Keikichi Fujikawa

Metal electrodes bonded on solid polymer electrolyte membranes (SPE)—The behaviour of platinum bonded on SPE for hydrogen and oxygen electrode processes

Abstract The electrochemical characteristics of a platinum electrode directly bonded to one side of a solid polymer electrolyte membrane (Pt-SPE) were studied. Current-potential relations of hydrogen and oxygen adsorbed on Pt-SPE were observed to be similar to those on a platinum metal electrode independently of the presence/absence of electrolytic solution at the platinum side of Pt-SPE. Hydrogen evolution, hydrogen ionisation, and oxygen reduction were examined at the condition that the side at the platinum of Pt-SPE was free from liquid but filled with the gases concerned

Hydrogenation of ethylene on metal electrodes. Part 4.—Electroreduction of ethylene at a platinum electrode

Electroreduction of ethylene was studied at a smooth platinum electrode in aqueous HClO4 and H2SO4 solutions. The reduction proceeds at a much higher rate than the hydrogen evolution reaction (≈ 103 times faster). Kinetic parameters of the reaction (the Tafel slope, ethylene pressure dependence and solution pH dependence) and the structure of the adsorption layer during the reaction lead to the same conclusion as previously obtained for the reduction by molecular hydrogen at Pt on open circuit; namely that the slowest surface step is C2H5(a)+ H(a)→ C2H6.

Catalytic gasification of carbon: method for the determination of the activity of alkali metal catalysts in the gasification of highly pure amorphous and graphitic carbons with steam

Abstract Gasifications of highly pure carbons in an amorphous state and graphite were studied in the presence of alkali metal catalysts and water. A detailed investigation was made with respect to the dispersion processes of the catalysts. It was found that (i) evaporation of the metal, (ii) condensation of the metal vapour into the amorphous carbon and (iii) dissolution of alkali metal into glass-wool occurred under the conditions of the gasification. Under certain conditions that minimized such dispersion phenomena during the gasification, both the gasification ratt and the amount of the effective catalyst were measured simultaneously. The order of the catalytic activity per unit catalyst composition and unit surface area of carbon was determined as follows: Li>Cs>Rb≥K>Na. A mechanism controlling such catalytic activity is discussed on the basis of the mediator action and the role of the carbon activator of the catalyst.

Hydrogenation of ethylene on metal electrodes. Part 2.—Structure of the adsorption layer on platinum at a working condition on open circuit

The structure of the adsorption layer was studied by the potential sweep method on a platinum electrode in 1 N HClO4 solution saturated with a reaction gas of various compositions (mixtures of H2, C2H4 and He). Results show that the structure of the adsorption layer satisfies previous predictions on the rate-determining step and that the open circuit potential is determined by the equilibrium condition of the step, H++ e– ⇌ H(ads). The large change in the hydrogen peak caused by standing the electrode in the solution is in agreement with the decay of the catalytic activity for the hydrogenation of ethylene.

Hydrogenation of ethylene on metal electrodes. Part 1.—Reduction of ethylene with hydrogen at a platinum electrode on open circuit

Reduction of ethylene with hydrogen was studied at a Pt electrode on open circuit in H2SO4 and HClO4 solutions. The reaction rate and the open circuit potential were followed as a function of the gas composition (C2H4+ H2+ He, 1 atm) and the acid concentration. The rate, v, is independent of the acid concentration and is expressed as v=kHkEPHPE/(kHPH+kEPE), where the k values are constants and PH and PE the partial pressures of H2 and C2H4. The ratio kE/kH is 5–6, which is the same value obtained for the ratio of the diffusion rate constant of C2H4 in solution to that of H2. The rate-determining step is the mass transfer step of H2 or of C2H4 depending on the condition, PH/PE 5–6, respectively. This conclusion is supported by the dependence of the open circuit potential on the gas composition. Time variation of the catalytic activity of the activated electrode is discussed in terms of the activity change of the adsorbed hydrogen atom. Newly formed adsorbed hydrogen atoms are reactive, but after remaining on the surface, some of them become stable and have a retarding effect on the reaction.

Hydrogenation of ethylene on metal electrodes. Part 5.—Reduction of light ethylene on Pt in deuteroperchloric acid solution and the dual-pathway mechanism

Electroreduction of light ethylene on a platinum electrode was conducted in a heavy-water solution of deuteroperchloric acid. Deuterium-atom distributions in the product, ethane, support the previous conclusion that ethylene diffusion is rate-controlling at potentials less positive than ca. 100 mV, whereas the surface reaction is rate-controlling at more positive potentials where the Tafel line holds. The D-atom distribution in the latter potential region reveals double maxima at [2H2]- and [2H6]-ethanes. This distribution is explained by the dual-pathway mechanism which assumes two reaction rates for the step C2H4(a)+ H(a)⇌ C2H5(a). The difference in the reaction rate will be attributed to the difference in the adsorption state of C2H4(a) but not of H(a), since only the weakly adsorbed hydrogen atoms are active in the hydrogenation.Reduction of light ethylene with D2 on platinum in deuteroperchloric acid solution gives the same results.A computer simulation based on the above mechanism can reproduce quantitatively not only the present distributions but also others given in the literature, even those observed for the gas-phase heterogeneous reduction.

Hydrogenation of ethylene on metal electrodes. Part 3.—Isotopic exchange and deuteration reactions at a platinum electrode on open circuit

Deuteration of ethylene and isotopic exchange of deuterium in the systems of D2/H2O, C2H4+ C2D4/H2O and D2+ C2H4+ C2D4/H2O are studied at a platinum electrode on open circuit in 1 N HClO4 solution. Results show that (i) ethylene undergoes molecular, irreversible adsorption, (ii) in region E where PH/PE > 5–6, deuterium atoms from D2 do not appear in the product ethane, whereas in region H, PH/PE < 5–6, deuterium atoms appear in the product at ca. 46 %, and (iii) the deuterium atoms in ethane are distributed almost randomly. These results are discussed in terms of the relative rates of the elementary steps of the mechanism.