Adsorbate-induced segregation in a PdAg membrane model system: Pd3Ag(1 1 1)
Thin PdAg alloy membranes with 20–25% Ag are being developed for hydrogen separation technology. Despite many investigations on such membranes as well as representative experimental and theoretical model systems, unresolved issues remain concerning the effect of the alloy surface structure and composition on adsorption and vice versa. Therefore, the interaction between hydrogen, carbon monoxide or oxygen with the surface of a PdAg model alloy was studied using periodic self-consistent density functional theory (DFT-GGA) calculations. In particular, the adsorption structure, coverage dependence and possible adsorption-induced segregation phenomena were addressed using Pd3Ag(1 1 1) model surfaces with varying degrees of surface segregation. In agreement with previous experimental and theoretical investigations, we predict Ag surface termination to be energetically favorable in vacuum. The segregation of Ag is then reversed upon adsorption of H, CO or O. For these adsorbates, the binding is strongest on Pd three-fold hollow sites, and hence complete Pd termination is favored at high coverage of H or CO, while 25% Ag may remain under oxygen because of the lower O-saturation coverage. CO adsorption provides a somewhat stronger driving force for Pd segregation when compared to H, and this may have implications with respect to permeation properties of PdAg alloy surfaces. Our predictions for high coverage are particularly relevant in underlining the importance of segregation phenomena to the hydrogen transport properties of thin PdAg alloy membranes.
Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (190 K)Download as PowerPoint slideHighlights► PdAg alloy membranes were studied using density functional theory calculations. ► Using Pd3Ag(1 1 1) model surfaces, adsorption and segregation phenomena are investigated. ► The Pd3Ag(1 1 1) surface is Ag terminated under vacuum conditions. ► The adsorption of H, CO and O is predicted to pull Pd atoms to the surface. ► The adsorbate-induced segregation is more prominent at high adsorbate coverage.
Journal: Catalysis Today - Volume 193, Issue 1, 15 October 2012, Pages 111–119