Thermodynamics of Hydrogen in fcc Pd-Au Alloys

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, Delta H-H(O), and Delta S-H(O), 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 Delta H-H(O) and Delta S-H(O) values differ. This is due to a compensating effect of the two thermodynamic parameters leading to (partial derivative Delta G(H)/partial derivative r) = RT(partial derivative In p(1/2)/partial derivative r,) approximate to constant for the alloys from X-AU approximate to 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.