The segregation behavior of the bimetallic alloys PtPd and CoCr in the case of bare surfaces and in the presence of an oxygen ad-layer has been studied by means of first-principles modeling based on density-functional theory (DFT). For both systems, change of the d-band filling due to charge transfer between the alloy components, resulting in a shift of the d-band center of surface atoms compared to the pure components, drives the surface segregation and governs the chemical reactivity of the bimetals. In contrast to previous findings but consistent with analogous PtNi alloy systems, enrichment of Pt atoms in the surface layer and of Pd atoms in the first subsurface layer has been found in Pt-rich PtPd alloy, despite the lower surface energy of pure Pd compared to pure Pt. Similarly, Co surface and Cr subsurface segregation occurs in Co-rich CoCr alloys. However, in the presence of adsorbed oxygen, Pd and Cr occupy preferentially surface sites due to their lower electronegativity and thus stronger oxygen affinity compared to Pt and Co, respectively. In either cases, the calculated oxygen adsorption energies on the alloy surfaces are larger than on the pure components when the more noble components are present in the subsurface layers.
翻译:PtPd和CoCr两金属合金在裸露表面和氧合金系统存在的情况下的隔离行为,已经通过基于密度功能理论(DFT)的一原则建模方法进行了研究。对两种系统来说,D波段填充由于合金组成部分之间电荷转移而导致D波段填充的变化,导致表层原子的d波段中心相对于纯成分的转移,驱动表层隔离并调节双金属的化学再活动。与以往的发现不同,但与类似的PtNi合金系统一致,在表层中的Pt原子和第一个表层下的Pd原子的浓缩都是通过第一次表层的一原则建模。尽管纯Pd与纯的Pt合金成分相比表面能量较低,但D波段的D波段的D波段填充填充量变化导致表层与纯成分的转移,导致表层的D波段与Cocr合金合金的化学再生分离。然而,与Pd和Cr相比,由于其较低的电子密度较低,表层和表层表面表面表面表面的Pt-原子表面表面表面表面表面的浓缩成分的浓缩程度比氧的含量更高程度要强。