Faceting energy change

One can consider two facets of the solvation process, the energetics and the kinetics. Clearly, the kinetics will not matter if reactions take place on a time scale that is much faster or much slower than the solvation process. However, if reaction and solvation occur on the same time scale, the considerable energy changes that the solvation process can engender will affect the reaction. In fact, exactly what solvent motions take place during the solvation process may well be important. Thus, it is of interest to understand the kinetics of the solvation process. 

Whereas the spot positions carry information about the size of the surface unit cell, the shapes and widths of the spots, i.e. the spot profiles, are influenced by the long range arrangement and order of the unit cells at the surface. If vertical displacements (steps, facets) of the surface unit cells are involved, the spot profiles change as a function of electron energy. If all surface unit cells are in the same plane (within the transfer width of the LEED optics), the spot profile is constant with energy. 

Figure 4.17. Measured sticking probability (relative to the clean surface) of a methane molecular beam on Ni(l 11) surfaces with varying amounts of Au alloying into the surface. The result of a model (prediction) based on DFT calculations of the change in the activation energy due to the addition of Au atoms is also shown. The beam data primarily measures methane sticking on the facets. Adapted from Ref. [59].

Recently, experimental and theoretical evidence for a model of the active site of industrial methanol synthesis that combines the role of ZnO and defects in Cu has been presented [58]. Planar defects have been shown to lead to changes in surface faceting of the Cu nanoparticles (Figure 5.3.8C) associated with formation of steps and kinks that were assumed to represent high-energy surface sites of special catalytic activity. For a series of Cu/ZnO-based catalysts, a linear correlation of the defect concentration with the intrinsic activity of the exposed Cu surface was observed. In addition, (partial) surface decoration of Cu with ZnOx by SMSI has been... 

It is known that adsorption decreases the surface energy [78]. Because the adsorption energy often depends on the type of facet, one expects a change of the equilibrium shape. Shi and Masel [106,107] have calculated the equilibrium shape of a crystal at 0 K in a presence of gases. The calculations show drastic changes of the equilibrium shape already at coverages around 0.1 ML. Then during a catalytic reaction the particle shape can evolve, which can affect the reaction kinetics, as shown recently by Monte Carlo simulations [108]. 

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