Adsorbate-induced segregation

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. 

A second example of the use of photoelectron spectroscopy to measure adsorbate induced segregation effects was the work of Rodriguez and Kuhn [76]... 

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... 

Several workers have attempted to use EXAFS to provide information on adsorbate induced segregation effects on bimetallic particles. Hills et al. [102] studied the effect of a hydrogen atmosphere on bimetallic Pt-Ru nanoparticles. Their interpretation of adsorbate induced segregation effects relied on comparing the coordination environments of Pt and Ru and trying to identify differences from what would be expected for a random distribtion of Pt and Ru within the particles. They concluded that hydrogen treatment at 673 K resulted... 

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. 

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

Adsorbate-Induced Segregation for Higher Temperature The T = 0 K results from above are complemented by canonical MC simulations for T > 0 K. They also cover the configurational entropy, give access to excited configurational states, and thus introduce a temperature scale into the Hamiltonian of Equation 11.47. Furthermore, a large MC superlattice allows adjustments to the coverage much finer than those possible with small DFT cells or comprehensive CE scans. 

Podloucky, R., and Muller, S. (2012) Adsorbate-induced segregation a first-principles study for C/Pt25Rh75(100). Phys. Rev. B, 86, 195420. 

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