Adsorbate-induced surface segregation

Mayrhofer KJJ, Juhart V, Hartl K, Hanzlik M, Arenz M (2009) Adsorbate-induced surface segregation for core-shell nanocatalysts. Angew Chem Int Ed 48(19) 3529-3531... 

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),... 

Later in this chapter, a considerable number of similar examples of room temperature adsorbate induced segregation will be shown. The role of particle size will be examined and the possibility will be discussed that near surface diffusion dominates over bulk diffusion. 

However, once the surface is covered by an adsorbate layer, it is difficult to use this method to quantify adsorbate induced segregation effects. If, for example, the adsorbate was to preferentially bond to element X, a relatively large attenuation of the XPS peak of element X may be observed due to the fact that this element is covered in an adsorbate layer while the peak intensity of element Y may be unaffected. This could be mistaken for the observation of segregation of element Y to the surface, which may be the complete opposite of the actual effect. 

LEIS has been used to investigate adsorbate induced segregation at the surfaces of bimetallic nanoclusters [84]. van den Oetelaar et al. showed that for Pt/Pd catalysts with low metal dispersions of about 0.3 and 0.8, Pd surface... 

One of the most revealing features of these experiments is that the composition of the top two layers of the bimetallic surface remains almost constant throughout. It appears that any segregation effect merely involves swapping atoms between the top two layers rather than any diffusion of material from the bulk. This may account for the fact that adsorbate induced segregation is observed even at room temperature and that diffusion of atoms from the bulk... 

Hammer et al. [55] used LEED 1(E) to study adsorbate induced variation of the stmcture and composition of the Mo75Re2s(100) surface under the influence of adsorbed H, C and O. They concluded that above 600 °C, 0(a) and C(a) induce segregation of Mo to the surface. 

Adsorbate induced segregation is a phenomenon which is underestimated by many researchers interested in the chemical properties of bimetallic surfaces. The existing database points to the driving force for such segregation being the different heat of adsorption of the adsorbate with each alloy component. 

The final group of chapters by Bertolini and Jugnet, Rogriguez, Hrbek and Baddeley address issues more directly related to the chemical properties of these surfaces, the first three of these chapters being concerned directly with the relationship of the nature of the surface alloys (and alloy surfaces) and their reactivity, while Baddeley turns the problem round in addressing the issue of adsorbate-induced modification of surface segregation not only does the alloy surface modify the reactivity, but also the reaction modifies the surface alloy. 

Menning CA, Chen JG (2009) General trend for adsorbate-induced segregation of subsurface metal atoms in bimetallic surfaces. J Chem Phys 130(17) 174709... 

The adsorbed H atoms vibrate around their equilibrium position (e.g. on top of a surface atom, or a bridge over two surface atoms) diffuse along the surface and form disordered or ordered surface structures according to temperature, coverage and substrate. The adsorption often induces a rearrangement of the surface atoms of the substrate, a surface reconstruction, or even a surface segregation if the substrate is a binary compound. Thus in a description of the topic H on metals" the following aspects should be considered ... 

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