Hydrogen and Deuterium Solubility in Commercial Pd–Ag Alloys for Hydrogen Purification

Abstract

Pd-Ag alloys with compositions close to 23-25% Ag are considered as a benchmark for hydrogen permeability. They are used in small scale reactors for hydrogen separation and purification. Permeability and solubility are strictly mathematically correlated, and the temperature dependence of solubility can provide useful information about the physical state of the material, the hydrogenation enthalpy, and the occurrence of different thermodynamic states. While the permeability of Pd-Ag alloys has been largely investigated, solubility measurements are available only in a restricted temperature range. In this paper, we extend solubility measurements up to 7 bar for Pd 77 Ag 23 in the temperature range between 25 • C and 400 • C and for Pd 30 Ag 70 for temperatures between 190 • C and 300 • C. The occurrence of solid solutions or hydride phases is discussed, and the hydrogenation enthalpy is calculated.

An Innovative Procedure to Evaluate the Hydrogen Diffusion Coefficient in Metals from Absorption Measurements

“…In Figure 1, one can observe that increasing the temperature, starting from similar values of p, the kinetics becomes faster as less time is needed to reach the steady state. However, it must be noted that the pressure drop at lower T is higher, in agreement with the higher hydrogen content measured at lower temperatures by the pressure-composition isotherms [27]. Moreover, considering the curves measured at the sample temperature of 250 • C starting with different pressures, one can note a faster kinetics at higher p. This behavior is the one expected for metal hydrides.…”

“…Before each single absorption experiment, the Pd-Ag sample was degassed pumping by means of a Pfeiffer Vacuum turbo pump (p ≈ 10 −4 mbar in the sample holder), until no more hydrogen desorption from the sample could be detected by a combined Pirani-Penning vacuum gauge. The amount of hydrogen absorbed in the sample measured by this new acquisition procedure was identical to that previously reported in [27].…”

“…The results of this model are in the lower limit of the diffusion values reported in the literature and then in agreement with the hypothesis at the basis of the numerical procedure adopted. The dependence of the diffusion coefficient on the pressure (i.e., on the hydrogen content) especially at lower temperatures is explained by the coexistence of the two phases of α and β as reported in a previous work [27].…”

“…Four pieces of the membranes (with typical dimensions of~20 mm × 7 mm and a total mass of~148 mg) were cut and used for collecting the data of the kinetics of hydrogen absorption in a homemade Sieverts apparatus [9,10] working up to a 200 bar pressure and a 500 • C temperature. These membrane pieces were already used for measuring the pressure-composition isotherms reported in [27]. The amount of hydrogen exchanged between the gas atmosphere and the solid samples was measured by the pressure variation in calibrated cylinders connected by Swagelok tubes to the reaction chamber, where the sample was placed.…”

“…In particular, hydrogen absorption measurements were performed in a standard Sieverts apparatus, which can be found in most laboratories working on hydrogen storage or permeation. According to a well-known experimental procedure, the hydrogen solubility into the metal was calculated from the steady-state values of the pressure in the gas phase [27]. The behavior of the pressure in the gas phase of the Sieverts' setup along time provides further information about the kinetics of the hydrogen diffusion through the metal lattice.…”