Surface atomic distribution and water adsorption on Pt-Co alloys

Density functional theory is used to study surface atomic distributions on slabs of PtCo and Pt₃Co overall compositions, as well as water molecule adsorption on PtCo(1 1 1) and Pt-skin structures. Pt-rich surfaces are energetically favored under vacuum in the PtCo and Pt₃Co alloys. The adsorption trend on the studied structures agrees with the d-band model, with stronger adsorption at higher surface Co composition. The most stable adsorption site for a water molecule on PtCo surfaces is on top of Co atoms, with the dipole vector parallel to the surface. This water/surface interaction is as strong as that of water molecule on Pt(1 1 1), whereas bonding to Pt-skin monolayers is found much weaker than that on Pt(1 1 1). It is found that water interacts mainly through its 1b₁ and 3a₁ orbitals with d orbitals of the Pt(1 1 1), PtCo(1 1 1) and Pt-skin surface atoms. Compared to the sum of the electron densities of the separated systems, the electron density of the water/surface gets depleted along O-Pt on Pt-skin surfaces while it becomes richer in the O-Co bonding region of PtCo.