Preparation, Characterization And Electrochemical Performance Of Tungsten Carbide Supported Platinum Catalysts

Since tungsten carbide（WC）bears an analogy of the catalytic capability of platinum and high ability to resist strong catalytic poisons,it is regarded as a promising material to take place of noble catalytic material of platinum.WC catalysts are widely used in many important chemical fields,such as fuel cells,organic electrosynthetic processes,and chemistry catalysis.The preparation,structure and application of catalytic materials of WC in the field of catalysis have been comprehensively reviewed in this dissertation,and the preparation of supported platinum catalyst is also discussed.With this understanding,tungsten carbide catalysts were prepared by spray drying sphere miniaturation—gas-solid reaction,and Pt nanoparticle was modified onto the surface of WC.SEM,EDS,XRD,HRTEM and XPS were used to investigate the morphology,chemical composition,surface structure and surface chemical states of the powders.The electrocatalytic properties of WC and Pt/WC catalysts and the catalytic mechanism were also investigated.In this work,the ammonium metatungstate（AMT）microspheres were prepared by using airflow spray drying method.The effects of solution concentration,feed rate and surfactant on the particle size of products were investigated.It was found that the solution concentration was the most important factor affecting the particle size.Then using AMT microspheres as precursors,the tungsten carbide catalyst was prepared by gas-solid reaction in the atmosphere of CH₄/H₂.The sample was mainly composed by hexagonal WC,and its morphology was hollow globe like with meso-porosity.The mechanism of bulk tungsten carbide catalysts synthesis from AMT in CH₄/H₂ was also studied using temperature programmed reactions.The results showed that the phase transformation was closely related to the reaction time,the temperature for reduction and carbonization and the flow ratio of CH₄ to H₂.Based on the above results,the phase transformation was as follows:AMTâ†'WO₃â†'WO₂â†'Wâ†'W₂Câ†'WC.Based on the above results,tungsten carbide powder microelectrode（WC-PME） was prepared from the above WC powder.The electrochemical catalytic behavior of α-nitronaphthalene on the WC-PME was investigated by using the electrochemical techniques of cyclic voltammetry and linear sweep voltammetry.The results showed that the powder microelectrode with WC prepared in the CH₄/H₂ atmosphere showed high catalytic activity and good chemical stability forα-nitronaphthalene electrochemical reduction.The peak current of NP on WC-PME remained nearly unchanged after 100 cycles.In the acidic media,the apparent activity energy for the electroreduction ofα-nitronaphthalene on WC electrode was 10.12 kJ/mol.The Pt/WC catalyst was prepared by impregnation-vapor phase deoxidation method in this paper.It was found that the average diameter of Pt particles which well dispersed on the surface of Pt/WC is about 13.5 nm.Furthermore,the（111）planes of the cubic structure of Pt metal seem to be the most favored and the interplaner spacing is d_{（111）}=0.227 nm.The Pt/WC catalyst was also prepared by impregnation-room temperature liquid phase reduction method.The average diameter of the Pt particles was 10.13 nm,which was smaller than that of Pt/WC powder prepared by vapor phase deoxidation method.The sample composed of WC and Pt,whose atom percentages of C,W and Pt are 52.96%,43.27%and 17.88%,respectively.The ratio of W to C is near 1.The surface area of Pt nanoparticle on Pt/WC powder was 25.4 m²/g.In addition,the Pt/WC catalyst was fabricated by spray drying-fixed bed method for the first time.The result indicated that the structure of the Pt/WC catalyst particles was hollow globe,which inherited the morphological feature of the H₂WO₃/H₂PtCl₆ microsphere precursor.The microstructure of Pt crystal is gelatinous and filament, which was inserted into the WC matrix.It was also found that the diffraction peak of Pt in the（111）direction was observed obviously,and no diffraction peak in the（200） direction and（220）direction appeared.The Pt/WC has been proved to have catalytic activity to HER.The catalytic property and reaction mechanism were studied.In acid solution,the exchange current density was 4.42×10^-3A/cm²,which was in the same magnitude order with Pt electrode.When the over-potential was 250 mV,the activation energy of hydrogen evolution was 26.20 kJ/tool.In basic media,The Pt/WC-PME exhibited an attractive catalytic activity in hydrogen evolution with a=0.32 V,which suggested that the electro-catalytic activity of tungsten carbide was between Pd electrode（0.31V）and Pt electrode（0.53V）.The exchange current density of hydrogen evolution was 2.23×10^-3 A/cm².When the over-potential was 263 mV,the activation energy of hydrogen evolution was 35.76 kJ/mol.The current of the Pt/WC-PME remained nearly unchanged after 100 cycles.However,the amount of degradation after 100 cycles in electrolyte solution was as much as 18.40%for the WC-PME.Pt/WC-PME showed a better catalytic stability than that of WC-PME.The Pt/WC prepared by impregnation-vapor phase deoxidation method was applied in catalytic electrode for methanol electro-oxidation reaction.The electrocatalytic property and reaction mechanism were studied systemically,and the effects of concentration of scan rate,temperature,concentration of H₂SO₄,concentration of methanol,and Pt loading on the electrochemical oxidation behaviors of the methanol on Pt/WC-PME have also been investigated.The results showed that the electrocatalytic property and catalytic mechanism of Pt/WC were similar to Pt and Pt-based electrode.And methanol was oxidized on Pt/WC in acidic media via a dual path reaction mechanism of dissociative adsorbate and reactive intermediates. Compared to Pt disk electrode,the methanol oxidation peak on Pt/WC-PME with 20wt%Pt loading shifted to more negative potential value.Furthermore,Pt/WC-PME showed a better catalytic efficiency.It is possible that Pt acts as the main catalyst for catalyzing the dehydrogenation of alcohol during the oxidation reaction,and oxygen containing species(OH_ads)could easier form on the surface of WC.The formation of oxygen-containing species at lower potential can transform CO-like poisoning species on Pt to CO₂,releasing the active sites on Pt for further alcohol oxidation,and finally indicating a synergistic effect between Pt and WC.The electrocatalytic property of Pt/WC-PME for methanol oxidation increased with increasing electrolyte temperature, methanol concentration and with decreasing H₂SO₄ concentration.With the increase of Pt loading,the peak current of methanol oxidation on Pt/WC-PME increased accordingly,and when the Pt loading reaches a certain value（20wt%）,the maximum catalytic activity can be obtained.