The role (or lack thereof) of nitrogen or ammonia adsorption-induced hydrogen flux inhibition on palladium membrane performance

The potential impact of nitrogen and ammonia exposure on hydrogen permeance through thin palladium membranes (1.3 mu m to 4.1 mu m thick) fabricated by electroless plating was studied. Additionally, a robust approach is introduced to quantify the pressure exponent which accounts for contributions to Knudsen flow through defects present in very thin membranes. In sharp contrast to previously published results, no flux inhibition was observed due to nitrogen or ammonia exposure. Studies included 24 h exposures to both pure gases and equimolar hydrogen/nitrogen or hydrogen/ammonia mixtures at trans-membrane pressures ranging up to 1.0 MPa and temperatures of 598 K to 773 K. One membrane did exhibit significant flux inhibition after helium exposure, but this was attributed to changes in surface microstructure associated with hydrogen departing the lattice. This apparent hydrogen flux inhibition behavior was permanently eliminated by air exposure which roughens the surface, and it is suggested that this surface structure mechanism is a more probable explanation for flux inhibition than adsorption of nitrogen based species. (C) 2016 Elsevier B.V. All rights reserved.