3 catalyst exhibits higher electrocatalytic activity and good stability during methanol electro-oxidation in comparison to Pt/WO3 and Pt/C catalysts. The enhanced catalytic performance is attributed to the unique hollow mesoporous structure and the double function mechanism which greatly accelerates the dehydrogenation of methanol on Pt.3. In this paper, the core-shell WO2/WO3 microspheres were prepared by the phase transformation route. It is found that the as-prepared samples also show well-defined hollow spherical structures with an average diameter of 3 um, the platinum particles, with a particle size of around 4-5 nm, are finely dispersed on the surface of the WO2/WO3 microspheres. The result of electrochemical method indicated the mass specific current of Pt-WO2/WO3(694 mA·mg-1 Pt) is also the highest among three catalysts, which is 2.3 and 3.0 times of that of JM Pt/C(298 mA·mg-1 Pt) and Pt/h-WO3(230 mA·mg-1 Pt). Moreover, the If/Ib ratio for Pt-WO2/WO3(1.04) is higher than that of JM Pt/C(0.72) and Pt/h-WO3(0.86), displaying that the Pt-WO2/WO3 has a better tolerance to carbonaceous species accumulation during the MOR. The Chronoamperometric curves(CA) recorded at 0.7 V for 6000 s further verifies that Pt-WO2/WO3 exhibits better electrocatalytic performance and stability in the MOR. Combined with the hydrogen spill-over effect and bifunctional mechanism, the improvement of electric conductivity by the modification of WO2 is responsible for the excellent electrocatalytic performance of Pt-WO2/WO3 catalyst.