Palladium interstitial hydride

The examples in Table III, show that the hydrogen atoms occupy tetrahedral holes at the beginning of the transition series. As we move along the transition series, we observe the interstitial hydride shift toward octahedral holes and the hydrides of the heavier elements become progressively unstable. Palladium is exceptional since it is the only heavy element of group VIII that gives a simple hydride. Hydride formation is accompanied in most cases by a change in metallic lattice type and in all cases by a considerable increase in metal-metal distances. 

Since platinum, rhodium, and ruthenium catalysts operate with similar activation energies, their differences in catalytic activity can be directly traced to differences in the A factor, which may be related to the % d-char-acter of the metal bond in the three metals above. Since the % d-character is 50, 50, and 44 for ruthenium, rhodium, and platinum, respectively (S), it is seen that this sequence is similar to that of the catalytic activity. During catalysis, the palladium surface becomes a chemical compound represented by various stages of interstitial hydride formation, whose d-charac-ter is essentially different from that of the metal. Therefore, the position of palladium in the % d-character sequence is not directly comparable to that of palladium in the catalytic activity sequence. 

Hydrogen forms an interstitial hydride with palladium, which behaves almost like a solution of hydrogen atoms in the metal. At elevated temperatures hydrogen atoms can pass through solid palladium other substances cannot. 

The third class of hydrides is the metallic, or interstitial, hydrides, which are formed when transition metal crystals are treated with hydrogen gas. The hydrogen molecules dissociate at the metal s surface, and the small hydrogen atoms migrate into the crystal structure to occupy holes, or interstices. These metal-hydrogen mixtures are more like solid solutions than true compounds. Palladium can absorb about 900 times its own volume of hydrogen gas. In... 

Assuming the composition of the hydride to be expressed by PdjH2 (which corresponds to PdH0. 7) and bearing in mind the interstitial positioning of the hydrogen in the palladium lattice, the authors postulate the existence of the following equilibrium at the surface of the j8-hydride phase... 

Nevertheless it does not change the principle of the mechanism proposed by Scholten and Konvalinka, i.e. the ability to act catalytically of only the superficial palladium centers released from the vicinity of the interstitial hydrogen. Bearing in mind the dynamic character of the equilibrium in a palladium-hydrogen system as a whole is to regard such centers as being mobile in the surface layer of the hydride. 

Interstitial Solid Solutions Interstitial solid solutions involve occupation of a site by introduced ions or atoms, which is normally empty in the crystal structure, and no ions or atoms are left out. Many metals form interstitial solid solutions in which small atoms (e.g., hydrogen, carbon, boron, nitrogen) enter empty interstitial sites within the host structure of the metal. Palladium metal is well known for its ability to absorb an enormous volume of hydrogen gas, and the product hydride is an interstitial sohd solution of formula PdH, 0 0.7, in which hydrogen atoms occupy... 

A striking property of many interstitial metal hydrides is the high rate of hydrogen diffusion through the solid at slightly elevated temperatures. This mobility is utilized in the ultra-purification of H2 by diffusion through a palladium-silver alloy tube. 

At that date, palladium hydride was regarded as a special case. Lacher s approach was subsequently developed by the author (1946) (I) and by Rees (1954) (34) into attempts to frame a general theory of the nature and existence of solid compounds. The one model starts with the idea of the crystal of a binary compound, of perfect stoichiometric composition, but with intrinsic lattice disorder —e.g., of Frenkel type. As the stoichiometry adjusts itself to higher or lower partial pressures of one or other component, by incorporating cation vacancies or interstitial cations, the relevant feature is the interaction of point defects located on adjacent sites. These interactions contribute to the partition function of the crystal and set a maximum attainable concentration of each type of defect. Conjugate with the maximum concentration of, for example, cation vacancies, Nh 9 and fixed by the intrinsic lattice disorder, is a minimum concentration of interstitials, N. The difference, Nh — Ni, measures the nonstoichiometry at the nonmetal-rich phase limit. The metal-rich limit is similarly determined by the maximum attainable concentration of interstitials. With the maximum concentrations of defects, so defined, may be compared the intrinsic disorder in the stoichiometric crystals, and from the several energies concerned there can be specified the conditions under which the stoichiometric crystal lies outside the stability limits. 

Since s does not remain constant with temperature, there are two unknown parameters in each equation. An IBM-7090 computer was used to obtain the best value of s and Evv (or Eu) at each temperature. Much better agreement was obtained with the assumption of hydrogen vacancies rather than metal interstitials for the case of uranium hydride. A comparison of the theoretically derived curves with the experimental points for uranium hydride is shown in Figure 3. For the case of palladium hydride, equally good agreement was obtained with both assumptions. However, on the basis of x-ray data, it was decided that the nonstoichiometry was due to hydrogen vacancies. The calculated... 

Palladium is a special case in the Group 10 metals in contrast to Ni and Pt it forms bulk hydrides under mild conditions. The schematic Pd-H phase diagram is shown in Figure 10.7. At low hydrogen concentrations the atoms occupy a small fraction of the interstitial octahedral positions in the palladium fee lattice (the diluted PdH or a-phase), up to about 0.015 H per Pd atom at room temperature. With increasing hydrogen gas pressure (and below the critical temperature T, which is 295 °C for... 

The dissolution of hydrogen in palladium to form a- and (i-phase palladium hydrides has been assumed to present problems in determining the surface area of palladium catalysts by hydrogen chemisorption, but under normal chemisorption conditions this is probably not a factor. The a-phase Pd-H forms initially by the migration of hydrogen atoms from the surface into the interstitial volume of the palladium crystals. Hydrogen pressures near atmospheric are... 

The hydrogen permeation process is influenced by the surface topography, the purity of the metal and its defect structure (e.g., grain boundaries and dislocations). Within the metal, the hydrogen occupies octahedral interstitial sites. At high hydrogen concentrations, above 20°C, the a phase of palladium hydride exists, and one of the problems associated with pure palladium as a membrane is hydrogen embrittlement and the distortion of the metal by repeated adsorption/desorption cycles of exposure. 

Palladium hydrides occurring in clusters, adsorbed surfaces, and interstitial sites will not be included here. Also excluded are reports of proposed structures with no sufficient characterization data. 

The introduction of other metals to form palladium based alloys has had promising results. In particular doping of the palladium with silver has been shown to improve the stability of the film and increase the solubility of hydrogen. Further, the temperature above which the a palladium hydride occurred was lowered with increasing silver content (Uemiya et al.,1991 Kikuchi Uemiya, 1991). The hydrogen permeability was optimized when the silver content of the alloy was aroimd 23 wt%. Silver occupies interstitial sites in the palladium lattice and so moderates the lattice expansion and contraction due to hydrogen absorption/desorption. 

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