Heterogeneous reactions, interfaces different phases

The solvent used in catalytic hydrogenation is chosen for its ability to dissolve the alkene and is typically ethanol, hexane, or acetic acid. The metal catalysts are insoluble in these solvents (or, indeed, in any solvent). Two phases, the solution and the metal, aie present, and the reaction takes place at the interface between them. Reactions involving a substance in one phase with a different substance in a second phase aie called heterogeneous reactions. 

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. 

Before a heterogeneous electron-transfer reaction can take place, be it oxidation or reduction, we must appreciate that the redox reaction occurs at the interface that separates the electrode and the solution containing the electroanalyte. Some electrochemists call this interface a phase boundary since either side of the interface is a different phase (i.e. solid, liquid or gas). An electrochemist would usually indicate such a phase boundary with a vertical line, . Accordingly, the interface could have been written as solution electrode . 

In Chapter 3 we described the structure of interfaces and in the previous section we described their thermodynamic properties. In the following, we will discuss the kinetics of interfaces. However, kinetic effects due to interface energies (eg., Ostwald ripening) are treated in Chapter 12 on phase transformations, whereas Chapter 14 is devoted to the influence of elasticity on the kinetics. As such, we will concentrate here on the basic kinetics of interface reactions. Stationary, immobile phase boundaries in solids (e.g., A/B, A/AX, AX/AY, etc.) may be compared to two-phase heterogeneous systems of which one phase is a liquid. Their kinetics have been extensively studied in electrochemistry and we shall make use of the concepts developed in that subject. For electrodes in dynamic equilibrium, we know that charged atomic particles are continuously crossing the boundary in both directions. This transfer is thermally activated. At the stationary equilibrium boundary, the opposite fluxes of both electrons and ions are necessarily equal. Figure 10-7 shows this situation schematically for two different crystals bounded by the (b) interface. This was already presented in Section 4.5 and we continue that preliminary discussion now in more detail. 

In spite of their seeming variety, theoretical approaches of different authors to the consideration of solid-state heterogeneous kinetics can be divided into two distinct groups. The first group takes account of both the step of diffusional transport of reacting particles (atoms, ions or, in exceptional cases if at all, radicals) across the bulk of a growing layer to the reaction site (a phase interface) and the step of subsequent chemical transformations with the participation of these diffusing particles and the surface atoms (ions) of the other component (or molecules of the other chemical compound of a binary multiphase system). This is the physicochemical approach, the main concepts and consequences of which were presented in the most consistent form in the works by V.I. Arkharov.1,46,47... 

However, the electric potential of the electrocatalyst at its interface with the electrolyte (and thus the facility for charge transfer) can be easily and extensively altered at will to control rate and selectivity. For instance, a decrease of electrode potential by about 0.15 V can change the product selectivity for vinyl fluoride and chloride reduction on palladium by as much as 80% (31). In contrast, gas phase parallel reductions, with 5 kcal/mol difference in activation energies, would require a temperature increase from 500 K to 730 K for a comparable selectivity change. We should note here that the electrocatalytic specificity of the above reductions is quite similar to that of conventional heterogeneous catalytic reactions, but differs from that of conventional electrolytic reduction on noncatalytic electrodes (32). 

Many different types of interfacial boundaries can be probed by SECM. The use of the SECM for studies of surface reactions and phase transfer processes is based on its abilities to perturb the local equilibrium and measure the resulting flux of species across the phase boundary. This may be a flux of electrons or ions across the liquid/liquid interface, a flux of species desorbing from the substrate surface, etc. Furthermore, as long as the mediator is regenerated by a first-order irreversible heterogeneous reaction at the substrate, the current-distance curves are described by the same Eqs. (34) regardless of the nature of the interfacial process. When the regeneration kinetics are more complicated, the theory has to be modified. A rather complete discussion of the theory of adsorption/desorption reactions, crystal dissolution by SECM, and a description of the liquid/liquid interface under SECM conditions can be found in other chapters of this book. In this section we consider only some basic ideas and list the key references. 

In the analysis of heterogeneous solubilization, the role of the solid-phase reaction in influencing the overall reaction is different from that for the usual gas-solid catalytic reaction. The most important situation is that the film and internal diffusion effects within the solid and at the solid-liquid interface are significant. 

In contrast to homogeneous catalysis, in the heterogeneous case reactants and catalyst do not exist in a single but in two different phases, separated by an interface, which is an additional area of high reactivity. The main principle of a catalyst is to accelerate a chemical reaction by diminishing its activation energy... 

The Boltzman factor e is of the order of 20 for a phosphatidylinositol membrane under the conditions of these experiments. The values of Kr and Kr indicate that the conditions for the reaction in solution and at the interface are rather different. In solution, the association takes place in a homogeneous medium on the other hand, the complex formation at the interface is a heterogeneous reaction in which the ion comes from the aqueous phase and combines with a carrier molecule that is bound to the membrane. The detailed mechanism of this heterogeneous reaction is not clear, however, and therefore we cannot explain why the reaction at the membrane is so much slower than in solution. It is possible that the carrier molecule at the interface is stabilized in a conformation that is less favorable for complex formation. 

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