Inhibition of hydrogen flux in palladium membranes by pressure-induced restructuring of the membrane surface

The effects of high pressure gas exposure on the hydrogen flux though Pd films were explored. It was observed that exposure of a Pd membrane to N2, Ar, or CO2 at 3.0 MPa and 500 °C caused a substantial decline in hydrogen flux within 24 h that only recovered to ca. 60% of its initial value after 24 h of subsequent hydrogen exposure. Atomic force microscopy images revealed that the Pd surface became smoother with a reduction in density of nanoscale features after high pressure exposure, consistent with an observed transition in rate limiting step from bulk diffusion to surface kinetics. The rate of flux loss was found to have an apparent activation energy of 39 kJ mol−1, which is consistent with values reported for Pd surface self-diffusion. This effect was not observed when the exposure gas was helium or at pressures <2 MPa, suggesting that impinging gas momentum is an important factor that promotes smoothening of the Pd surface. Lastly, it was observed that the flux loss could be completely reversed by air treatment or prevented by the presence of O2 impurities (ca. 1%) in the feed.