Mechanistic studies of hydrogen transport through _Mo2C/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 _Mo2C 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 >20nm. 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^-8molm/m²sPa^0.5 at 750°C.