Pt surface segregation in bimetallic Pt3M alloys: A density functional theory study

A simplified 5-layer slab model is used to study Pt segregation at Pt3M(1 1 1) surfaces (M = Ag, Au, Co, Cr, Cu, Fe, Ir, Mn, Mo, Ni, Pd, Re, Rh, Ru, Ti, V). The calculated segregation energies are generally in good agreement with the experimental results. However, for Pt3Ti(1 1 1) and Pt3Mn(1 1 1), the model is unable to give the correct prediction because the experimental subsurface compositions are very different from those of the model. Pt segregation is found to be correlated with the subsurface atomic structure, the size of metal atoms and the surface energy. Interestingly, the calculations show that the displacement of M atoms at the topmost Pt3M layer can be used to predict the surface segregation trend. The local geometries for the segregation and non-segregation structures are carefully compared and the surface geometric effect on the oxygen reduction reaction (ORR) activity is discussed. In particular, it is determined that the enhanced ORR activity for the Pt-skin surfaces of Pt3Ni(1 1 1) and Pt3Co(1 1 1) alloys are not caused by geometric effects.