The hydrogen permeability of Pd4S

Hydrogen permeates rapidly through pure Pd membranes, but H2S, a common minor component in hydrogen-containing streams, produces a Pd4S film on the Pd surface that severely retards hydrogen permeation. Hydrogen still permeates through the bi-layered Pd4S/Pd structure, indicating that the Pd4S surface is active for H2 dissociation; the low hydrogen permeability of the Pd4S film is responsible for the decreased rate of hydrogen transport. In this work, the hydrogen permeability of Pd4S was determined experimentally in the 623-773 K temperature range. Bi-layered Pd4S/Pd foils were produced by exposing pure Pd foils to H2S. H2 fluxes through the bi-layered Pd4S/Pd foils were measured during exposure to both pure H2 and a 1000 ppm H2S in H2 gas mixture. Our results show that H2S slows hydrogen permeation through Pd mainly by producing a Pd4S film on the Pd surface that is roughly an order-of-magnitude less permeable to hydrogen (kPd4S = 10−7.5 exp(−0.22 eV/kBT) molH2/m/s/Pa1/2) than pure Pd. The presence of H2S in the gas stream results in greater inhibition of hydrogen transport than can be explained by the very low permeability of Pd4S. H2S may block H2 dissociation sites at the Pd4S surface.