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 gas phase method. In these studies, an increase of low pressure hydrogen solubility with increasing Ag content and a decrease of it with increasing Rh content was observed. The plateau pressure (pplat) of the Pd–Rh–H system showed remarkably higher value than that of the Pd–Ag–H system. The pplat value decreased with increasing Ag content, whereas it increased with increasing Rh content. Increase in lattice size of the Pd–Ag alloy with increasing 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 and pplat value. In the Pd–Ag–H system, the miscibility gap value decreased remarkably with increasing Ag content, whereas it increased a little with the increment of Rh. Such a variation of miscibility gap value with changing the alloy component can be understood from the available vacant d-band holes of Pd–Ag and Pd–Rh alloys. Higher hydrogen solubility was attained for the higher Rh content alloy. The studied systems showed remarkable pressure-composition (p-c) hysteresis. The hysteresis factor (pa/pd) and losses in useful energy due to hysteresis were determined from the absorption–desorption pplat values. The pa/pd and energy loss due to hysteresis gradually decreased with increasing temperature, and Ag and Rh content. At a given temperature, the pa/pd and loss in energy of a given Pd–Ag–H system 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 than those of the Pd–H system. At and above 365 K, Pd–Ag and Rh–H systems seemed 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.