Palladium is widely used in membrane applications because of its extreme selectivity for hydrogen. However, conventional palladium membranes can be damaged at the high temperatures required for many processes that generate hydrogen for various subsequent uses, including the manufacture of fertilizers, food products and semiconductors. A team of researchers from Massachusetts Institute of Technology (MIT; Cambridge, Mass.; www.mit.edu) have devised a novel fabrication method for palladium membranes that results in better resilience and expands membranes’ application into a wider range of processes where separation via membranes was not previously feasible. “Conventional palladium membranes are made of a thin film of the metal. When subject to high temperatures, metal atoms will diffuse and the metal film will slowly deform to minimize surface energy. Since a thin metal film has high surface energy, usually it will tend to break up and thicken to reduce its surface area, which destroys the membrane. The new ‘plug’ membrane does not employ a thin, continuous film. Instead, it employs discrete ‘plugs’ of metal that sit inside pores of another membrane. Such a ‘plug’ nanostructure has already minimized its surface energy and cannot deform to further reduce its surface area, and therefore it can tolerate high temperatures without deforming,” explains Rohit Karnik, Abdul Latif Jameel professor of mechanical engineering and director of the Abdul Latif Jameel Water and Food Systems Lab (J-WAFS) at MIT.
To fabricate the membrane, the team started with a porous silicon support membrane and thermally oxidized it to make a porous silica membrane. Then, an extremely thin film of gold was deposited on the membrane via a sputtering technique. Finally, the palladium layer was added via electroless deposition, seeded by the gold layer. “Electroless deposition continues inside the silica pores by exposing the silica support to the electroless plating solution from the opposite side, which deposits more metal inside the silica pores starting from the previously deposited metal. Then, the palladium metal that is outside the silica pores is removed by mechanical polishing to leave palladium only inside the silica pores,” continues Karnik.
Using chip-sized membrane samples, the MIT team validated the membrane’s stability and hydrogen-separation capabilities at temperatures as high as 1,000 K for over 100 hours. Typical palladium membranes begin to break down at around 800 K, and expanding membrane operating range presents many advantages for industrial hydrogen users. “For high-temperature hydrogen-generating reactions, such as ammonia cracking or methane reforming, conventional approaches include pressure-swing adsorption, cooling to condense and separate the non-condensable hydrogen, or operating at very high temperatures so that the reaction practically goes to completion and separation is not required. The plug membranes have the potential to enable process intensification with simpler and more efficient operation. Additionally, integration of plug membranes with reactors could open a wider process design space, allowing reactor operation at somewhat lower temperatures or conditions where the reaction may not go to completion, thus improving energy efficiency,” notes Karnik.