Corey, Robert L. and Ivancic, Timothy M. and Shane, David T. and Carl, Erik A. and Bowman, Robert C., Jr. and von Colbe, Jose M. Bellosta and Dornheim, Martin and Bormann, Rüdiger and Huot, Jaques and Zidan, Ragaiy and Stowe, Ashley C. and Conradi, Mark S. (2008) Hydrogen Motion in Magnesium Hydride by NMR. Journal of Physical Chemistry C, 112 (49). pp. 19784-19790. ISSN 1932-7447 http://resolver.caltech.edu/CaltechAUTHORS:CORjpcc08b
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In coarse-grained MgH2, the diffusive motion of hydrogen remains too slow (<10^5 hops s^−1) to narrow the H NMR line up to 400 °C. Slow-motion dipolar relaxation time T1D measurements reveal the motion, with hopping rate ωH from 0.1 to 430 s^−1 over the range of 260 to 400 °C, the first direct measurement of H hopping in MgH2. The ωH data are described by an activation energy of 1.72 eV (166 kJ/mol) and attempt frequency of 2.5 × 10^15 s^−1. In ball-milled MgH2 with 0.5 mol % added Nb2O5 catalyst, line-narrowing is evident already at 50 °C. The line shape shows distinct broad and narrow components corresponding to immobile and mobile H, respectively. The fraction of mobile H grows continuously with temperature, reaching ∼30% at 400 °C. This demonstrates that this material’s superior reaction kinetics are due to an increased rate of H motion, in addition to the shorter diffusion paths from ball-milling. In ball-milled MgH2 without additives, the line-narrowed component is weaker and is due, at least in part, to trapped H2 gas. The spin−lattice relaxation rates T1^−1 of all materials are compared, with ball-milling markedly increasing T1^−1. The weak temperature dependence of T1^−1 suggests a mechanism with paramagnetic relaxation centers arising from the mechanical milling.
|Additional Information:||Copyright © 2008 American Chemical Society. Received: September 5, 2008; Revised Manuscript Received: October 27, 2008. Publication Date (Web): November 13, 2008. The authors gratefully acknowledge funding from the Office of Basic Science (OBS), U.S. Department of Energy, under Grant DE-FG02-05ER46256. J.H. thanks J. Lang for assistance with sample preparation and NSERC of Canada for funding. R.Z. and A.C.S. appreciate support from DOE-OBS.|
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