Apparent activation energy for hydrogen permeation and its relation to the composition of homogeneous PdAu alloy thin-film membranes

Abstract

Abstract PdAu alloy composite membranes are often fabricated by sequential electroless plating of Pd and Au, but this forms a layered structure that must be annealed to form a homogenous alloy. Using ex situ analysis techniques such as X-ray diffraction (XRD) or energy dispersive X-ray spectroscopy (EDS) to judge the extent of homogenization can be time-consuming as the composite membranes must be cooled to ambient temperatures prior to analysis. However, an in situ alternative is to use the apparent activation energy for H2 permeation (Eact). In this work, 27 – 41 wt% Au films were fabricated to expand upon a range of 0 – 21 wt% that has been previously reported. Homogenization (confirmed by XRD and EDS) was achieved by heat treatment in 3 MPa H2 at 550 °C for up to 120 h. The apparent Eact was found to initially decrease with increasing Au composition from 12.2 kJ mol−1 (pure Pd) to 7.5 kJ mol−1 (21 wt% Au) after which it slowly increased to 9.0 kJ mol−1 (41 wt% Au). The two distinct regimes can be explained using the relation Eact = ΔHH + ED, wherein Eact is the sum of the partial enthalpy of solution of H into the PdAu alloy (ΔHH) and the activation energy for diffusion of H in the alloy (ED). Essentially, the two components of Eact have opposite signs that counteract each other and cause a non-linear trend with increasing Au compositions.