Hydrogen absorption and thermodynamic properties of hydrogen in low rh content Pd‐Rh alloys

Abstract

AbstractThe hydrogen absorption characteristics of fully annealed Pd‐Rh alloys containing up to 10.0 at.% Rh have been investigated at temperatures between 273 K and 433 K and hydrogen pressures up to 1000 Torr by means of pressure‐composition isotherm measurements. For the p‐c isotherms of Pd‐7.5 at.% Rh alloy, in addition to the normal (α+β) two‐phase plateaus, “anomalous plateaus” are observed near the normal βmin phase boundaries. The formation of this anomalous plateau implies that in the fully annealed alloys some precursory phase separation exists, together with the metastable, homogeneous solid solution phase. The low pressure hydrogen solubilities decrease with an increase of Rh content and the main plateau pressures increase with the solute metal content. The hydrogen solubilities exhibit a maximum at about 7.5 at.% Rh at 1000 Torr. The derived relative partial molar thermodynamic quantities at infinite dilution, and also the plateau thermodynamic parameters are in agreement with the previously reported results. These thermodynamic quantities are not affected by the formation of the anomalous hydride phase, because the hydride does not form in the dilute phase region, and because the amount of hydrogen involved in the anomalous hydride phase is small compared to the main hydride phase.