A thermodynamic study of dilute solutions of hydrogen and deuterium in Pd0.9Ag0.1 at 555 and 700 K

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Abstract
Precise determinations of the partial molar thermodynamic properties of hydrogen and deuterium in the alloy Pd0.9Ag0.1 have been carried out at 555 and 700 K, using a combined calorimetric‐equilibrium method. These quantities are compared with the corresponding values for hydrogen and deuterium in pure palladium. Our measurements confirm the significant increase in Δ?H with increasing temperature, which was observed already for solutions in pure palladium. It is found that the enthalpy of solution of hydrogen is about 900 cal mol−1 more exothermic in the alloy than in pure palladium. This difference depends somewhat on temperature. The partial entropies of solution of hydrogen are about 0.4 cal mol−1 K−1 smaller in Pd0.9Ag0.1 than in pure palladium. Because of the large number of different physical measurements that have been carried out previously on palladium and on palladium–silver alloys, it is possible to give a detailed analysis and interpretation of these entropy differences, which may be understood in terms of vibrational, electronic, and configurational entropy contributions. The major effect is configurational, arising from a preference of the hydrogen atoms for interstitial sites surrounded by only palladium atoms. It also is shown that very simple models are not capable of explaining the magnitude of the H–H interaction energies and our thermodynamicmeasurements of the isotope effects.