Surface segregation, in alloys

Ab-initio studies of surface segregation in alloys are based on the Ising-type Hamiltonian, whose parameters are the effective cluster interactions (ECI). The ECIs for alloy surfaces can be determined by various methods, e.g., by the Connolly-Williams inversion scheme , or by the generalized perturbation method (GPM) . The GPM relies on the force theorem , according to which only the band term is mapped onto the Ising Hamiltonian in the bulk case. The case of macroscopically inhomogeneous systems, like disordered surfaces is more complex. The ECIs can be determined on two levels of sophistication ... 

It is obvious that the surface segregation in alloys is suppressed by increasing the dispersion D of alloys Small particles might not have enough Group IB metals to achieve a pronounced segregation and, moreover, the... 

Simple criteria for surface segregation in alloys (relative melting points, enthalpies of sublimation, metal atom radii, surface free energies of the pure metals) all indicate that surface segregation of titanium should occur on Pt/Ti alloys in vacuo. However, this is inadequate because of the large departures from ideality in Pt/Ti alloys. Analysis (11) of a broken bond model of the system, especially with the use of data directly determined with Pt/Ti alloys, gives a more reliable result. 

Most theories of surface segregation in alloys deal with one or more of the following points ... 

As we have seen, surface segregation in alloys is a fascinating and diverse effect It is not restricted to the surfaces of random alloys (solid solutions) and also occurs in... 

Wang G, Van Hove MA, Ross PN, Baskes MI. 2005. Quantitative prediction of surface segregation in bimetaUic Pt-M alloy nanoparticles (M = Ni, Re, Mo). Prog Surf Sci 79 28-45. 

Bozzolo G, Noebe RD, Khalil J, Morse J. 2003. Atomistic analysis of surface segregation in Ni-Pd alloys. Appl Surf Sci 219 149-157. 

As alluded to before, the adsorption of atoms and molecules may also induce segregation in alloys. Upon revisiting the thermodynamic behavior of the improved Cu-Ag alloy catalysts for ethylene epoxidation synthesized by Linic et al, (section 2.1) Piccinin et al. calculated that, while in the absence of oxygen Cu prefers to stay in the subsurface layers, oxygen adsorption causes it to segregate to the surface which then phase-separates into clean Ag(lll) and various Cu surface oxides under typical industrial conditions (Fig. 7). This casts doubt on the active state of the previous Cu-Ag catalysts being a well-mixed surface Ag-Cu alloy. 

The BFS method has been applied to a variety of problems, ranging from the determination of bulk properties of solid solution fee and bee alloys and the defeet strueture in ordered bee alloys [28] to more speeifie applieations ineluding detailed studies of the strueture and eomposition of alloy surfaees [29], ternary [30] and quaternary alloy surfaees and bulk alloys [31,32], and even the determination of the phase strueture of a 5-element alloy [33]. Previous appheations have foeused on fundamental features in monatomie [26] and alloy surfaces [29] surface energies, reconstructions, surface structure and surface segregation in binary and higher order alloys [34,35] and multilayer relaxations [36,37]. While most of the work deals with metallic systems, the lack of restrictions on the type of system that can be studied translated into the extension of BFS to the study of semiconductors [38]. 

Compared to SRO effects on surface segregation in solid solutions, the role of LRO should be naturally more prominent and common. Its elucidation requires calculations that take into account various factors contributing to the net segregation characteristics in ordered alloys including the temperature dependence the crystal bulk structure and surface orientation, effective bulk and surface interatomic interactions (NN, non-NN) in relation to segregation driving forces, deviation from exact stoichiometry, possible surface relaxation and reconstruction, atomic vibrations, etc. This section attempts to quantify some of these factors and present several possible scenarios of segregation/order interplay. 

Fig.18. Phase diagram of Al-Ag [81]. Insert schematics of processes pertinent to surface segregation in bi-phase alloys (a - solid solution, S- ordered compound).

S.M. Foiles, Calculation of the surface segregation of alloys using the embedded atom method in P. A. Dowden and A. Miller Eds. Surface segregation phenomena, CRC, Boston, 1990. 

As mentioned in the introduction, measurements of surface segregation on alloys are possible by several methods. As Table 1 shows, first-layer concen-... 

In this paper we present results related to the atomic structure and catalytic properties of Pd overlayers on various substrates. A reaction has been chosen to test the catalytic properties of these systems, it is the 1,3-butadiene hydrogenation, a reaction for which Pd is known to be the best catalyst. In the following, after a short description of the experimental approach, the 1,3-butadiene hydrogenation reaction and the specific properties of Pd for this reaction will be presented. Then the reactivity of several Pd overlayers obtained either by surface segregation in Pd-based alloys or by atomic beam deposition on a metal will be investigated and discussed in terms of structure, composition related to surface segregation and surface stress. The influence of the surface orientation of the substrate will be discussed. 

Similar investigations were carried out by Hansen et al. [104], who investigated alloy formation and surface segregation in zeolite supported Pt-Pd bimetallic catalysts and concluded that hydrogen treatment at 573 K results in Pd enrichment at the particles surface. 

The aim of this chapter is to review our understanding of the fundamental processes that yield improved electrocatalytic properties of bimetallic systems. Three classes of bimetallic systems will be discussed bulk alloys, surface alloys, and overlayer(s) of one metal deposited on the surface of another. First, we describe PtjM (M=Ni, Co, Fe, Cr, V, and Ti) bulk alloys, where a detailed and rather complete analysis of surface structure and composition has been determined by ex situ and in situ surface-sensitive probes. Central to our approach to establish chemisorption and electrocatalytic trends on well-characterized surfaces are concepts of surface segregation, relaxation, and reconstruction of near-surface atoms. For the discussion on surface alloys, the emphasis is on Pd-Au, a system that highlights the importance of surface segregation in controlling surface composition and surface activity. For exploring adsorption and catalytic properties of submonolayer and overlayer structures of one metal on the surface of another, we summarize the results for Pd thin metal films deposited on Pt single-crystal surfaces. For all three systems, we discuss electrocatalytic reactions related to the development of materials... 

Kitchin JR, Reuter K, Scheffler M (2008) Alloy surface segregation in reactive environments first-principles atomistic thermodynamics study of AgjPd(l 11) in oxygen atmospheres. Phys... 

The theoretical analysis (11) of surface segregation in PtjTi, on the same basis as the analysis of dilute solid solutions, predicts no surface segregation at exact stoichiometry. The broken bond model of surface segregation is also successful in the prediction of surface segregation in some other non-ideal platinum alloys (17). 

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