Catalysts electron transfer processes

An important aspect of hydrogen transfer equilibrium reactions is their application to a variety of oxidative transformations of alcohols to aldehydes and ketones using ruthenium catalysts.72 An extension of these studies is the aerobic oxidation of alcohols performed with a catalytic amount of hydrogen acceptor under 02 atmosphere by a multistep electron-transfer process.132-134... 

The reduction of dioxygen to its fully reduced form, H20, requires the transfer of 4 electrons, and the transfer may proceed via a series of intermediate oxidation states, such as 02 /H00, HOO /HOOH, 0 /OH. These reduced forms of oxygen exhibit different redox properties and in the presence of substrate(s) and/or catalyst(s) may open different reaction paths for the electron transfer process. Fast proton transfer reactions between the corresponding acid-base pairs can introduce composite pH dependencies into the kinetic and stoichiometric characteristics of these systems. 

The kinetic models for these reactions postulate fast complex-formation equilibria between the HA- form of ascorbic acid and the catalysts. The noted difference in the rate laws was rationalized by considering that some of the coordination sites remain unoccupied in the [Ru(HA)C12] complex. Thus, 02 can form a p-peroxo bridge between two monomer complexes [C12(HA)Ru-0-0-Ru(HA)C12]. The rate determining step is probably the decomposition of this species in an overall four-electron transfer process into A and H202. Again, this model does not postulate any change in the formal oxidation state of the catalyst during the reaction. 

All the surface recombination processes, including back reaction, can be incorporated in a heavy kinetic model [22]. The predicted, and experimentally observed, effect of the back reactions is the presence of a maximum in the donor disappearance rate as a function of its concentration [22], Surface passivation with fluoride also showed a marked effect on back electron transfer processes, suppressing them by the greater distance of reactive species from the surface. The suppression of back reaction has been verified experimentally in the degradation of phenol over an illuminated Ti02/F catalyst [27]. 

Figure 4. Schematic of electron transfer processes for 2,6-disubstituted phenol. The ligand groups are indicated as Am and the intermediate polymer chain segments as straight lines, (a) Hydroxo-bridged catalyst (b) chloro-bridged catalyst.

Steric constraints dictate that reactions of organohalides catalysed by square planar nickel complexes cannot involve a cw-dialkyl or diaryl Ni(iii) intermediate. The mechanistic aspects of these reactions have been studied using a macrocyclic tetraaza-ligand [209] while quantitative studies on primary alkyl halides used Ni(n)(salen) as catalyst source [210]. One-electron reduction affords Ni(l)(salen) which is involved in the catalytic cycle. Nickel(l) interacts with alkyl halides by an outer sphere single electron transfer process to give alkyl radicals and Ni(ii). The radicals take part in bimolecular reactions of dimerization and disproportionation, react with added species or react with Ni(t) to form the alkylnickel(n)(salen). Alkanes are also fonned by protolysis of the alkylNi(ii). 

Szwarc, M. 1968. Carbanions, Living Polymers and Electron-Transfer Processes. Wiley, New York. Ziegler, K. 1952, 1955. Ziegler catalysts. Angew. Chem., 64 323 67 541. 

As demonstrated in this review, photoinduced electron transfer reactions are accelerated by appropriate third components acting as catalysts when the products of electron transfer form complexes with the catalysts. Such catalysis on electron transfer processes is particularly important to control the redox reactions in which the photoinduced electron transfer processes are involved as the rate-determining steps followed by facile follow-up steps involving cleavage and formation of chemical bonds. Once the thermodynamic properties of the complexation of adds and metal ions are obtained, we can predict the kinetic formulation on the catalytic activity. We have recently found that various metal ions, in particular rare-earth metal ions, act as very effident catalysts in electron transfer reactions of carbonyl compounds [216]. When one thinks about only two-electron reduction of a substrate (A), the reduction and protonation give 9 spedes at different oxidation and protonation states, as shown in Scheme 29. Each species can... 

Significant progress in the development of such artificial photosynthetic systems, particularly aimed at the photolysis of water, has been reported in recent years. Several approaches to resolve the problems involved in controlling the photoinduced electron transfer process as well as the development of catalysts for multi-electron fixation processes will be discussed in this paper. 

Different aspects involved in the design of artificial photosynthetic systems have been discussed. Charged colloids and water-oil microemulsions provide effective organized media for controlling photosensitized electron transfer processes. Development of catalysts capable of utilizing the photoproducts in chemical routes, particularly in multi-electron fixation processes is of major... 

The catalytic cycle of Cu-catalyzed oxidation of phenol involves the coordination of the substrate to the Cu(II) complex and an electron transfer process from the substrate to Cu(II). The reduced catalyst may be reoxidized to the original Cu(II) complex by 02. The details of the catalysis are still undetermined because of the complexity of its elementary steps. 

The d-d absorption of the copper complex differs in each step of the catalysis because of the change in the coordination structure of the copper complex and in the oxidation state of copper. The change in the visible spectrum when phenol was added to the solution of the copper catalyst was observed by means of rapid-scanning spectroscopy [68], The absorbance at the d-d transition changes from that change the rate constants for each elementary step have been determined [69], From the comparison of the rate constants, the electron transfer process has been determined to be the rate-determining step in the catalytic cycle. 

Ruthenium catalysts can participate in electron-transfer processes. Thus, a variety of radical reactions of organic halides have been catalyzed by ruthenium complexes, as in the following example [ 126] (Eq. 95). 

It was found, that also Ru and Os colloids can act as catalysts for the photoreduction of carbon dioxide to methane [94, 95]. [Ru(bpy)3]2+ plays a role of a photosensitizer, triethanolamine (TEOA) works as an electron donor, while bipyridinium electron relays (R2+) mediate the electron transfer process. The production of hydrogen, methane, and small amounts of ethylene may be observed in such a system (Figure 21.1). Excited [Ru(bpy)3]2+ is oxidized by bipyridinium salts, whereas formed [Ru(bpy)3]3+ is reduced back to [Ru(bpy)3]2+ by TEOA. The reduced bipyridinium salt R + reduces hydrogen and C02 in the presence of metal colloids. Recombination of surface-bound H atoms competes with a multi-electron C02 reduction. More selective reduction of C02 to CH4, ethylene, and ethane was obtained using ruthenium(II)-trisbipyrazine, [Ru(bpz)3]2+/TEOA/Ru colloid system. The elimination of hydrogen evolution is thought to be caused by a kinetic barrier towards H2 evolution in the presence of [Ru(bpz)3]2+ and noble metal catalysts [96]. 

In such processes the metal ion acts (in combination with ROOH) as an initiator rather than a catalyst. It is important to note that homolytic decomposition of alkyl hydroperoxides via one-electron transfer processes is generally a corn-... 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

Single-electron transfer processes have been implicated." When combined with asymmetric hydrosilylation using a chiral catalyst, optically active alcohols can be generated. ... 

Compared to the multistep electron-transfer process shown in Scheme 3.4, more simple aerobic oxidations of alcohols were reported with various homogeneous and heterogeneous ruthenium catalysts. The aerobic oxidation of alcohols with metal catalysts is an attractive method for economical and environmental reasons. Aerobic oxidation of alcohols can be carried out using RUCI3 catalyst with moderate conversion and selectivities (Eq. 3.13) [31]. Since this reaction was first reported, an arduous search for suitable catalysts has been continuing using various ruthenium complexes (Table 3.1). 

B. Energy and Electron Transfer Processes on Catalysts 1. Energy Transfer... 

Dynamics of Electron Transfer Processes on Semiconducting Catalysts... 

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