J. F. Lynch

The effect of lattice defects on hydrogen solubility in palladium: I. Experimentally observed solubility enhancements and thermodynamics of absorption

Abstract Measurable, and reproducible solubility enhancements have been observed for hydrogen in cold-worked palladium in the low content α-phase (273–363 K). It is shown that the solubility of hydrogen is unaffected in palladium samples which had been quenched from near their melting points. It is concluded that the stress field about the dislocation array is the principal cause of the solubility enhancement. Absorption of hydrogen at the cores of dislocations is believed to be insignificant. From absorption isotherms measured at various temperatures, the relative partial molar enthalpies and entropies of solution of hydrogen into deformed palladium have been determined. The heat of solution of hydrogen by 78% deformed palladium is approximately 880 J/H more exothermic than for well-annealed palladium in the low hydrogen content α-phase.

The effect of lattice defects on Hydrogen solubility in palladium: II. Interpretation of solubility enhancements

Abstract It is proposed that the stress-field of the dislocation array is the principal cause of the solubility enhancement observed in the low hydrogen content, α-phase of cold-worked palladium. With an assumed uniform dislocation density of 9 × 10 11 cm −2 , the observed solubility enhancement of 1.65 (298 K) for heavily cold-worked palladium can be reproduced if the core radius is assumed to be 2 × Burgers vector. The temperature dependence of the solubility enhancement is also reasonably well predicted. The significance of measured relative partial molar enthalpies and entropies of absorption of hydrogen into cold-worked palladium is examined.

The effect of the α-β phase change on the α phase solubility of hydrogen in palladium

Abstract Following the α-β phase change in palladium, there is an enhanced solubility of hydrogen at a given equilibrium pressure within the low hydrogen content α phase. There are marked similarities between the absorption isotherms for hydrogen following the phase change and those exhibited by heavily cold-worked palladium. Partial thermodynamic parameters for hydrogen absorption are reported for palladium plastically deformed by the phase change.

Absorption of hydrogen by vanadium-palladium alloys

Abstract Pressure-composition isotherms (273–373K) have been determined for the absorption of hydrogen by a series of six palladium alloys (f.c.c.) in the composition range from 1 to 8 at.% vanadium. At a given hydrogen content, the equilibrium hydrogen pressure progressively increases with vanadium content. Thermodynamic parameters for the absorption of hydrogen are reported at infinite dilution of hydrogen, and for the formation of the nonstoichiometric hydride phase from the hydrogen-saturated alloy. The relative, partial molar enthalpy of solution of hydrogen at infinite dilution increases slightly with vanadium content, e.g., for V(5%)-Pd, Δ H ° H = −8,500 j H compared with −10,040 j H for palladium. The presence of vanadium, which absorbs hydrogen itself in its normal b.c.c. structure, greatly inhibits the ability of palladium to absorb hydrogen. For example, the isobaric solubility of hydrogen (1 atm, 298 K) decreases from H Pd = 0.7 (palladium) to 0.024 (V(6%)-Pd). The lattice expansion due to the presence of interstitial hydrogen has been determined by X-ray diffraction. From these data it can be concluded that the formation of two non-stoichiometric hydride phases does not occur at vanadium contents greater than 5 at.% (298 K). Electrical resistance has been measured as a function of the hydrogen content of the alloys. The electrical resistance increases more markedly with hydrogen content for these alloys than for any of the palladium alloys previously examined.

Absorption of Hydrogen by Substitutional fcc Lead/Palladium Alloys

The recent observation1 of relatively high superconductive transition temperatures for deuterium-charged Ag/Pd alloys, e.g., 16 K, has rekindled interest in the characteristics of hydrogen (deuterium) absorption by palladium alloys. Prior to the discovery of superconductivity2, interest in these systems derived from the fact that they are used as hydrogen purification membranes3 and because their behavior could offer clues about the nature of hydrogen in pure transition metals4. An illustration of the information about pure metals derivable from studies of hydrogen absorption by palladium alloys is the recent evidence from studies on Rh/Pd alloys that pure

Weak chemisorption of hydrogen by platinum black

Abstract Following the titration of an oxygenated platinum black surface with hydrogen, chemisorption isotherms have been determined in the range of hydrogen pressures from 0.001 to 50 mm Hg from 77.4 to 303 K. These data are shown to be reversible over most of the pressure range and isosteric heats for chemisorption have been derived from the Clausius-Clapyron equation. These heats decline with coverage from 69 to 23 kJ/mole H 2 . BET surface area measurements of the sample (Kr: 77.4 K) are compared with the area derived from the titration of the oxygenated surface. The maximum error which can be introduced into surface area measurements (by the titration technique) due to the presence of weak, reversible chemisorption is estimated to be 12%.

The formation of voids in palladium, metal by the introduction and removal of interstitial hydrogen

Abstract It is shown that voids are created in palladium by the introduction (H-to-Pd, atomic ratio 0·68) and subsequent removal (50°C) of interstitial hydrogen. At a given low pressure the dehydrogenated palladium sorbs more hydrogen than the original palladium sample. The quantities of this additionally sobbed hydrogen are too large to be attributed to the accessible surface area. Further evidence is cited to show that it corresponds to hydrogen chemisorbed onto the surface of voids.

Thermodynamics of hydrogen dissolved in palladium/rhodium and palladium/platinum alloys

Partial thermodynamic parameters have been determined for the solution of hydrogen in Pd/Pt and Pd/Rh alloys. The partial excess entropies of absorption of hydrogen at infinite dilution decrease regularly with atom fraction platinum but not with atom fraction rhodium. This suggests that interstices are not excluded for hydrogen occupation by rhodium atoms neighbouring the interstices but are excluded by the platinum nearest neighbours. Other thermodynamic parameters for hydrogen absorption have been determined and are compared for these alloys. These results are discussed in terms of the observation that rhodium will absorb hydrogen at high pressures of hydrogen when situated within the palladium lattice whereas platinum will not.

Calorimetric determination of differential heats of absorption of hydrogen by palladium

Differential heats of absorption of hydrogen by palladium have been measured with an adiabatic calorimeter. For the first time calorimetric heats have been determined for bulk palladium in the single phase, α and β, regions. The heat of absorption in the α-phase (25°C) was found to be 23.32 ± 4.45 kJ (mol H2)–1 and in the β-phase the isosteric heats decrease with hydrogen content from 46.5 (H-to-Pd = 0.61) to 27.3(H-to-Pd = 0.69). (The measured differential (adiabatic) heats have been corrected for compression work to give isosteric heats.) Heats of sorption determined in the α-phase of palladium black reflect contributions due to both absorption and chemisorption of hydrogen. Some preliminary heats of sorption are also reported at – 78°C and at –195°C with palladium black.

Absorption of hydrogen by iridium/palladium substitutional alloys

Thermodynamic parameters for absorption of hydrogen by Ir/Pd alloys have been determined by p, c, T techniques in an u.h.v. system and by electrochemical measurements. The equilibrium solubility of hydrogen in these alloys (1 atm, 25°C) decreases to low values at small contents of substituted iridium in comparison to other palladium-rich alloys which have been investigated. Formation of a second, non-stoichiometric hydride phase ceases above approximately 8 atomic percent of substituted iridium (25°C). Somewhat anomalous behaviour is observed for the values of the heat of solution at infinite dilution: Δtext-decoration:overlineH°H=–10.04 kJ (g-atom H)–1 for Pd; –8.03 kJ (g-atom H)–1 for Ir(2.82 %)/Pd; –8.36 kJ (g-atom H)–1 for Ir(5.77 %)/Pd and –8.78 kJ (g-atom H)–1 for Ir(8.86 %)/Pd. Lattice parameters of the hydrogen-containing alloys are reported. The electrical resistances of these alloys have been measured as a function of their hydrogen-contents and the observed increases are greater than those reported for other palladium alloy systems.