Tetsuhiko Kobayashi

Gold Catalysts Prepared by Coprecipitation for Low-Temperature Oxidation of Hydrogen and of Carbon Monoxide

Novel gold catalysts were prepared by coprecipitation from an aqueous solution of HAuCl4 and the nitrates of various transition metals. Calcination of the coprecipitates in air at 400 °C produced ultrafine gold particles smaller than 10 nm which were uniformly dispersed on the transition metal oxides. Among them, Auα-Fe2O3, AuCo3O4, and AuNiO were highly active for H2 and CO oxidation, showing markedly enhanced catalytic activities due to the combined effect of gold and the transition metal oxides. For the oxidation of CO they were active even at a temperature as low as −70 °C.

Electrochemical reactions concerned with electrochromism of polyaniline film-coated electrodes

Abstract Mechanisms of the electrochromic reactions of polyaniline film-coated electrodes were investigated in acid solutions in the potential range between −0.2 V and 0.6 V vs. SCE, where the color of the film changes from transparent yellow to green, depending on the potential. It was suggested from spectro-electrochemical measurements and rotating ring-disk electrode studies that two kinds of redox reactions were involved in the electrochromic reaction of polyaniline. One of these is concerned with proton addition/elimination reactions which give a couple of well-defined voltammetric waves at potentials less positive than 0.3 V vs. SCE, and relates to the color change from yellow to yellowish green or vice versa. The other reaction, which occurs mainly at potentials more positive than 0.3 V, does not give any appreciable voltammetric waves, but gives solely, large capacitive currents, and relates to the color change from yellowish green to green or vice versa. The large capacitive currents cause insertion of the electrolyte anions into the polyaniline film in the course of its oxidation and elimination of the inserted anions during its reduction.

Optical recognition of CO and H2 by use of gas-sensitiveAu–Co3O4 composite films

Gold–cobalt oxide (Au–Co 3 O 4 ) composite films have been prepared by the sputter-deposition of gold onto a glass plate substrate followed by pyrolysis of spin-coated cobalt 2-ethylhexanoate. The films comprise small Au particles and Co 3 O 4 nanocrystals, and exhibit different and independent optical responses to CO and H 2 in air. Carbon monoxide caused only a decrease in absorbance, while H 2 caused both a decrease and an increase in absorbance at different wavelengths. Among solid-state gas sensor materials, this Au–Co 3 O 4 composite film is probably the first example of an inorganic material which can be used for the recognition of CO and H 2 molecules through optical absorbance changes.

Influence of dry operating conditions: observation of oscillations and low temperature CO oxidation over Co3O4 and Au/Co3O4 catalysts

The effect of dry operating conditions upon the oxidation of CO over Co3O4 and Au/Co3O4 has been studied. Under dry conditions (with pretreatment and reaction gases dried at −76°C using molecular sieve traps) oxidation of CO over Co3O4 can be observed at temperatures as low as −54°C. However, without sufficient drying Co3O4 rapidly deactivates. On the other hand, the Au/Co3O4 catalyst is resistant to the presence of moisture even at low temperatures. For both the Co3O4 and Au/Co3O4 catalytic systems, strong and periodic oscillations in percentage conversion and catalyst bed temperature have been observed.

Investigation of PEM type direct hydrazine fuel cell

Abstract Hydrazine was examined as a fuel in a direct-liquid-fueled fuel cell employing proton exchange membrane (PEM) for the electrolyte. Hydrazine showed better performance than methanol in the direct fuel cell; the cell using hydrazine gave voltage twice as high as that using methanol in the low-current density region. The I – V characteristics were drastically changed depending on the surface area of the anode catalyst. Compositions of the exhaust materials from each electrode were analyzed in order to investigate the reaction that occurred at the electrodes. The analysis revealed that the catalytic decomposition reaction of hydrazine proceeded further than the electro-oxidation reaction on the anode side using a high specific surface area catalyst. The crossover of hydrazine and ammonia through the PEM was confirmed and the reduction of the hydrazine crossover is important in developing further high performance.

A selective CO sensor using Ti-doped α-Fe2O3 with coprecipitated ultrafine particles of gold

Abstract Titanium-doped α-Fe 2 O 3 with ultrafine deposits of Au was prepared by the coprecipitation method. A thick-film sensor fabricated from the above semiconductor had high sensitivity and excellent selectivity to CO against ethanol and H 2 in the temperature range 30 °C to 100 °C. This unique sensing property was found to originate from a high catalytic activity of the material, which could catalyse CO oxidation even at a temperature as low as −70 °C. High-resolution electron microscopic observation has revealed that gold ultrafine particles with a mean diameter of 36 A are homogeneously dispersed and that they are not merely supported on but strongly held by the host oxide α-Fe 2 O 3 -Ti 4+ . The sensing mechanism and the origin of the high selectivity to CO are discussed.

Oxidative removal of CO contained in hydrogen by using metal oxide catalysts

Oxidative removal of a small quantity of CO from hydrogen atmosphere has been attempted by using catalysts consisting of 3d transition metal oxides. The oxidation of CO takes place in preference to that of H2 on catalysts containing Co or Mn in a temperature range from 323–423 K. At higher temperatures, the oxidation of H2 becomes to coincide and then the selectivity to the CO oxidation decreases. Hydrogenation of CO giving methane also occurs over the catalysts containing Ni or Co. Reduction of catalysts at high temperatures might be the reason for the observed change in the catalytic properties. Among the catalysts tested, CoO showed the best performance and was considered to be one of the candidates for the refinement of the hydrogen fuel for the polymer-electrolyte type fuel cells (PEFCs).

Preparation of Highly Dispersed Gold on Titanium and Magnesium Oxide

Abstract Gold could be highly dispersed on titanium oxide and magnesium oxide in their aqueous dispersion containing Mg citrate. The mean diameter of gold particles are smaller than 5nm. These gold catalysts are active for the oxidation of CO even at a temperature below 0°C. On magnesia support, Mg citrate acts not as a reducing agent but as a sticking agent which blocks the coagulation of gold particles. On titania support dispersed in neutral solution Mg2+ ions instead of citrate ions are mainly adsorbed. It is likely that Mg2+ ion suppresses the transformation of amorphous titania to anatase during calcination and prevent gold particles from coagulation caused by earthquake effect.

Combined effects of small gold particles on the optical gas sensing by transition metal oxide films

The combination of small Au particles with NiO, CuO and CO3O4 films led to improved or new optical gas sensitivity via different mechanisms. The deposition of small Au particles on NiO film enhanced the detection of CO in air through a decreasing change of absorbance in the whole visible-near IR region. This can be explained by the change in positive hole density change of NiO, which was enhanced by an increase in catalytic formation and decomposition of surface carbonates with Au. In the case of CO-insensitive CuO film, gold deposition made it sensitive only at plasmon absorption region. This might be because the absorbance by small Au particles in plasmon band was influenced by the change in the dielectric constant of copper oxide which surrounded small Au particles. A Au-Co3O4 composite film showed a novel function to recognize CO and H2, probably for the first time as inorganic materials for optochemical sensors. This owes to the fact that CO and H2 bring about the specific absorbance change of the film at different wavelengths. Carbon monoxide decreased the absorbance of CO3O4 moiety, while H2 not only decreased the absorbance of CO3O4 moiety but also increased plasmon absorbance of Au.

Photoinduced hydrogen production from an aqueous solution of ethylene glycol over ultrafine gold supported on TiO2

Abstract Photocatalytic H 2 production from aqueous solutions of ethylene glycol was studied using a catalyst consisting of gold supported on TiO 2 . The catalysts were prepared by the deposition-precipitation method, using TiO 2 powder and an aqueous solution of chloroauric acid. Transmission electron microscopy showed that Au was deposited as highly dispersed particles with mean diameters of 2 to 5 nm, depending on the method of pretreatment of the precursor. The effects of precursor calcination temperature, duration of irradiation, gold loading, concentration of ethylene glycol, initial pH and temperature on H 2 production were investigated. The gas phase products of the reaction were mainly H 2 and CO 2 . The precursor calcined in air at 673 K proved to be the most active photocatalyst. The yield of H 2 exhibited a weak maximum at a gold loading of about 2 wt.%. The rate of H 2 production was strongly dependent on the initial pH of the suspension; it increased with pH up to neutral pH and decreased thereafter. Increasing the concentration of ethylene glycol and the temperature had a positive effect on the H 2 yield. The activation energy of H 2 production, as given by an Arrhenius plot over the temperature range 284–340 K, was 15 kJ mol −1 .