On the Promoting Role of Ag in Selective Hydrogenation Reactions over Pd−Ag Bimetallic Catalysts:  A Theoretical Study

The surface structure of Pd−Ag alloy and its alteration in the presence of atomic hydrogen have been studied using density functional calculations on Pd1-xAgx(111) (x ≈ 0.2) models. In the absence of an adsorbate, silver atoms are found to segregate on the surface, in line with previous experimental observations under vacuum conditions. At equilibrium, the surface is predicted to expose mainly Ag atoms. Isolated Pd atoms incorporated in this Ag-rich layer appear to be slightly preferred over the Pd2 dimers. Increasing the coverage of adsorbed H atoms on the Pd−Ag substrate gradually suppresses surface segregation of silver, such that migration of all surface Ag atoms into the subsurface region becomes favorable at a H coverage of ∼0.25 ML. For the latter structures, with solely Pd atoms in the surface layer and Ag atoms in the subsurface layer, the propensity of H to be accommodated in interstitial sites below the surface layer essentially vanishes: subsurface H atoms are predicted to be energetically driven to escape to the surface without an activation barrier. These results might have strong implications in understanding the promoting role of Ag in selective hydrogenation reactions over Pd−Ag catalysts. The presented adsorbate-induced resegregation in bimetallic systems is a general concept applicable to a broad variety of catalytic systems and advanced materials.