| The effect of surface impurities (in particular carbon and sulphur) on the permeation of atomic or molecular hydrogen through a palladium membrane was examined. The elemental coverages on one of the membrane surfaces was measured by in-situ Auger spectroscopy. The upstream, driving, pressure for molecular permeation ranged from {dollar}sim10sp{lcub}-5{rcub}{dollar} to 3 {dollar}times{dollar} 10{dollar}sp{lcub}2{rcub}{dollar} Pa. For atomic permeation the incident sub-eV atomic flux varied from {dollar}sim{dollar}1 {dollar}times{dollar} 10{dollar}sp{lcub}15{rcub}{dollar} to 3 {dollar}times{dollar} 10{dollar}sp{lcub}20{rcub}{dollar} H{dollar}sp{lcub}rm o{rcub}{dollar}/m{dollar}sp2{dollar}.s. The palladium temperature ranged from 355 to 630K. Atomic hydrogen was used to measure the membrane asymmetry in the surface limited permeation regime. The permeation rates were observed to change by a factor of {dollar}sim10sp4{dollar}. The phenomenological recombination rate coefficient, K{dollar}sb{lcub}rm r{rcub}{dollar}, decreased by {dollar}sim10sp{lcub}-3{rcub}{dollar} for an increase in the impurity coverage from 0.13 to 0.45. The change observed is compatible with a mechanism which views the reduction in the permeation rate as resulting from a blockage of surface adsorption sites. Molecular sticking coefficients measured at 625K ranged from 3.2 {dollar}times{dollar} 10{dollar}sp{lcub}-6{rcub}{dollar} to 7.0 {dollar}times{dollar} 10{dollar}sp{lcub}-2{rcub}{dollar}, depending on impurity coverage. Observed permeabilities in the diffusion limited regime were found to be in agreement with literature values. |
