First-principles calculations on sulfur interacting with ternary Pd-Ag-transition metal alloy membrane alloys

Pd-alloy membranes are promising for use in fossil-fueled power plants with integrated carbon capture, but for some applications this requires resistance to performance degradation in the presence of sulfur containing gases like H₂S. By use of atomistic modeling based on density functional theory, the role of transition metals (TMs) in ternary Pd–Ag–TM alloys was in the current article used to identify new Pd-alloys with potentially less performance degradation in contact with H₂S. A number of slab models were created, giving a wide and representative span of compositions of the two upper atomic layers of the Pd-alloys. All TMs in the 4th, 5th and 6th rows of the periodic table were included, as well as some of the poor metals. By comparing energies of pure alloy slabs, the tendency of different TMs to segregate to the surface in vacuum was quantified; this turned out to depend only weakly on the presence of silver. In order for the TM to be of any benefit for the surface properties of the membrane, the segregation energy towards the bulk should not be too large. The free energy of different compositions was then calculated for two systems: a clean surface in a gas mixture of H₂ containing a fraction of 20×10–6 H₂S, and a slab with adsorbed sulfur in pure H₂ atmosphere. This was used to calculate a typical release temperature for sulfur from the surface and a maximal H₂S concentration before the membrane surface can be expected to be blocked by adsorbed S. When using a tentative threshold for the segregation energy (0.2 eV) and the release temperature (600 °C), we arrived at the following list of potential TM additives to sulfur-resistant Pd–Ag membranes: Cu, Zn, Ga, Cd, In, Sn, Pt, Au, Hg, Tl, Pb, and Bi.