Segregation and H2 Transport Rate Control in Body-Centered Cubic PdCu Membranes

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• 作者：Lixiang Yuan ; Andreas Goldbach ; Hengyong Xu
• 刊名：Journal of Physical Chemistry B
• 出版年：2007
• 出版时间：September 20, 2007
• 年：2007
• 卷：111
• 期：37
• 页码：10952 - 10958
• 全文大小：181K
• 年卷期：v.111,no.37(September 20, 2007)
• ISSN：1520-5207

文摘

The H₂ permeation of a supported 2 m thick Pd₄₈Cu₅₂ membrane was investigated between 373 and 909 Kat P = 0.1 MPa. The initial H₂ flux was 0.3 mol·m^-2·s^-1 at 723 K with an ideal H₂/N₂ selectivity betterthan 5000. The membrane underwent a bcc-fcc (body-centered cubic to face-centered cubic) phase transitionbetween 723 and 873 K resulting in compositional segregation. After reannealing at 723 K the alloy layerreverted to a bcc structure although a small fcc fraction remained behind. The mixed-phase morphology wasanalyzed combining X-ray diffraction with scanning electron microscopy-energy-dispersive spectroscopicanalysis (SEM-EDS) measurements, which revealed micrometer-scale Cu-enriched bcc and Cu-depleted fccdomains. The H₂ flux J_H2 of the fcc Pd₄₈Cu₅₂ single phase layer prevailing above 873 K could be describedby an Arrhenius law with J_H2 = (7.6 ± 4.9) mol·m^-2·s^-1 exp[(-32.9 ± 4.5) kJ·mol^-1/(RT)]. Thecharacterization of the H₂ flux in the mixed-phase region required two Arrhenius laws, i.e., J_H2 = (1.35 ±0.14) mol·m^-2·s^-1 exp[(-10.3 ± 0.5) kJ·mol^-1/(RT)] between 523 and ca. 700 K and J_H2 = (56.1 ± 9.3)mol·m^-2·s^-1 exp[(-25.3 ± 0.6) kJ·mol^-1/(RT)] below 454 K. The H₂ flux exhibited a square root pressuredependence above 523 K, but the pressure exponent gradually increased to 0.77 upon cooling to 373 K. Theactivation energy and pressure dependence in the intermediate temperature range are consistent with a diffusion-limited H₂ transport, while the changes of these characteristics at lower temperatures indicate a desorption-limited H₂ flux. The prevalence of desorption as the permeation rate-limiting step below 454 K is attributedto the pairing of an extraordinarily high hydrogen diffusivity with a marginal hydrogen solubility in bccPdCu alloys. These result in an acceleration of the bulk diffusion rate and a deceleration of the desorptionrate, respectively, allowing the bulk diffusion rate to surpass the desorption rate up to relatively hightemperatures.