The permeability of hydrogen selective Pd-based membranes was tested in different experimental conditions. The membranes were obtained by depositing palladium-silver films onto ceramic porous supports, with film composition of about 20 wt% of silver and thicknesses of about 2.5 mu m. Their permeance was measured at 400 C at total trans-membrane pressures between 0.2 and 6 bar, using pure feeds of H-2 and N-2, as well as H-2/N-2 and H-2/CO mixtures; the temperature dependence of permeability was investigated using pure H2 feeds at 300, 400 and 500 degrees C. The membranes exhibit a very attractive behavior, maintaining a virtually infinite selectivity throughout the testing, with permeance values among the highest values reported in literature for similar membranes. Permeation of pure hydrogen accurately follows Sieverts' law and confirms the presence of a chemisorption-dissociation-diffusion mechanism, characterised by the transport of atomic hydrogen through the Pd-Ag layer as the limiting step. In the case of H-2/N-2 mixtures, the high membrane permeance originates also significant concentration polarization phenomena resulting in apparent deviations from Sieverts' behavior; the presence of CO in the feed may reduce hydrogen permeability even by 75%, although this effect is shown to be fully reversible after a subsequent air treatment at 400 degrees C. The temperature dependence of the membrane permeability is of Arrhenius type, with an activation energy of about 17 kJ/mol, that is, close to what is reported for Pd-Ag membranes following Sieverts' behavior. (c) 2008 Elsevier B.V. All rights reserved.
