Electron heterogeneous systems

Electron transfer reactions involving alkali metals are heterogeneous, and for many purposes it is desirable to deal with a homogeneous electron transfer system. It was noticed by Scott39 that sodium and other alkali metals react rapidly with aromatic hydrocarbons like diphenyl, naphthalene, anthracene, etc., giving intensely colored complexes of a 1 to 1 ratio of sodium to hydro-... 

Electron Transfer Far From Equilibrium. We have shown how the Marcus Theory of electron transfer provides a quantitative means of analysis of outer-sphere mechanisms in both homogeneous and heterogeneous systems. It is particularly useful for predicting electron transfer rates near the equilibrium potential,... 

Marcus, R. A. (1975), "Electron Transfer in Homogeneous and Heterogeneous Systems", in E. D. Goldberg, Ed., The Nature of Seawater, Dahlem Konferenz, Berlin. 

Metal ions in heterogeneous systems are expected to be solvated by the different types of water, leading to a partition of the ions between the various phases present. If the guest is paramagnetic, its solvation can be studied using the technique of Electron Spin Resonance (ESR) and used to derive information on the network-solvent interactions. 

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. 

Metal sulfides belong to the most important classes of compounds because they are of general significance for geochemistry (they are the most important ores for many metals), analytical and structural chemistry, and biochemistry (metal sulfide systems act as electron transfer systems) as well as catalysis (a high percentage of industrially used heterogeneous catalysts are sulfides) and materials science. 

It was originally thought that the sonication approach would promote miscibility at the interface of the heterogeneous system and also would raise the electronic states of a small portion of the reactants above their ground state, which would hopefully enhance the addition reaction. We later found, that only the simultaneous photoexcitation and ultrasonic irradiation plus the addition of an appropriate gas (e.g., hexafluoropropane), which Increased the pressure in the reaction vessel, formed the fully saturated products. The added pressure above the liquid phase is another parameter required to form the desired saturated product and should have affected the temperature and pressure at the localized hot spots created by sonication. 

However, this commonly accepted theory is incomplete and applies with much difficulty to systems involving nonvolatile substances. The most relevant example is metals. For a heterogeneous system, only the mechanical effects of sonic waves govern the sonochemical processes. Such an effect as agitation, or cleaning of a solid surface, has a mechanical nature. Thus, ultrasound transforms potassium into its dispersed form. This transformation accelerates electron transfer from the metal to the organic acceptor see Chapter 2. Of course, ultrasonic waves interact with the metal by their cavitational effects. 

Mother nature has resolved the various limitations involved in multi-electron processes. Unique assemblies composed of cofactors and enzymes provide the microscopic catalytic environments capable of activating the substrates, acting as multi-electron relay systems and inducing selectivity and specificity. Artificially tailored heterogeneous and homogeneous catalysts as well as biocatalysts (enzymes and cofactors) are, thus, essential ingredients of artificial photosynthetic devices. 

In addition to described heterogeneous systems a homogeneous photocatalytic C02 reduction was tested using heteroleptic rhenium complexes [99, 100] and supramolecular ruthenium and rhenium bi- and tetranuclear complexes their excited states were quenched by 1-benzyl-1,4-dihydronicotinamide (BNAH) and C02 was reduced by the electron donor intermediate species [101]. 

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