Transition multielectron transfers

Alonso-Vante N, Schubert B,TributschH (1989) Transition metal cluster materials for multielectron transfer catalysis. Mater Chem Phys 22 281-307... 

All these results indicate that one is just at the beginning of understanding the function of catalysts being deposited on a semiconductor. There is still quite a confusion in many papers published in this field. Therefore the catalytic properties depend so much on the procedure of deposition . It seems to be rather difficult to produce a catalyst for 02-formation, as shown by results obtained with Ti02 (see e.g.) . Rather recently new concepts for the synthesis of new catalysts have been developed applicable for multielectron transfer reactions. Examples are transition metal cluster compounds such as M04 2RU1 gSeg and di- and trinuclear Ru-complexes . 

As will be discussed in Sect. 4.1, multielectron transfer reactions at electrodes are most likely to occur in a series of single one-electron steps. For the present discussion, a general single n-electron transfer reaction is considered (only one transition state) with n most probably one... 

In order to produce higher-order products, there has been a focus on transition-metal-based electrocatalysts containing multiple metal centers to facilitate multielectron transfers. This approach is based on the concept that a multielectron mechanism is required to produce highly reduced species [31], However, while multielectron charge-transfer catalysts have been demonstrated to affect the 2 e reduction of CO2 to CO and formate, more highly reduced products are only sporadically observed. 

But also the gs deviates from the uncorrelated limits, with f, 0, in Fig. 11, panel b), even if, quite predictably, deviations are much smaller than for excited states. Finite near the discontinuous neutral-zwitterionic crossover show that the gs cannot be described as the product of local molecular states and therefore does not coincide with the mf or excitonic vacuum state the very same gs is collective in nature and 1-droplet states contribute to the gs. This is the key to understand multielectron transfer wave functions with 1-droplet character have, in C-clusters, very large permanent dipole moments, so that their finite amplitude in the gs is the origin of sizeable transition dipole moments towards states characterized by a high 1-droplet character. 

Solorza-Eeria O, EUmer K, Giersig M, Alonso-Vante N (1994) Novel low-temperature synthesis of semiconducting transition metal chalcogenide electrocatalyst for multielectron charge transfer Molecular oxygen reduction. Electrochim Acta 39 1647-1653... 

Attempts have been made to estimate quantitatively the various effects possible from the theoretical viewpoint on an electrochemical interface for superconductors have been made. For example, it was established [154, 156, 158] that the probability of the electron-pair tunneling is, in principle, always substantially lower than that for usual electrons (all other factors being equal), a result that implies the prediction of inhibition near 7. Kuznetsov [158] considered in detail the mechanisms of the processes with the participation of Cooper pairs. For instance, the energy barriers were estimated for a variety of mechanisms, including the transfer to one and the same particle (capable of multielectron transformation) to two spatially separated particles, and also the transfer of the pair to one particle with the simultaneous transition of one of the pair s electrons to the normal state. It was found that the properties of the system can vary substantially, depending on the relationship between the band gap, the medium reorganization energy, and the overpotential. 

Another problematic point appears in the treatment of electron loss due to heavy (neutral) targets. In this case, unrealistic capture processes come into play where the projectile electron is transferred into populated bound target states. In principle, this problem may be circumvented by using the multielectron anti-symmetrization method, where the Pauli exclusion principle is enforced for the transitions amplitudes. Thus, an explicit and time-consuming treatment of these occupied bound states would then be necessary. 

The reduction of C02 requires electron transfer in one-electron or multielectron steps either from reducing agents, for example, H2, or electrochemically. H2 can also be produced by water splitting either electrochemically or photochemically. For efficient electrochemical reduction of dissolved CO2, electron transfer catalysts (electron relays, mediators), usually transition metal complexes, are required while photochemical systems need also a photosensitizer. The two approaches can be combined to photoelectrochemical systems, as well. 

The mechanism of electrochemical reduction of seven-coordinate MoO complexes is illustrated in Figure 6. Each electron transfer is accompanied by a change in coordination number of the metal. This alters the energy of the redox orbital in such a way that transfer of two electrons becomes possible at neatly identical potentials. The mechanism in Figure 6 may not be relevant to tite behavior of molybdenum hydroxylases, but it does illustrate that coodination reactions can sustain multielectron redox events at transition metal centers. 

In order to effect the conversion of radiation energy into chemical energy, multielectron processes will frequently be necessary. As a means to achieve such processes, photoredox at a multiplicity of metal centers may be necessary. Such complexes have been synthesized using a cyanide bridge from cobalt to other metal centers. One of the earliest examples is the complex (NH3)5Co° 0/-NC)Ru°(CN)5. This complex shows an intervalence transfer (IT) band due to a Ru° Co° transition at 375 nm, and irradiation into this band results in photoredox. Similar photoredox reactions are observed in complexes containing cyanide-bridged Co Os°, Cr Fe° and Fe°Os° combinations. ... 

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