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Published December 11, 2008 | public
Journal Article

Hydrogen Motion in Magnesium Hydride by NMR

Abstract

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

© 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.

Additional details

Created:
August 20, 2023
Modified:
October 17, 2023