Fast hydrogen ion diffusion in solid hydrogen uranyl phosphate (HUP)

NMR relaxation time measurements and pulsed field gradient diffusion measurements have been made for ¹H in hydrogen uranyl phosphate (HUP). The results confirm that for T > 274 K HUP is a fast proton conductor exhibiting self-diffusion coefficients greater than 10^-11m²s^-1.     

Time-resolved luminescence studies in hydrogen uranyl phosphate intercalated with amines

Time-resolved luminescence decays of intercalated compounds of hydrogen uranyl phosphate (HUP) with p-toluidinium (HUPPT), benzylaminium (HUPBZ), α-methylbenzylaminium (HUPMBZ) and hydroxylaminium (HUPHAM) were studied. The prepared compounds belong to the tetragonal P4/ncc space group and showed 00 l reflections shifted to lower angles relative to HUP, indicating that the intercalation increases the c parameter of the unit cell. The luminescence decays of the compounds with 100% of intercalation ratio (HUPHAM and HUPBZ) were analyzed by Global Analysis, assuming Lianos’ stretched exponential as the model function, which can be applied to compounds with restricted geometry and mobile donor and quencher molecules. It was remarkable that the luminescence decays showed that the quenching of the emission of the uranyl ions by the intercalated protonated amines is not restricted by low dimensionality of the host uranyl phosphate, and that a diffusion mechanism occurs. Benzylaminium cation efficiently quenches the excited energy of the uranyl ions at close distance, but the long-range and long-lifetime quenching is hindered. A different situation is found in the case of the small hydroxylaminium cation, where the long distance diffusion of the species is fast, playing an important role in the quenching of the excited uranyl ions at longer times.     

The measurement of the conductivity of hydrogen uranyl phosphate tetrahydrate and the determination of the thermodynamics of the hydrogen-tungsten oxide system

The ac impedance method has been used to investigate the conductivity of pressed HUP pellets. The activation energy for the ionic conduction process at high frequencies (30 kJ mol^?1) was attributed to a simple proton hopping step. At lower frequencies (32 Hz) a Warburg diffusional impedance was observed with a much higher activation energy. This was attributed to the higher energy process of vacancy rotations needed for ion transfer over significant distances. Using a concentration cell, with HUP as the electrolyte, the thermodynamics of the hydrogen-tungsten oxide system was investigated.     

A novel hydrogen gas sensor based on hydrogen uranyl phosphate

The layered hydrate HUO₂PO₄·4H₂O is known to be a good selective proton conductor at room temperature. We have shown that the conductivity is dependent on the pressure of water vapour and good long term stability is obtained even at current densities as high as 5 mA/cm². The construction of porous composite electrodes of Pt and Pd is described and their use in hydrogen concentration cells is demonstrated. Furthermore, it is shown that these concentration cells give the correct voltage according to Nernst's law.     

NMR studies of electronic structure and hydrogen diffusion in transition metal hydrides

Nuclear magnetic resonance spectroscopy has had extensive applications for the characterization of numerous metal-hydrogen systems. Although the greatest emphasis of proton NMR has been to evaluate diffusion behavior, increasing attention has been addressed upon the correlation of proton Knight shifts and the conduction electron contributions to proton spin-lattice relaxation times to the electronic structure properties of the hydride. The general principles of NMR, that pertain to the usual situations for most transition metal hydrides, will be briefly reviewed. Several specific examples from some recent research will be discussed in greater detail. In particular, the roles of host crystal structure and hydrogen site occupancy to hydrogen diffusion behavior are examined for the Ti_1-y Cu _y H _x and Zr_1-y Pd _y H _x systems. The proton hyperfine parameters in TiH _x and ZrH _x , as well as several related ternary hydrides, are used to qualitatively assess the character of the Fermi level electronic states. The relationship between the tetragonal distortions of the Ti and Zr dihydrides and a solid-state Jahn-Teller mechanism will also be examined. Mound is operated by the Monsanto Research Corporation for the U.S. Department of Energy under Contract No. DE-AC04-76-DP00053.     

NMR studies of hydrogen diffusion in β-LaNi5−yAly hydrides

Proton relaxation times have been measured in β-phase LaNi_5?yAl_yH_x, where 0<y<1.2, to determine the role of Al on hydrogen diffusion. Al substitution significantly increases the observed activation energy with a corresponding 100-fold decrease in the room temperature (300K) diffusion constant between y=0 and y=1.2. Comparisons are also made with previous studies of β-LaNi₅H_x and possible diffusion mechanisms are suggested.     

NMR study of hydrogen diffusion in uranium hydride

The diffusion of hydrogen in uranium hydride is studied employing the NMR technique. From measurements of spin-spin relaxation time T₂, the activation energy for hydrogen diffusion in β-UH₃ is determined to be $\text{E}{}_{\mathrm{a}}\text{= (19.25 ± 0.4)}\text{kcal}\text{mole}$ and the preexponential factor to be A₀ ≈ 5 × 10¹⁴ Hz. It is shown that these results are in fair agreement with spin-lattice relaxation time T₁ data. Assuming that hydrogen diffusion proceeds via vacancies whose concentration is temperature dependent, it is concluded that E_a is the sum of the energies of vacancy formation and barrier height, and that A₀ contains an entropy change factor. Using vacancy concentration data calculated by Libowitz, we estimate the barrier height energy to be E_b ≈7 kcal/mole. Using a value for the frequency of hydrogen vibration v₀ determined from inelastic neutron scattering by Rush et al., we estimate the entropy change due to vacancy formation and the hydrogen atom jump to be about $\text{S}\text{k}{}_{\mathrm{B}}\text{≈3}$. Similar measurements on samples containing less hydrogen than is needed to compose stoichiometric UH₃, show that the rate of diffusion is enhanced by the presence of excess metal in the sample. The jump frequency at 500°K in UH₃ is found to be approximately 10⁶ Hz while for the two-phase samples of H/U = 2.8 and 2.5, it is larger by a factor of about 3 and 3.5, respectively.     

NMR studies of restricted diffusion in lyotropic systems

Tortuosity, 1/alpha, and surface-to-volume ratio, S/V, were determined in aqueous solutions of decylammonium, dodecylammonium and tetradecylammonium chlorides of various concentrations by measuring the apparent diffusion coefficient of water, D(app)(delta). This was found to be much smaller than in the bulk state. Such restricted diffusion is interpreted in terms of the Mitra model, where D(delta) depends on diffusion time and is controlled primarily by S/V. The samples exhibit lamellar (L), hexagonal (H) and isotropic (I) liquid crystalline phases. We observed changes in S/V upon phase transition. In the lamellar and hexagonal phases, the system is ordered, resulting in relatively small S/V ratios, compared to the micellar-isotropic phase. We did not observe a dependence on the diffusion time, delta, in the isotropic phase, because the duration of the experiment was not sufficiently short to observe the change from D(app)(delta) to D(eff). We observed the effective diffusion coefficient of water, which directly probes the tortuosity of the system. The S/V ratios were obtained by fitting the Mitra model, using known values of the bulk water diffusion coefficients, and the assumption that D(app) --> D0 for delta --> 0. S/V is correlated with the type of structure, increasing on transition to the isotropic phase and decreasing on transition to other phases. The change in tortuosity is small, but slightly larger for the isotropic phase.     

Optical probes of interlamellar redox chemistry: Intercalation of the Creutz-Taube complex into hydrogen uranyl phosphate

Hydrogen uranyl phosphate (HUP) can be intercalated by the Creutz-Taube (C-T) complex, {[Ru(NH₃)₅]₂(μ-pyrazine)}⁵⁺, to yield hydrated, partially-substituted lamellar solids, H(C-T)UP, of approximate composition H_1−5x(C-T)_xUO₂PO₄ (0.001<x<0.01). The more crystalline samples exhibited an X-ray powder pattern similar to that of HUP: the structure can be indexed in tetragonal symmetry and possesses an interlamellar spacing of ∼8.69 Å. The absorption spectrum of H(C-T) UP appears to be a simple superposition of bands due to the UO²⁺₂ moiety and the C-T complex. Photoluminescence (PL) of H(C-T) UP is quenched relative to HUP; PL decay curves are nonexponential and span a shorter time domain relative to HUP. Exposure of H(C-T) UP to Br₂ vapor causes the purple solid to become yellow. The spectral changes are consistent with redox chemistry wherein the intercalated C-T complex undergoes oxidation. For most samples examined, the PL decay times were enhanced upon oxidation.     

Energetics of atomic hydrogen diffusion on Si(100)

We present first principles calculations of the potential energy surface for the diffusion of a single hydrogen atom on Si(100)2 × 1. Surface relaxation is found to be very important for the energetics of diffusion. A strong anisotropy is predicted for hydrogen motion: H should diffuse mainly along dimer rows, where activation energies are ~ 1.3 eV, while the barrier for row-to-row hopping is ~ 0.5 eV higher. Our results indicate that diffusion can be considered a fast process compared to H₂ recombinative desorption.     

铀氢化反应过程中氢扩散行为的第一性原理研究

铀氢化反应动力学及产物形式受氢在铀氧化物及氢化物中的固溶、扩散等过程控制.本文利用第一性原理计算方法,研究氢在UO_2、α-UH_3、β-UH_3、PdH_x等介质中的固溶、扩散行为,从微观角度解释氢化反应各个阶段氢化反应速率的主要特征,并对α/β相生成的条件作一番探讨.研究表明,UO_2中较高的氢扩散势垒导致氢扩散到达氧化物-金属界面非常困难,这是氢化反应起始阶段存在孕育期的主要原因;与此相反,β-UH_3的氢具有非常低的溶解能和扩散势垒,且实际状态下β-UH_3的表面积非常大,对氢化反应具有显著的促进作用;PdH_x具有和α-UH_3相近且均略高于β-UH_3的溶解能和扩散势垒,这使得氢在其中的扩散速率较低,是Pd膜覆盖下铀氢化主要产物为α-UH_3的主要原因.     

Combining macroscopic and microscopic diffusion studies in zeolites using NMR techniques

In this study the zero length column (ZLC) technique is extended to the case where the decay of the adsorbed phase concentration is observed directly by nuclear magnetic resonance (NMR). An adsorption-desorption apparatus compatible with a 400-MHz NMR spectrometer was developed. It operates with nitrogen or helium as the inert purge gas. The column of the adsorbent material is placed in the sensitive region of the superconducting magnet and the rf coil of the NMR spectrometer. The time scales of the adsorption and desorption processes depend on concentration, temperature and crystal shape and are found to be in the range of 1-10 min. From the desorption branch, the non-equilibrium ZLC-NMR measurements yield intracrystalline diffusion coefficients in the range of 10(-13) to 10(-11) m2/s for different alkanes in silicalite-1. These values are always found to be smaller than the values measured by pulsed field gradient NMR under equilibrium condition indicating that there must be additional transport resistance at the external surface of these silicalite-1 zeolite crystals.     

A pulsed NMR study of hydrogen diffusion in hydrides of group IVa transition metals

Activation energies E_A for hydrogen diffusion in hydrides of Group IVa transition metals have been determined by ¹H nuclear magnetic resonance measurements of spin lattice relaxation in both the laboratory (T₁) and rotating (T_1p) frames. For the HfHx system both activation energies obtained from T₁ data, and the value of T₁ at the minimum appear to be insensitive to hydrogen content x in the range 1.58 ? x ? 1.98. For hydrides of titanium and zirconium of approximately stoichiometric composition MH₂ (where M = metal), there is excellent agreement between activation energies obtained from T₁ and T_1p data. Mean activation energies obtained were 0.51 eV for TiH_1.98 and 0.83 eV for ZrH_1.96, consistent with a single diffusion mechanism in each case over the temperature range 260–600K and 400–800K respectively. In the case of HfH_1.98 the agreement was less good, values of 0.64 and 0.55 eV being obtained from T₁ and T_1p respectively.     

Stable isotope-enhanced two- and three-dimensional diffusion ordered C NMR spectroscopy (SIE-DOSY C NMR)

The feasibility of obtaining high quality homonuclear or heteronuclear diffusion-ordered ¹³C NMR data is shown to be greatly improved by using ¹³C isotopically-enriched samples. Stable isotope-enhanced diffusion ordered (SIE-DOSY) ¹³C NMR has been applied to ¹³C-enriched carbohydrates, and has been used to determine diffusion coefficients for pentose and hexose monosaccharides, and a disaccharide and trisaccharide. These 2D spectra were obtained with as little as 8 min of acquisition time. Fully resolved 3D DOSY-HMQC NMR spectra of [U-¹³C]xylose, [U-¹³C]glucose, and [1-¹³C^gal]lactose were obtained in 5 h. Sample derivatization with [carbonyl-¹³C]acetate (peracetylation) extends the usefulness of the technique to included non-labeled sugars; the ¹³C-carbonyl – carbohydrate ring proton ¹H–¹³C correlations also provide additional structural information, as shown for the 3-D DOSY-HMQC analysis of a mixture of maltotriose and lactose per-[carbonyl-¹³C]acetates.     

NMR studies on hydrogen bonding and proton transfer in Mannich bases

A review of studies on the ortho Mannich bases containing various substituents in the phenyl ring on the basis of¹H,¹³C and¹⁵N nuclear magnetic resonance (NMR) spectra in various solvents over the temperature range 110–298 K is presented. Some new results are also included. The data gathered so far show that there is some critical (inversion) range of ΔpK _a (= pK _a(NH⁺) − pK _a(OH)) in which the proton transfer equilibrium appears. This inversion range is well reflected in the behaviour of secondary deuterium isotope effect in¹³C NMR spectra. A strong temperature effect on the strength of hydrogen bonding should be emphasized. The¹H chemical shift for trichloroderivative increases from 13.5 at room temperature up to 17 ppm at 130 K when the proton is equally shared between the bridging atoms (¹ J(¹H,¹⁵N) = 30–40 Hz). The potential for the proton motion in such bridges is discussed taking into account the behaviour in the ultraviolet and infrared spectra. The role of dimerization in proton transfer equilibria is shown. In addition the rotation of OH groups involved in hydrogen bond formation and nitrogen pyramidal inversion was studied by the¹H dynamical NMR spectra.     

PAC studies of hydrogen diffusion in amorphous hydrides of binary metallic alloys

The diffusive motions of hydrogen in amorphous Zr₂Ni hydrides were observed by¹⁸¹Ta PAC relaxation measurements. The comparison with the experimental data obtained for crystalline Zr₂Ni hydride lead to the conclusion that hydrogen loading induce a short-range ordering into the amorphous structure. The absence of a large distribution of activation energies may be a consequence of such ordering and/or of collective effects in the hydrogen diffusion processes Affiliated to the Centre National de la Recherche Scientifique.     

NMR studies of water diffusion and relaxation in hydrating slag-based construction materials.

The NMR relaxation properties of hydrating blast-furnace slag cements have recently been shown to be dominated by the effect of water self-diffusion in internal magnetic field gradients in the pastes. While this was suggested on the basis of NMR relaxometry and magnetic susceptibility data, we report here the results from first direct studies of the water self-diffusion in the hydrating paste using a specialized PFG sequence and very intensive magnetic field gradient pulses.     

Solid-state NMR studies of some tin(II) compounds

High-resolution NMR spectra of [Formula: see text] nuclei, particularly (119)Sn and (31)P, in solid tin(II) phosphite, SnHPO(3), and tin(II) phosphate, SnHPO(4), are presented. The results are discussed in relation to the crystal structures. Spinning sideband analysis has been carried out for both nuclei, giving information on the shielding tensors. Satellite peaks allow the indirect Sn,Sn coupling constants to be determined. Surprisingly large values of 2600+/-200Hz and 4150+/-200Hz are reported for SnHPO(3) and SnHPO(4) respectively. The satellite peaks were investigated by using a single Hahn echo for each refocusing time, which showed that the observed splittings result from (119)Sn, (117)Sn coupling. For SnHPO(3), the calculated relative intensities of the satellites for six intra-layer coupling interactions are in agreement with the experiment values, but for SnHPO(4) the coupling appears to be inter-layer in nature. Tin-119 (and in one case phosphorus-31) shielding tensor data derived from MAS NMR are also reported for four other crystalline tin(II) compounds, namely tin diphosphate, tin oxalate, tin sulphate and calcium tin ethylenediamine tetraacetate.