Hydrogen isotherms have been measured for a series of solid solution Pd-Au alloys in the temperature range from 393 to 523 K. Standard partial thermodynamic parameters at infinite dilution of H, DeltaH(H) degrees, and DeltaS(H) degrees, have been determined from these equilibrium data; both standard values for H(2) absorption become more negative with increase of atom fraction Au, X(Au). An interesting result is that the dilute phase isotherms at 423 and 523 K are all very similar for alloys with X(Au) = 0.15 to about 0.30 although their DeltaH(H) degrees and DeltaS(H) degrees differ. This is due to a compensating effect of the two thermodynamic parameters leading to (partial partial differentialDeltaG(H)/partial partial differentialr) = RT(partial partial differential ln p(1/2)/partial partial differentialr) approximately constant for the alloys from X(Au) approximately 0.15 to 0.30 at low r where r = H-to-metal atom ratio. Calorimetric enthalpies and isotherms at 303 K have been determined for a series of Pd-Au alloys over a range of H contents including, for some of the low Au content alloys, the plateau regions. These calorimetric data are the most complete reported for the Pd-Au-H system.

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In this study, Pd−Mo alloys have been internally oxidized to form Pd/MoO3 composites, and it is shown for the first time that H2 reacts with these composites to form H-bronzes, for example, H x MoO3, within the Pd matrix. The Pd matrix is the source of mobile H atoms, which react with the oxide precipitates to form H-bronzes. The kinetics of H-bronze formation are extremely fast. Solubilities of H2 in the Pd/MoO3 composites have been measured, and because H2 solution in the oxide is so much more exothermic than that in the Pd matrix, the former can be separated from the latter. The intercept of the solubility plot along the H content axis gives the stoichiometry of the H-bronze formed. Some irreproducibility was found in the p H 2 versus r relationships, which is also found in H-bronzes prepared by other techniques such as H spillover. The relative partial enthalpies of H2 solution, ΔH H, in the H-bronzes have been determined by reaction calorimetry as a function of the H content of the bronze. The ΔH H values decrease sharply in exothermicity with increasing x in H x MoO3.

Specific permeabilities and diffusion constants for H have been measured in a series of Pd-Al alloy membranes using a new method for activation of the membranes. The membranes have been partially internally oxidized at an elevated temperature before inserting into the apparatus for measuring H 2 permeabilities. If the percent internal oxidation is small relative to the membrane thickness, the diffusion takes place essentially through the unoxidized alloys. The activation energy for diffusion of H increases with atom fraction Al from 23.2 kJ/mol 1/2H 2 (X Al =0) to 25.1 J/mol 1/2H 2 (X Al =0.08). It is shown that the partially oxidized alloys are more resistant to CO poisoning than unoxidized alloys. The resistance of the partially internally oxidized alloys towards poisoning by CO (g) increases and the diffusion constants decrease with X Al .

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A frequently employed boundary condition for H permeation of membranes is p down ≈ 0 and, for this case, the flux is given by J = (KD H/d)p up n , where K is a constant and d is the membrane thickness. Values of n can then be obtained from slopes of plots of ln J against ln p up. When the H solution is ideal, n = 0.5, and D H is concentration-independent, but when the H solution is nonideal, i.e., when the H atoms interact, n ≠ 0.5 and D H is concentration-dependent. An equation is derived for n, which includes the effects of nonideality of H on both D H and the solubility. The terms in this equation for n can be obtained from the appropriate equilibrium H2 isotherms for Pd or its alloys. Good agreement is found between the derived values of n and the experimental values for the pressure dependence of H2 permeation through Pd, Pd0.77Ag0.23, and Pd0.50Ag0.50 membranes. By comparing experimental n ≠ 0.50 values to those calculated from the equilibrium isotherms, a decision can be made about whether bulk diffusion is the slow step.

The solution of hydrogen and hydride formation in Pd/Cr2O3 composites has been investigated. Internal oxidation of Pd−Cr alloys is employed to prepare the composites consisting of nanosized Cr2O3 precipitates within Pd matrices. The dissolved H segregates to the internal Pd/Cr2O3 interfaces. Plots of p H2 1/2 versus H content for internally oxidized Pd−Cr alloys exhibit small positive intercepts along the H/Pd axis which can be attributed to trapping of H by Pd/Cr2O3 internal interfaces. The amount of trapping is found to be directly proportional to the atom fraction Cr in the alloys after internal oxidation at the same temperature. In addition to the enhanced solubility noted from these intercepts, H2 solubilities in the dilute phase of these internally oxidized alloys are larger than in pure Pd or in other internally oxidized Pd-rich alloys and these solubilities also increase with atom fraction Cr, X Cr. In contrast with internally oxidized Pd−Al alloys studied earlier, internally oxidized Pd−Cr alloys with X Cr ≥ 0.02 have an initial plateau p H2 lower than those of Pd−H. The H2 isotherms for internally oxidized alloys with X Cr ≥ 0.05 exhibit gradual phase transitions to the two phase coexistence region rather than an abrupt one as for annealed Pd.

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