I. Nikolov

Tungsten carbide-based electrochemical sensors for hydrogen determination in gas mixtures

An amperometric study of gas-diffusion electrodes (GDE) catalysed by two types of tungsten carbide, WC(1) and WC(2), which differ considerably in their specific surface area (0.5 and 6 m2 g−1), was carried out. The H2–air gas mixture (H2 1–4%) measurements show that for this range of hydrogen concentration the hydrogen limiting diffusion current (id(H2)) may be attained so that a curve of limiting current density against hydrogen concentration can be obtained. The response and stability of the electrode performance were compared to those of platinum catalysed GDEs. The most promising for use in amperometric hydrogen sensing is the WC(1) catalyst of small specific surface area. Electrodes catalysed with this catalyst show inferior response time in comparison to electrodes catalysed with the other two catalysts (WC(2) and Pt) but their overall stability is much better.

The effect of the method of preparation on the catalytic activity of tungsten carbide for hydrogen evolution

Abstract The catalytic activity of tungsten carbide for the evolution of hydrogen in acid medium has been investigated. Specific activities were compared by taking current density values at a given potential in the straight line portion of Tafel curves. The real surface area of the electrodes was determined from capacitance measurements. It was shown that, depending on the starting material used, the activity of the carbides varied by a factor of up to three times. A relation was observed between catalyst activity and the average values of the Tafel slope. Thus, the results obtained confirm the assumption previously made that the different catalytic activity of tungsten carbides prepared from various starting materials for the anodic reaction is due to the different adsorption behaviour of the surface.

Effect of preparation conditions on the structure and catalytic activity of carbon-supported platinum for the electrooxidation of methanol

Carbon-supported platinum catalysts (Pt/C) were prepared by treatment of PtO2/C under different conditions: (a) heating at 380°C in air, argon and hydrogen; (b) electrochemical reduction in H2SO4; and (c) reduction with NaBH4. The effect of the preparation conditions on the structure and the catalytic activity of the catalysts for the electrooxidation of CH3OH in acid media was studied. The highest activity was achieved for the catalyst treated in air. The activity is determined by the crystal faces exposed at the particle surface as well as particle size and the partial oxidation of the carbon support.

The effect of method of preparation on the catalytic activity of tungsten carbide

Abstract The catalytic activity of tungsten carbide for the oxidation of hydrogen in an acid medium has been investigated on sedimented powder electrodes. Specific activities were compared by taking current density values at a given potential in the straight line portion of Tafel curves. The real surface area of the electrodes was determined from capacitance measurements. It was shown that, depending on the starting material employed, the activity of the carbides varied by half an order of magnitude. This observation is discussed in terms of differences in surface properties. No effect on activity was observed when changing the carburization atmosphere from carbon monoxide to a mixture of carbon monoxide and hydrogen or argon.

The effect of method of preparation on the corrosion resistance and catalytic activity during corrosion of tungsten carbide I. Corrosion resistance of tungsten carbide in sulfuric acid

Abstract The corrosion resistance in 4.5N and 9.9N H 2 SO 4 of tungsten carbide synthesized under different conditions has been investigated. The amount of oxides in both electrolytes increases linearly with time, suggesting that during the corrosion process the catalyst surface is not passivated. The corrosion rate depends on the method of preparation of the tungsten carbide. Under identical preparation conditions, carbides synthesized from H 2 WO 4 displayed the lower corrosion resistance. The corrosion resistance of carbides synthesized from identical starting materials depends on the carburation temperature: carbides synthesized at elevated temperatures display higher corrosion resistance. The different corrosion rates are explained by the variations in the specific surfaces, the grain size, and the different extent of segregation of the tungsten carbide particles, respectively. It is shown that the corrosion rate in 4.5N H 2 SO 4 is twice that in 9.9N H 2 SO 4 . This observation is discussed in terms of differences in the rate of the anodic process and the lower solubility of the corrosion process depolarizer (oxygen) in highly concentrated solutions.

Effect of pyrolysis temperature of the catalytic activity of active carbon + cobalt phthalocyanine in sulfur dioxide oxidation by oxygen

Structural changes during the pyrolysis of the catalyst ‘active carbon+cobalt phthalocyanine’ (Ac+CoPc) in the temperature range 500–800°C are investigated using infrared spectroscopy, XRD and DTG analysis. The decomposition products contain organic residues of the phthalocyanine rings and separate phases of CO3O4 and β-Co. The effect of pyrolysis temperature on: (i) the chemical stability of Co in the pyrolyzed CoPc in 0.5M H2 SO4; (ii) the electrocatalytic activity for oxidation of SO2 by oxygen; and (iii) the partial electrochemical reactions of SO2 oxidation and oxygen reduction, has been studied. The rate of electrochemical oxidation of SO2 is affected only by the active centres formed on pyrolysis of the chelate rings. The presence of Co at lower pyrolysis temperatures decreases the oxygen diffusion limitations. It is shown that the rate of SO2 oxidation by oxygen is limited by mass transport of oxygen.

Low temperature electrochemical oxidation of sulfur dioxide

The low temperature electrochemical oxidation of sulfur dioxide on carbon gas diffusion electrodes modified with cobalt phthalocyanine (CoPc) was studied. It was found that when operating with mixtures of air containing up to 20% by volume SO2 a basic problem of the electrooxidation of SO2, namely its permeation into the electrolyte, can be resolved. It is demonstrated that sulfur dioxide can be oxidized in the presence of air on active carbon, modified with cobalt phthalocyanine without external current at room temperature. The electrocatalytic character of the process is established and the rate dependence on the concentrations of H2SO4 and SO2 up to 0.30 vol % in air is displayed. An original method for the ‘cold oxidation’ of SO2 to sulfuric acid is proposed.

Influence of composition on the activity of tungsten carbide gas diffusion hydrogen electrodes

Abstract Efforts were aimed at full utilization of tungsten carbide in double-layer electrodes operating on hydrogen in sulphuric acid. By varying the proportions of WC:PTFE:active carbon:NH 4 Cl, a current density of 580 mA/g of the catalyst was obtained with an electrode consisting of 180 mg/cm 2 WC, 14 mg/cm 2 PTFE, 10 mg/cm 2 carbon and 10 mg/cm 2 NH 4 Cl at 350 mV (R.H.E. in the same solution) and 60 °C. The addition of NH 4 Cl as a pore forming agent had a beneficial effect on the activity of the electrodes. The electrodes had a service of 2000 hours.

Activity and corrosion of tungsten carbide recombination electrodes during lead/acid battery operation

Abstract The oxidation of the tungsten carbide (WC) catalyst in recombination electrodes partially immersed in H 2 SO 4 solution was investigated when the electrodes operated in an atmosphere of oxygen and hydrogen. It has been established that after a long operation period (400 h) 60 to 70% of the catalysts, depending on the initial active surface of WC, may be oxidized to WO x , whereby the rate of recombination decreases about three times. It is assumed that the oxidation of WC is due to the H 2 O 2 formed as an intermediate product of the recombination of hydrogen and oxygen. Silver accelerates the decomposition of H 2 O 2 and hence the use of a WC—Ag mixture as catalyst in the recombination electrodes reduces strongly the carbide corrosion.

The effect of method of preparation on the corrosion resistance and catalytic activity during corrosion of tungsten carbide II. Changes in the catalytic activity of tungsten carbides during the corrosion process

Abstract Sedimented electrodes were used for the investigation of the changes in the catalytic activity during corrosion in 4.5 and 9.9N H 2 SO 4 of tungsten carbides synthesized under different conditions. It is shown that during corrosion the specific activity of all the samples studied reaches a maximum after a certain exposure time. The same phenomenon is observed also during the periodic contact of the corroding samples with hydrogen. The catalytic activity decreases with increase in the quantity of tungsten oxides formed during corrosion. In 4.5N H 2 SO 4 the specific activity of the tested samples is subjected to more pronounced changes. Corrosion processes lead to some exceptions in the general rule that carbides synthesized from WO 3 display a higher specific activity than those synthesized from H 2 WO 4 (w).