Platinum group metals play a fundamental role in oxidation catalysis, and clarification of
the interaction of their surfaces with oxygen is crucial to understand the catalytic process
at atomistic level. We have investigated formation of adsorption structures and surface
oxides on selected Pd and Pt surfaces by first-principles techniques as complement to experimental investigations. In particular we have calculated phase diagrams for surface phases by means of first-principles thermodynamics. On Pd(111), a large variety of metastable structures has been observed to coexist with the thermodynamically stable Pd5O4 surface oxide. On palladium surfaces, surfaces oxide monolayers PdOx form with x between 0.8 and 1, whereas on Pt surfaces α-PtO2-like structures are most stable. Consistently, as in the bulk case, platinum in surface oxides reaches higher oxidation states than Pd under same conditions of temperature and pressure. Although atomic structure of surface oxides may be not directly related to the bulk oxide structure, local coordination of individual atoms shows regular patterns in all structures considered.