Controllable Synthesis And Electrochemical Performances Of Tungsten Carbide Materials

Tungsten carbide (WC) has attracted considerable interest due to the diverse physical and chemical properties that are of significance not only for fundamental investigations but also for practical application. In particular, WC bears an analogy of the catalytic capability of platinum and high ability to resist strong catalytic poisons. Preparing WC under facile and practical conditions, investigating their general growth mechanism, and realizing the tailored synthesis in size, shape and porous structure are fundamentally important to study the relationship between material structure and properties. In this dissertation, three types of WC samples, including nanosized WC, mesoporous WC and WCs with varying crystals, have been developed under several novel methods. And the relationship between the structure of WC and its electrocatalyze activity has been discussed. The investigation of preparation processes could help to improve the electrocatalyze performance of WC materials.The carburization process has been developed and improved to carburize the precursor. The parameters were investigated by identifying the samples prepared by different external factors such as carburizing time, temperature, and velocity of flow. The results reveal that the optimal parameters which can obtain proper constructive state are the following:the carburization atmosphere is a mixture of carbon monoxide and hydrogen at a ratio of 1:2 (v:v CO:H2; the velocity was 375 mL/min), the carburization temperature is 800℃ and the holding time is 6 h. Based on the above results, the phase transformation was as follows:ammonium metatungstate (AMT)â†'WO3â†'WO2â†' Wâ†'W2Câ†'WC.Nanosized WC samples with low and high carbon content were synthesized by carburizing the freeze drying treated ammonium metatungstate (AMT) and polymeric self-assembly treated mixture, respectively. The pure nanosized WC sample with low carbon content was well-dispersed and its particle size was about 50-150 nm. Comparatively, WC particles of the high carbon content nanosized WC sample were distributed in carbon particles and their size is about 20 nm. The electrochemical catalytic behavior of nitrobenzene on these electrodes was investigated by using the electrochemical techniques of cyclic voltammetry. Nanosized WC samples showed high catalytic activity and good chemical stability for nitrobenzene electrochemical reduction. The peak current of nitrobenzene on these electrodes remained nearly unchanged after 50 cycles. The results of electrocatalytic test exhibited that the activity for WC nanoparticles was strongly dependent on the precursor preparation method, which was probably due to the fact that the carbon content in the sample could lead to changes in active surface area.The mesoporus structure of WC could be tuned during precursor preparing. Precursors of two mesoporous WC samples were prepared by ultrasonic-assisted method and a novel self-assembly approaches, respectively. The former mesoporous WC sample exhibited two main mesopore distributions, and high thermo stablity in the air (<410℃). On the other hand, the second type of mesoporous WC sample with varying pore size distribution was synthesized by controlling the components of precursor. The mesoporous structure, crystal phase, and amount of deposited graphitic carbon can be tuned by controlling carburizing atmosphere. The resultant mesoporous WCs are of high specific surface areas (11.3～20.4 m2/g) and adjustable pore-size distributions (average pore size:15.3～22.3 nm). The results of electrocatalytic test exhibited that the activity of WC with different mesoporous structure was strongly dependent on the precursor preparation method, which was probably due to the fact that pore size distribution could lead to changes in surface area utilization. WC samples with developed mesoporous structure exhibited higher activities than nano-sized WC samples.Three attempts have been made to improve the catalytic activity of WC by improving the particle morphology and changing the chemical composition of the surface layer. Firstly, ultrafine mesoporous WC was prepared from as-synthesized mesoporous WC using high-energy ball milling treatment. BET surface area of WC sample increased with the increasing ball milling time and kept constant at 10～11 m2/g for over 9 h. Secondly, W@WC sample with shell/core structure was synthesized by carburizing hollow microspherical AMT with an atmosphere mixture of CO, CO2 and H2O. Characterization results indicate that the morphology of the particles is dodecahedral crystal. The W@WC sample exhibited high thermo stability in the air (< 600℃).Thirdly, using Co and Fe as templates, WC-M (M:Co or Fe) with different structure were built up. With a further treatment by acid, WCs with different crystal plane distribution were successfully obtained. The electro-reduction test exhibited that ball milled mesoporous WC sample exhibited low electro-reduction activity for the WOx component. Interestingly, W@WC revealed high activity for its special surface properties, although the BET surface area of this sample is only 0.52 m2/g. Furthermore, acid treated WC-Fe sample exhibited almost 10 times higher catalytic activity than other former WC samples. Electrocatalyze test results suggested that particle morphology and crystal habit have strong influences on the electrocatalysis properties of WC. It could be concluded that the difference in catalytic activities results from the different surface properties such as chemical composition of the surface layer and crystal plane distribution. Thus, it is important to achieve effective active sites on the WC surface layers.Impregnation-vapor phase deoxidation method was used to prepare the platinum supported WC samples for discussing the support capacity of WC samples and the possibility to be an active methanol electrooxidation catalyst. The result exhibited that high carbon content and surface oxygen composite favor the Pt loading. Furthermore, a new Pt/WC sample was synthesized by developing a replacing reaction to the WC-Fe sample. TEM results showed that Pt particles (2 nm) were well dispersed on the surface of WC after the replacement procedure. The (111) planes of the cubic structure of Pt metal seem to be the most favored and the interplaner spacing is d(111)=2.27 A. The electrochemical performance of methanol oxidation was also evaluated and compared with Pt/C and Pt black. The results showed that the Pt/WC exhibited an attractive catalytic activity. Compared to 40 wt% Pt/C electrode, the methanol oxidation peak in negative-going on Pt/WC with 10% Pt loading shifted to more positive potential value. It is possible that replaced Pt particles acts as the main catalyst for catalyzing the dehydrogenation of methanol during the oxidation reaction, and oxygen containing species (OHads) could easier form on the surface of WC. The formation of oxygen containing species at lower potential can transform poisoning species on Pt to CO2, releasing the active sites on Pt for further methanol oxidation, and finally indicating a synergistic effect between Pt and WC. Furthermore, the Pt/WC catalyst exhibited high stability, and methanol oxidation process in Pt/WC electrode is affected by liquid-phase mass transfer.