Absorption of hydrogen by palladium-nickel-rhodium ternary alloys

Abstract

The hydrogen absorption characteristics of PdNiRh alloys ((1) Pd97.5−xNi2.5Rhx with x (at.% Rh) = 0, 2.5, 5.0, 7.5, (2) Pd95.0−xNi5.0Rhx with x = 5.0, (3) Pd90.0−xNi10.0Rhx with x = 7.5, and (4) Pd87.5−xNi12.5Rhx with x = 2.5, 5.0) have been investigated at temperatures between 273 K and 433 K and hydrogen pressures up to 1000 Torr, and for some alloys up to 10 000 Torr, by means of pressure-composition isotherm measurements, together with X-ray measurements of the lattice parameters of hydrogen-free and hydrogenated alloys. The dependence of the lattice parameters of the βmin phase boundaries of the hydrogenated low Ni content PdNiRh alloys on the Rh content follows a similar trend to that exhibited by hydrogenated PdRh alloys; these display an increase in the aβmin values with increasing Rh content up to about 7.5 at.% Rh. The low pressure solubilities of the ternary alloys decrease with Ni and Rh content, and the α + β plateau pressures increase. The hydrogen capacities of the PdNiRh alloys with a given Ni content exhibit a maximum around 7.5 at.% Rh. The relative chemical potential ΔμHo of dissolved hydrogen at infinite dilution in the low Ni content PdNiRh alloys increases with Rh content from that of PdNi alloys with the specified Ni content, and the values lie almost on the mid-points between the values of PdNi and PdRh binary alloys where the ΔμHo values for PdRh binary alloys are slightly larger than those for PdNi alloys. The ΔμHo values for high Ni content PdNiRh alloys are a little smaller rather than those for PdNi binary alloys. The standard free energy change ΔGplato for β-hydride formation in PdNiRh alloys increases with the Rh content from that of the PdNi binary alloys with constant Ni content. In particular, for low Ni content PdNiRh alloys, the rate of increase in the ΔGplato values with Rh content is almost the same as that for PdRh alloys, where the stability of β-hydride for PdRh binary alloys is slightly greater than that of PdNi alloys.