Material and operational considerations for effective use of palladium composite hydrogen separation membranes in ammonia decomposition membrane reactors

Abstract

In this work, the effects on hydrogen permeation of various operating conditions and support materials are explored. The primary topic explored was focused on the viability of pure Pd films to be used in ammonia decomposition catalytic membrane reactors. In such a reactor, the Pd membrane is surrounded by an ammonia decomposition catalyst, which allows hydrogen to be removed simultaneously as the decomposition occurs. To this end, the first section details effects on hydrogen permeance from nitrogen and ammonia adsorption. Essentially, no effects were observed and Pd is proposed as a potential candidate without any alloying or protective actions. The second topic then explores a peculiar effect on hydrogen permeation due to high-pressure gas exposures. It is observed that at pressures of 3.0 MPa, inert gases such as argon can assist in surface morphological changes that affect hydrogen permeation by smoothening. This appears to anneal surface defects, which negatively affects the hydrogen rate of sorption into the palladium films. The document then proceeds to study the effects of support materials, wherein permeating hydrogen was observed to reduce a MgO/MgAl2O4 support and cause Mg to alloy with the Pd film. The Mg was observed to segregate to the surface of the Pd film and essentially block all hydrogen permeation by not allowing contact with Pd. Finally, a strategy for sealing defects in supported Pd films is detailed wherein a low melting point ceramic frit is mixed with water and dripped onto the defect. This is then fired under an inert atmosphere at the frit melting temperature whereat the frit melts into the defect and completely seals the hole. This allows large defects on a fabricated Pd membrane to be sealed, thus salvaging an otherwise ruined membrane. Furthermore, the sealed membrane was tested for an extended duration in an ammonia decomposition membrane reactor to prove its utility as a sealing technique for realistic applications.