An investigation of thermal stability of thin palladium–silver alloy membranes for high temperature hydrogen separation

The thermal stability of palladium (Pd)-based alloy membranes is the integral part for their practical applications in hydrogen recovery from mixed gas. While many research groups have developed porous ceramic supported Pd-based alloy membranes, their long-term thermal stability at elevated temperatures are not almost investigated so far. We examined the change in hydrogen permeability and gas selectivity with time of thin palladium–silver (Pd–Ag) alloy membranes (less than 5-μm thick) supported on porous α-Al2O3 substrates at temperatures of 300–850 °C. When the composite membranes were exposed to hydrogen below 550 °C, they showed excellent permselectivity for hydrogen during long-term gas permeation tests. The hydrogen permeation flux across the Pd membranes significantly enhanced by Ag alloying, reaching 1.85 mol m−2 s−1 at 550 °C in an alloy membrane containing 20 wt.% Ag. However, an appreciable decrease in the hydrogen permeation flux was observed above 600 °C. SEM-EDX and XPS analyses indicated that a considerable amount of aluminum (Al) penetrated the Pd–Ag alloy membrane layer. Highly active atomic hydrogen present at the interface between the alloy membrane and the porous α-Al2O3 substrate induced the reduction of Al2O3 to Al and caused significant migration of Al atoms into the alloy layer. Consequently, the Pd–Ag/α-Al2O3 composite membranes lost their hydrogen permeability during operations at 600 °C.