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 research, the thermodynamics of H2 solution and hydride formation in a series of disordered Pd-Ag alloys has been determined using both reaction calorimetry and equilibrium PH2-composition-T data. Trends of DeltaHH and DeltaSH with both H and Ag concentration have been determined. For the Pd0.76Ag0.24 alloy, which does not form a hydride phase, DeltaHH and DeltaSH both exhibit minima with H/(Pd0.76Ag0.24) followed by a linear increase of the former. A linear increase of DeltaHH is found for all of the alloys in the high H content region beyond the two-phase region or, if, there is no two-phase region, in the high H content region. DeltaHH degrees at infinite dilution of H decreases with atom fraction Ag, XAg, up to about 0.40 and then increases. Enthalpies for hydride formation/decomposition, 1/2H2(g) + dilute <--> hydride, have been determined calorimetrically for alloys which form two phases (303 K). The enthalpies for hydride formation become more exothermic with XAg while the corresponding entropy magnitudes are nearly constant, 46 +/- 2 J/K mol H.

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.

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