Hydrogen Motion in Magnesium Hydride by NMR

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 degrees C. Slow-motion dipolar relaxation time T-ID measurements reveal the motion, with hopping rate omega(H) from 0.1 to 430 s(-1) over the range of 260 to 400 degrees C, the first direct measurement of H hopping in MgH2. The omega(H) data are described by an activation energy of 1.72 eV (166 kJ/mol) and attempt frequency of 2.5 x 10(15) s(-1). In ball-milled MgH2 with 0.5 mol % added Nb2O5 catalyst, line-narrowing is evident already at 50 degrees 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 similar to 30% at 400 degrees 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 H-2 gas. The spin-lattice relaxation rates T-1(-1) of all materials are compared, with ball-milling markedly increasing T-1(-1). The weak temperature dependence of T-1(-1) suggests a mechanism with paramagnetic relaxation centers arising from the mechanical milling.