Catalytic coatings for vanadium-based hydrogen membranes

Abstract

Vanadium is a cheaper and highly permeable alternative to palladium as a dense metallic hydrogen membrane. However, catalytic surface coatings must be applied to vanadium surfaces to enable the transport of molecular hydrogen. These coatings facilitate dissociation and recombination kinetics of molecular hydrogen and protect vanadium from oxidation. Palladium is the most common coating, but suffers from high material costs and interdiffusion with vanadium in the presence of hydrogen at and above 400 °C. In this work, alternative surface catalysts that operate via a spillover transport mechanism were investigated for membrane operation at temperatures > 400 °C. Membrane performance of the various group V metals (vanadium, niobium, and tantalum) was studied with the application of Mo2C. Vanadium provided the best hydrogen permeability and mechanical stability, and therefore remained the focus of this thesis. Low temperature operation (< 600 °C) of Mo2C/V membranes revealed a robust resistance to embrittlement which was onset by transport limitations in the Mo2C. TiC was also tested as a coating and produced high fluxes up to 0.71 mol m-2 s-1 at 650 °C and 1 MPa transmembrane pressure. The effect of hydrogen isotopes on permeation at high temperature (600 – 700 °C) was tested on the TiC/V membranes. Protium permeated 1.12 – 1.34 times faster than deuterium, yet no isotopic effect on solubility was measured. A challenge with both Mo2C and TiC coatings was the competitive adsorption of CO2 and N2 which inhibited hydrogen transport. However, this was resolved with the addition of thin Pd films over the carbide layers. Aside from transition metal carbides, a simple air treatment produced catalytically active V2O3 on the surface of vanadium. The surface oxide yielded similar fluxes to Mo2C, yet, was only stable at 550 °C. Density functional theory simulations revealed superior energetic properties for hydrogen adsorption and dissociation on the V2O¬3 (0001) surface compared to other V oxide states. Lastly, vanadium sputter conditions and fabrication of composite Pd/V/Pd membranes on porous ceramic supports were studied to enable tubular geometries.