Michio Enyo

Electro-oxidation of CO and methanol on graphite-based platinum electrodes combined with oxide-supported ultrafine gold particles

Abstract Graphite-based Pt electrodes were combined with metal-oxide-supported ultrafine Au particles (diameter C Fe 2 O 3 + Au + Pt electrodes exhibited much higher catalytic activity than the electrodes without Au particles, namely graphite-based Pt ( C Pt ) or graphite-based Pt + α- Fe 2 O 3 ( C Fe 2 O 3 + Pt ) electrodes. The electro-oxidation of CH3OH on the C Fe 2 O 3 + Au + Pt and C Fe 2 O 3 ( Ti-doped ) + Au + Pt electrodes was deactivated to much less than that on either C Pt or C Fe 2 O 3 + Pt electrodes by continuous bubbling of CO. Rather, the apparent activity was increased due to superimposed CO electro-oxidation, particularly at potentials below 500 mV/RHE. The enhancement of activity by oxide-supported Au particles can be attributed to the electro-oxidative removal of CO-like carbonaceous intermediates which were produced by the partial oxidation of CH3OH.

Electrooxidation of formate and formaldehyde on electrodes of alloys between Pd and Group IB metals in alkaline media: Part II. The possibility of complete oxidation of formaldehyde in weak alkali

The possibility of complete electrooxidation of formaldehyde on Pd + IB alloy electrodes was studied in K2CO3 solution. Formaldehyde is oxidized first to formate and hydrogen intermediate, and the latter is oxidized simultaneously on the electrodes provided that their Pd content is more than ~ 20 at.%. The rates of individual oxidation of HCOO− and HCHO become roughly equal to each other at a bulk Pd content of 50–60, 20–30 and 0–10 at.% for Pd + Cu, Pd + Ag and Pd + Au alloys, respectively. The oxidation of formate is, however, strongly retarded by the presence of formaldehyde. This retardation is least serious on Pd + Au alloy electrodes with 40–70 at % Pd, and thus they exhibit the highest electrocatalytic activity among Pd + IB alloys towards the oxidation of HCOO− to carbonate in HCOO− + HCHO mixed solution.

Amorphous PdZr alloys for water electrolysis cathode materials

Amorphous Pd0.35Zr0.65 alloys prepared by a melt—quench technique were investigated for their possible use as water electrolysis cathodes. The Tafel—Volmer reaction route of the hydrogen electrode reaction was concluded. Kinetic parameter values of the constituent steps were evaluated by a transient technique. The electrocatalytic activity for cathodic hydrogen evolution on the as-obtained electrode was 102 times lower than Pd foil electrodes, but it was improved after the electrodes were treated in acids, typically aqueous HF. The activity in the highest state exceeds that of Pd by one order of magnitude. SEM and XPS observations indicated that the improvement in activity is due to increased surface Pd concentration after removal of a Zr-enriched surface layer which was produced during fabrication of the amorphous alloy specimens. Crystalline alloy electrodes prepared by heat treatment of the amorphous alloy were very brittle but showed electrocatalytic activity close to that of the amorphous electrodes.

On-line mass spectroscopy applied to electroreduction of nitrite and nitrate ions at porous Pt electrode in sulfuric acid solutions

Abstract On-line mass spectroscopy was applied to detect volatile products of the electroreduction of NO−2 and NO−3 at Pt in 0.5M H2SO4. It provided important information, particularly on the transient behavior. Nitrite is reduced stepwise to NO (at 0.6–1 V, rhe), N2O (0.2–0.8 V) and N2 (0.1–0.6 V) in the potential ranges indicated. On the other hand, nitrate reduction takes place at much lower potentials (E

Study of methanol electrooxidation on RhSn oxide, PtSn oxide, and IrSn oxide in comparison with that on the Pt metals

Abstract Catalytic activities of RhSn oxide, IrSn oxide, and PtSn oxide were studied with respect to their tin oxide effect in comparison with those of the Pt metals and each other. Surface chemical states were observed using XPS for the RhSn and PtSn oxides. Electrochemical and XPS results were compared to each other. Tin oxide showed a pronounced enhancement of the catalytic activity toward methanol electrooxidation on PtSn oxide with respect to Pt itself in acidic solutions, but not in alkaline solutions. In the case of RhSn oxide, tin oxide had a negative catalytic activity effect, with respect to Rh, and had no effect in the case of IrSn oxide, with respect to Ir. This difference was considered to be correlated to the presence or the absence of redox coupling of the respective Pt group metal species. Formaldehyde electrooxidation was also studied on PtSn oxide in acidic solution. In alkaline solution, PtSn oxide was also used for not only formaldehyde, but also formate electrooxidation and its catalytic activity was discussed in comparison with Pt itself.

Hydrogen absorption in palladium electrodes in alkaline solutions

Abstract The entry of hydrogen into a palladium electrode in alkaline solutions was investigated. It is shown, from analysis of the overpotential components observed in transient polarization measurements, that the hydrogen electrode reaction on palladium obeys the Volmer-Tafel mechanism, similar to that in acidic solutions studied earlier. Hydrogen pressures equivalent to hydrogen overpotentials are evaluated. They increase with the overpotential but the dependence is much less than that calculated by applying a Nernst type equation to the total overpotential; this is also similar to the behaviour observed in acidic solution. The equivalent hydrogen pressure is practically independent of the concentration of alkali but slightly dependent on the kind of alkali, being highest in KOH solution.

The overpotential components on the palladium hydrogen electrode

Abstract When a polarization current is applied or switched off, the Pd hydrogen electrode exhibits characteristic overpotential transients due to extensive dissolution of hydrogen in Pd. The physical significance of the overpotential components are discussed in some detail. This was shown to be an excellent system to investigate the detailed mechanism of the hydrogen electrode reaction. Individual affinity values and kinetic parameters of the constituent steps were readily obtainable. Some examples are presented.

Electrocatalytic activity of CoII TPP-pyridine complex modified carbon electrode for CO2 reduction

Abstract A cobalt(II)tetraphenylporphyrin (CoIITPP)-pyridine complex modified glassy carbon electrode (CoTPP/py/GC) was prepared and characterized in the form of CoIITPP-py-NHCO-GC. The catalytic activity for the electrochemical reduction of CO2 was studied in phosphate buffer; the electrode showed a high catalytic activity for CO2 reduction to CO at potentials more negative than −1.0 V vs. SCE (∮.4 V vs. RHE), with a current efficiency of 92% for CO production at −1.1 V vs. SCE. CoTPP/py/GC also showed a high durability of the catalytic activity and the overall turnover number (mol of CO produced/mol of CoIITPP on GC) exceeded 107. The role of the pyridine in CoTPP/py/GC is discussed in connection with the activity and stability as an electrode.

The hydrogen electrode reaction characteristics of thin film electrodes of Ni-based hydrogen storage alloys

Abstract Film-type electrodes of hydrogen absorbing intermetallic compound and alloys, LaNi 5 , Ni 0.11 Ti 0.89 , Ni 0.50 Ti 0.50 and Ni 0.76 Ti 0.24 were prepared by a flash evaporation method. The hydrogen electrode reaction characteristics of the LaNi 5 and NiTi alloy films in 1 M NaOH are very similar to each other. The reaction proceeds via the Volmer-Tafel reaction route with mixed rate-determining characteristics. The exchange current densities of the constituent steps, as well as the overall reaction, are in the range of 10 −6 A cm −2 (true). Surface analysis by an XPS technique has shown that La or Ti on the electrode surface exists as an electrocatalytically innert oxide of La 2 O 3 or TiO 2 . Close similarities of these electrodes with pure Ni electrodes indicate that Ni is responsible for the electrocatalytic activity. No synergistic effect is thus noticeable.

Underpotential deposition of Zn2+ ions on platinum, palladium and gold at various pH values

Abstract The underpotential deposition (UPD) of Zn 2+ ions on Pt, Pd and Au was observed in solutions at various pH values. The UPD peak potential on Pt and Pd shifted to more positive potentials with increase in Zn 2+ ion concentration, the shift being 85–95% of the equilibrium shift of the Nernst equation for Zn 2+ + 2e − = Zn. This suggests that the peak is due to UPD of Zn 2+ ions and is associated with electron charge transfer of ca. 2. With Au, the shift of UPD peak potential with change in Zn 2+ ion concentration varied with the change in pH and the electron transfer numbers were 1.5 and 2 at pH 5.0 and 6.0, respectively. The UPD shift for Zn 2+ -Pt and Zn 2+ -Pd systems was approximately the same, being ca. 1.0 V at pH 0.69–6.9, whereas that for Zn 2+ -Au was ca. 0.6 V. The half-width of the UPD peak increased with increase in pH; the interaction parameter of the Temkin adsorption isotherm was estimated as being repulsive for UPD of Zn 2+ on Pt and Pd.