Strong reactant-surface interactions

Outer-sphere pathways are normally expected to be of the weak-overlap type since reactant-electrode bond formation is, by definition, absent. Nevertheless, substantial energy differences between 0 and P, and R and S may occur (so-called "double-layer effects, Sect. 3.5.2), at least as a result of electrostatic reactant-surface interactions. Inner-sphere pathways may be either of the weak- or strong-overlap type, depending on the strength and specificity of the reactant-surface binding (Sect. 3.5.1). 

INFLUENCE OF REACTANT-SURFACE INTERACTIONS 3.5.1 Strong-overlap pathways... 

A further complication is the recent indication that even small amounts of strongly bound surface-adsorbed water may play a critical, indeed determining, role in the interaction of gases with surfaces traditionally thought to be solids. For example, in the NaCl-HN03 reaction, there is evidence that the reaction even in laboratory vacuum systems occurs on sites holding adsorbed water. As a result, the surface does not become saturated as one would expect for a solid surface, since the underlying reactant salt continues to dissolve in the surface water (Beichert and Finlayson-Pitts, 1996). 

It also is important to note that the aforementioned treatments of free-energy barriers refer only to weak-overlap reactions. This corresponds to the curve PAS in Fig. 1, where the transition-state energy is essentially unaffected (at least in a specific manner) by the proximity of the metal surface. When these reactant-electrode interactions become sufficiently strong and specific, marked decreases in both the inner- and outer-shell intrinsic barriers can be anticipated. This is discussed further in Sects. 3.5.1 and 4.6. 

The enhancement of rs was strongly interrelated with the enantiomeric excess and was explained by the interactions between reactant and modifier on the catalyst surface. Interactions responsible for the enantiodifferentiation result also in the enhancement of regioselectivity. The coverage dependent adsorption modes of the modifier and reactant are responsible for the dependence of the rs and enan-tioselectivity on the modifier concentration. 

Bonding modifiers are employed to weaken or strengthen the chemisorption bonds of reactants and products. Strong electron donors (such as potassium) or electron acceptors (such as chlorine) that are coadsorbed on the catalyst surface are often used for this purpose. Alloying may create new active sites (mixed metal sites) that can greatly modify activity and selectivity. New catalytically active sites can also be created at the interface between the metal and the high-surface-area oxide support. In this circumstance the catalyst exhibits the so-called strong metal-support interaction (SMSI). Titanium oxide frequently shows this effect when used as a support for catalysis by transition metals. Often the sites created at the oxide-metal interface are much more active than the sites on the transition metal. 

Most catalytic reactions take place via the formation of intermediate compounds between the reactants or products and the surface. The surface atoms of the catalyst form strong chemical bonds with the incident molecules, and it is this strong chemical surface-adsorbate interaction which provides the driving force for breaking high-binding-energy chemical bonds (C—C, C—H, H—H, N—N, and C=0 bonds), which are often an important part of the catalytic reaction. 

In a simple ion transfer reaction, the distance of the reactant to the surface changes, and it becomes quite strong when it is actually in contact with the metal. Thus, a full description requires a good treatment of the interaction both with the solvent and with the metal. Nevertheless, the energy of activation is mainly determined by the partial... 

Reactants AB+ + CD are considered to associate to form a weakly bonded intermediate complex, AB+ CD, the ground vibrational state of which has a barrier to the formation of the more strongly bound form, ABCD+. The reactants, of course, have access to both of these isomeric forms, although the presence of the barrier will affect the rate of unimolecular isomerization between them. Note that the minimum energy barrier may not be accessed in a particular interaction of AB+ with CD since the dynamics, i.e. initial trajectories and the detailed nature of the potential surface, control the reaction coordinate followed. Even in the absence (left hand dashed line in Figure 1) of a formal barrier (i.e. of a local potential maximum), the intermediate will resonate between the conformations having AB+ CD or ABCD+ character. These complexes only have the possibilities of unimolecular decomposition back to AB+ + CD or collisional stabilization. In the stabilization process,... 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

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