Monolayer model, heterogeneous catalysts

Theoretical and experimental studies of model bimetallic catalysts in recent years have distinguished between thermodynamically stable bulk alloys and so-called near surface alloys. Near surface alloys are materials where the top few surface layers are created in a chemically heterogeneous way, for example, by depositing a monolayer of one metal on top of another metal. These structures are often not the thermodynamic equilibrium states of the material. To give one example, Ni and Pt form an fee bulk solid solution under most (but not all) conditions,73 so if a monolayer of Ni is deposited on Pt and the system comes to equilibrium, all of the deposited Ni will dissolve into the bulk. There is, however, a considerable kinetic barrier to this process, so the near surface alloy of a monolayer on Ni on Pt(lll) is quite stable provided a moderate temperature is used.191 If the deposited monolayer in systems of this type has a tendency to segregate away from the surface, a common near surface alloy structure is the formation of a subsurface layer of the deposited metal.85 The deposition of V on Pd(lll) is one example of this behavior.192... 

It is difficult to deduce what gold particle morphologies arise from heterogeneous chemical reduction of HAuCU. Understanding of the model catalysts is much easier. In brief, a) nucleation of gold clusters occurs at surface defects that act as traps b) on AI2O3, there are two kinds of traps at <0.8 and >1.6eV c) the defect density is ca. 3 x 10 sites per cm (10 monolayer) and d) when the clusters grow to >600 atoms, they leave the traps. This can explain the bimodal size distribution of the clusters. Atomistic definition of these traps is needed. 

FIGURE 3.8 Mapping of a 3D nanoparticle model onto a 2D surface model with active and inactive sites. The active site concept is the integral part of the heterogeneous surface model of nanoparticle activity, explored here for CO d electro-oxidation. Immobile adsorbs, preferentially, on specific sites called the active sites. The mobile CO d is found on the remaining inactive catalyst sites. (Reprinted with permission from Andreaus, B.et al. 2006. Kinetic modeling of COad monolayer oxidation on carbon-supported platinum nanoparticles. J. Phys. Chem. B, 110, 21028-21040, Figure 2,7,8,9, American Institute of Physics.)... 

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