Thermal stability, glass-forming ability and hydrogen permeability of amorphous Ni64Zr36-XMX (M = Ti, Nb, Mo, Hf, Ta or W) membranes

Nickel-zirconium-based amorphous alloys are promising hydrogen-selective membrane materials. These membranes can potentially be combined with a suitable water-gas-shift catalyst to form a catalytic membrane reactor, which can produce high-purity H-2 and CO2 streams from coal-derived syngas at elevated temperatures. One shortcoming of amorphous alloys is that their temperature of operation is limited by the onset of crystallization, which in many cases occurs at a temperature below that demanded by typical WGS catalysts. It is hypothesized that the substitution of zirconium with other elements of higher bond valance will increase the crystallization temperature of these amorphous alloys. Systematic substitutions (M = Ti, Nb, Mo, Hf, Ta and W) have been made to the eutectic alloy Ni64Zr36-XMx. Results have shown that niobium, hafnium and tantalum increase the crystallization temperature of these alloys, while Ti decreases the crystallization temperature. Mo and W are problematic in that they have limited solubility, and lead to brittle alloys that are unsuitable for the gas separation application. Of the alloys studied, those from the Ni-Nb-Zr system are most prospective for use in a hydrogen-selective catalytic membrane reactor. Hydrogen permeability studies of three alloys showed that partial substitution of Zr with Nb reduced permeability but improved the thermal stability. An increase in thermal stability not only increases the resistance to crystallization, but also reduces interdiffusion between the membrane and I'd catalyst. Thus, the operating temperature of Ni64Zr36 membranes can be increased through partial substitution of zirconium with niobium. (c) 2008 Elsevier B.V. All rights reserved.