The surface stability and compositions of catalysts under varied conditions play an important role in its activity and selectivity toward various reactions. In this paper, density functional theory based first principles calculations were used to investigate the stability and compositions of the first two layers of Pt–Pd alloys on Pd substrate under the electrode-potential dependent oxygen reduction conditions. The adsorption of O and OH have different preference surface compositions of Pt: Pd 50: 50 and Pt: Pd 0: 100, respectively. However, at high electrode potential, it is found that O should be dominant adsorbate on the surface. Therefore, the surface composition should favor the Pt: Pd 50: 50. Moreover, this oxygen covered surface is characterized by weakened surface Pt–Pt bonds, which is attributed to the increase in the population of the Pt–Pt antibonding state. These findings support the experimentally observed Pd segregation from the as-prepared Pt=Pd(111) to the composition of Pt: Pd ¼ 60: 40 during ORR.
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