Metal-hydrogen Interactions Research Articles

Heterogeneous catalysis

This short communication endeavours to explain the activity of platinum toward molecular hydrogen, employing some new developments in research of metal-hydrogen surface interactions.

Hydrogen in Nb/Ta superlattices

The structural and thermodynamic properties of hydrogen dissolved in Nb/Ta superlattices are studied by in-situ x-ray scattering techniques. From the x-ray satellite intensities, it is found that H induces a strain modulation exhibiting a Curie-Weiss temperature dependence. The intensities are analyzed in terms of a mean field model of the modulated lattice gas, yielding quantitative information on the hydrogen-metal and hydrogen-hydrogen interaction energies. Critical behavior associated with a gas-liquid transition is also observed. Hydrogen density fluctuations with wavelengths shorter than a superlattice period are, however, suppressed by the superlattice, which represents a novel manifestation of a coherent phase transition. These experiments provide new and fundamental insight into the role of spacially varying two body interactions in critical phenomena.

Etude comparative des hexahydrido-et des hexadeuteridoaluminates de lithium et de sodium. I - Spectres raman et infrarouge de Li 3-et Na 3AlH 6, et Li 3AlD 6

Raman and Infrared spectra of Lithium and Sodium hexahydrio- and hexadeuteridoaluminates Li 3AlH 6, Na 3AlH 6 and Li 3AlD 6 have been investigated in order to study metal-hydrogen interactions. A vibrational analysis has been performed. Several force field models (Urey-Bradley, Orbital Valence force field, Shimanouchi, Murrell).have been used in order to compute the force constants of Al-H bonds. No significative difference has been observed between the results obtained from the various models, and the environnement of aluminum has been thus well determined : AlH 6 3− ions form an almost perfect octahedron in Li 3AlH 6 and a distorted one in Na 3AlH 6.

Analysis by the recursion method of the electronic transfers involved in the dihydrogen chemisorption on a platinum cluster

As a first example of a possible analysis of elementary steps of a heterogeneous catalytic reaction, the electronic transfers and the associated energy costs involved in the dihydrogen adsorption on various sites of a Pt 13 cluster have been analysed in terms of the changes occurring in the local densities of states computed by the recursion method. The Hamiltonian defining the various metal-metal, hydrogen-hydrogen, and metal-hydrogen interactions is of extendedHückel-type. This method gives local information on the behaviour of the surface orbitals along the reaction path. In particular, it shows the role of a late electron transfer from d-type to s or p-type metal orbitals, and it exhibits the role of electronic reservoir of the platinum atoms not directly bound to the adsorbate, in the electron transfer. Furthermore, it shows that the height of the activation barrier and the energy difference of the whole process are mainly related to the energy positions of the local HOMO and LUMO contributions of the metal atoms in direct interaction.

Influence of surface impurities on hydrogen permeation through palladium

In order to improve the understanding of the hydrogen metal interaction, we have studied the influence of the surface state of a thin film of palladium on the hydrogen permeation process. Energetic desorption values E d have been calculated by thermal programmed desorption experiments for various pretreatments of the sample. Auger spectral analysis shows that carbon, sulphur and oxygen are present at the surface of the metal. Results allow us to conclude that the increase in E d is certainly connected with an increase in concentration of sulphur at the surface.

Electronic structure and bonding in metal hydrides, studied with photoelectron spectroscopy

We present photoemission spectra of various binary and ternary metal hydrides and of some intermetallic compounds. An analysis of these data with respect to the known heats of hydrogen solution in the metals demonstrates two important properties of the metal-hydrogen bond: First we find that core level, shifts in ternary systems are not simply related to those in binary ones. In contrast to a frequently used assumption, metal-hydrogen interaction in a ternary hydride cannot be a pair interaction between the atomic constituents. Secondly, we find from our studies of the valence band spectra of some intermetallic compounds an inverse correlation between the heat of hydrogen solution and the density of states at the Fermi level.

Theory of hydrogen interaction with metals

The present understanding of hydrogen interacting with metals as provided by the effective medium theory and self-consistent model calculations is reviewed. An extremely simple picture of the hydrogen-metal interaction is developed capable of describing in a quantitative way a vast number of experimentally observed phenomena such as the molecular adsorption and dissociation process, the properties of chemisorbed hydrogen, surface and bulk diffusion, interstitial hydrogen and the trapping of hydrogen at defects.

Trends in hydrogen heats of solution and vacancy trapping energies in transition metals

Heats of solution and impurity-induced lattice relaxations are calculated for hydrogen in all the transition metals. The hydrogen-metal interaction is described using the effective-medium theory and the metal-lattice relaxations are included in a harmonic-continuum model. The authors find that the trends in the heats of solution can be related to the magnitude of the electron density of the interstitial metal. The trends in the hydrogen trapping energies for vacancies are estimated from the difference between the calculated heats of solution and chemisorption energies. A very simple conceptual picture of the hydrogen-metal interaction results, which can easily be extended to treat for instance hydrogen in metal alloys.

Theory of superconductivity based on direct electron-phonon coupling. II.

The origin of a superconducting interaction in metallic hydrogen computed in the preceding paper is shown to be an electronic screening of the zero-point oscillations of the nuclei as a result of electron-phonon interaction, where the superconducting wave function allows more effective screening than the normal-state wave function. Virtual phonons are involved in the screening process, and the phonons of interest are the bare longitudinal phonons with small wave vector. The screening results in a correlation energy between electrons and nuclei where the correlation is of long range. The screening length is shown to be the same as the coherence length defined by the uncertainty principle.

A new superconducting interaction in metallic hydrogen

A new theory of superconductivity is suggested from an examination of the energy levels of metallic hydrogen, where the levels are computed from a perturbation expansion in the configuration space of N electrons and N nuclei. The leading terms in the expansion indicate that states exist which lie lower than the normal state if the nuclei are sufficiently heavy. The interaction which leads to the superconducting state is quite different from that incorporated in current models, since it arises from a long-range, electronic screening of the zero-point oscillations of the nuclei. The energy is lowered because the superconducting state is more effective in screening long-wavelength virtual phonons.

NMR study of hydrogen adsorption on rhodium/titanium dioxide

The adsorption of hydrogen on reduced Cl3Rh/TiO2 catalysts originates a new line in the NMR spectrum, shifted toward high fields, that has been ascribed to hydrogen adsorbed on metal particles. The analysis of the intensity and position of this line vs. the H2 pressure indicates the presence of two types of adsorbed hydrogen on the metal and the modification of the metal-hydrogen interaction with the extent of the adsorption. The reduction of the sample at temperatures above 723 K produces a drop of the shifted line intensity, indicating the appearance of the strong metal-support interaction, but a subsequent reoxidation treatment permits to recover the intensity of this line and eliminates the SMSI effect. The adsorption of the most labile hydrogen on the metal is accompanied by a hydrogen transfer to the support, which is reflected in the NMR spectra of the oxidized sample as an intensity increase of the unshifted line with the H2 pressure. The incorporation of hydrogen to the support is favored when the temperature of the sample is increased above 400 K. 14 references, 8 figures, 1 table.

Thermodynamics of ternary PdPtH solid solutions

The solubility of hydrogen in PdPtH ternary solutions in equilibrium with hydrogen gas at atmospheric pressure was measured in the temperature range 625–1250 K and in PdPt binary solvents containing up to 61 at.% Pt. Concomitant elastic measurements provided data which enable the partial thermodynamic functions of the hydrogen atoms, deduced from the solubility measurements, to be converted to refer to a hypothetical PdPt lattice of constant specific volume. The resulting volume-corrected functions are discussed in terms of the cell model for ternary solutions and are shown to vary with temperature and platinum concentration in a manner in accord with this model. In contrast with Pd(Au, Ag, Cu)H solid solutions, the isobars for the PdPtH system always show a decreasing hydrogen solubility at any given temperature as the platinum content of the PdPt matrix increases, whereas isobars for the Pd(Au, Ag, Cu)H solutions show well-defined solubility maxima. This marked difference in thermodynamic behavior is almost entirely due to volume effects caused by changes in the host lattice and not to differences in the hydrogen-metal interactions in a rigid lattice system.

A model for atomic hydrogen-metal interactions — application to recycling, recombination and permeation

A model for the interaction of hydrogen with metals is discussed which includes the surface concentration and the surface potential and is based on the observation that molecular hydrogen is dissociated before it is chemisorbed at a metal surface. It is shown that an activation barrier for dissociation of molecular hydrogen at the metal surface leads to an enhanced solubility and increased permeation rate of hydrogen in a metal exposed to atomic hydrogen as compared to molecular hydrogen. The sticking probability of molecular hydrogen on clean transition metals has been measured in the past and shown to be of the order of unity. As no activation barrier is present no difference is expected between atomic and molecular hydrogen. Whenever large differences have been observed in the past they must be attributed to an impurity contamination of the metal surface. Substantial reductions of the sticking probability due to dissociation barriers have been found for metals contaminated with less than one monolayer of electronegative impurities. We compare our model with earlier descriptions of hydrogen metal interactions. A theoretical expression is derived for the “recombination rate constant”. Furthermore we discuss implications of our model on hydrogen permeation (“super permeation”) as well as hydrogen pumping and pumping panels in tokamaks.

Electronic structure of intermetallic hydrides: Mg2FeH6 and Ca2RuH6

Abstract Two ternary metal hydrides of the family M 2 M'H x , where M is a divalent simple or rare earth metal and M' is a transition metal, are studied using the augmented plane wave band structure method. The nature and the relative strength of the transition metal-hydrogen, divalent metalhydrogen and hydrogen-hydrogen interactions are inferred from our partial wave analysis of the wavefunctions at the metal and hydrogen sites. The compounds are found to be semiconductors, and the gap originates from a splitting of the transition metal M' d bands. The t 2g manifold, which does not interact with the H s states, forms narrow filled bands which are separated by a gap from the antibonding empty (M′ d(e g ))-(H s) states. The electronic origin of the maximum hydrogen capacity in this family of compounds is discussed.

The role of metal-hydrogen systems in fusion research

A summary is given of some of the areas of hydrogen-metal interactions which play a major role in fusion devices. Selected areas are described in some detail, especially those where experimental and theoretical work is required and which promise to be useful to fusion technology in the short term.

Time-of-flight analysis of direct recoils applied to the study of hydrogen-metal interactions

Time-of-flight analysis of direct atomic recoils (TOF-DR) produced by pulsed rare gas ion irradiation of a solid surface provides a means of identifying surface hydrogen (and its isotopes), as well as other surface atoms. This newly developed surface analysis technique is described and its suitability for the study of hydrogen-metal interactions is emphasized. The type of information obtainable by this method is classified into 1. (i) surface composition, 2. (ii) surface chemistry and 3. (iii) surface structure. Some recent examples of the application of TOF-DR measurements to the study of H-Mg and H-La systems are presented. Possible directions for future hydrogen-metal research are discussed.

First-Principles Investigation of Metal-Hydrogen Interactions in NbH

First-principles total-energy calculations have been performed for NbH in the $\ensuremath{\beta}$- and $\ensuremath{\gamma}$- phase structures. Lattice constants, bulk moduli, and heats of formation as well as the frequencies and second harmonics of the optical vibrations of hydrogen and deuterium in the lattice have been calculated and found to be in excellent agreement with experiment. Our calculated electronic densities of states resolve the discrepancy between previous band calculations and photoemission experiments.

Electronic structure and bonding in CaH 2: Experiment and theory

Synchrotron radiation photoemission studies of Ca and CaH 2 reveal changes in the electronic structure and bonding which directly reflect metal-hydrogen interactions. The results indicate similarities between the saline dihydrides and the transition metal dihydrides, particularly the energy location and width of the bonding band states. Combined with APW band calculations for CaH 2 in the CaF 2 structure, they provide new insight regarding bonding in hydrides.

Electronically induced trapping of hydrogen by impurities in niobium

The binding energies of hydrogen and its isotopes to substitutional impurities Ti, Cr, and V in niobium have been calculated. The hydrogen-metal interaction is based on the effective-medium theory. The wave mechanics of the hydrogenic interstitials are explicity dealt with, and the lattice distortion created by the hydrogen is incorporated through the method of lattice statics. The difference in the electronic structure between impurity and host atoms is shown to be largely responsible for the binding of hydrogen to the impurities. The results are in agreement with recent inelastic neutron scattering experiments.

Differential scanning calorimetry and Mossbauer spectroscopy of hydrogenated Pd-C, Ni-C and Fe-Ni-C alloys

Palladium hydride formed in the Pd-C alloy is less stable than in carbon-free palladium, while the hydrides in FCC Fe-Ni-C and Ni-C alloys are more stable than in carbon-free metals. To investigate the effect of carbon on the stability of hydrides, the outgasing process of the hydrogenated alloys is studied by differential scanning calorimetry and Mossbauer spectroscopy. The former measurement gives the values of the heat of reaction and the latter clarifies the carbon-metal, hydrogen-metal and carbon-hydrogen interactions in the alloys. The observed effect of carbon on hydride formation is interpreted by combining lattice distortion theory and electron theory.