Thermodynamic characterization of hydrogen interaction with iridium polyhydride complexes
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Thermodynamic characterization of hydrogen interaction with iridium polyhydride complexes Ragaiy A. Zidan and Richard E. Rocheleau Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii 96822 (Received 5 March 1998; accepted 28 May 1998)
Hydrogen interaction with solid iridium complexes IrXH2 (PPr i 3 )2 (X Cl, I) was investigated. Gaseous hydrogen was found to react reversibly with solid chloro-iridium complex IrClH2 (PPr i 3 )2 , forming IrClH2 (PPr i 3 )2 H2 . The relative partial molal enthalpy and entropy were obtained from equilibrium isotherms at different hydrogen concentrations. The decrease in entropy with increasing hydrogen concentration and the absence of plateaus in the equilibrium isotherms were consistent with a single phase solid solution with two chemical components. Hydrogen release from solid iodo-iridium complex IrIH2 (PPr i 3 )2 H2 was not observed at temperatures up to 350 K, indicating stronger hydrogen bonding.
I. INTRODUCTION
Elemental and intermetallic metal hydrides have been explored for many years as hydrogen storage materials. Although many intermetallic compounds show potential as hydrogen storage media, (e.g., Mg2 Ni, FeTi, LaNi5 , etc.),1–4 unfavorable kinetics and/or unfavorable thermodynamics often limit the release of hydrogen from those materials exhibiting the highest capacities, requiring high dissociation temperatures that limit their practical application. In addition, many of the intermetallic alloys suffer from disproportionation when subjected to repeated cycling which reduces their capacity for hydrogen storage. A new class of metal-hydrogen compounds, dihydrogen complexes, in which H-H bonding is preserved was first identified by Kubas et al. in 1984.5 This discovery has been followed by extensive work to synthesize and characterize similar compounds. The interaction between hydrogen and many of these polyhydride complexes, including iridium, has been described as a weak covalent chemical bond. Electron donation from the (s) orbital of H2 to the metal (d) orbital weakens but does not break the H-H bond of H2 , forming a dihydrogen complex. Sufficient back donation, however, from the (d) orbital into the antibonding (s*) of H2 results in breaking the HH bond, forming a dihydride complex.6–9 The relatively weak metal-dihydrogen bond allows dehydrogenation to occur under mild conditions. Studies in solution have shown that the strength of the metal-dihydrogen bond can be modulated significantly with only a slight change in the ligand environment.9 Because of these properties, some complexes such as the iridium dihydrogen complexes IrXH2 (PPr i 3 )2 (H2 ) (X Cl, I), which are the subject of this paper, have been proposed as an alternative to conventional metal hydrides for hydrogen storage. To our 286
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 1, Jan 1999
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knowledge, however, gas-solid equilibrium isotherms have been reported only for the dihydrogen comp
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