Tatsuya Kodama

High-temperature solar chemistry for converting solar heat to chemical fuels

This paper reviews the recent developments on thermochemical conversion of concentrated solar high temperature heat to chemical fuels. The conversion has the advantage of producing long term storable energy carriers from solar energy. This conversion also enables solar energy transportation from the sunbelt to remote population centers. The thermochemical pathway is characterized by a theoretical high efficiency. However, there are solar peculiarities in comparison to conventional thermochemical processes—high thermal flux density and frequent thermal transients because of the fluctuating insolation—, and conventional industrial thermochemical processes are generally not suitable for solar driven processes. Therefore, the adaptation to such peculiarities of solar thermochemical processes has been the important R&D task in this research field. Thermochemical water splitting, steam or CO2 gasification of coal, steam or CO2 reforming of methane, and hydrogenetive coupling of methane, are industrially important, endothermic processes to produce useful chemical fuels such as hydrogen, synthesis gas and C2-hydrocarbons, which have been examined as solar thermochemical processes. The technical developments and feasibilities to conduct these endothermic processes by utilizing concentrated solar radiation as the process heat are discussed here. My recent experimental results to improve the advanced solar thermochemical technologies are also given.

Selective oxidation of liquid hydrocarbons over photoirradiated TiO2 pillared clays

Abstract Selective photo-oxidation of benzene and cyclohexane were investigated by using TiO2 pillared clays (mica, montmorillonite and saponite). The characterization results indicated that all the TiO2 pillared clays contain TiO2 with similar structure, anatase-like small particles, in the different silicate layers of clays. The solvent effect on the photo-oxidation of benzene on TiO2 pillared clay was significant; both the activity and selectivity were increased by an addition of 10% water in acetonitrile solvent and they were further increased when the reaction was performed in an aqueous environment. In the latter condition, TiO2 pillared clays showed higher selectivity to oxygenates than TiO2 (Degussa P25). The product distribution among oxygenates also depends on the type of the clay host, indicating that the photocatalytic properties of TiO2 depend strongly on the type of clay host. For the cyclohexane photo-oxidation, TiO2 pillared clay showed much higher selectivity than TiO2, possibly because of the hydrophobic nature of pillared clay.

A Two-Step Thermochemical Water Splitting by Iron-Oxide on Stabilized Zirconia

The thermochemical two-step water splitting cycle was examined by using an iron oxide supported on yttrium-stabilized, cubic zirconia (YSZ) as the working material with a view toward direct conversion of solar high-temperature heat to clean hydrogen energy. In the first step of the cycle, the YSZ-supported Fe3O4 was thermally decomposed to the reduced phase at 1400°C under an inert atmosphere. The reduced solid phase was oxidized back to the original phase (the YSZ-supported Fe3O4) with steam to generate hydrogen below 1000°C. A new redox pair, which is different from the Fe3O4–FeO pair previously examined by others, served as the working solid material on this YSZ-supported Fe3O4. Our new redox reaction proceeded as follows. The Fe3O4 reacted with YSZ to produce an Fe2+-containing ZrO2 phase by releasing oxygen molecules in the first step: The Fe2+ ions entered into the cubic YSZ lattice. In the second step, the Fe2+-containing YSZ generated hydrogen via steam splitting to reproduce Fe3O4 on the cubic YSZ support. This cyclic reaction could be repeated with a good repeatability of the reaction below 1400°C.

Photocatalytic water splitting on hydrated layered perovskite tantalate A2SrTa2O7.nH2O (A = H, K, and RB)

A series of layered perovskite tantalates, A2SrTa2O7 (A=H, Li, K, and Rb), were prepared as novel photocatalysts for photocatalytic water splitting into H2 and O2 under UV irradiation. The layered perovskite tantalates with hydrated interlayer space, A2SrTa2O7·nH2O (A=H, K, and Rb), showed higher H2 formation rate than anhydrous layered tantalate, Li2SrTa2O7, and anhydrous perovskite tantalate, KTaO3. H2SrTa2O7·nH2O and K2SrTa2O7·nH2O showed high activity for overall splitting of water without loading co-catalysts. The reaction over H2SrTa2O7·nH2O proceeded steadily more than 70 h, demonstrating a high durability of the catalyst. Effects of hydrated interlayer space on the catalytic activity were discussed on the basis of the results of photoluminescence spectra and the hydrogen evolution from aqueous solution of n-butylamine as a test reaction. The results indicate that the availability of interlayer space of layered tantalate as reaction sites is an important factor to improve the photocatalytic activity of Ta-based semiconductor materials.

Thermochemical methane reforming using a reactive WO3/W redox system

The methane reforming process combined with metal oxide reduction was evaluated, for the purpose of converting solar high-temperature heat to chemicals below 1273 K. The metal oxide was endothermically reacted with methane, to produce CO, hydrogen and the component metal in the temperature range of 1173–1273 K. Of the metal oxide candidates, WO3 and V2O5 were found to be reactive and selective metal oxides for the purposes of methane reforming. The metallic tungsten produced by methane reforming could be used to split water and to generate hydrogen at a lower temperature of 1073 K. To improve the reactivities of WO3 for methane reforming and the subsequent splitting of water, supported tungsten oxides were examined in the temperature range of 1073–1273 K. The reactivities were much improved with the ZrO2-supported WO3, giving a methane conversion of 70% and a CO selectivity of 86%. Our findings indicate the possibility that the proposed two-step process using a WO3/W redox system may be a potentially new thermochemical path that produces useful energy carriers of processed metal, syngas and methanol for storing and transporting solar energy from the sun belt to remote population centers.

Suzuki cross-coupling reaction catalyzed by palladium-supported sepiolite

Abstract Palladium-supported sepiolite clay have effectively catalyzed the Suzuki cross-coupling reaction of phenylboronic acid with aryl halide including less reactive electron-rich aryl bromides.

Synthesis of Titania Pillared Saponite in Aqueous Solution of Acetic Acid

The preparation of TiO2-pillared saponite was carried out in a CH3COOH aqueous solution. Titanium ion species to intercalate into the interlayer of saponite were obtained by an addition of Ti(C3H7O)4 to an aqueous solution of CH3COOH and by subsequent aging of the solution for a prescribed time. Ti4+-intercalated saponite including organic materials was obtained by ion exchange. After the sample was calcined at 500°C in air, TiO2-pillared saponite was obtained. The resulting TiO2-pillared saponite (Ti-Sapo) possessed surface areas in the range 300–400 m2/g and a sharp pore size distribution with the pore radius of 1.2 nm. The basal spacing of the product heated at temperature >250°C was about 2.45 nm. The pillar height of TiO2 in the Ti-Sapo was estimated to be 1.5 nm.

Stepwise production of CO-rich syngas and hydrogen via methane reforming by a WO3-redox catalyst

A two-step cyclic steam reforming of methane by a WO3/ZrO2 redox catalyst was performed to produce CO-rich syngas and hydrogen. The two-step cyclic processes could be repeated using a redox system of WO3 below 1273 K. The produced CO-rich syngas had the H2/CO ratios of about two, which was suitable for methanol production. This endothermic syngas-production process will be used as a solar thermochemical process for converting concentrated solar radiation to chemical liquid fuel of methanol in the sun-belt regions. The solar syngas-production process using the WO3/ZrO2 catalyst was demonstrated in a laboratory scale under direct irradiation of the catalyst by a solar-simulated, high-flux visible light.

Self-aldol condensation of unmodified aldehydes catalysed by secondary-amine immobilised in FSM-16 silica

Self-aldol condensation of unmodified aldehydes was catalysed effectively by N-metlyl-3-aminopropylated FSM-16 mesoporous silica, whose activity was higher than that of homogeneous amine catalyst.

Preparation of large surface area nickel magnesium silicate and its catalytic activity for conversion of ethanol into buta-1,3-diene

The ternary oxide catalysts of nickel magnesium silicate with a large surface area (498–784m2/g) can be obtained by calcining a mixture of Ni(NO3)2, Mg(OH)2 and SiO2. The large surface area of the catalyst is attributed to its layer structure. The selectivity for buta-1,3-diene formation from ethanol on the catalysts was 90% or more and the yield of buta-1,3-diene was 53 mol%.