Preparation, Characterization And Electrochemical Properties Of Catalytic Materials Of High Activity Tungsten Carbide

The preparation, property and application of the catalytic materials of tungsten carbides (WC) in the field of catalysis have been comprehensively reviewed in this dissertation, and the various factors affecting the catalytic activity of tungsten carbides and reaction mechanism for the preparation of tungsten carbides are also discussed. With this understanding, the preparation, characterization, forming mechanism, chemical and electrochemical stabilities and electrochemical activities of tungsten carbides are investigated from the point view of preparing electrocatalysts with high activities.First of all, the preparation processes using yellow H₂WO₄ or (NH₄)₆(H₂W₁₂O₄₀)·4H₂O as tungsten source and CO/CO₂ as carbon source at continuous mode or batch mode were investigated. The experimental results showed that three stages of pyrolysis, reduction and carbonization were occurred in turn during the preparation of tungsten carbides; and the phase composition, surface composition and specific surface area were greatly affected by the preparing conditions. The better conditions for preparing tungsten carbides at batch mode were as follows: flux of CO and CO₂ were 480 mL/h-g H₂WO₄ and 48 mL/h·g H₂WO₄, respectively; the tungsten source materials were first heated at 500 ℃ for 1 h to get rid of crystalline water, then reacted at 750 ℃ for 12 h. The better conditions for preparing tungsten carbides at batch mode were as follows: flux of CO was 1.5³ m³/h, the rest time of the solid materials in the reactor was 9-12 h; the temperatures at the inlet of solid materials, wall at the middle of the reactor and inlet of gas were 400-500 ℃, 850± 20 ℃ and 400-600 ℃, respectively.Based on the above results, an experimental setup for preparing nano-size tungsten carbides with the spray drying-fixed bed method was successfully constructed. And a kind of tungsten carbide powders with mesopores and hollow ball shapes were made for the first time. These powders were formed by pillar-like species with the length of 1000-800 run and width of 50-150 nm, and mesopores were constructed by these pillar-like species. The further studies indicated that the carbonization temperature and cooling rate of the products had remarkable influence on the surface profile and structure of tungsten carbides. The tungsten carbide prepared with rapid cooling rate had porous structure and was mainly composed by hexagon WC. W, C and O were contained in this sample with the atomic ratio of W to C+O of 0.977. WC with such composition exhibited good electrocatalyticactivities.In situ XRD technique combined with SEM was introduced to elucidate the phase transformation and profiles of the samples during the process for preparing WC, and the forming mechanism of WC particles with mesopores and hollow ball shapes were also discussed. The results showed that the phase transformation was closely related to the temperature for reduction and carbonization and the rising rate of temperature for the preparing tungsten carbides from (NH4)6(H2Wi204o)-4H20 in the atmosphere of CO/CO2. For lower rising rate of temperature, the phase transformation was as follows: AMT—'WO3 -"â– WO2->-W2C-*WC; For stair case rising of temperature, the phase transformation was as follows: AMT— WO3—WO2—WC. In addition, the dependence of the profiles of tungsten carbides on the nature of precursor, spray-drying process, gas released during the reaction and the sublimation of WO3 were also observed.The electro-oxidation behavior and stability of WC in different electrolytes were first investigated by galvanostatic anodic charging method. High electrocatalytic activity for hydrogen oxidation and better electrochemical stability of WC in acidic solutions were found for the potential below 800 mV; For the potential over than 800 mV, W contained in WC became to oxidized and the active sites at the surface of the electrode was destroyed. In basic solution the main reaction occurred at the WC anode was the oxidation of WC itself or the evolution of gas. This indicated that WC was not suitable as anode material for hydrogen oxidation in basic solution. It was found that the products of the oxidation of WC were W2O5 in 2.0 mol/L H2SO4 above 800 mV, W8O23 in 3.5mol/L HC1, WO3 in 2.5 mol/L KOH, respectively.In addition, the performance of gas diffusion electrode catalyzed with WC on hydrogen oxidation in acidic solutions was also evaluated. High electrocatalytic activity was found for the anodic oxidation of hydrogen. The apparent activation energies were 23.3 kJ/mol in 3.5 mol/L HC1, 14.5 kJ/mol in 2.0 mol/L H2SO4 and 13.7 kJ/mol in 85%H3PO4, respectively. Under the same conditions, 33.4 kJ/mol was commonly reported in the documents and 16.7 kJ/mol was the best one. Furthermore, different reaction mechanisms were observed for anodic oxidation of hydrogen at WC electrode and W2C...