Promising Isotope Effect in Pd77Ag23 for Hydrogen Separation

Pd-Ag alloys are largely used as hydrogen separation membranes and, as a consequence, the Pd-Ag-H system has been intensively studied. On the contrary, fewer information is available for the Pd-Ag-D system; thus, the aim of this work is to improve the knowledge of the isotope effect on the commercial Pd77Ag23 alloy, especially for temperature above 200 degrees C. In particular, deuterium absorption measurements are carried out in the Pd77Ag23 alloy in the temperature range between 79 and 400 degrees C and in the pressure range between 10(-2) and 16 bar. In this exploited pressure (p) and composition (c) range, above 300 degrees C the pc isotherms display the typical shape of materials where only a solid solution of deuterium is present while at lower temperatures these curves seem to be better described by the coexistence of a solid solution and a deuteride in a large composition range. The obtained results are compared and discussed with the ones previously measured with the lightest hydrogen isotope. Such a comparison shows that the Pd77Ag23 alloy exhibits a clear inverse isotope effect, as the equilibrium pressure of the Pd-Ag-D system is higher than in Pd-Ag-H by a factor of approximate to 2 and the solubility of deuterium is about one half of that of hydrogen. In addition, the absorption measurements were used to assess the deuteration enthalpy that below 300 degrees C is Delta H-deut = 31.9 +/- 0.3 kJ/mol, while for temperatures higher than 300 degrees C, Delta H-deut increases to 43 +/- 1 kJ/mol. Additionally, in this case a comparison with the lighter isotope is given and both deuteration enthalpy values result lower than those reported for hydrogenation. The results described in this paper are of practical interest for applications operating above 200 degrees C, such as membranes or packing column, in which Pd77Ag23 has to interact with a gas stream containing both hydrogen isotopes.