Takashi Nagai

Synthesis of single crystal SnS2 by chemical vapor transport method at low temperature using reverse temperature gradient

Abstract Using the chemical vapor transport technique at lower temperature than that ever reported, single crystals of SnS 2 were successfully grown. The reverse temperature gradient (the transport zone at T 1 =419–438° C and growing zone at T 2 =449–455° C ) was another characteristic feature in this method. The reasons why SnS 2 single crystals can be grown at low temperatures were investigated from the view point of impurities, particle diameter, chemical binding energy and crystalline condition of the starting materials. It was found that lattice defects generated by grinding polycrystalline SnS 2 and decomposition products of SnCl 4 or SnCl 4 ·5H 2 O as transport agents were necessary for the growth of single crystalline SnS 2 .

Electrode behaviour of spinel-type Co-Fe cobaltites

Abstract Anodic and cathodic behaviour of spinel-type Fe x Co 3− x O 4 (0 ⩽ x ⩽ 2) prepared in thin film form by pyrolysis of Fe(NO 3 ) 3 and Co(NO 3 ) 2 aqueous mixtures spread on a titanium plate was investigated. Catalytic activities for both the oxygen evolution and the oxygen reduction reactions increase with increasing cobalt content in the oxides. The iron component in the oxides inhibits oxidation of the oxides but accelerates reduction, causing changes in the surface composition.

Insertion of lithium into vanadium molybdenum oxides

Abstract Chemical insertion of lithium into V 2 O 5 , Mo 0.1 V 1.8 O 4.8 and MoV 2 O 8 by LiI has been investigated. The limiting lithium content in all oxides is Li/V = 0.5, suggesting that only the V component is responsible for the lithium insertion. For V 2 O 5 , two potential plateaux correspond exactly to two-phase coexistent crystal structures with different van der Waals gap distances; the interaction energy between inserted lithium is attractive. By contrast, as the ratio of V substitution in Mo increases, the two-phase behaviour decreases, and the interaction between Li + cations becomes repulsive. These insertion characteristics have been analyzed in relation to structural changes in VO 5 polyhedra.

Cathodic dissolution of nickel ferrous ferrites in acid solutions

Abstract The cathodic dissolution behaviour of nickel ferrous ferrites in acid solutions is presented. Dissolution of the ferrites occurs cathodically similarly to that of magnetite, but the dissolution rate decreases markedly with increasing nickel content in the ferrites. Preferential dissolution of iron over the nickel component occurs in the region where cation removal controls the rate and also in both regions where anion removal controls the rate. Changes in the composition of the surface by the preferential dissolution also affect the dissolution rates.

Degradation behaviour of a titanium supported RuO2TiO2 electrode

Abstract The degradation behaviour of a titanium supported RuO 2 TiO 2 electrode in HClH 2 SO 4 solutions is investigated. In the early stage of electrolysis cracks on the electrode surface grow by dissolution of oxide in the strongly acidified medium, the proton concentration of which is controlled by the main electrolytic reactions. It is concluded that the titanium component in the oxide is responsible for the dissolution of the oxides in the cracks.

Cathodic reduction of oxygen on transition metal sulphides

Abstract For the oxygen reduction reaction on transition metal sulphides, the reaction rate decreases in the order β-NiS > CoS > α-NiS > FeS, and this order is the inverse for the Volmer reaction on the sulphides. It is deduced from these relations that the reaction sites for the oxygen reduction reaction and the Volmer reaction are metal and sulphur sites respectively on the surface. The stoichiometric ratio of metal to sulphur at the surface has an important role in the catalytic activities for both reactions.

Anodic behaviour of cobalt ferrous ferrites in acid solutions

Abstract The anodic dissolution behaviour of cobalt ferrous ferrites in acid solutions was investigated using rotating ring disk electrodes and the results were compared with those for nickel ferrous ferrites. The ions that dissolve preferentially to form the surface layer are deduced to be Co 2+ ions; this contrasts with the results obtained for nickel ferrous ferrites, where Fe 3+ ions dissolve preferentially. The difference in the resulting composition of the surface layer affects the rates and the composition dependences of both chemical dissolution and diffusion of the divalent cations.

Effect of substitution of metal atoms in nickel sulphides and cobalt sulphides on their anodic behaviours

Abstract The anodic behaviours of the iron-substituted nickel sulphides Fe x Ni 1− x S (0 x ⩽0.05) and the nickel-substituted cobalt sulphides Ni x Co 1− x S 90 x ⩽0.07) were investigated. In the anodic process, in which the metal components dissolve to leave a sulphur-rich surface layer, the reaction rate increases when the percentage of substituted metal atoms is increased, and marked preferential dissolution of the minority component is observed. In more noble potential regions the sulphides dissolve with the formation of metal ions and sulphate ions, and the rate of this process is also accelerated by the substitution of the metal components.

Anodic oxidation of nickel ferrous ferrite electrodes

The anodic oxidation behaviour of corrosion-resistant nickel ferrous ferrite electrodes is presented. Oxidation of the ferrites proceeded preferentially at FeII ions in the B sites of the oxides, as in the anodic oxidation of magnetite on iron to maghemite. Preferential dissolution of nickel over the iron component was observed in weak acid and neutral solutions, in contrast with the case of acid solutions where the iron component dissolved preferentially.

Untersuchungen über die anodische auflösung von V2O5-elektroden in wässrigen sauren lösungen

Zusammenfassung Experimentelle Untersuchungen uber das anodische Auflosungsverhalten von V 2 O 5 - sowie mit Li(I) dotierten V 2 O 5 - Einkristallelektroden zeigen, dass die Auflosungsgeschwindigkeit der Elektroden unter anodischer Polarisation massgeblich durch den Abtransport der gebildeten Ionen von den V 2 O 5 -Oberflache in die Losung bestimmt wird. Der ionische Charakter des anodischen V 2 O 5 -Auflosung bleibt auch bei der unter UV-Lichtbestrahlung sowie erhohten Elektrodenspannungen uber (VO + 2 ) ads verlaufenden Sauerstoffbildungsreaktion erhalten. Die bereits bei fruheren Versuchen ohne aussere Stromquelle festgestellte bevorzugte selektive Auflosung von V(IV)-gegenuber V(V)-Gitterplatzen scheint auch bei der anodischen Auflosung von Vanadiumoxid einen massgeblichen Einfluss auszuuben. Sie durfte insbesondere den moglichen Auflosungsort bestimmen, an dem dann die charakteristischen Atzgrubchen gebildet werden.