In the April 1992 issue of this journal two papers from Russia were concerned with palladium alloy membrane technology (1,2). Now this topic has again been featured, in a useful review of high temperature membrane catalysis by John N. Armor of Air Products & Chemicals, Inc., Allentown, Pennsylvania (3).

High temperature membranes are those that can be used at temperatures above 200°C, and suitable materials include: inorganic oxides, carbon, palladium and its alloys, and composites. While the use of palladium-based membranes is limited to reactions that involve hydrogen, the high solubility of hydrogen in palladium and the fact that it is currently being fabricated into thin foils makes it particularly suitable for these reactions. To avoid problems associated with the α/β phase transformation, the operating temperature should be above 310°C.

The benefits that are encountered when palladium is alloyed with silver, ruthenium, rhodium and rare earths, are considered, as are the limitations of such membranes. For the future, the potential for commercial exploitation is seen to be with catalytic materials deposited on monomodal sub–8 Å inorganic membranes, and with thin metal alloy coatings on meso-porous supports. Already studies have been made of composite membranes consisting of palladium and silver-palladium deposited on the outer surface of porous glass tubes and porous alumina cylinders by electroless plating techniques.

While many problems have still to be overcome, it is concluded that the progress made to-date in membrane catalysis encourages further fundamental research on the topic.