Solubility of Hydrogen in Palladium‐Yttrium‐Silver Alloys

Abstract

Thermodynamic quantities for absorption of hydrogen by Pd95–xYxAg5 solid solution alloys with up to x = 6.3 at.% Y have been determined from measurements of pressure‐composition isotherms at temperatures between 273 K and 473 K and hydrogen pressures up to 1000 Torr. The hydrogen absorption characteristics have been compared with those of the previously reported Pd–Ag and Pd–Y binary alloys. The relative partial molar enthalpy, ΔHH0, at infinite dilution of hydrogen in Pd95–xYxAg5 alloys increases in exothermicity with increasing Y content from that of Pd–5.0 at.% Ag, almost parallel to that in Pd–Y binary alloys. The relative partial molar entropy, ΔSH0, at infinite dilution in the ternary alloys decreases with Y content from that of Pd–5.0 at.% Ag alloy. The hydrogen solubility behavior at low hydrogen concentration in the Pd–Y–Ag alloys under a “constant volume” condition can be described as due to mainly the strong H–Y attractive pair interaction in the Pd host lattice. The stability of β‐hydride and hydrogen concentration at αmax phase boundary increase following alloying with Y from that of Pd–5.0 at.% Ag alloy, respectively, and the changes in the properties with Y content are similar to that in Pd–Y binary alloys.

Thermodynamics of H in Disordered Pd−Ag Alloys from Calorimetric and Equilibrium Pressure−Composition−Temperature Measurements

“…Salomons et al determined p − c − T data for some of these alloys and described it, following the lead of Oates and Ramanathan, by preferential occupation of interstices with different energies due to different numbers of Ag atom nearest neighbors. There have been other determinations of solubilities in these alloys, but in some, only one or two compositions were examined, e.g., Loos determined very detailed thermodynamic data for the X Ag = 0.10 alloy and Sakamoto et al determined data for the X Ag = 0.05 alloy while Kishimoto et al measured H 2 solubilities in the X Ag = 0.05 and 0.10 alloys in a low-temperature range, 200−250 K.…”

Thermodynamics of H2 Solution and Diffusion of H in (Pd0.77Ag0.23)1–xYx Alloys

“…That is, at low p H 2 , the solubility is greatest in the (Pd 0.77 Ag 0.23 ) 0.94 Y 0.06 alloy but it is the lowest at higher p H 2 . The enhanced solubilities at small H contents in the (Pd 0.77 Ag 0.23 ) 1– x Y x alloys are most likely due to the larger interstitial volumes arising from the presence of Y and, after these initial solubility regions, other effects become dominant causing decreases in hydrogen solubility compared to the Pd 0.77 Ag 0.23 alloy (Figure ), for example, electronic effects related to the donation of the Y electrons to the d-band of the Pd 0.77 Ag 0.23 alloy and its effect on the subsequent donation of the absorbed hydrogen electrons to the alloy.…”

“…If it is assumed that both Ag and Y each block for occupation by H the adjacent six octahedral interstices surrounding them, then the calculated values of ΔS H °(β = (1 − (X Ag + X Y )) 6 are those shown in the last column of Table 1, −50.5 and −52.8 J/ K mol H for the (Pd 0.77 Ag 0.23 ) 0.97 Y 0.03 and (Pd 0.77 Ag 0.23 ) 0.94 Y 0.06 alloys, respectively, which are in reasonably good agreement with the value for this temperature range for H dissolved in pure Pd of −51.2 J/K mol H. 11 On the other hand, this gives an unrealistic value for the fraction of interstices which can be occupied, that is, for the (Pd 0.77 Ag 0.23 ) 0.94 Y 0.06 alloy (β = (1 − (X Ag + X Y )) 6 , β = 0.128 whereas Figure 6 shows experimental H contents greater than this. It seems that β may increase at the higher pressures.…”

“…Sakamoto and co-workers examined the H 2 solubilities in a series of (Pd 0.95 Ag 0.05 ) (1–x) Y x alloys where x was varied up to X Y = 0.067 . These workers also determined some H diffusion parameters in the (Pd 0.95 Ag 0.05 ) (1–x) Y x alloys .…”

“…Sakamoto and co-workers examined the H 2 solubilities in a series of (Pd 0.95 Ag 0.05 ) (1−x) Y x alloys where x was varied up to X Y = 0.067. 6 These workers also determined some H diffusion parameters in the (Pd 0.95 Ag 0.05 ) (1−x) Y x alloys. 7 Because of the importance of the Pd 0.77 Ag 0.23 alloy for hydrogen purification, rather than employing the Pd 0.95 Ag 0.05 alloy for alloying with Y, the Pd 0.77 Ag 0.23 alloy will be alloyed with Y.…”