Multi-electron processes

Compound 6 contains seven iron-based units [ 12], of which the six peripheral ones are chemically and topologically equivalent, whereas that constituting the core (Fe(Cp)(C6Me6)+) has a different chemical nature. Accordingly, two redox processes are observed, i.e., oxidation of the peripheral ferrocene moieties and reduction of the core, whose cyclic voltammetric waves have current intensities in the 6 1 ratio. Clearly, the one-electron process of the core is a convenient internal standard to calibrate the number of electron exchanged in the multi-electron process. In the absence of an internal standard, the number of exchanged electrons has to be obtained by coulometry measurements, or by comparison with the intensity of the wave of an external standard after correction for the different diffusion coefficients [15]. 

Because the charge separation is a one-electron process but the watersplitting reactions are multi-electron processes (although they have been written above as one-electron processes for simplicity), suitable catalysts are needed to accelerate these multi-electron processes so they can be brought about during the lifetime of the photoinduced species. 

In an aqueous medium the reduction of inorganic ions (for example, Cu2+, Zn2+, Cd2+) to their respective metallic states takes place by a single two-electron process. In effect, however, the process only apparently involves a two-electron step, in that it is assumed that multi-electron processes proceed by a sequence of elementary one-electron steps. Every elementary step is characterized by its own rate constant and its own standard potential. 

Therefore, given that a multi-electron process can be described as a series of one-electron transfers, more or less separated from each other, the shape of the cyclic voltammogram depends on the following factors 14... 

Excitation to the repulsive electronic state may also involve a multi-electron process. For example, creation of a core hole on a metal atom in an oxide may lead to an interatomic Auger transition which ultimately results in a positive oxygen ion which desorbs because it is now in a strongly repulsive Madelung well. Knotek and Feibelman have reported results which they interpret in this manner. Core ionization in the adsorbed molecule can also lead to an Auger process which leads to desorption. 

These potentials theoretically allow water photolysis. However, multi-electron processes have to occur at the catalyst in order to photolyze water with this complex. The lifetime of the excited state is 650 ns, and the excited state is quenched efficiently through electron transfer with redox reagents. The conversion model with this complex is described in Chapter 4. 

As described in Section 3 of Chapter 2, multi-electron processes are important for designing conversion systems. Noble metals are most potent catalysts to realize a multi-electron catalytic reaction. It is well known that the activity of a metal catalyst increases remarkably in a colloidal dispersion. Synthetic polymers have often been used to stabilize the colloids. Colloidal platinum supported on synthetic polymers is attracting notice in the field of photochemical solar energy conversion, because it reduces protons by MV to evolve H2 gas.la)... 

These metal and metal oxide catalysts must work as a kind of electron pool which brings about multi-electron process for H2 and 02 generation. Silver colloids were studied as electron pool for H2 formation under y-irradiation in the aqueous system composed of Ag° colloids, acetone, 2-propanol and SDS S9). The colloids (average diameter 140 A) of 2.5 x 10 4 M can store 1 coulomb/1, corresponding to the storage of 450 electrons/particle 60 ... 

A colloidal polynuclear metal complex was proved to be an effective catalyst for water photolysis in combination with Ru(bpy)3 probably because its capability of multi-electron process. The details are described in the next chapter. 

Polymers play important roles in water photolysis. For multi-electron processes, polymer supported metal colloids or colloidal polynuclear metal complexes are very useful as catalysts. Unstable semiconductors with a small bandgap which photolyse... 

The polyneclear and mixed valent structure of the PB allow multi-electron processes of Eqs. (22) and (23) as well as both the reduction and oxidation catalyses. The reactions are schematically shown in Fig. 18. 

Multi-electronic processes (like those consisting of two-electron transfers, EE mechanism) have been widely treated in the literature, both in their theoretical and applied aspects [4, 10, 56-68]. This high productivity measures in some way the great presence and relevance of these processes in many fields, and hence the importance of understanding them. 

In this section, the general analytical expression for the current-potential response (Eq. (6.15)) is particularized for the electrochemical techniques Cyclic Staircase Voltammetry (CSCV) and Cyclic Voltammetry (CV). Thus, the expression for the CSCV and CV currents of multi-electron processes at electrodes of any geometry and size is... 

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. 

Here, the longer arrow indicates the direction of the preferred electron transfer from the metal to the substrate (S), and the shorter arrow indicates the direction of the reverse transfer. It is obvious that four protons accompanied by the water molecule rearrangement cannot be transferred in one synchronous step. Owing to the high degree of electron delocalization in the polynuclear metal complexes, these complexes are more suitable for multi-electron processes. 

As shown previously in Fig. 9, compound 16 is a promising addition to this family of complexes capable of photodriven multi electron processes. Flash photolysis reveals a stepwise three electron oxidation at the MnnMn" center to yield the MnlnMnIV complex 17.196,197 Unlike the preceding examples, multiple oxidizing equi valents or holes are stored during the photochemical reaction making this system complimentary to those that collect multiple electrons. 

This mechanism with coupled one-electron intermolecular electron transfer from the external donor and intramolecular multi-electron transfer from the catalyst to coordinated N2 is, presumably, more efficient than the simpler mechanism considered above with one-electron and multi-electron transfers separated in time. In this mechanism the strongest reductant, which is of necessity the external reducing agent, is used for direct reduction of the substrate, whereas for consecutive one-electron and multi-electron processes its reducing power is used only to prepare the reduced form of the catalyst. 

Formation of the hydrazobenzene product is strongly favored when platinized zinc or cadmium sulfide is used as the photocatalyst (Table 3). In both cases the rate decreases considerably and hydrazobenzene becomes the major product. It is known that platinum favors multi-electron processes [34]. 

Higher-order processes contribute in both the excitation of the core electron and relaxation of the core hole. These multi-electron processes are significant because of the strong perturbation caused by the creation or the annihilation of a core hole. In these processes, additional electrons are excited (shake-up) or ionized (shake-off). Two electron processes, double autoionization and double Auger decay, were mentioned above. The final states reached in core hole decay may be excited states and also may autoionize. It is clear that excitation of a core electron and the relaxation of the core hole provide many paths leading to multiple-electron excited states. These states have a unique chemistry relative to the single-electron excited states produced by arc lamp, laser, or vacuum ultraviolet (VUV) excitation. 

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