The effect of alloying of palladium with silver and rhodium on the hydrogen solubility, miscibility gap and hysteresis

The variation of hydrogen solubilities in Pd-5.0 and 10.0 at.% Ag and Pd-5. 0 and 10.0 at.% Rh alloys has been studied at different temperatures by a g as phase method. In these studies, an increase of low pressure hydrogen sol ubility with increasing Ag content and a decrease of it with increasing Rh content was observed. The plateau pressure (p(plat)) of the Pd-Rh-H system showed remarkably higher value than that of the Pd-Ag-H system. The p(plat) value decreased with increasing Ag content, whereas it increased with incr easing Rh content. Increase in lattice size of the Pd-Ag alloy with increas ing Ag content and a decrease of it with increasing Rh content in the Pd-Rh alloy was responsible for the opposite appearance of hydrogen solubility a nd p(plat) value. In the Pd-Ag-H system, the miscibility gap value decrease d remarkably with increasing Ag content, whereas it increased a little with the increment of Rh. Such a variation of miscibility gap value with changi ng the alloy component can be understood from the available vacant d-band h oles of Pd-Ag and Pd-Rh alloys. Higher hydrogen solubility was attained for the higher Rh content alloy. The studied systems showed remarkable pressur e-composition (p-c) hysteresis, The hysteresis factor (p(a)/p(d)) and losse s in useful energy due to hysteresis were determined from the absorption-de sorption p(plat) values. The p(a)/p(d) and energy loss due to hysteresis gr adually decreased with increasing temperature, and Ag and Rh content. At a given temperature, the p(a)/p(d) and loss in energy of a given Pd-Ag-H syst em showed lower values than those of a Pd-Rh-H system. The values for Pd-5. 0 and 10.0 at.% Ag-H and Pd-10.0 at.% Rh-H systems are remarkably lower tha n those of the Pd-H system, At and above 365 K, Pd-Ag and Rh-H systems seem ed better for industrial application than the Pd-H system. The Pd-10.0 at.% Ag-H system showed the lowest loss of energy for hysteresis. (C) 2000 Inte rnational Association for Hydrogen Energy. Published by Elsevier Science Lt d. All rights reserved.