Theoretical investigations on the potential-induced formation of Pt-oxide surfaces

Using the extended ab initio atomistic thermodynamics approach together with density functional theory calculations the interfacial structure and composition of Pt-electrodes in electrochemical environments at elevated electrode potentials was studied. Focusing on the electrode potential region, at which the oxide-formation occurs, the bulk systems and all low-index surfaces of alpha-PtO,, beta-PtO,, and PtO bulk-oxides were calculated. On the basis of the bulk-oxide formation energies we first deduced the stability ranges at which the bulk-oxides are the thermodynamically stable phases. In agreement with experimental observations, we find that at experimental temperature and pressure conditions alpha-PtO2 and beta-PtO2 bulk-oxides are stable above 1.2 V, while PtO requires Delta phi > 1.3 V. Afterwards the corresponding (p, T, phi)-phase diagrams of surface structures were obtained, showing a preference for alpha-PtO2O(001), beta-PtO2(110), and PtO(100), respectively, having bulk-like compositions even on their surfaces. However, in case of thin oxide layers a NO composition might also be present. (c) 2007 Elsevier B.V. All rights reserved.