%0 journal article
%@ 1932-7447
%A Corey, R.L.,Ivancic, T.M.,Shane, D.T.,Carl, E.A.,Bowman, R.C.,Bellosta von Colbe, J.M.,Dornheim, M.,Bormann, R.,Huot, J.,Zidan, R.,Stowe, A.C.,Conradi, M.S.

%D 2008
%J The Journal of Physical Chemistry  C
%N 49
%P 19784-19790 
%R doi:10.1021/jp807900r
%T Hydrogen Motion in Magnesium Hydride by NMR
%U https://doi.org/10.1021/jp807900r
49 
%X In coarse-grained MgH2, the diffusive motion of hydrogen remains too slow (<105 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−1over 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 × 1015 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.