Fumitaka Hayashi

Ammonia synthesis using a stable electride as an electron donor and reversible hydrogen store

Industrially, the artificial fixation of atmospheric nitrogen to ammonia is carried out using the Haber-Bosch process, but this process requires high temperatures and pressures, and consumes more than 1% of the worlds power production. Therefore the search is on for a more environmentally benign process that occurs under milder conditions. Here, we report that a Ru-loaded electride [Ca(24)Al(28)O(64)](4+)(e(-))(4) (Ru/C12A7:e(-)), which has high electron-donating power and chemical stability, works as an efficient catalyst for ammonia synthesis. Highly efficient ammonia synthesis is achieved with a catalytic activity that is an order of magnitude greater than those of other previously reported Ru-loaded catalysts and with almost half the reaction activation energy. Kinetic analysis with infrared spectroscopy reveals that C12A7:e(-) markedly enhances N(2) dissociation on Ru by the back donation of electrons and that the poisoning of ruthenium surfaces by hydrogen adatoms can be suppressed effectively because of the ability of C12A7:e(-) to store hydrogen reversibly.

Ammonia decomposition by ruthenium nanoparticles loaded on inorganic electride C12A7:e−

The use of ammonia as a hydrogen carrier has received much attention due to its high hydrogen content and liquid state under mild conditions, which could lead to fuel cell applications. This study demonstrates facile ammonia decomposition on ruthenium nanoparticles loaded on inorganic electride, C12A7:e−. A high turnover frequency (∼12 s−1 at 400 °C) and low activation energy (64 kJ mol−1) for recombinative N2 desorption were obtained for Ru/C12A7:e−. N2-temperature programmed desorption (N2-TPD) and kinetic analyses indicate that the high catalytic performance is due to the low work function of chemically stable C12A7:e−, which enables electron injection to the antibonding orbital of the Ru–N bond formed transiently through the reaction by raising the Fermi level of Ru metal.

Superconductivity and phase instability of NH 3 -free Na-intercalated FeSe 1- z S z

The discovery of ThCr2Si2-type A(x)Fe(2-y)Se2 (A = K, Rb, Cs and Tl) with Tc ~30 K make much progress in iron-based superconducting field, but their multiple-phase separations are disadvantageous for understanding the origin. On the other hand, for small alkali metals, studies on (Li,Na)FeCu(S,Se)2 and NaFe(2-δ)S2 show that these compounds possess CaAl2Si2-type structure, implying that ThCr2Si2-type structure is unstable for small alkali metal-intercalated FeSe under high temperature. Here we report a new intercalate Na0.65(1)Fe1.93(1)Se2 with Tc ~37 K, synthesized by low-temperature ammonothermal method. The notable finding is that the Na0.65(1)Fe1.93(1)Se2 shows a ThCr2Si2-type structure, which is the first instance of small-sized alkali metal intercalates without NH3 co-intercalation. Besides, the NH3-poor Na0.80(4)(NH3)0.60Fe1.86(1)Se2 and NH3-rich phase with Tcs at 45 and 42 K are identified by tuning the concentration of Na-NH3 solutions. The modulation of interlayer spacing reveals the versatile evolution of structural stability and superconductivity in these intercalates.

Modulation effect of interlayer spacing on the superconductivity of electron-doped FeSe-based intercalates.

FeSe-based intercalates are regarded as promising candidates for high-critical temperature (Tc) superconductors. Here we present new Na- and Sr-intercalated FeSe superconductors with embedded linear diamines (H2N)CnH2n(NH2) (abbreviated as DA; n = 0, 2, 3, or 6) prepared using a low-temperature ammonothermal method to investigate the effect of interlayer spacing on the superconductivity of electron-doped FeSes. The embedded DA formed a monolayer or bilayer in the interlayer of FeSe. The interlayer spacing between nearest FeSe layers could be tuned from 0.87 to 1.14 nm without significant change in the Na/Sr content or the ratio of Fe to Se. Importantly, bilayer phases Na/ethylenediamine- and Sr/hydrazine-FeSe show improved structural stability compared to that of Na/NH3-FeSe. The series of Na- and Sr-intercalated FeSe samples exhibited nearly the same high Tc values of 41-46 and 34-38 K, respectively, irrespective of rather different interlayer spacing d. The peculiar insensitivity for both series can be ascribed to the negligible dispersions of bands along the c axis; i.e., Fermi surfaces are nearly two-dimensional when d is larger than a certain threshold value (dsat) of ∼0.9 nm. The Fermi surface shape is already optimal for Tc, and a larger d will not enhance Tc further. On the other hand, the difference in Tc between two series may be explained by the higher carrier doping level in Na/DA-FeSes compared to that in Sr/DA-FeSes, resulting in the increased density of states at the Fermi level and superconducting pairing strength.

Surface Treatment for Conductive 12 CaO⋅7 Al2O3 Electride Powder by Rapid Thermal Annealing Processing and Its Application to Ammonia Synthesis

The inorganic electride [Ca24Al28O64]4+(e−)4 (C12A7:e−) has unique properties, that is, chemical stability and a low work function comparable to that of metal K. However, its surface area is low (≈1 m2 g−1) because sintering occurs during high‐temperature annealing, which is needed to remove CaO layers formed by reaction with metal Ca that works as a reductant. We report a simple synthesis method for moderate‐surface‐area C12A7:e− (9–19 m2 g−1) by using a rapid thermal annealing (RTA) technique. The synthesis consists of 1) high‐temperature evacuation, 2) reaction with Ca metal, and 3) RTA. The influence of these synthesis conditions was first studied to achieve both high surface area and high electron concentration. Next, C12A7:e− samples were examined as Ru catalyst supports for NH3 synthesis. The activity of Ru‐loaded as‐prepared C12A7:e− was moderate but was increased fivefold by RTA processing prior to Ru loading as a result of surface structure reconstruction. The improvement in the Ru dispersion degree that results from the increased surface area trebled the overall activity (3550 μmol  NH 3  gcat−1 h−1 at 340 °C) compared with that of conventional Ru/C12A7:e−.

NH2– Dianion Entrapped in a Nanoporous 12CaO·7Al2O3 Crystal by Ammonothermal Treatment: Reaction Pathways, Dynamics, and Chemical Stability

Inorganic imides are useful for hydrogen storage and base-catalyzed reactions but are extremely unstable under ambient conditions, which hinders their practical use as functional materials. Here, we demonstrate that NH2(-) and H(-), as well as NH(2-), can be incorporated into the nanocages of the mayenite crystals, [Ca24Al28O64](4+)(e(-))4 and [Ca24Al28O64](4+)(O(2-))2, by ammonothermal treatment. We evaluated the reaction conditions and found that the anion exchange reaction proceeded at higher than 500 °C. Raman spectroscopy showed that the N-H band position of encaged NH(2-) was close to that of CaNH and MgNH crystals. We also studied the reaction pathways that yield NH2(-) and NH(2-) anions and their dynamic motions by (1)H NMR spectroscopy. Successive reactions of encaged e(-) and O(2-) ions with NH3 yielded NH2(-), NH(2-), and H(-) or OH(-), in which the O(2-) ion reacted more efficiently with NH3. The maximum NH(2-) concentration and content were ∼2.7 × 10(20) cm(-3) and ∼0.25 (wt %)NH, respectively. The short spin-lattice relaxation time found in (1)H NMR suggests that the incorporated NH2(-) and NH(2-) rotate or librate in the cage near room temperature. Stability tests showed that the encaged NH(2-) ions are chemically stable under ambient conditions and in organic solvents. These results are attributed to the encapsulation of active anions within subnanometer-sized cages composed of Ca-O-Al oxide frameworks. The encaged NH(2-) desorbed as NH3 at higher than 500 °C under vacuum (Ea = 172 kJ mol(-1)). It is thus expected that C12A7:NH(2-) will function as a reactive nitrogen source for nitrogen transfer reactions by in situ cage degradation.

Emergence of magnetism and controlling factors of superconductivity in Li/Na-ammonia cointercalatedFeSe1−zTez

The discovery of superconductivity in alkali-ammonia co-intercalated FeSe has generated intensive interest because of their highest Tc (~ 45 K) among iron-chalcogenide superconductors with bulk form. Here, we report the phase diagrams of two series of Li/Na-ammonia co-intercalated FeSe1-zTez. When superconductivity is suppressed by Te doping, the magnetic ordering states are emergent. Moreover, a novel phase Lix(NH3)yFe2-{\delta}Te2 with possible antiferromagnetism is discovered. This strongly indicates the intimate relation between superconductivity and magnetism in these materials, like in other iron-based superconductors. On the other hand, comparative structure analysis with FeSe1-zTez suggests that although there is remarkable similarity in phase diagrams for both iron-chalcogenide and iron-pnictide superconductors, the different types of anions with different charges lead to the dissimilar controlling factors in superconductivity for both classes of materials. This opens up an opportunity to tune the superconductivity in iron-chalcogenide superconductors in more ways than just one.

Flux-boosted coating of idiomorphic CuInS2 crystal layers on Mo-coated glass substrate

Cu–In–Ga–S–Se (CIGSSe) is a promising light-absorber in thin-film photovoltaic cells, as well as a photocathode for solar H2 production, but the fabrication of layers of CIGSSe crystals on substrates is both time- and cost-intensive. Here, we report the fabrication of CuInS2 crystal layers on various precursor-loaded Mo/soda-lime glass (SLG) substrates using a flux coating method. X-ray diffraction analysis indicated that the main phase was CuInS2, while MoS2 which formed through the sulfurization of the Mo substrate was present as a minor phase. Top-surface field-emission scanning electron microscopy (FE-SEM) images indicated that the CuInS2 crystals were sparsely formed on the bare Mo/SLG, In/Mo/SLG, and Cu/Mo/SLG substrates. In contrast, closely packed CuInS2 crystals were formed on Cu2S/Mo/SLG. Smaller CuInS2 crystals 0.5–1 μm in size were grown on Cu2S/Mo/SLG compared to those on other substrates. This sharp difference in crystal population and size could be attributed to the pre-loading effect of the precursors. Cross-sectional FE-SEM analysis with energy-dispersive X-ray spectroscopy mapping revealed the homogenous fabrication of idiomorphic CuInS2 crystals on the partially sulfurized Mo substrate, yielding a CuInS2/MoS2/Mo heterostructure amenable to photovoltaic applications. The formation mechanism of this unique heterostructure was discussed based on the experimental results.

Almost Complete Nitridation of Mesoporous Silica to Mesoporous Silicon (Oxy)Nitride with Ammonia

Mesoporous silicon oxynitride and nitride were prepared through nitridation of various mesoporous silica, MCM-41, -48, and SBA-15 with ammonia in a plug flow reactor. The nitrogen contents were dependent on the reaction temperature and the amount of ammonia supplied per sample weight. The appropriate nitridation temperature was 1273 K and the maximum contents of nitrogen were 35-39 wt % which correspond to 88-98% of that of Si3N4. Various physicochemical characterization of the resulting silicon (oxy)nitiride indicated that the pore structures were not changed upon the nitridation though the lattice constants and the pore diameters decreased and the wall thickness increased. The nitridation mechanism was discussed on the basis of 29Si MAS NMR and XPS experiments.

Growth of {100}-faceted NaFeTiO4 crystals with a tunable aspect ratio from a NaCl–Na2SO4 binary flux

The controlled growth of needle-shaped and planar bar-shaped NaFeTiO4 crystals, a CaFe2O4-type structure, was carried out by a flux method using a NaCl–Na2SO4 binary flux. NaCl fluxes have been empirically investigated for growing unique anisotropic crystal shapes. However, strategies for controlling the crystal morphology based on NaCl fluxes have not been established. In this study, Na2SO4 was added to a NaCl flux to supply O2− ions, which is essential for the dissolving ability of a metal oxide into ions, and the growth manner was systematically investigated as a function of flux composition. As a result, needle-shaped crystals were obtained from the pure NaCl flux with exposed {100} facets. Meanwhile, with the binary flux, the morphology of the crystals changed from a needle shape to a planar bar shape depending on the Na2SO4 content, where the aspect ratio of the {100} facets was increased by about ten times. It was found out that the aspect ratio of the {100} planes of NaFeTiO4 crystals can be controlled kinetically by the cooperative effect of Na+ ions and anionic species in the flux; Na+ ions stabilize the {100} facets and a high O2−/Cl− ratio increases the concentration of ions as a precursor for crystal growth to promote the growth in the direction, resulting in planar bar-shaped crystals. We believe that the morphological control regime demonstrated here in the growth of NaFeTiO4 crystals in a NaCl–Na2SO4 binary flux could be a useful idea in high temperature chemistry and for their desirable applications.