Tetsuo Umegaki

Bimetallic Au–Ni Nanoparticles Embedded in SiO2 Nanospheres: Synergetic Catalysis in Hydrolytic Dehydrogenation of Ammonia Borane

Gold-nickel nanoparticles (NPs) of 3-4 nm diameter embedded in silica nanospheres of around 15 nm have been prepared by using [Au(en)(2)Cl(3)] and [Ni(NH(3))(6)Cl(2)] as precursors in a NP-5/cyclohexane reversed-micelle system, and by in situ reduction in an aqueous solution of NaBH(4)/NH(3)BH(3). Compared with monometallic Au@SiO(2) and Ni@SiO(2), the as-synthesized Au-Ni@SiO(2) catalyst shows higher catalytic activity and better durability in the hydrolysis of ammonia borane, generating a nearly stoichiometric amount of hydrogen. During the generation of H(2), the synergy effect between gold and nickel is apparent: The nickel species stabilizes the gold NPs and the existence of gold helps to improve the catalytic activity and durability of the nickel NPs.

Porous Materials for Hydrolytic Dehydrogenation of Ammonia Borane

Hydrogen storage is still one of the most significant issues hindering the development of a “hydrogen energy economy”. Ammonia borane is notable for its high hydrogen densities. For the material, one of the main challenges is to release efficiently the maximum amount of the stored hydrogen. Hydrolysis reaction is a promising process by which hydrogen can be easily generated from this compound. High purity hydrogen from this compound can be evolved in the presence of solid acid or metal based catalyst. The reaction performance depends on the morphology and/or structure of these materials. In this review, we survey the research on nanostructured materials, especially porous materials for hydrogen generation from hydrolysis of ammonia borane.

Hydrogen Production via Steam Reforming of Ethyl Alcohol over Palladium/Indium Oxide Catalyst

We report the synergetic effect between palladium and indium oxide on hydrogen production in the steam reforming reaction of ethyl alcohol. The palladium/indium oxide catalyst shows higher hydrogen production rate than indium oxide and palladium. Palladium/indium oxide affords ketonization of ethyl alcohol with negligible by-product carbon monoxide, while indium oxide mainly affords dehydration of ethyl alcohol, and palladium affords decomposition of ethyl alcohol with large amount of by-product carbon monoxide. The catalytic feature of palladium/indium oxide can be ascribed to the formation of palladium-indium intermetallic component during the reaction as confirmed by X-ray diffraction and X-ray photoelectron spectroscopic measurements.

Development of Plasmonic Cu2O/Cu Composite Arrays as Visible- and Near-Infrared-Light-Driven Plasmonic Photocatalysts

We describe efficient visible- and near-infrared (vis/NIR) light-driven photocatalytic properties of hybrids of Cu2O and plasmonic Cu arrays. The Cu2O/Cu arrays were prepared simply by allowing a Cu half-shell array to stand in an oxygen atmosphere for 3 h, which was prepared by depositing Cu on two-dimensional colloidal crystals with a diameter of 543 or 224 nm. The localized surface plasmon resonances (LSPRs) of the arrays were strongly excited at 866 and 626 nm, respectively, at which the imaginary part of the dielectric function of Cu is small. The rate of photodegradation of methyl orange was 27 and 84 times faster, respectively, than that with a Cu2O/nonplasmonic Cu plate. The photocatalytic activity was demonstrated to be dominated by Cu LSPR excitation. These results showed that the inexpensive Cu2O/Cu arrays can be excellent vis/NIR-light-driven photocatalysts based on the efficient excitation of Cu LSPR.

Metallic ruthenium nanoparticles for hydrogenation of supercritical carbon dioxide

Metallic ruthenium nanoparticles prepared in a methyl alcohol solution under solvothermal conditions showed high activity for hydrogenation of supercritical carbon dioxide to formic acid. The activity of the metallic nanoparticles was drastically improved by addition of an appropriate amount of water to the suspension of methyl alcohol and triethylamide.

Effect of Oxide Coating on Performance of Copper-Zinc Oxide-Based Catalyst for Methanol Synthesis via Hydrogenation of Carbon Dioxide

The effect of oxide coating on the activity of a copper-zinc oxide–based catalyst for methanol synthesis via the hydrogenation of carbon dioxide was investigated. A commercial catalyst was coated with various oxides by a sol-gel method. The influence of the types of promoters used in the sol-gel reaction was investigated. Temperature-programmed reduction-thermogravimetric analysis revealed that the reduction peak assigned to the copper species in the oxide-coated catalysts prepared using ammonia shifts to lower temperatures than that of the pristine catalyst; in contrast, the reduction peak shifts to higher temperatures for the catalysts prepared using L(+)-arginine. These observations indicated that the copper species were weakly bonded with the oxide and were easily reduced by using ammonia. The catalysts prepared using ammonia show higher CO2 conversion than the catalysts prepared using L(+)-arginine. Among the catalysts prepared using ammonia, the silica-coated catalyst displayed a high activity at high temperatures, while the zirconia-coated catalyst and titania-coated catalyst had high activity at low temperatures. At high temperature the conversion over the silica-coated catalyst does not significantly change with reaction temperature, while the conversion over the zirconia-coated catalyst and titania-coated catalyst decreases with reaction time. From the results of FTIR, the durability depends on hydrophilicity of the oxides.

Control of Shell Thickness of Hollow Silica-Alumina Composite Spheres and their Activity for Hydrolytic Dehydrogenation of Ammonia Borane

Herein, we investigated the influence of the shell thickness of hollow silica-alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane (NH3BH3). Silica-alumina composite shells were fabricated by coating on polystyrene (PS) template particles and then completely removing the PS by calcination. Based on the transmission electron microscopy images, the shell thicknesses of the hollow spheres prepared using 5, 10, and 15 g of PS suspensions and coating times of 24.0, 7.0, and 1.5 h were 20, 13, and 5 nm, respectively. The results indicated that the shell thickness of homogeneous hollow spheres was controlled by adjusting the amount of PS suspension and the coating time. In the presence of the hollow spheres with shell thicknesses of 5, 13, and 20 nm, 10.0, 9.5, and 9.0 mL of hydrogen was evolved from aqueous NH3BH3 solutions for 13, 12, and 13 min, respectively. The molar ratios of the hydrolytically generated hydrogen to the initial NH3BH3 in the presence of the hollow spheres with shell thicknesses of 5, 13, and 20 nm were 2.6, 2.5, and 2.3, respectively. From these results, it can be inferred that the activity for the hydrolytic dehydrogenation of NH3BH3 increase with decrease of the shell thickness of the hollow spheres. The result of the temperature-programmed desorption profile of ammonia showed that in the number of amount of acid sites of the hollow spheres increase with decrease of the shell thickness of the hollow spheres. The results indicate the activity depends on the number of acid sites of the hollow spheres.

Fabrication of hollow silica–nickel particles for the hydrolytic dehydrogenation of ammonia borane using rape pollen templates

Hollow silica–nickel particles were fabricated by coating silica–nickel shell on rape pollen templates following calcination. The samples obtained by various preparation steps and the resulting products were characterized by scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectroscope (EDX) and Fourier transform infrared spectroscopy (FTIR). The as-prepared porous hollow silica–nickel particles were obtained by coating, calcination, and supporting process of nickel species. The morphology and pore structure of the hollow silica support depend on the coating time. The morphology and pore structure affected the catalytic activity of the hollow silica–nickel particles for hydrogen generation from an aqueous NaBH4/NH3BH3 solution, and the hollow particles using silica support prepared for the coating time = 6 h showed the highest activity. The hollow particles possessed good recycling stability without significant loss of activity up to the fifth run.

The influence of the pore structure of hollow silica–alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane

The effect of the pore structures of hollow silica–alumina composite spheres on their activity for the hydrolytic dehydrogenation of ammonia borane has been studied. Following a calcination process that resulted in hollow spheres, the spheres’ shells were coated on spherical polystyrene particle templates. The pore structures of the hollow spheres were controlled by adding a surfactant, cetyltrimethylammonium bromide (CTAB); the volume and homogeneity of the mesopores in the shell of the hollow spheres increased with increasing amount of aqueous CTAB solution. The hollow spheres with a large volume and homogeneous mesopores showed high rates of hydrogen evolution from aqueous ammonia borane solution. The highly active hollow spheres prepared with 5 mL of aqueous CTAB solution showed a considerably high turnover frequency (TOF), although the hollow spheres did not have a significantly high BET surface area or a large quantity of acid sites. This suggests that the activity for hydrogen evolution from aqueous ammonia borane solution in the presence of the hollow spheres with homogeneous mesopore structures was unexpectedly high.

Influence of preparation conditions on morphology of in-situ synthesized hollow ruthenium-silica composite spheres for hydrolytic dehydrogenation of ammonia borane

Hollow ruthenium-silica composite spheres were synthesized from spherical ruthenium-silica composite particles prepared by sol-gel method, followed by in-situ activation in an aqueous sodium borohydride (NaBH4)/ammonia borane (NH3BH3) solutions. Through the preparation of the spherical particles, we investigated influence of promotors (L(+)-arginine and ammonia) on the sol-gel reaction in terms of the morphology of the spherical particle precursors and the hollow spheres. Average particle size of the precursors drastically increased by increasing the amount of L(+)-arginine used, though this also increased the solution pH. Average particle size of the precursors did not significantly increase when concentration of ammonia increased. These results indicate that L(+)-arginine promotes particle growth more effectively than ammonia. The spherical particles prepared with L(+)-arginine shows a higher hydrogen evolution rate and a higher quantity of evolved hydrogen from the aqueous NaBH4/NH3BH3 solution than the spherical particles prepared with ammonia. The spherical particles resulting from in-situ synthesis with sizes ranging from 100 to 950 nm possess hollow voids. UV-Vis spectra of the in-situ synthesized samples indicated that the activity depends on the reducibility of the active ruthenium species. The ruthenium species included in the sample prepared using L(+)-arginine was more metallic than that included in the sample prepared using ammonia.Graphical Abstract