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

Abstract

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 ( 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 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 p plat 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 ( p a/ p d) and losses in useful energy due to hysteresis were determined from the absorption–desorption p plat values. The p a/ p d and energy loss due to hysteresis gradually 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 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.