Electronic origins for sulfur interactions with palladium alloys for hydrogen-selective membranes

Atomic modeling was conducted to investigate the origin of S interactions with Pd alloy H selective membrane candidates selected from the Pd–Cu, Pd–Ag, and Pd–Au binary systems, as well as their constitutive metals. The electronic characteristics of these alloy/metal systems played a more predominant role in controlling S bonding behavior than surface site geometries. The electronic coupling of S p orbitals bonding with alloy/metal d-bands in the adsorbate/slab density of states split the lower energy p bonding state and the d-band center further apart with increasing S bonding strength. A universal linear correlation was established for increasing adsorption strength (decreasing adsorption enthalpy) of 0.25 monolayer S with the increasing density of states energy difference: [d-band center – S p bonding peak]. The S interactions predicted at higher coverage provided indications of alloy susceptibility to irreversible S corrosion. The reversible adsorption of 1.0 monolayer S was only the most stable configuration on the more open Pd0.5Cu0.5Im3¯m and P4mmm (1 1 0) surfaces. The most competitive configuration for the interaction of a full S monolayer with the Pd0.75Cu0.25Pm3¯m and Pd0.875Au0.125Fm3¯m surfaces was the partial desorption and coupling of S. Partial incorporation of S to form a mixed absorbed/adsorbed S monolayer was more favorable for the Pd Fm3¯m (1 1 1) surface, and also on the Pd0.5Cu0.5P4mmm (1 0 1) and Pd0.75Ag0.25Pm3¯m (1 1 1) surfaces when accompanied by Pd segregation. The combination of S incorporation and Pd segregation was interpreted to be the first step towards nucleation of irreversible Pd4S formation.

► Electronic factors predominately control S interactions with Pd alloys. ► Adsorption enthalpy of 0.25 monolayer S on Pd alloys inversely correlates with density of states [d-band center – S p bonding peak]. ► Sulfur incorporation and Pd segregation at high S coverages leads to Pd4S formation.