Hydrogen Dissociation on Pd4S Surfaces

Exposure of Pd-based hydrogen purification membranes to H 2 S, a common contaminant in coal gasification streams, can cause membrane performance to deteriorate, either by deactivating surface sites required for dissociative H 2 adsorption or by forming a low-permeability sulfide scale. In this work, the composition, structure, and catalytic activity of Pd 4 S, a surface scale commonly observed in Pd−membrane separation of hydrogen from sulfur-containing gas streams, were examined using a combination of experimental characterization and density functional theory (DFT) calculations. A Pd 4 S sample was prepared by exposing a 100 μm Pd foil to H 2 S at 908 K. Both X-ray photoemission depth profiling and low energy ion scattering spectroscopic (LEISS) analysis reveal slight sulfur-enrichment of the top surface of the sample. This view is consistent with the predictions of DFT atomistic thermodynamic calculations, which identified S-terminated Pd 4 S surfaces as energetically favored over corresponding Pd-terminated surfaces. Activation barriers for H 2 dissociation on the Pd 4 S surfaces were calculated. Although barriers are higher than on Pd(111), transition state theory analysis identified reaction pathways on the S-terminated surfaces for which hydrogen dissociation rates are high enough to sustain the separation process at conditions relevant to gasification applications.