Preparation Of Tungsten Carbide Composites And Its Catalytic Performance On The Methanol Electrooxidation

At present, one of the major hurdles for commercialization of system with direct methanol fuel cell (DMFC) is still the requirement of a significant amount of platinum catalyst to achieve an acceptable power density. Pt and Pt-based metals are expensive due to limited supplies. In addition, strong chemisorption of CO on Pt makes the electrocatalyst susceptible to CO poisoning, blocking the active sites for methanol oxidation. Accordingly, there have been considerable researches in recent years to develop new non-noble catalysts for DMFC electrodes. Tungsten carbide-based materials have received extra attention in recent years since tungsten carbide (WC) has been found possessing a catalytic behavior similar to the more expensive Pt metal and a promising catalytic activity for methanol oxidation. Some reports also prove that tungsten carbide could activate water and form surface hydroxyls easily on WC surface which will enhance the CO oxidation. However, the electro-catalytic activity of WC is far inferior to Pt and Pt-group metal. In order to find the catalyst alternative to the Pt metal, we selected WC and its composites as the study object. We present some approaches to prepare the WC composites by surface modification. We also synthesize WC composites with other functional materials as the suitable support to modify the electronic structure and increase the specific surface area of WC. The main achievements can be summarized as follows:(1) Nobel metal supported WC composites, especially the Pt supported WC particles have the better performance than that of commercial Pt/C. The synergic catalytic effect between the Pt and WC is depended on the distribution of Pt on WC surface. In this chapter, two types of Pt/WC catalysts were prepared by microwave-assisted (MW) polyol method and impregnation-reduction (IR) method. These catalysts were denoted as MW-Pt/WC and IR-Pt/WC respectively. Using microwave-assisted polyol method, the Pt particles could be loaded on the surface of WC with small size and uniform distribution. The cyclic voltammogram (CV) of the methanol oxidation showed that MW-Pt/WC catalyst had the better performance than the IR-Pt/WC for methanol oxidation in terms of the onset potential and peak current. The chronoamperometry results demonstrated that the MW-Pt/WC yielded higher electrocatalytic activity comparing with IR-Pt/WC catalyst.(2) Based on the above, the surface pretreatments of WC were carried out with different conditions and PtRu/WC catalysts were prepared by microwave-assisted polyol method. We discussed the effects of pretreatments on the structure and activity of catalysts after loading noble metal. The results indicated that treating WC in KOH at pH=14 was conducive to the nucleation and crystal growth of PtRu grains on WC, and this PtRu/WC showed the highest electrocatalytic properties. Moreover, the effect of pretreatment time was also investigated and 5 h in KOH was found to be the preferable condition to obtain the higher activity of PtRu/WC. After 5 h treatment, the W phase in composites was removed so that more active sites of WC and more crystal defects were exposed which led to the better dispersion of nobel metals.(3) Tungsten carbide/titanium oxide (WC/TiO2) particles were prepared by hydrothermal method, and the platinum particles were loaded on the WC/TiO2 to prepare PtWC/TiO2-G. We found that the performance of the PtWC/TiO2-G catalysts for methanol electro-oxidation was better than which of Pt/WC. Tungsten carbide/titanium oxide (WC/TiO2) particles were also prepared by the microwave-assisted heating in conjunction with ionic liquid and the subsequent reduction-carbonization. The platinum particles were loaded on the WC/TiO2 to prepare PtWC/TiO2. The electro-catalytic performances of the PtWC/TiO2 toward methanol oxidation were evaluated by cyclic voltammetry, chronoamperometry and the CO stripping tests. It is found that elctrocatalytic activity of PtWC/TiO2 by adding ionic liquid (IL) during sample preparation was better than that of PtWC/TiO2-G. Adding of IL can give a significant effect on the morphology and structure of WC/TiO2 nanoparticles. It showed that the WC/TiO2-1.5 catalyst had better dispersion and uniform interface between WC and TiO2. The results indicated better performance toward methanol oxidation and higher tolerance to CO were achieved with an optimal addition of 1.5 ml [BMIM][BF4] for preparing PtWC/TiO2. It was proved that the ionic liquid in this reaction system provides the multiple functions for preparing a promising catalyst for methanol oxidation. Moreover, the concentration of IL can be an effective factor to control the morphology and microstructure of WC/TiO2, thus result in the higher electro-catalytic property of PtWC/TiO2-1.5. The thesis can provide important information for structure design and morphology controlled synthesis of oxides.(4) We had developed a microwave-assisted synthesis method for fabricating tungsten carbide/carbon (WC/C) by using resorcinol-formaldehyde polymer as in situ carbon source. Pt-WC/C electro-catalysts were prepared by loading Pt particles on the WC/C which acted as a support. The electro-catalytic performances of Pt-WC/C and commercial Pt/C catalysts toward methanol oxidation were compared by cyclic voltammetry, chronoamperometry and the CO stripping test. It was found that Pt-WC/C exhibited improved catalytic activity for methanol oxidation than those of commercial Pt/C catalyst. The enhancement is due to the larger specific surface areas of Pt-WC/C. We also studied the formation mechanism of WC/C sample by adjusting the ratio of resorcinol-formaldehyde polymer to tungsten (C/W). C/W value chose as 9:1 was found to be the best condition to synthesize WC/C particles with high dispersion.(5) In order to obtain the larger specific surface area, a novel ordered mesoporous tungsten carbide/carbon (m-WC/C) composite with platinum-like behavior was cooperatively constructed by soft-templating method. The m-WC/C composites have higher specific surface area (up to 414.16m2/g). The electro-oxidation of methanol test showed that Pt-m-WC/C catalyst had better performance for methanol oxidation than those of commercial Pt/C catalysts because WC with smaller size was well dispersed on the surface of carbon. The experimental conditions including the stirring time of phenol formaldehyde polymer, the amount of tungsten source and template and carburization temperature are systematically studied. The results indicated the composites had optimized crystal phases and ordered arrangement of pores structures.