Hydrogen In Palladium Research Articles

Molecular dynamics of a dilute solution of hydrogen in palladium.

Molecular-dynamics results on a dilute solution of H in Pd are presented and compared with available incoherent inelastic neutron-scattering results. The embedded-atom model adopted here does a good job of describing the H-Pd atomic forces probed by incoherent inelastic neutron scattering. The time correlation functions associated with the computed spectra are strongly damped and indicative of the anharmonicity that has been suggested as the principal contribution to the anomalous isotope dependence of the superconducting transition temperature in PdH. These results highlight the fact that the H-atom vibrations in Pd-H solutions are low-frequency, large-amplitude vibrations relative to vibrations of H atoms in usual covalent interactions. The rms displacement of the H atom from its mean position in the center of the Pd octahedron compares favorably with the available neutron-diffraction results.

Investigation of hydride formation and decomposition in palladium using the electrochemical permeation technique

In addition to providing hydrogen transport data, the electrochemical permeation technique can be used to determine the terminal solid solubility of hydrogen in palladium, and to assess in situ the dislocation density resulting from stresses associated with the formation and decomposition of palladium hydride. It is suggested that this method may prove useful in the evaluation of solid-state hydrogen storage materials.

The two-state solubility isotherm of hydrogen in Pd xZ y-alloys (Z = P, Si)

The effect of microstructure on the behaviour of hydrogen in polycrystalline palladium can be satisfactorily interpreted within the framework of Oriani's two-state model. Indeed, in dilute ..cap alpha..-phase, both the reverse deviation of the solubility isotherm from Sieverts'law and the variation of the diffusion coefficient as a function of hydrogen concentration have been predicted and good agreement with experiment has been found. These results seem to be somewhat unexpected in view of the present theory on hydrogen behavior in polycrystalline and amorphous metals. The results recently reported on the hydrogen diffusion measurements in some amorphous (Pd/sub x/,Si/sub y/)-alloys also seem to be quite unexpected. Quasi-elastic neutron scattering studies have shown that the diffusion process is characterized by two well-separated regimes of jump rates, in disagreement with the present ideas on diffusion in amorphous metals. This behaviour has been interpreted in terms of a two-state diffusion-trapping model and some possible analogies with the crystalline Pd/sub 3/P/sub 0.80/ alloy have been suggested. Owing to the rather complicated structures of these alloys and the inherent presence of impurity atoms, even chemically identical interstices may be of different sizes or be differently distorted. Therefore they could contain a whole spectrum of interstitial sites whichmore » are energetically different for hydrogen. But to a first approximation and within a narrow pressure range, these energetically different interstices could be considered as a system with mainly two types of holes and the hydrogen solubility in these alloys interpreted in terms of the two-state model. In this paper an attempt has been made to discuss the hydrogen solubility data in crystalline Pd/sub x/P, alloys, already published in the literature within the framework of Oriani's two-state model.« less

Hydrogen and deuterium depth profiling by elastic recoil detection analysis

The determination of hydrogen and deuterium depth profiles was investigated using alpha beams, with energies between 2.5 and 3.0 MeV and with the detector placed at a forward angle of 30°. Hydrogen loss from polystyrene (C 8H 8), used as a standard, was found to occur but could be corrected for. The depth profiling sensitivity was about 5 × 10 19 atoms/cm 3 (0.1 at.%). The technique was used to determine hydrogen in palladium, silicon nitride, amorphous silicon and obsidian, while both hydrogen and deuterium were determined simultaneously in polymers. Considerable improvement of the depth resolution was obtained by using 16O ion beams.

Neutron spectroscopic evidence for subsurface hydrogen in palladium.

The interaction of hydrogen with palladium black, a high-surface-area metal powder, has been investigated by incoherent inelastic neutron scattering. The vibrational spectrum associated with chemisorbed surface hydrogen occupying threefold sites has been identified. A vibrational mode at 58 meV, which is found to be inconsistent with either surface or bulk palladium-hydrogen species, is assigned to chemisorbed hydrogen occupying subsurface sites.

Effect of trapping on the solubility and diffusivity of hydrogen in palladium (?-phase)

The solubility and diffusivity data of hydrogen in palladium with different concentrations of defects are discussed within the framework of the two-level model with local equilibrium (Oriani's model). Analytical expression of the solubility isotherm and of the variation of the apparent diffusion coefficient depending on the total bulk concentration are derived. A good agreement is found between the experimental results on the solubility and diffusivity of hydrogen and the calculated values using the theoretical expressions.

Absorption of hydrogen in ultrathin palladium and palladium-based alloys

We observe that the critical temperature T c in PdH x thin films decreases below room temperature for a thickness d < 600 A ̊ . Absorption-desorption cycling restores, in part, the bulk properties. These effects, also observed in a Pd-Ag alloy, are attributed to a decrease in the elastic H-H interactions by coherency stresses mainly related to the clamping of the films on the substrate. Effects possibly related to modification of these stresses are also observed in ferromagnetic Pd-Ni films.

A simulation model for hydrogen in palladium. II. Mobility and thermotransport

For pt.I see ibid., vol.19, p.6169 (1986). The author uses molecular dynamics simulation to study the transport properties of the model for hydrogen in palladium established earlier. The main purpose is to determine the heat of transport Q*' characterising diffusion driven by a temperature gradient, although the calculations also yield results for the hydrogen mobility B and the thermal conductivity of the model. He discusses the various methods available for computing the transport coefficients and shows that the Green-Kubo relations and one of the forms of non equilibrium molecular dynamics provide practicable routes to the calculation of Q*' and B for the present system. The two techniques give identical numerical results within the statistical errors. Calculations for different sizes of simulated system show that the influence of size is small. He finds that Q*' is positive, but much smaller than the migration energy, in qualitative agreement with experiment.

The thermodynamics of dilute solutions of hydrogen in palladium and its substitutional alloys

A review has been given of the thermodynamic and kinetic behavior of hydrogen in dilute solid solution in palladium and in binary solvents consisting of palladium containing a substitutional solute element. The properties of the binary Pd-H system can be understood in terms of simple mixing statistics and classical rate theory. The situation with respect to the ternary systems is not so unequivocable. A large volume of thermodynamic and kinetic data has been analyzed in terms of the cell model for such ternary solid solutions. In some cases the statistical theory is in impressive agreement with both sets of experimental data. In other cases, although the diffusion and thermodynamic behavior of the H-atoms can be described by modified forms of the cell theory, the modifications required are somewhat arbitrary in nature. Modern developments in fundamental calculations of the interactions between H-atoms and both ideal crystals and lattices containing defects have been briefly reviewed. Impressive agreement between the calculated H-atom binding energies to lattice defects and experimental values have been achieved.

Pressure modulated absorption-desorption: Permeation, diffusion and surface rate coefficients for hydrogen in nickel and palladium

Periodic flows result when a hot solid is held in a diffusant gas whose pressure is subject to modulation. If the solid is placed in a sealed chamber whose volume is caused to vary periodically the consequential pressure modulation will be attenuated and advanced relative to that expected with an inert solid. The frequency variations of the attenuation and phase which result from harmonic modulation prove to be characteristic of the processes which control flow into the solid. In consequence diffusion limited, surface modified, and trap modified flows are mathematically distinguishable. Experiments with nickel under diffusion limited conditions and with palladium under surface limited conditions show the predicted variation with frequency and provide coefficients in line with those reported elsewhere.

A simulation model for hydrogen in palladium. I. Single-particle dynamics

The author constructs and examines the properties of a simulation model for Pd:H. The metal-metal potential is based on the polynomial model of van Heugten (1979), and the metal-hydrogen potential is represented by a parameterised exponential form. The molecular dynamics calculations show that the temperature-dependent tracer diffusion coefficient of hydrogen is in semi-quantitative agreement with experiment, and that the residence site of the hydrogens is the octahedral position, as required by diffraction measurements. Diffusion occurs by jumps between these sites, but the jumps are not uncorrelated, so that the Chudley-Elliott model does not provide a satisfactory description. The relation between the model and the real system is discussed with particular reference to the correlation of jumps, and a physical mechanism which might reduce this correlation is suggested.

Kinetics of hydrogen absorption by Pd(110).

A coupled high-pressure--ultrahigh-vacuum technique is employed to make the first kinetic measurements of high-pressure hydrogen absorption through an atomically clean palladium surface, Pd(110). The hydrogen uptake kinetics in the \ensuremath{\alpha}-phase region are found to be rate limited by diffusion of atomic H into the bulk rather than by dissociative chemisorption of ${\mathrm{H}}_{2}$ at the surface as previously reported. Our findings strongly suggest that previous high-pressure measurements of hydrogen absorption by palladium may be suspect due to uncharacterized surface contamination. The bulk diffusivity of hydrogen in palladium determined in this study has an activation barrier of 5.39\ifmmode\pm\else\textpm\fi{}0.30 kcal/mol with a preexponential factor of (2.83\ifmmode\pm\else\textpm\fi{}0.05)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{cm}}^{2}$/s.

Influence of the subsurface layer on the measurements of the diffusion coefficient of hydrogen in polycrystalline palladium

Effects of the roughness factor ƒ w of the permeation surface and of the thickness l on the galvanostatic desorption of hydrogen from a finite polycrystalline palladium electrode in 0.1 N H 2SO 4 are reported. The I aπ functions (where I a is the anodic current and π is the transition time) were measured under different conditions and further used to establish the influence of the subsurface layer on the measurements of the diffusion coefficient of hydrogen in polycrystalline palladium electrodes. The relative weight of the equilibrium processes involving the subsurface hydrogen in the overall desorption process depends on the ratio of the permeation surface area to the geometrical volume of the finite palladium electrode. The diffusion coefficient of hydrogen in palladium estimated by the galvanostatic desorption method was D = (3.98±0.09) × 10 -7 cm 2 s -1 at25 °C for a thickness l much greater than 10 -2 cm.

The influence of CO on hydrogen sorption by Pd(111) single crystals

A novel method of calibration based on the solubility data of hydrogen in palladium is applied to both the analysis of thermal desorption spectra of hydrogen from Pd(111) and to studies on the influence of adsorbed carbon monoxide on the behaviour of adsorbed and absorbed hydrogen, at room temperature. For clean Pd(111) ( θ CO < 0.04 ML) the comparison of experimental and theoretical spectra shows that part of the hydrogen originally adsorbed on the surface dissolves. This hydrogen appears at higher temperatures as a diffusion tail. A strong influence of coadsorbed CO on the maximum hydrogen coverage was observed in the low-coverage region where usually the existence of isolated CO molecules on the surface can be expected. For hydrogen sorption comparison of experimental data with those calculated on the basis of the solution of Fick's second law for a plane sheet indicates that this process is solely diffusion controlled. On the other hand, a strong barrier for desorption of dissolved hydrogen is observed at θ CO = 0.33 ML. It is concluded that the influence of CO on the behaviour of dissolved hydrogen may be consistently explained if there is a CO-induced change in the polarization state of adsorbed hydrogen atoms.

Transport coefficients and energetics of hydrogen in palladium

A bielectrode cell was employed to determine the mass transport coefficients of hydrogen in annealed and deformed palladium membranes. In the annealed samples the diffusivity was evaluated as 3.41×10 -11 m 2 s -1 at 23°C with a concomitant activation energy Q D as 23.0 kJ mol -1. The variation in diffusivity and chemical potential with concentration is shown for the deformed membranes.

Diffusion and equilibrium properties of hydrogen in palladium measured by the stripping potentiostatic method

Theory and experiments on the (non-equilibrium) stripping potentiostatic method for evaluation of the diffusion coefficient and desorption isotherms of hydrogen in α-Pd are discussed. Diffusion coefficient of hydrogen D can be evaluated either under transient conditions, from the anodic stripping current ( I a ) or under steady-state condition, from the anodic charge corresponding to the stripping of hydrogen held initially in Pd membrane ( Q a ). Following data are reported: D = (3.60±0.06) × 10 −7 cm 2 s −1 (transient) and D = 3.41 × 10 −7 cm 2 s − (steady-state) at 20°C in 0.1 N H 2SO 4. Equilibrium concentration and equivalent pressure of hydrogen at the entrance side of Pd membrane can be calculated from the steady-state permeation current ( I ∞ a or the anodic stripping charge ( Q a ) and its open-circuit potential just after stopping hydrogen generation, E 0 c . Reliable Sieverts' constants ( K s ) have been calculated within 15 and 60°C, and the following thermodynamic values obtained: Δ H o = −(2.6±0.1) kcal (mol H) −1 and Δ S o = −(14.5±0.4) cal K −1 (mol H) −1. These data agree well with those obtained in gas phase measurements.

Nitrogen spill-over effect in the palladium-tungsten system

The system N 2-Pd-W has been used as a model of a bimetallic catalyst for ammonia synthesis. The role of the individual components can be fairly well seen in this system: tungsten is responsible for trapping and dissociation of nitrogen molecules, and palladium for hydrogenation of the chemisorbed species. The transfer of reactant particles from trapping centers to reaction sites - i.e. in our case the spill-over of nitrogen from tungsten to the palladium surface - has been studied by means of a field emission microscope (FEM). A field ion microscope (FIM) has been used for the sample characterization.

The Resistivity of Dissolved Hydrogen in Palladium and Amorphous Pd80Si20

The Resistivity of Dissolved Hydrogen in Palladium and Amorphous Pd80Si20

The use of palladium to obtain reproducible boundary conditions for permeability measurements using galvanostatic charging

The diffusion current of hydrogen through palladium in an electrochemical cell initially rises linearly with the charging current, reaches a steady “plateau” value, and then rises again. The diffusivity of hydrogen in palladium was measured using standard transient techniques in the initial region of low current density. Combining this value with the measured value of diffusion current at the plateau level gave a concentration of hydrogen at the entrance surface of the palladium that was the same for three different palladium thicknesses, and was equal to the saturation value in α palladium. It is proposed that this can be used as a known and reproducible effective hydrogen pressure (0.019 atm) if palladium is plated onto other metals before measuring their permeability in an electrochemical cell. Experimental evidence for this was obtained from permeability measurements made on several thicknesses of iron. Permeation studies were also made on AISI 410 stainless steel and tin plated mild steel. The measured value for electrolytic tinplate was 107 times that expected from extrapolation of high temperature data. This could be attributed to grain boundaries or porosity covering 0.003 pct of the area. The permeability values of iron and stainless steel are 8.4 x 1012 and 2.8 x 1013 H atom/cm • s • √atm, respectively.

Multidilatometric methods for measurements of the diffusion and electromigration of hydrogen in metals

A new technique for studying the diffusion and electromigration of hydrogen in metals is presented. In this technique the evolution of the hydrogen concentration profile during the diffusion process is detected by local dilatation measurements (at several positions along the diffusion direction) on the sample by means of a capacitance technique. This multidilatometric method is applied to the study of the diffusion of hydrogen in palladium and the electromigration of hydrogen in vanadium, niobium and tantalum. Representative experimental results are in good agreement with those obtained by using other methods.