Multi-electron charge-transfer reactions

The need to convert chemical energy to electricity, efficiently and at low temperature, has increased the development of materials with electrocatalytic activity toward multi-electron charge transfer. Reactions of technical relevance are, for example, cathodic processes, such as the oxygen reduction reaction (ORR),3 13 and anodic processes, such as small organics (R-OH, where R = CH3-3-12 or CH3CH-13-17) and sugars.18-20 These complex electrochemical processes are useful in low-temperature systems such as the direct methanol FC (DMFC) or biofuel cell systems. 

This multi-electron charge transfer reaction (1.23 V/SHE) depends on the electrolyte nature and the catalysts in form of a well defined surface or faceting nanoparticles, and represents a substantial cathodic overpotential loss of ca. 300 mV on the best catalyst, Pt. This means that its four-electron ORR kinetics is slow with an exchange cnrrent density between 10 to lO ttiA cm depending on the natnre of the exposed surface and electrolyte. The consequence is the generation of intermediate species of the overall reaction (3) that can be depicted by a series of reactions, namely ... 

It is evident that a single electron transfer photoproduct is transformed into a doubly reduced charge relay in two phase systems. The primary processes in the natural photosynthetic apparatus involve single electron transfer reactions that proceed in hydrophobic-hydrophilic cellular microenvironment. Thus, we suggest similar induced disproportionation mechanisms as possible routes for the formation of multi-electron charge relays, effective in the fixation of CO2 or N2. 

Another advantage of SWV over CV can be seen when dealing with a separate multi-electron transfer reaction. The CV current wave of each or each group of electrons always contains the contribution from the previous electron transfer, particularly the diffusion-controlled current. Separating currents from different electron transfers can be tedious, if not impossible. It can be even worse when we have to take into account the capacitive charging current. Since both capacitive and diffusion-controlled currents are absent or at least minimized on the 7net vs E curve of an SW voltammogram, current waves from each electron transfer are much better resolved and more accurate information can be obtained. 

Electrocatalysis is, in the majority of cases, due to the chemical catalysis of the chemical steps in an electrochemical multi-electron reaction composed of a sequence of charge transfers and chemical reactions. Two factors determine the effective catalytic activity of a technical electrocatalysts its chemical nature, which decisively determines its absorptive and fundamental catalytic properties and its morphology, which determines mainly its utilization. A third issue of practical importance is long-term stability, for which catalytic properties and utilization must occasionally be sacrificed. 

The design of such artificial photosynthetic systems suffers from some basic limitations a) The recombination of the photoproducts A and S+ or D+ is a thermodynamically favoured process. These degra-dative pathways prevent effective utilization of the photoproducts in chemical routes, b) The processes outlined in eq. 2-4 are multi electron transfer reactions, while the photochemical reactions are single electron transformations. Thus, the design of catalysts acting as charge relays is crucial for the accomplishment of subsequent chemical fixation processes. 

Voltcoulommetry (DSCVC) and Square Wave Voltcoulommetry (SWVC) are also considered, since they are very valuable tools for the analysis of fast electrochemical reactions between surface-confined molecules. First, a simple mono-electronic electrochemical reaction is analyzed and, after that, the cases corresponding to multi-electron electrochemical reactions and chemical reactions coupled to the surface charge transfer, including electrocatalytic processes, are discussed. 

The topic of this section is strictly related to those of the previous sections. In fact whereas multi-step electron transfer is the basic reaction leading to charge separation, very often energy transfer reaction can occur, and not necessarily detrimentally, in arrays designed to achieve charge separation. 

This is in contrast to the results obtained following selective excitation of the PH2 unit discussed above, and yielding a multi-step electron transfer leading to charge separation. The different outcome can be discussed on the basis of the overlap of the HOMO and LUMO orbitals involved in the electron transfer reaction for the Ir acceptor unit and the PH2 donor unit, with the aid of semi-empirical calculations [48]. Remarkably, the zinc porphyrin based array PZn-Ir-PAu, 254+, displays an efficient electron transfer with the formation of a CS state with unitary yield also upon excitation of the iridium complex. This happens because the selective excitation of the zinc porphyrin chromophore discussed above, and the deactivation of the excited state PZn-3Ir- PAu, follow the same paths as those reported in Scheme 8. 

In the MO theory, the most reliable approach for the study of reaction pathways perhaps is CASSCF [12, 13], but multi-VBSCF is essentially at the same level with CASSCF [14]. Since a VB wave function can be expanded into the combination of numerous Slater determinants that are used to define configurations in the MO theory, the VB theory provides a very compact, accurate description for chemical reactions. While both MO and VB theories have their own advantages as well as disadvantages, in our opinions, there are some areas where the VB theory is particularly superior to the MO theory 1) the refinement of molecular mechanics force field 2) the development of empirical or semi-empirical VB approaches 3) the impact of intermolecular charge transfer or intramolecular electron delocalization on the structure and properties 4) the validation of classical chemical theories and concepts at the quantitative level 5) the elucidation of chemical reactions and excited states intuitively. 

Bocarsly, Pfennig and co workers reported interesting multi electron photoreac tions for the trimeric M" Ptlv M" complexes 25a-c.212 215 In this system, a single photon excitation into the intervalence charge transfer band results dissociation of the trimer into [Pt(NH3)4]2+ and two equivalents of a M111. The initial photoexcited complex is though to dissociate first to a Mm complex and Ptni-Mn intermediate. The latter dimer subsequently undergoes a thermal electron transfer reaction to yield the final products. 

The frequency of the minimum in the semicircle is equal to the sum of the rate constants for charge transfer and recombination cum/n = k,r + krec- Light modulated microwave measurements therefore provide the sum of the two rate constants, but since it is possible to measure /c,r at high band bending where recombination is negligible, the rate constants can be separated if it is assumed that k,r is independent of potential (this assumption may not be valid for multi-electron transfer reactions as noted in section 4.2). 

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