Segregation and H2 transport rate control in body-centered cubic PdCu membranes

The H-2 permeation of a supported 2 mu m thick Pd48Cu52 membrane was investigated between 373 and 909 K at Delta P = 0.1 MPa. The initial H-2 flux was 0.3 mol center dot m(-2)center dot S-1 at 723 K with an ideal H-2/N-2 selectivity better than 5000. The membrane underwent a bcc-fcc (body-centered cubic to face-centered cubic) phase transition between 723 and 873 K resulting in compositional segregation. After reannealing at 723 K the alloy layer reverted to a bcc structure although a small fcc fraction remained behind. The mixed-phase morphology was analyzed combining X-ray diffraction with scanning electron microscopy-energy-dispersive spectroscopic analysis (SEM-EDS) measurements, which revealed micrometer-scale Cu-enriched bcc and Cu-depleted fcc domains. The H-2 flux J(H2) of the fee Pd48CU52 single phase layer prevailing above 873 K could be described by an Arrhenius law with JH2 = (7.6 +/- 4.9) mol center dot m(-2)center dot s(-1) exp[(-32.9 +/- 4.5) kJ center dot mol(-1)/(R7)]. The characterization of the H2 flux in the mixed-phase region required two Arrhenius laws, i.e., J(H2) = (1.35 +/- 0.14) mol center dot m(-2)center dot s(-1) exp[(-10.3 +/- 0.5) kJ-mol(-1)/(R7] between 523 and ca. 700 K and J(H2) = (56.1 +/- 9.3) mol center dot m(-2)center dot s(-1) exp[(-25.3 +/- 0.6) kJ center dot mol(-1)/(R7)] below 454 K. The H-2 flux exhibited a square root pressure dependence above 523 K, but the pressure exponent gradually increased to 0.77 upon cooling to 373 K. The activation energy and pressure dependence in the intermediate temperature range are consistent with a diffusion-limited H-2 transport, while the changes of these characteristics at lower temperatures indicate a desorption-limited H-2 flux. The prevalence of desorption as the permeation rate-limiting step below 454 K is attributed to the pairing of an extraordinarily high hydrogen diffusivity with a marginal hydrogen solubility in bcc PdCu alloys. These result in an acceleration of the bulk diffusion rate and a deceleration of the desorption rate, respectively, allowing the bulk diffusion rate to surpass the desorption rate up to relatively high temperatures.