Surface segregation basics

In the context of alloys, segregation is the enrichment of one element on the surface, where it reaches a higher concentration than in the bulk. As the theory of surface segregation is covered in more detail in other chapters of this book as well as a previous book devoted to the subject [41], here we just mention the basics. In thermodynamic equilibrium, the most simple description of segregation is the Langmuir-McLean equation. 

A big amount of experimental studies of stability of many component systems Pt Me (where Me - transition metals Cr, Fe, Co, Ni, Ru) indicates about the formation of nanoclusters with core-shell structures [11-13], where mechanisms of the processes (including corrosive) with the formation of such structures are described. Firstly this is a surface segregation during the process of multicomponent nanocluster preparation [14], Due to such segregation nanocluster surface becomes enriched by one of the components, especially by platinum with the reduction of surface energy in segregated binary nanocluster [75]. In the process of corrosive influence (in model conditions or in tests of fuel cells) a prevailing dissolution of one component from basic metal Me and surface enrichment by platinum with the formation of a core-shell system. 

New method of dispersed perovskites synthesis based upon mechanochemical activation of the solid starting compounds is elaborated. The influence of defect structure of these compoimds as well as surface segregation on their catalytic properties is discussed. Basic stages of the monolith perovskite catalysts preparation are optimized. The experimental samples of monolith catalysts of various shapes are obtained, possessing high activity, thermal stability and resistance to catalytic poisons. 

Figure 10.5a illustrates basic flow patterns in a circular drum partway filled with a mixture of particles and rotated at intermediate speeds. The majority of particles move in solid-like rotation with the drum walls, and a thin free-surface layer flows in laminar-like shear flow over the top surface. Segregation occurs in this thin layer and, as in shaken systems, large particles typically rise to the top. 

Surface segregation takes place in practically all metal alloys and is controlled by the chemical equilibrium between the near-surface layers and the bulk. Consequently, a successful theoretical description of this phenomenon demands a consideration of both bulk and surface properties in order to understand correlations between segregation profile, atomic structure, SRO, and temperature. For this reason, the basics of the alloy s bulk properties have to be discussed (Section 11.2) before considering the surfaces and their experimental (Section 11.3.1) as well as theoretical characterizations (Sections 11.3.2 and 11.3.3). In Section 11.3, we will introduce the methods that are in general applied to alloy surfaces. Special focus will be on a very new ab initio-based description that allows for a direct prediction of the segregation profile and the mentioned correlated parameters. This concept will then be applied to two different classes of alloy phases an intermetallic compound and a disordered alloy. The last example will demonstrate which possible effects will take place if an adsorbate comes to the surface. Besides changes in the atomic position of the surface atoms (the so-called adsorbate-induced surface reconstruction),... 

From the calculations it turned out as well, that the p(2xl) chain structure at the sub-surface location is 86 meV per Ir atom more favorable as compared with the c(2x2) array of Ir and Cu atoms, which is basically due to directional forces of the straight d-d hybridization between Ir atoms along the chains. These forces are obviously absent in a c(2x2) situation. For the p(2xl) Cu-Ir structure, an energy increase of 49 meV has been determined before segregation of Ir into deeper layers sets in. Such an energy barrier can evidently be overcome by temperature augmentation. Therefore, the experimentally observed diffusion of Ir at T > 650 K into the bulk (cf. Fig. 11) becomes plausible too. 

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