Hayao Imamura

Composites for hydrogen storage by mechanical grinding of graphite carbon and magnesium

Novel hydrogen storage Mg/G nano-composites obtained by mechanical grinding of magnesium (Mg) and graphite carbon (G) with organic additives (benzene, cyclohexane or tetrahydrofuran) have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC) and temperature programmed desorption (TPD) techniques. The occurrence of various effects as a result of the formation of Mg/G composites ground with benzene, cyclohexane or tetrahydrofuran (designated hereafter as (Mg/G)BN, (Mg/G)CH or (Mg/G)THF, respectively) is expected. Upon mechanical grinding with benzene or cyclohexane for 4–40 h, new hydrogen-storing sites, other than those due to the magnesium component, were formed in the Mg/G composites and they took up hydrogen reversibly. The cleavage-degraded graphite in the composites plays an important role in such hydrogen uptake and release. The formation of Mg/G composites upon grinding with the organic additives led to not only a drop in the onset temperature of MgH2 decomposition, but the formation of additional hydrogen uptake sites. In marked contrast to (Mg/G)BN and (Mg/G)CH, the composites ground without any additives (referred as (Mg/G)none) did not show such behavior. The effective nano-composites are those in which there are synergetic interactions between magnesium and graphite as a result of mechanical grinding with the organic additives.

Hydrogen absorption of Mg-Based composites prepared by mechanical milling: Factors affecting its characteristics

Abstract Hydriding and dehydriding properties of Mg-based composites (MgG and MgPdG) which are prepared by mechanical milling of magnesium powder and graphite (G) or graphite supporting 5 wt.% Pd (5 wt.% Pd-G) in the presence of various additives (tetrahydrofuran, benzene or cyclohexane) have been studied. Such composites were effective as hydrogen storage materials even under mild reaction conditions (500 Torr, 453 K). Factors affecting the hydriding characteristics of the composites were extensively investigated in connection with the preparative conditions (additives and their amounts, milling times, Mg:G component ratios, etc.). In particular, the presence of tetrahydrofuran in the milling process strongly affected the hydriding and dehydriding kinetics of the resulting composites. The active composites for hydrogen storage are those in which finely divided magnesium is in intimate contact with graphite. It is expected that their activity results from certain synergetic interactions between Mg and aromatic carbon atoms of graphite containing charge transfer to some extent.

Characterization and hydriding properties of Mg-graphite composites prepared by mechanical grinding as new hydrogen storage materials

Abstract The characteristics and hydriding properties of novel Mg/G composites as prepared by mechanical grinding (MG) of magnesium metal powder and graphite (G) in the presence of cyclohexane (CH) or tetrahydrofuran (THF) have been studied. The use of graphite led to chemical modifications that improved the hydriding properties of magnesium. The Mg/G composites were more active and effective for hydrogen absorption than reported previously. The additives CH and THF in the MG process influenced strongly the hydriding activity of (Mg/G)CH and (Mg/G)THF, respectively. The quantities of CH and THF required for optimal activity of (Mg/G)CH and (Mg/G)THF are quite different. For (Mg/G)THF, MG of 1 h and THF of 6.1 cm3 gave the best results while the formation of good (Mg/G)CH composites required MG of 20 h and the presence of CH of 12 cm3. The different influence of CH and THF on the characteristics of the Mg/G composites are also discussed. The composites formed upon grinding in the presence of CH or THF contain finely divided magnesium which is in intimate contact with the aromatic carbon of graphite. This leads to synergetic effects which result in the formation of an effective hydrogen-absorbing material.

Hydrogen-absorbing magnesium composites prepared by mechanical grinding with graphite: effects of additives on composite structures and hydriding properties

Abstract Novel Mg/G composites were prepared by mechanical grinding of magnesium (Mg) and graphite (G) with benzene as an additive. The addition of benzene was very important in determining the composite structures and hydriding properties. The composites prepared without benzene (designated hereafter as (Mg/G)none) showed negligible activity for hydriding, whereas the use of benzene during grinding led to drastic changes in composite structures, leading to much improved hydriding. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the structures of the Mg/G composites, especially for the mode of degradation of graphite structure during grinding. In the course of the composite formation in the presence of benzene (referred to as (Mg/G)BN), the graphite structure was predominantly degraded by cleavages along graphite layers, while the graphite for (Mg/G)none was broken irregularly and disorderly, leading to rapid amorphization. Moreover, the additive for the composite formation plays an important role in promoting synergetic actions induced during the mechanical grinding of magnesium and graphite, in which the flaked graphite formed by fracture along graphite layers interacts with divided magnesium with charge-transfer. X-ray photoelectron spectroscopy (XPS) of (Mg/G)BN proved the charge-transfer from magnesium to graphite carbons.

Hydriding-dehydriding behavior of magnesium composites obtained by mechanical grinding with graphite carbon

Abstract Novel Mg/G composites were prepared by mechanical grinding of magnesium (Mg) and graphite carbon (G) with cyclohexadiene, cyclohexene, cyclohexane, benzene or tetrahydrofuran as an additive. The presence of organic additives during the grinding was very important in determining the composite structures and hydriding–dehydriding properties. The composites prepared without additives [designated hereafter as (Mg/G)none] showed negligible activity for hydriding, whereas the use of additives led to drastic changes in composite structures, leading to much improved hydriding and dehydriding behavior. The effectiveness of organic additives in the initial hydriding was in the order: cyclohexadiene≈tetrahydrofuran≈cyclohexene>benzene>cyclohexane. In the course of the composite formation in the presence of organic additives, the graphite was predominantly degraded by cleavages along graphite layers, resulting in the occurrence of synergetic interactions with magnesium. The graphite for (Mg/G)none was broken irregularly and disorderly to rapid amorphization with negligible interactions with magnesium. Various metal-doped Mg/G composites obtained by grinding of magnesium and graphite with organometallic solutions (Al(C2H5)3, Ti(OC3H7)4, Fe(C5H5)2, Ni(C5H5)2 or Zn(C2H5)2) in benzene have been further examined. Ti-doped Mg/G composites using Ti(OC3H7)4 among others showed an excellent activity; the initial hydriding activity increased above 10-fold relative to that for the metal-free composites.

Hydriding characteristics of Mg-based composites prepared using a ball mill

Abstract Mechanical milling of magnesium and 5 wt.% Pd-supporting graphite (5 wt.% Pd/G) in the presence of tetrahydrofuran (THF) gave rise to the formation of novel Mg-based composites (Mg/Pd/G) as effective hydrogen storage materials. The Mg-composite storage materials were extremely active toward hydrogen absorption even at 300 K and showed excellent reversibility for hydriding-dehydriding. The characteristics of the samples were investigated extensively in connection with the preparative conditions. In particular, the presence of THF strongly affected the hydriding properties of the resulting composites. Upon milling of Mg and Pd/G in the presence of THF the graphite layer structure was almost decomposed and the magnesium component was highly dispersed in the particle sizes of about 24–27 nm. The specific surface area of the samples increased to reach a value of 34–65 m2 g−1. The Mg-composites as a hydrogen storage material are those in which such a finely divided magnesium is in intimate contact with graphite containing synergism between Mg and aromatic carbons in graphite.

Rare earth metals as hydrogenation catalysts of unsaturated hydrocarbons

Abstract To elucidate the characteristics of rare earth metallic catalysts the hydrogenation of unsaturated hydrocarbons (ethene, propene, 1-butene, 1,3-butadiene, ethyne, propyne, and benzene) were carried out around ambient temperature using samarium and ytterbium particles formed by clustering metal atoms in frozen organic matrices by metal vapor techniques. In the hydrogenation reactions the rare earth metallic catalysts discriminated between the CC double bonds and triple bonds; alkenes, dialkenes, and aromatic compounds were readily hydrogenated, whereas alkynes were not hydrogenated at all. However, enhanced isomerization activity of propyne to propadiene was observed. The addition rates of hydrogen to alkenes were represented on coordinates of a first-order equation: v = kP H 2 . Preliminary kinetic studies suggest that the reaction is controlled by the hydrogen adsorption process. This identification is reinforced by the H 2 -D 2 isotope scrambling measurements. The hydrogenation of 1,3-butadiene by the rare earth catalysts was completely selective for alkene formation, and the yield of 2-butene was relatively high (>80%) with a high trans:cis ratio (2 ~ 20). The mode of hydrogen addition to the diene was examined using isotope techniques, indicating that 1-butene and 2-butene were formed by 1: 2- and 1:4-addition of hydrogen to 1,3-butadiene, respectively. In addition, it was found that the molecular identity of hydrogen was conserved during the hydrogenation of unsaturated hydrocarbons.

Hydrogen absorption in modified intermetallic compound systems

Abstract The modification of Mg 2 Ni by various organic compounds was investigated with a view to improving its hydrogen absorption properties. In particular Mg 2 Ni reversibly absorbed hydrogen under more moderate conditions when it had been modified with tetracyanoethylene or phthalonitrile. Studies of the treated materials using electron spin resonance and electronic spectra showed the formation of electron donor-acceptor (EDA) complexes owing to the high electron affinity of the organic species used. It is believed that the EDA complexes formed on the surface layer provide sites for hydrogen activation which is followed by the diffusion of excess hydrogen into the underlying intermetallic phase. This is consistent with the results of X-ray analysis and thermodynamic measurements.

Exceptionally active magnesium for hydrogen storage: Solvated magnesium clusters formed in low temperature matrices

Abstract It was found that the cocondensation of magnesium atoms with tetrahydrofuran vapour on a cooled surface (77 K) leads to the formation of magnesium aggregates which are exceptionally active for hydrogen absorption under mild conditions. The active magnesium clusters immediately began to absorb hydrogen without the conventional activation treatments even at about 473 K under atmospheric pressures. The present study is concerned with the evaluation of the magnesium clusters formed in low temperature matrices as a hydrogen storage medium. Further, attention is directed towards changes in the surface properties of magnesium caused by treating it with aromatic molecules in connection with the process of hydrogen absorption.

Effects of lanthanide metals in Eu–Ni and Yb–Ni bimetallic catalysts

Lanthanide metals (Eu and Yb) dissolved in liquid ammonia and reacted readily with reduced Ni powders to form Eu–Ni and Yb–Ni bimetallic catalysts with different lanthanide contents. The catalysts were characterized by the hydrogenolysis of ethane and cyclohexane, the hydrogenation of ethene, hydrogen chemisorption and temperature-programmed desorption (TPD) measurements. The rate of hydrogenolysis decreased markedly when Yb and Eu were added to the Ni surface, whereas the hydrogenation activity showed a tendency to increase, especially in the Eu–Ni system. The lanthanide itself showed a low activity for the hydrogenolysis and hydrogenation reactions under the reaction conditions. An analysis of the decrease in the rate of hydrogenolysis with lanthanide addition suggested a decrease in concentration of surface nickel available for structure-sensitive reactions caused by lanthanide coverage. For the hydrogenation of ethene the surface was gradually covered with the lanthanide metals and simultaneously certain interactions occurred to produce active centres; the presence of lanthanide metals on Ni strongly influenced the state of adsorption of hydrogen in the subsequent activation processes, resulting in enhanced capacity of this surface to dissociate hydrogen. For the hydrogenation of ethene and adsorption characteristics, lanthanide and transition metals were more efficient when they were used together.