Characteristics of Hydrogen Absorption-Desorption Reaction in R-M (R=Y, La, Ce; M=Co, Rh, Ir, Ni, Pd, Pt) Binary Systems
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Characteristics of Hydrogen Absorption-Desorption Reaction in R-M (R=Y, La, Ce; M=Co, Rh, Ir, Ni, Pd, Pt) Binary Systems H.Mizutani1, K.Hirano2, J.Kadono3, S.Nishiuchi3, S.Yamamoto1, T.Tanabe1 and J.Kawai1 1 Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, 606-8501 Kyoto, Japan. 2Asahi Seiren Co.Ltd., 9 Ota, Yao, 581-0037 Osaka, Japan. 3Kyoto Municipal Institute of Industrial Research, Shimogyo-ku, 600-8813 Kyoto, Japan. ABSTRACT The amount of absorbed hydrogen, the absorption rate and the reversibility of hydrogen absorption-desorption reaction were measured for binary systems R-M (R= Y, La, Ce; M=Co, Rh, Ir, Ni, Pd, Pt). These experimental results were discussed by comparing the number of states unoccupied by electrons, the cohesive energy and the energy fluctuation, which were calculated by the extended Hückel method. The main results are as follows. (a) The more the number of unoccupied electronic states in the compounds, the more hydrogen is absorbed, (b) the critical concentrations of hydrogen in the R-M compounds where the energy fluctuation decreases remarkably correspond to the inflection or saturation points in the absorption curve, and (c) when the cohesive energy of a compound decreases linearly with hydrogen concentration, the compound easily desorbs hydrogen. On the other hand, when a sharp knickpoint is observed in the curve of cohesive energy - hydrogen concentration, the desorption reaction is hard to occur. INTRODUCTION The hydrogen energy is recently considered as major one among various types of clean energy for the future. The hydrogen storage materials are received much attention due to their importance in applications such as hydrogen and thermal reservoirs, heat pumps, compressors, batteries, hydrogen purification and recovery devices, catalysts in hydrogenation and dehydrogenation of organic compounds, etc.[1-5] And for the hydrogen absorption-desorption mechanism of hydrogen storage materials, hydrogen atoms in the materials are usually supposed to occupy the interstitial or substitutional sites in the solid solution of alloys (alloy model) or to exist as bonded atoms in the hydrides (compound model)[6,7]. Neither of these models can comprehensively explain the capacity, rate and reversibility of hydrogen absorbing-desorbing reaction which are indispensable factors to elucidate the mechanism. In this work, we will propose a new idea for the elucidation of the hydrogen absorption-desorption mechanism from a new perspective of using the results calculated by the extended Hückel method. With this method, we can gain the three calculated results, the number of states unoccupied by electrons, the energy fluctuation and the cohesive energy[8,9]. EXPERIMENTALS The commercial rare earth metals of 99.97 % purity and platinoid metals of 99.99 % purity were used in this work. The intermetallic compounds of R-M systems were made by arc melting; Y3Co, YCo, Y2Co3, YCo2, YCo3, YCo5, Y3Ni, YNi, YNi2, YNi3, YNi5, La3Co, La2Co3, La2Co7, LaCo5, LaCo13, LaRh, LaRh3, La7Ir3, LaIr2, L
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