Density functional theory is used for the evaluation of surface segregation, trends for dissolution of Pt surface atoms in acid medium, and oxygen reduction reaction activity of core-shell materials, containing a monolayer of platinum over a monometallic or bimetallic core. Two groups of cores are investigated: Pt/X with X = Ir, Au; Pd, Rh, Ag; Co, Ni, Cu; and Pt/Pd(3)X, with X = Co, Fe, Cr, V, Ti, Ir, Re. It is found that all the 4d and 5d pure cores may serve as stable cores, and their beneficial effect on the Pt monolayer may be further tuned by alloying the core to another element, here chosen from 3d or 5d groups. The Pd(3)X cores enhance the stability of the surface Pt atoms both in vacuum and under adsorbed oxygen; however the high oxygen philicity of some of the X elements induces their surface segregation that may cause surface poisoning with oxygenated species and their dissolution in acid medium.

No abstract

Classical molecular dynamics simulations of surface oxidation in the presence of atomic oxygen and water are performed on Pt(111) and Pt/PtCo/Pt3Co(111) to determine the surface evolution during the oxidation process. On the basis of density functional theory calculations, the model considers electrostatic interactions between the adsorbates and the two topmost layers of the surface as a function of oxygen coverage and it is able to reproduce the main features of the oxidation phenomena observed experimentally. The results indicate that oxygen and water induce changes in the structure and local composition of the near-surface layers which may affect the activity and stability of the catalyst. Such changes are strongly dependent on the amount of adsorbed oxygen, and include oxygen absorption in the subsurface layers at high coverages, and significant atomic buckling and surface reconstruction. In the Pt-skin alloy surface, migration of cobalt atoms to the surface is accompanied by atomic buckling and detachment. The reverse “reduction” process is also simulated by gradually removing the electrostatic interactions between the top layers and adsorbates.

No abstract

Density functional theory is used to evaluate geometric and electronic effects of the presence of vacancies in subsurface layers of Pt-based alloys composed by a monolayer of Pt on top of an alloy core for bimetallic Pt/PtM/Pt 3 M (M = Co, Pd, Ir, Cu) and trimetallic Pt/M1 3 M2 (M1, M2 = Pd, Cu; M1 ≠ M2) systems. Our model simulates metal porous structures arising after dealloying due to exposure of metal nanoparticles to oxidative conditions in acid medium. Enhanced oxygen reduction reaction activity experimentally observed in these structures is tested through the calculation of binding energies of O and OH. It is found that subsurface vacancies induce relatively weaker binding energies of O and especially of OH, and reduced Pt−Pt surface distances, which can explain the observed activity enhancement.