Enhancing the catalytic activity of Ni-based amorphous alloy membrane surfaces

The poor surface properties of alloy membranes formed primarily from non-Pd elements (including many Ni-based amorphous alloys and body-centred cubic alloys formed from V, Nb, Zr, Ti and Ni) necessitate the application of a Pd overlayer to promote the rate of H2 exchange between the surrounding atmosphere and the bulk alloy. One exception is rapidly solidified Ni64Zr36, which exhibits the ability to permeate hydrogen without need for a catalytic Pd overlayer. Elimination of Pd would dramatically decrease the cost of these membranes, but the slower surface kinetics relative to Pd mean hydrogen permeability is significantly reduced. In this study we determined that heat treatment of these materials under hydrogen promotes the development of sub-micron Ni particles at the surface, resulting in a dramatic increase in the catalytic activity of the alloy surface to the dissociation and recombination of H2. This treatment can also result in bulk crystallisation of the amorphous alloy which degrades hydrogen permeability. An appropriate treatment regime which promotes surface enhancement while minimising bulk crystallisation was shown to result in improved hydrogen permeability. This research confirms that surface morphology, not just elemental composition, is vital for achieving rapid hydrogen exchange between solid and gas phases.