Segregation energy

Here, we will first briefly recall the principles of this method in the case of transition metals. Then we will apply it to two illustrative examples the surface segregation energy of an impurity is a pure host and the growth of adislands on FCC(lll) surfaces of the same chemical species. 

We will limit ourselves to the surface segregation energy of an impurity of atomic number Z + 1 in a BCC matrix of atomic number Z and study the variation of this energy as a function of the number Nj of d electrons per atom in the d band of e transition metal Z. 

This case is particularly interesting since the surface segregation energy can be directly compared to surface core level binding energy shifts (SCLS) measurements. Indeed, if we assume that the excited atom (i. e., with a core hole) is fully screened and can be considered as a (Z + 1) impurity (equivalent core approximation), then the SCLS is equal to the surface segregation energy of a (Z + 1) atom in a Z matrixi. in this approximation the SCLS is the same for all the core states of an atom. 

Figure 2. Segregation energy in layer Sp (p = 0 surface layer...) of a transition metal impurity of atomic number Z + 1 (d band-filling (Nj + l.l)e /atom, full curves (Nj + l)e /atom, dashed curve) in a BCC transition metal matrix of atomic number Z (d band-filling Nje" /atom) for various crystallographic orientations of the surface...

In the case of W(H0) (Nd=4.4eVatom), we have also calculated the modification of the surface segregation energy of a Re impurity when a p(2 x 1) overlayer of oxygen is present at the surface (Eig. 3). Then, there are two geometrically inequivalent atomic rows, labelled a and b, of W atoms on the surface (and in the sublayers). However, the modification of their effective atomic levels relative to the bulk is vanishingly small beyond the second... 

M. Said, M. C. Desjonqudres and D. Spanjaard, Surface Core Level Shifts in BCC Transition Metals Deduced from Segregation Energy Calculation, Phys. Rev. B 47 4722 (1993)... 

G. Abramovici, M. C. Desjonqu res and D. Spanjaard, W Surface Core Levels Shifts of O/W(110) Deduced from Surface Segregation Energies, /. de Physique 15 907 (1995)... 

Using results from the DFT calculations, combined with databases of segregation energies, estimate the stability of the alloys in working reaction environments. 

Ruban AV, Skriver HE, Nprskov JK. 1999. Surface segregation energies in transition-metal alloys. Phys Rev B 59 15990-16000. 

Pt skin catalysts are prepared by high-temperature annealing and are therefore expected to be thermodynamically stable structures Thermochemical studies of the metal segregation energies of various metal alloys [114] suggest that Pt-rich alloys prefer to segregate Pt atoms to the surface and form Pt skins. 

Figure 9. Pt/Pd (shell/core) system of increasing thickness, where we examine the tendency of Pd atoms to segregate from the core to the surface. In the absence of oxygen, we observe small anti-segregation of Pd atoms from the core (positive energy values) and the tendency to form alloy in subsurface layers, as shown by the negative values of segregation energies to move a Pd atom (blue) from the core to the subsurface layers. Gray spheres are Pt atoms, blue spheres are Pd atoms.

Segregation Energies of Core Ir Atoms to the Snrface under 1/3 ML of Adsorbed Oxygen Aeeording to the Geometries in Fig. 13. The Segregation Site Shown in the Table is the Surfaee Loeation Where Ir Moves from the Core. 

Figure 14. Segregation of Ir atoms (green) from the core to the shell in the presence of surface vacancies (red circle in the inset) for three different Pt-shell (gray) thicknesses. Segregation energies in the 2- and 3-layer shells are calculated in two and three steps respectively. Negative segregation energies mean favorable segregation ofir.

Tlic results also suggest that substitutional formation is most favorable on the (110) surface. This supports the view that the (110) surface will be more catalytically active than the (111) surface, as impurities segregate preferentially to this surface. Sayle et al. note that the segregation energies (i.e. the differences between bulk and surface energies) are larger for the Af cations than for cations due to elec-... 

Another distinctive feature of the surface energy curve is its slope. In fact the slope of is simply the segregation energy of the deposited... 

If the segregation energy is negative, as in the case of Ag on Pt(Ill), the deposited element stays at the surface. If the segregation energy is positive, as in the case of Pt on Cu(l 11) and Ru on Au(l 11), the deposited element should go into the deeper layers of the surface region (if the transfer of deposited element into the bulk is kinetically hindered). Usually, the deposited element... 

GENERAL TRENDS FOR SURFACE SEGREGATION ENERGIES IN TRANSITION METAL ALLOYS... 

In most cases the experimental techniques used to study surface phenomena do not seem to yield consistent values for the surface segregation energies. One important exception is the special case of an atom of atomic number Z+1 in a host of atoms of atomic number Z, where the surface segregation energy may in fact be extracted with a high degree of accuracy from X-ray photoemission spectroscopy (XPS) measurements of surface core-level shifts (SCLS) [39]. In contrast they may be calculated quite accurately by modern first-principles methods [18,25,40]. 

A) in the host, respectively, N the number of d-electrons in the host and the impurity, and 0=O.O5[l — ]zjz ], where z and zj> are the coordination numbers at the surface and in the bulk, respectively. The dependence on the surface coordination number means that the segregation energy in transition... 

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