Tsuyoshi Hatamachi

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.

Coal Coke Gasification in a Windowed Solar Chemical Reactor for Beam-Down Optics

Solar thermochemical processes, such as solar gasification of coal, require the development of a high temperature solar reactor operating at temperatures above 1000°C. Direct solar energy absorption by reacting coal particles provides efficient heat transfer directly to the reaction site. In this work, a windowed reactor prototype designed for the beam-down optics was constructed at a laboratory scale and demonstrated for CO2 gasification of coal coke using concentrated visible light from a sun-simulator as the source of energy. Peak conversion of light energy to chemical fuel (CO) of 14% was obtained by irradiating a fluidized bed of 500–710 μm coal coke size fraction with a power input of about 1 kW and a CO2 flow-rate of 6.5 dm3 min−1 at normal conditions.

Pillaring of Ruddlesden–Popper perovskite tantalates, H2ATa2O7 (A = Sr or La2/3), with n-alkylamines and oxide nanoparticles

The Ruddlesden–Popper-type layered perovskite tantalates, H2ATa2O7 (A = Sr or La2/3), were pillared with n-alkylamines and oxide nanoparticles for the first time. H2SrTa2O7 can accommodate n-alkylamines (carbon numbers = 4, 8, 12, 16) to form intercalation compounds. A linear relationship is observed between the interlayer distance and the number of carbon atoms in n-alkyl chains, indicating the formation of a bilayer of the n-alkylamines. Porous metal oxides were synthesized by pillaring the n-octylamine intercalated H2ATa2O7 perovskites with Fe2O3 or Fe–Si mixed oxide (FeSi). These materials were well characterized by XRD, TEM, N2 adsorption, and Fe K-edge XAFS (XANES and EXAFS). FeSi pillared H2SrTa2O7 and H2La2/3Ta2O7 have a high surface area (52 and 130 m2 g−1) and microporosity. Fe2O3 pillared H2SrTa2O7 has a macroporous structure with a relatively low surface area (14 m2 g−1). XAFS analysis reveals that the Fe species in FeSi pillared perovskites are tetrahedral Fe3+ species incorporated in a silica matrix or highly dispersed on silica particles, while those in the Fe2O3 pillared one are α-Fe2O3 nanoparticles. The acidic property is tested by temperature programmed desorption (TPD) of ammonia, and the result shows that pillaring increases the number of acid sites in perovskites.

Carbonate Composite Catalyst with High-Temperature Thermal Storage for Use in Solar Tubular Reformers

The composite materials of Ni-applied, porous zirconia balls with molten Na 2 CO 3 salt were examined for use in solar thermochemical reforming of methane as the catalyst with high-temperature thermal storage. The millimeter-sized composite balls were tested on the heat discharge property and the catalytic activity for CO 2 reforming of methane in a laboratory-scale reactor. The high heat capacity and large latent heat (heat of solidification) of the composite molten salt circumvented the temperature dropping of the catalyst bed, which resulted in the alleviation of rapid decay in chemical conversion during cooling mode of the reactor. The composite catalyst is expected to realize stable operation in the solar reformer under fluctuation of insolation by a cloud passage.

Comparison Studies of Reactivity on Nickel-Ferrite and Cerium-Oxide Redox Materials for Two-Step Thermochemical Water Splitting Below 1400°C

A two-step thermochemical water splitting cycle using a redox system of non-volatile metal oxide is one of the promising processes for converting concentrated solar high-temperature heat into clean hydrogen in sun-belt regions. In the 1st step of the cycle or the thermal reduction step, metal oxide is thermally reduced to release oxygen molecules in an inert gas atmosphere at a higher temperature above 1400°C. In the second step or the water-decomposition step at a lower temperature, the thermally-reduced metal oxide reacts with steam to produce hydrogen. As the reactive redox metal oxide materials to be capable of working below 1400°C, nickel-doped iron oxides or Ni-ferrites supported on zirconia, and non-stoichiometric cerium oxides are the promising working materials. In the present work, a series of the nickel-ferrite redox materials of monoclinic-zirconia-supported, cubic-YSZ(yttrium-stabilized zirconia)-supported, and non-supported Ni-ferries and non-stoichiometric cerium oxide were compared on reactivity for two-step thermochemical water splitting cycle. The monoclinic-zirconia-supported Ni-ferrite produced the most quantity of hydrogen in the repeated cycles when the thermal reduction step was performed for 30 min at 1400°C and the water decomposition step for 60 min at 1000°C.© 2011 ASME

Molten-Salt Tubular Absorber/Reformer (MoSTAR) Project: Reforming Performance of Reactor Tubes During Intermittent Heating

Reforming performances for the double-walled reactor tubes with Na2 CO3 /MgO composite thermal storage was examined by an intermittent heating. The intermittent heating of the reactor tubes is composed of the heat-discharge (or cooling) mode and the subsequent heating mode. The heat-discharging mode simulates a fluctuating insolation for cloud passages. The heating mode simulates a heating of reactor due to concentrated solar radiation by using an electric furnace. The internal tube of the reactor was packed with the 2wt%Ru/Al2 O3 catalyst balls while the thermal storage materials were filled in the annular region of the reactor tubes. The reactor was heated up to 920°C in the cylindrical electric furnace and the CH4 /CO2 mixture was fed into the internal catalyst tube at gas hourly space velocity (GHSV) of 12500 h−1 . Through the cooling mode and the subsequent heating mode, temperature variations of reactor tubes, catalyst and composite material, H2 /CO ratio variations of effluent gas from the reactor, higher heating value (HHV) power of reformed gas were respectively examined for the double-walled reactor tubes and a single-wall reactor tube without the thermal storage.Copyright

Molten-Salt Tubular Absorber/Reformer (MoSTAR) Project: Metal-Plate-Bridged Double Tube Reactor

The Molten-Salt Tubular Absorber/Reformer (MoSTAR) Project, which is jointly conducted by Niigata University, Japan, and Inha University, Korea, aims to develop a novel-type of “double-walled” tubular absorbers/reformers with molten-salt thermal storage at high temperature for use in solar natural-gas reforming and solar air receiver, and to demonstrate their performances on sun with a 5-kWt dish-type solar concentrator. The new concept of “double-walled” reactor tubes was proposed for use in a solar reformer by Niigata University, Japan, and involves packing a molten salt in the annular region between the internal catalyst tube and the exterior solar absorber tube of the double reactor tube. In this work, “metal-plate-bridged” double reactor tubes are newly proposed for use in a solar reformer. Two different sized reactor tubes are constructed, and tested on chemical reaction performance for dry reforming of methane during cooling or heat-discharge mode of the reactor tube using an electric furnace. The experimental results obtained under feed gas mixture of CH4 /CO2 = 1:3 at a residence time of 0.36 s and at 1 atm showed that the double reactor tube with the heat storage medium Na2 CO3 in the annular region successfully sustained a high methane conversion above 90% with about 0.7-kW output power of the reformed gas based on HHV for 40 min of the heat-discharge mode. The application of the new reactor tubes to solar tubular reformers is expected to help realize stable operation of the solar reforming process under fluctuating insolation during a cloud passage.Copyright

Ferrite-Loaded Ceramic Foam Devices Prepared by Spin-Coating Method for a Solar Two-Step Thermochemical Cycle

A two-step water-splitting thermochemical cycle using redox working material of iron-based oxide (ferrite) particles has been developed for converting solar energy into hydrogen. The two-step thermochemical cycle for producing a solar hydrogen from water requires a development of a high temperature solar-specific receiver-reactor operating at 1000–1500°C. In the present work, ferrite-loaded ceramic foams with a high porosity (7 cells per linear inch) were prepared as a water splitting device by applying ferrite/zirconia particles on a MgO-partially stabilized Zirconia (MPSZ) ceramic foam. The water splitting foam device was prepared using a new method of spin coating. A spin coating method we newly employed that has advantages of shortening preparation period and reducing of the coating process in comparison to previous preparation method reported. The water-splitting foam devices, thus prepared, were examined on hydrogen productivity and reactivity through a two-step thermochemical cycle. NiFe2 O4 /m-ZrO2 /MPSZ and Fe3 O4 /c-YSZ/MPSZ foam devices were firstly tested for thermal reduction of ferrite using the laboratory scale receiver-reactor by a sun-simulator to simulate concentrated solar radiation. Subsequently, with another quartz reactor the light-irradiated device was reacted with steam by infrared furnace. As a result, it was possible to perform cyclic reactions over several times and to produce hydrogen through thermal-reduction at 1500°C and water-decomposition at 1100–1200°C. In further experiments, the NiFe2 O4 /m-ZrO2 /MPSZ foam device was successfully demonstrated in a windowed single reactor for cyclic hydrogen production by solar-simulated Xebeam irradiation with input power of 1 kW. The NiFe2 O4 /m-ZrO2 /MPSZ foam device produced hydrogen of 70–190μmol per gram of device through 6 cycles and reached ferrite conversion of 60% at a maximum.Copyright

Thermal storage/discharge performances of Cu-Si alloy for solar thermochemical process

The present authors (Niigata University, Japan) have developed a tubular reactor system using novel “double-walled” reactor/receiver tubes with carbonate molten-salt thermal storage as a phase change material (PCM) for solar reforming of natural gas and with Al-Si alloy thermal storage as a PCM for solar air receiver to produce high-temperature air. For both of the cases, the high heat capacity and large latent heat (heat of solidification) of the PCM phase circumvents the rapid temperature change of the reactor/receiver tubes at high temperatures under variable and uncontinuous characteristics of solar radiation. In this study, we examined cyclic properties of thermal storage/discharge for Cu-Si alloy in air stream in order to evaluate a potentiality of Cu-Si alloy as a PCM thermal storage material. Temperature-increasing performances of Cu-Si alloy are measured during thermal storage (or heat-charge) mode and during cooling (or heat-discharge) mode. A oxidation state of the Cu-Si alloy after the cyclic ...

Kinetics of CO2 Reforming of Methane by Catalytically Activated Metallic Foam Absorber for Solar Receiver-Reactors

Ni-Cr-Al alloy foam absorber with high porosity was catalytically activated using a Ru/γ-Al2 O3 catalyst, and was subsequently tested with respect to CO2 reforming of methane in a small-scale volumetric receiver-reactor by using a sun simulator. A chemical storage efficiency of about 40% was obtained for a mean light flux of 325 kWm−2 . Furthermore, the activity and the stability of the metallic foam absorber were compared with those of a SiC foam absorber activated with the same Ru/γ-Al2 O3 catalyst for 50 h of light irradiation, and it was found that the metallic foam absorber has superior catalytic stability in comparison to the SiC form absorber. In addition, unlike ceramic foams such as SiC, metallic foams feature superior plasticity, which prevents the emergence of cracks caused by mechanical or thermal shock. The kinetics of CO2 reforming of methane over metallic foam absorbers were also examined for temperatures of 600–750°C using a quartz tube reactor and an electric furnace. The experiments were performed by varying the methane/CO2 ratios of 0.5–2.3. Moreover, the kinetic data were fitted to four different types of kinetic models, namely the Langmuir-Hinshelwood, Basic, Eley-Rideal, and Stepwise mechanisms. The kinetic model which provided the best prediction of the experimental reforming rates was the Langmuir-Hinshelwood mechanism.Copyright