Ken-ichi Machida

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.

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

Electrocatalysis of Pd + Au alloy electrodes: Part IV. IR spectroscopic studies on the surface species derived from formaldehyde and formate in alkaline solutions

Abstract The formation of a bridge-type adsorbed CO species during the electro-oxidation of formaldehyde on Pd+Au alloy electrodes, but not on a Au electrode, in alkaline media was investigated by means of infrared reflectance spectroscopy. The CO species is probably formed from HCHO which is predominant in the solution at lower pH, but not from HOCH 2 O − . The species acts as a catalytic poison in formaldehyde electro-oxidation, but the effect is weaker at higher pH where oxidation of HOCH 2 O − takes place more readily. The rate of CO formation is faster at lower pH and on electrodes of higher Pd content. No CO species was detected in the HCOO − system. In mixed solutions of HCOO − and HCHO, the oxidation of formate on both Pd and Pd+Au alloy electrodes is suppressed greatly by the CO species originating from HCHO. The poisoning effect becomes less pronounced on the alloy electrodes of higher Au content.

Electrooxidation of formate and formaldehyde on electrodes of alloys between Pd and Group IB metals in alkaline media: Part I. Electrocatalytic properties of component metals

The electrocatalysis of alloys between Pd and Group IB metals (Pd + IB alloys) towards the oxidation of formate and formaldehyde in 1.0M NaOH has been investigated. Pd + IB alloy electrodes exhibit roughly an additivity character with respect to the component metals in their electrocatalytic activity. Pd-rich alloy electrodes are highly active towards oxidation of formate, while those rich in Group IB metals, especially Au and Cu, are active towards formaldehyde. Hydrogen gas is liberated from Group IB metals during the oxidation of formaldehyde even at potentials positive to the RHE, but it is oxidized simultaneously on the Pd+IB alloy electrodes with a bulk Pd content of more than 70 at.% for Pd + Cu or 50 for both Pd + Ag and Pd + Au. A moderate synergistic effect is seen with a maximum activity at 50 to 70 and 20 at.% Pd in the oxidation of formate on Pd+Au alloys and in the oxidation of formaldehyde on Pd + Cu alloys, respectively. The higher the electrocatalytic activity, the lower is the activation energy, but the effect is compensated partly by an accompanying decrease of the pre-exponential factor. The compensation is more extensive in the oxidation of HCHO.

Surface composition of binary alloys between Pd and Group IB metals as studied by Auger electron spectroscopy and cyclic voltammetry

The surface composition of alloys between Pd and Group IB metals (Pd+IB alloys) was investigated on the basis of Auger electron spectroscopy (AES) and cyclic voltammetry. A correction factor for the extinction of Auger peak intensities by surface contaminants, mainly carbon, in deducing the surface composition was determined first. The applicability of this method was substantiated in the case of Pd + Au alloys by the empirical rule in the electrochemical observation that the potential of the oxygen reduction peak in the voltammograms varies linearly with the surface composition. Both AES and electrochemical analysis revealed that in Pd + Au alloy specimens treated by electrochemical oxidation-reduction cycles in 0.1 M NaOH within the potential range 0.05–1.50 V (vs. RHE) the original surface composition was maintained whereas a significant amount of surface Au enrichment took place on those specimens which were anodically polarized in 0.5 M H2SO4. AES analyses of the Pd+Ag and Pd+Cu alloy systems revealed that a weak degree (less than 0.1 in atomic fraction) of surface enrichment in Ag and Cu took place when they were treated by oxidation-reduction cycles in the potential ranges 0.05–1.0 and 0.05–0.50 V (vs. RHE), respectively, in 1.0 M NaOH.

Competitive oxidation of formaldehyde and formate on amorphous copper-palladium-zirconium alloy electrodes in alkaline media

Raney-type Cu−Pd alloy electrodes were prepared from amorphous Cu−Pd−Zr ternary alloys by treatment with aq. HF, and competitive anodic oxidation reactions of HCHO and HCOO− were studied on these electrodes in alkaline media. The initial HCHO oxidation product was HCOO− even on Pd or Pd-rich alloy electrodes which should be more active to the HCOO− oxidation than to HCHO. The product HCOO− was oxidized only after a large decrease of the HCHO concentration in the electrolyte. The oxidation rate of HCOO− was considerably lowered by the existence of even a small amount of HCHO, as well as by the introduction of CO. These results suggest that the HCHO electro-oxidation is accompanied by production of a surface contaminant such as adsorbed CO. The optimum nominal Pd atomic fraction in the Cu−Pd alloy electrodes suitable for the steady simultaneous oxidation of HCHO and HCOO− in mixed solution was shown to be 0.25 and 0.4 in 1.0 M NaOH (M=moldm−3) and 0.5 M K2CO3, respectively.

Methanol electro-oxidation and surface characteristics of amorphous Pt-Zr alloys doped with tin or ruthenium

Abstract Methanol electro-oxidation on electrodes of amorphous Pt-Zr alloys doped with tin or ruthenium was studied in 0.5 M H 2 SO 4 by means of potentiostatic polarization and the results were correlated with surface characteristics determined by surface area measurements, scanning electron microscopy observations and X-ray photoelectron spectroscopy (XPS) analyses. The electrocatalytic activity was considerably enchanced by brief treatment with aqueous HF, which yielded a porous surface layer on the alloy electrodes. The layer was effectively in a higher state of dispersion than ordinary platinum black. The XPS analyses indicated that Raney-type porous platinum layers are formed in the surface region, and that a suitable concentration of tin or ruthenium dopant is essential in realizing a high electrocatalytic activity.

Surface characteristics and electro-oxidation of formaldehyde and formate ion on amorphous Cu-Pd-Zr ternary alloys

Abstract A series of amorphous Cu-Pd-Zr alloys were fabricated by a rapid quench technique. Surface characterization before and after the treatment with aqueous HF was made mainly by surface area measurements and X-ray photoelectron spectroscopy. It was found that zirconium was leached out by the acid treatment and a Raney-type porous Cu-Pd alloy layer was effectively formed at the surface. Electrocatalytic properties for HCHO and HCOO− electro-oxidation were investigated in a 1.0 M NaOH solution at room temperature. Surface composition analyses showed that surface enrichment in copper or palladium was small even after the acid treatment. It was seen that the alloys rich in palladium were active both for HCHO and HCOO− electro-oxidation, while those rich in copper were active only for HCHO. The alloy with composition [Cu]:[Pd] = 1:3 had nearly the same activity for HCHO and HCOO− electro-oxidation.

Dual solution-type HCHO fuel cell systems using an anion exchange resin membrane with electroless-plated Cu or Pd anode

Loading characteristics of a prototype HCHO fuel cell systems with an anion exchange membrane which separates the anolyte from the catholyte were investigated. Electrodes of Cu or Pd, deposited by an electroless-plating technique onto the membrane, showed high electrocatalytic activity to the anodic oxidation of HCHO in 1 M NaOH solution. The system with Cu anode and 1 M NaOH for both anolyte and catholyte showed high loading characteristics but poor durability, whereas that with 1 M K2CO3 showed low characteristics because of lowered pH of the anolyte. It was shown that a dual solution-type cell with 1 M K2CO3 anolyte and 1 M NaOH catholyte yielded improved characteristics as compared with the simple K2CO3 system. The output level was, however, at an unsatisfactory level owing to poor membrane conductance. The temperature dependence of the output performance was studied in the range 7–55°C.

Investigations on the Fabrication of Active Electrocatalysts for Methanol Electrooxidation

The fabrication of direct-type methanol fuel cells relies upon the development of high-performance electrodes such that the rest (open-circuit) potential should be as negative and the current as high as possible. The theoretical potential of methanol oxidation in acidic solution is 0.05 V (versus RHE), but the rest potential actually observed on a Pt electrode may be 0.4–0.5 V at room temperature. Due also to additional difficulties involved in the oxygen counter electrode, the working output voltage of methanol fuel cells at the present stage is only about 0.5 V, as compared with the theoretical value of 1.18 V.