Mechanistic studies of hydrogen transport through image/V composite membranes

Composite membranes comprised of vanadium foils coated with molybdenum carbide catalyst layers were recently introduced as alternatives to palladium for high temperature separation of H2. Experiments using D2/H2 mixtures unambiguously show that the mechanism involves dissociation, proton transport, and subsequent recombination. Temperature-dependent measurements of H2 flux were performed on sets of membranes in which the thickness of both the V foil and the image layers were varied in order to provide insight into the underlying transport mechanisms. It is shown that hydrogen transport through the carbide itself can be limiting for catalyst layers >20 nm. At temperatures <750 °C the flux is highly activated, suggesting that dissociation of H2 on the carbide surface is the rate limiting step. At higher temperature the permeability decreases with temperature in good agreement with the theoretical predictions of the permeability of the underlying V metal. The permeability of these composite membranes significantly exceeds that of pure Pd, with values as high as 5.9×10−8 mol m/m2 s Pa0.5 at 750 °C.