Interaction substrate-adsorbate

The balance between these different types of bonds has a strong bearing on the resulting ordering or disordering of the surface. For adsorbates, the relative strength of adsorbate-substrate and adsorbate-adsorbate interactions is particularly important. Wlien adsorbate-substrate interactions dominate, well ordered overlayer structures are induced that are arranged in a superlattice, i.e. a periodicity which is closely related to that of the substrate lattice one then speaks of commensurate overlayers. This results from the tendency for each adsorbate to seek out the same type of adsorption site on the surface, which means that all adsorbates attempt to bond in the same maimer to substrate atoms. 

Conventional bulk measurements of adsorption are performed by determining the amount of gas adsorbed at equilibrium as a function of pressure, at a constant temperature [23-25], These bulk adsorption isotherms are commonly analyzed using a kinetic theory for multilayer adsorption developed in 1938 by Brunauer, Emmett and Teller (the BET Theory) [23]. BET adsorption isotherms are a common material science technique for surface area analysis of porous solids, and also permit calculation of adsorption energy and fractional surface coverage. While more advanced analysis methods, such as Density Functional Theory, have been developed in recent years, BET remains a mainstay of material science, and is the recommended method for the experimental measurement of pore surface area. This is largely due to the clear physical meaning of its principal assumptions, and its ability to handle the primary effects of adsorbate-adsorbate and adsorbate-substrate interactions. 

The first four facets are rotationally equivalent to each other as are the final four. The two sets are related by reflectional symmetry to each other. When a chiral adsorbate, for example, S-lysine, is used, the reflectional symmetry is no longer valid and only rotationally equivalent facets should be formed. This was demonstrated elegantly by Zhao with STM [53], The driving force for facet formation is proposed to be a three-point interaction involving the carboxylate group, the a-amino group, and the amino-terminated side chain. The simultaneous optimization of adsorbate-adsorbate and adsorbate-substrate interactions determines the stereochemistry of the facet. 

Preferred adsorption of the unsaturated bond of the substrate occurs at that face which presents the least steric interactions between the adsorbed substrate and the surface. Since some amazingly sterically hindered molecules can be hydrogenated, at least some active sites must look like corners or edges or some other protuberances. 

The adsorbed substrate adopts that conformation which best balances the least strain within the adsorbed substrate and the fewest interactions... 

In non-electrochemical heterogeneous catalysis, the interface between the catalyst and the gas phase can often be characterized using a wide variety of spectroscopic probes. Differences between reaction conditions and the UHV conditions used in many studies have been probed extensively 8 as have differences between polycrystalline and single-crystalline materials. Nevertheless, the adsorbate-substrate interactions can often be characterized in the absence of pressure effects. Therefore, UHY based surface science techniques are able to elucidate the surface structures and energetics of the heterogeneous catalysis of gas phase molecules. 

The electrochemical interface between an electrode and an electrolyte solution is much more difficult to characterize. In addition to adsorbate-substrate and adsorbate-adsorbate interactions, adsorbate-electrolyte interactions play a significant role in the behavior of reactions on electrode surfaces. The strength of the adsorbate-substrate interactions is controlled by the electrode potential, which also determines the configuration of the electrolyte. With solution molecules, ions, and potential variation involved, characterization of the electrochemical interface is extremely difficult. However, by examining solvation, ion adsorption, and potential effects as individual components of the interface, a better understanding is being developed. 

An enantioselective catalyst must fulfill two functions (1) activate the different reactants (activation) and (2) control the stereochemical outcome of the reaction (controlling function). As an accepted general model, it is postulated that this control is achieved by specific interactions between the active centers of the catalyst, the adsorbed substrates, and the adsorbed chiral auxiliary (Figure 14.4). Experience has shown that most substrates that can be transformed in useful enantiomers have an additional functional group that can interact with the chiral active center. 

At low adsorbate coverages the surface structure of the deposited metal is determined by the substrate periodicity. Thus, under these conditions the adsorbate-substrate interaction is predominant. At higher coverages the adsorbate may continue to follow the substrate periodicity or form coincidence structures with new periodicities that are unrelated to the substrate periodicity. The ordering geometry of large-radius metallic adatoms (especially K, Rb and Cs) shows relatively little dependence on the substrate lattice they tend to form hexagonal close-packed layers on any metal... 

Deposition of Cu-phthalocyanine on a Pt(l 11) surface resulted in only poorly ordered monolayer structures and no ordering of multilayer structures. This demonstrates the importance of the details of the adsorbate-substrate interaction even for very large adsorbates which overlap tens of surface atoms. The absence of an ordered multilayer structure on this substrate indicates the role of an initially ordered mono-layer in controlling epitaxial growth. 

When adsorption takes place on an ordered metal-crystal surface, the adsorbed material forms ordered surface structures. The root cause of this is in mutual atomic interactions, which may be categorized into adsorbate-adsorbate and adsorbate-substrate interactions. In case of chemisorption, the former is considerably the weaker of the two. The possible long-range ordering of the overlayer formed is dominated by adsorbate-adsorbate interaction, however. Ordering of the adsorbed material is also dependent on the degree of surface coverage. Thus, for instance, at... 

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