Abstract
Pd–Cu alloys are of potential interest for use as hydrogen purification membranes, but have relatively low permeability compared to the commercially used alloys such as Pd–Ag. In this work, the effects of partial Ag substitution on the hydrogen diffusivity, solubility and the permeability of Pd–Cu membranes with a bcc structure have been investigated. With the addition of 2.3 and 3.9at% Ag to Pd–Cu, lattice expansions of 0.11% and 0.35% were observed. Structural analyses by in-situ XRD showed that the bcc structure of the 2.3at% Ag alloy is retained upon heating to 600°C, whereas an fcc phase forms in the 3.9at% Ag alloy resulting in a mixed (bcc+fcc) structure. Whilst the diffusion coefficients between 350 and 400°C for both Pd–Cu–Ag ternary samples were shown to be lower than their binary alloys (which had similar structures), higher solubility values were obtained. The lower diffusion coefficients of the ternary alloys are related to an increase in the diffusion activation barrier in the presence of Ag, and the higher solubility values may be attributed to the lattice expansion and high Ag–H chemical interaction. Hydrogen permeation measurements showed that an enhancement in the hydrogen solubility of the bcc phase Pd45.8Cu51.9Ag2.3, does not have a substantial effect on the permeability of the membrane. In contrast, for the Pd45.1Cu51Ag3.9 sample with a mixed (bcc+fcc) phase, higher hydrogen solubility can lead to a remarkable improvement in permeability. Hence, it is suggested that the hydrogen permeability in the bcc phase is mainly controlled by hydrogen diffusion, and the solubility enhancement can only significantly improve the hydrogen permeability when the fcc phase is present.
Highlights
The commercial hydrogen separation by metallic membranes is mainly focused on palladium alloys
In addition Density Functional Theory (DFT) calculations for hydrogen diffusion in the Pd–Cu–Ag fcc phase showed that the presence of Ag in the window forming Transition State (TS) leads to a significant increase in its energy
Whilst the bcc structure was retained during heating of the Pd45.8Cu51.9Ag2.3 ternary alloy up to 600 1C, formation of the fcc phase was promoted at higher temperatures in the Pd45.1Cu51Ag3.9 ternary alloy
Summary
The commercial hydrogen separation by metallic membranes is mainly focused on palladium alloys. Transition between the α and β phases is accompanied by a volume increase of 10%, which results in lattice distortion, the formation of high internal stresses, deformation, and failure of the membrane [1,2] This problem, in conjunction with susceptibility of the pure Pd to surface poisoning by impurity gases [3,4] and the high cost of Pd have led to the exploration of a wide variety of Pd-alloy membranes, such as Pd–Ag, Pd–Cu, and Pd–Y [5,6,7,8,9,10].
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