Kiyochika Yahikozawa

Size effects of platinum particles on the electroreduction of oxygen

Abstract Electrocatalytic reduction of oxygen is one of the most important reactions for electrochemical science and technology. Although many reports have been presented on the effects of platinum particle size on the reaction, considerable discrepancies exist which seem to be caused mainly by the influence of the complex pore structure of supporting carbon materials both on the electric field of the double layer and diffusion of electrolyte and gases. This study has tried to reveal the “real size effect” by use of a carbon-supported platinum catalyst electrode model with a flat and dense carbon substrate. With a decrease in the platinum particle size, the specific activity decreases and the Tafel slope increases, the rest potential falls, and the oxygen coverage at any electrode potential in the Tafel region increases. Effects of electronic states and surface crystallography of the platinum particles on the catalytic properties have also been discussed.

Electrocatalytic properties of ultrafine platinum particles for oxidation of methanol and formic acid in aqueous solutions

Abstract The effect of particle size of ultrafine platinum particles, which were prepared by vacuum evaporation onto a glassy carbon rod, on the electrochemical oxidation of methanol and formic acid has been examined by a cyclic voltammetry. The specific activity for the oxidation of methanol decreased by decreasing the platinum particle size, whereas that of the formic acid increased as the particle size decreased. Features of the cyclic voltammograms of the latter reaction suggested that the ratio of the exposed Pt(100) plane of platinum particles increased with decreasing the platinum particle size, while that of Pt(110) decreased.

Preparation of Ultrafine RuO2 ‐ TiO2 Binary Oxide Particles by a Sol‐Gel Process

Ultrafine binary oxide particles of RuO 2 -TiO 2 have been prepared by a sol-gel process consisting of hydrolysis and condensation of metal alkoxides in the liquid phase. This process enabled us to obtain desired rutile-type solid solution oxides of ruthenium and titanium with well-defined stoichiometry, except for the Ti-rich oxide composition, which appear not to form a homogeneous solid solution

Electrochemical properties of ultra-fine palladium particles for adsorption and absorption of hydrogen in an aqueous HClO4 solution

Ultra-fine palladium particles were prepared by vacuum evaporation onto a flat plane of a glassy carbon rod. Their diameters were estimated to be 2–10 nm by means of a high resolution scanning electron microscopy (SEM). Properties of Pd/GC electrodes in hydrogen adsorption were characterised by cyclic voltammetry. Adsorption strength of hydrogen species on the Pd particles of Pd/GC did not depend on the particle size, whereas amounts of the adsorbed hydrogen species and the absorbed ones into the Pd particles were dependent. The X-ray photoelectron spectroscopy (XPS) clearly revealed that the energy level of Pd 3d electron band increased with a decrease in the Pd particle size.

Preparation of ultrafine RuO2IrO2TiO2 oxide particles by a sol—gel process☆

Six specimens of ultrafine RuO2IrO2TiO2 ternary oxide particles with different content ratios have been prepared by a sol—gel method using metal alkoxides. Only a RuO2-rich specimen was composed of a mixture of rutile-type oxide phase and a metallic phase, while the other specimens were composed of only rutile-type oxide phase after calcination of them in air at 450°C for 3 h. The comparison of the XRD spectra of these ternary oxide particles with those of ternary oxide films prepared by a conventional dip-coating method on titanium plate substrates suggested that the oxide particles tended to attain more stable state than the oxide films.

Hydrogen electrode reaction on electrodes of glassy carbon-supported ultrafine Pd particles in alkaline media

Abstract The hydrogen electrode reaction on electrodes of ultrafine Pd particles supported on flat glassy carbon substrates (Pd/GC) was investigated by means of cyclic voltammetry. Reduction and oxidation currents of hydrogen were resolved into those due to adsorption, absorption and evolution of hydrogen, as well as their corresponding re-oxidation currents. Two types of hydrogen species adsorbed at Pd/GC electrodes were identified; one was a weakly bound species which was readily oxidized, and the other was a strongly bound species which was removed at a higher potential. The capacity of Pd/GC for hydrogen absorption diminished with the decrease of particle size, presumably because of the decrease in the fraction of bulk Pd atoms in Pd particles.

Preparations of ultrafine IrO2-SnO2 binary oxide particles by a sol-gel process

Ultrafine particles of IrO2-SnO2 binary oxides have been prepared by a sol-gel process consisting of hydrolysis and condensation of metal alkoxide alcoholic solutions. The rutile-type oxide solid solution of binary oxides,(ir,Sn)O2 was formed after the calcination at 450°C. The a and c lengths of the lattice parameters linearly decreased with the increase of the iridium content. The Ir-Sn binary oxides were crystallized to the (Ir,Sn) rutile-type at lower temperature than the iridium oxide to IrO2.

Surface Characterization of RuO2 ‐ IrO2 ‐ TiO2 Coated Titanium Electrodes

coatings on titanium electrodes prepared by thermal decomposition have been characterized by scanning electron microscopy, x‐ray diffraction analysis (XRD), x‐ray photoelectron spectroscopy (XPS), thermal desorption spectrometry (TDS), and cyclic voltammetry (CV). The XRD results indicated that the mixed oxide coatings formed a three‐component solid solution. The TDS and the CV were applied for quantitative analysis of surface content. The chemical composition examined by XPS, TDS, and CV showed that the surface content was lower than the nominal content. The XPS results, however, deviated considerably from those of TDS and CV. The XPS method can detect the elements only at the outer surface of rough textures, whereas the other two methods can reflect the content of both the outer and the inner surface, including pores and intergrain regions, of the coatings. This means that TDS and CV are useful in the investigation of the surface composition of Ru‐base mixed oxide electrodes.

Surface Characterization of RuO2 ‐ SnO2 Coated Titanium Electrodes

RuO 2 -SnO 2 coatings supported on titanium were characterized in morphology using a high resolution scanning electron microscope and an atomic force microscope and in composition using a scanning Auger microscope. Ruthenium was segregated in island-like deposits in the case of RuO 2 -rich electrodes and along cracks for SnO 2 -rich electrodes where tin was localized in the macroscopically flat plane consisting of ultrafine particles. The ultrafine particles found on SnO 2 -rich electrodes, which were smaller than those found for RuO 2 -rich electrodes, contributed to the large active surface area of the electrode

Electrocatalytic properties of ultrafine gold particles supported onto glassy carbon substrates toward formaldehyde oxidation in alkaline media

Abstract Ultrafine Au particles were supported on flat glassy carbon substrates with fine dispersion, by means of the vacuum evaporation technique. Highest activities toward the HCHO oxidation in alkaline media were obtained with the electrodes of intermediate amounts of Au deposited.