Study of n value and α/β palladium hydride phase transition within the ultra-thin palladium composite membrane

The gas permeation of a 2 mu m thick Pd membrane was examined between 363 K and 773 K with p(feed) ranging from 1.4 bar to 3.8 bar and p(perm) fixed at atmospheric pressure. The pressure exponent value ranged from 0.68 to 1.0, and it gradually decreased with increasing temperature and pressure. The transformation of rate-limiting step and enhancement of surface process were suggested to the main reason for the fluctuation of n value, and the influence of contaminant species on the n value can be excluded in this study. The H-2 permeation flux of the membrane gave a marked change forming a peak at 413 K and 443 K with p(feed) = 2 bar and 3 bar, respectively. These two peaks can be ascribed to the alpha/beta palladium hydride phase transition. The metal membrane delaminated from the support after hydrogen embrittlement but it still maintained the nanostructure. The phase transition temperature range (20 K) for our 2 mu m thick membrane was smaller than that of a 25 mu m thick one (above 40 K) described in the literature. (C) 2008 Elsevier B.V. All rights reserved.