Nuclear magnetic resonance in metal hydrogen systems

Barnes, R. G.

doi:10.1007/bfb0103402

Cited by 33 publications

(18 citation statements)

References 229 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The activation energy for H diffusion in MgH 2 found here may be compared to other systems. In metallic hydrides (interstitial hydrides), activation energies − are typically between 0.2 and 1.0 eV. Of course, such low energies and correspondingly high diffusivities are advantageous features for hydrogen storage in metallic systems.…”

Section: Resultsmentioning

confidence: 99%

Hydrogen Motion in Magnesium Hydride by NMR

et al. 2008

View full text Add to dashboard Cite

In coarse-grained MgH 2 , 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 T 1D measurements reveal the motion, with hopping rate ω H from 0.1 to 430 s -1 over the range of 260 to 400 °C, the first direct measurement of H hopping in MgH 2 . The ω H 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 MgH 2 with 0.5 mol % added Nb 2 O 5 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 MgH 2 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.

show abstract

Section: Resultsmentioning

confidence: 99%

Hydrogen Motion in Magnesium Hydride by NMR

et al. 2008

View full text Add to dashboard Cite

show abstract

“…In bulk PdH x with x = 0.70, corresponding to a H 2 pressure 1,19 of about 1 atm at 300 K, T 1 data at several frequencies have been reported previously as a function of temperature. [6][7][8][9] As T 1 is known only to increase with increasing frequency, the 47 MHz data 7 can be used to estimate a T 1 of approximately 100 ms at our 202.1 MHz. Hence, the observation of hydride-resonance T 1 values in our widely dispersed palladium black (on glass wool) samples as short as 12 ms is strong evidence for the exchange-driven relaxation process.…”

Section: ' Results and Discussionmentioning

confidence: 99%

“…We suppose the hydrogen nuclear spins are inverted with an rf pulse at time t = 0 in both phases. Now, the intrinsic relaxation time T 1 of the hydride − is typically much longer than that of the gas, so we may approximate the gas-phase T 1 as zero. Thus, the gas spins instantly return to equilibrium and remain so throughout the process.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen NMR of Palladium Hydride: Measuring the Hydride−Gas Exchange Rate

et al. 2011

View full text Add to dashboard Cite

We consider small PdH x particles in equilibrium with the surrounding H2 gas. Because the hydrogen longitudinal nuclear spin relaxation rate R 1 = 1/T 1 of the gas is so much larger than that of the bulk hydride, the apparent hydride relaxation rate will be increased by rapid exchange of H between the gas and the hydride phases. Under a wide range of conditions, the apparent hydride relaxation rate R 1 will equal the rate of hydride-to-gas exchange events. This method is demonstrated for well-dispersed PdH x using commercial Pd black where the hydride and gas signals are distinguished by the Knight frequency shift. The method is an equilibrium measurement of the hydride-to-gas exchange rate and should find application in high surface area metallic hydrides for characterization of the surface reactivity toward H2.

show abstract

“…Lots of works focus on the study on the hydrogen diffusion [5e10]. Since hydrogen atom has a much lower mass than other interstitials, quantum diffusion can be the dominant mechanism for hydrogen atom diffusion in metals [11], especially at low temperatures [12,13]. At the temperature of absolute zero, there is still the special vibration mode of the atoms, whose energy is named zero point energy (ZPE).…”

Section: Introductionmentioning

confidence: 99%