Reduction thermodynamics

From equation (2.36), it can be seen that E-E0 when 0OX = 1/2 this defines E0, it is the potential at which the surface coverage by the oxidised form is 1/2, it is not the thermodynamic reduction potential of the O/R couple. Differentiating (2.36) ... 

Thermodynamic reduction potentials of numerous aromatics were first measured by Hoijtink and van Schooten in 96% aqueous dioxane, using polarography [15, 16]. These fundamental works were decisive tests of the HMO theory, showing that the polarographic half-wave potentials vary linearly with the HMO energies of the lowest unoccupied molecular orbitals (LUMO) of the hydrocarbons [1]. Hoijtink etal. had already noticed that most aromatics can be further reduced to their respective dianions [17]. They proposed a... 

This represents electrodeless electrochemistry, whereby the only process is electron transfer and the only product is the one-electron adduct of 02, the superoxide ion (02). Through the use of redox mediator dyes and spectrophotometry, the electron-transfer thermodynamic reduction potential for 02 has been evaluated ... 

It is necessary to know the thermodynamic reduction potentials of the active metals in chloroaluminate melts. Scordilis-Kelley et al. [451,467] have studied standard reduction potentials in ambient temperature chloroaluminate melts for lithium and sodium, and they have calculated those of K, Rb, Cs. The values are, respectively, -2.066 V, -2.097 V, -2.71 V, -2.77 V and -2.87 V [versus A1(III)/A1 in a 1.5/1.0 A1C13/MEIC reference melt]. 

The product combination (H - - HO ) represents the ultimate thermodynamic reductant for aqueous systems. In the absence of an H atom (and the stabilization afforded to HO by formation of the 119kcalmol H-OH bond), the hydroxide ion becomes a much less effective reductant (equation 160). 

The reduction mechanism changes for complexes containing sterically hindered danphen or catphen ligands. The 4-coordinated species [Ni(danphen)2] + and [Ni(catphen)2] + are reduced by one electron to the corresponding Ni complexes [108]. The distorted tetrahedral coordination required by these ligands stabilizes Ni complexes both thermodynamically (reduction occurs at ca -0.57 V) and kineti-cally towards ligand loss and, for [Ni(catphen)2] +, even towards reoxidation by O2 [108]. [Ni(catphen)2] +, on the other hand, cannot be oxidized to a Ni complex. 

Hoijtink et al. [27] also developed an alternative method of generating anionic species, which was improved by Szwarc et al. [28]. The technique involves potentiometric titration of aromatic compounds with a standard solution of Na-biphenylide. The extremely negative reduction potential of biphenyl assures that most of the common aromatics can be reduced to at least their respective radical anions. The values of the thermodynamic reduction potentials are generally obtained from the potentiometric titration curve. As all experiments are usually carried out in ethereal solutions, such as tetrahydrofuran (THF) or dimethoxyethane, problems of follow-up processes are less severe. Later, Gross and Schindewolf [29] reported on the potentiometric titration of aromatics using solvated electrons in liquid ammonia. 

Advances in nanostructured conducting materials for DET have resulted in impressive current densities for the ORR, and application of these three-dimensional materials to DET from MCOs other than CueO may provide biocathodes with the characteristics suitable for an implantable EFC. While a DET approach using MCOs can provide for ORR at potentials approaching the thermodynamic reduction potential for oxygen, the current density achievable in this approach still relies upon intimate contact, and correct orientation, ofthe MCO to a conducting surface. Use of a mediator, capable of close interaction with the TI site of the MCOs, and with a redox potential tailored to permit rapid electron transfer to the TI site, can eliminate the requirement for direct contact in the correct orientation between MCO and electrode, and offer the possibility of a three-dimensional biocatalytic reaction layer on electrodes for higher ORR current densities. 

The existence of reduced volatile phosphorus compounds in aquatic systems has been in question for several decades (Morton and Edwards, 2005). Similar to nitrate reduction to ammonia (dissimi-latory nitrate reduction, see Chapter 8) and sulfate reduction to sulfide, thermodynamic reduction of phosphate to phosphine is possible. Under highly anaerobic conditions, phosphate (oxidation number of +5) can be reduced by obligate anaerobes to phosphine (oxidation number of-3). 

For instance, with a PPy film deposited on a zinc electrode in aqueous salicylate solution, the reduction potential of the doped polymer is around 0.9 V/SCE [43], whereas the thermodynamic reduction potential of O2 varies considerably with the pH, and can shift from 1.2 V/NHE (at pH 0, when the reduction involves four electrons) to 0.4 V in basic medium at pH 12 [44]. 

The theoretical thermodynamic reduction potential for oxygen is + 1.23 V vs NHE at pH 0, or +0.82 V vs NHE at pH 7, and it is thus, like peroxide, a strong oxidant [1-3]. The reduction of oxygen at electrodes is, again like peroxide, hampered by large overpotentials, with direct electrochemical reduction occurring only at----0.1 V vs NHE... 

Analysis of high-frequency square-wave voltammograms showed that the putative electrode product [4Fe-4S] did not survive for any measurable length of time. Failure to detect the reverse of Eq. (15) even at a frequency of 200 Hz meant that its half-life was certainly less than 1.6 ms at 5 °C. With the coupled, irreversible reaction Eq. (16) taken into account, a lower limit of 860 mV was assigned for the thermodynamic reduction potential of the [4Fe-4S] couple. This is much higher than the reduction potential, E = 350 mV, that is established for Chromatium vinosum HiPIP [182]. 

The efficiency of this electrocatalytic hydrogenation is self-evident since it occurs close to the thermodynamic reduction potential calculated for the fumarate/... 

Because of the favorable rates for reductive eliminations involving a hydride ligand noted in the introductory section, examples of complexes that undergo reductive elimination to form the C-H bonds in alkanes and arenes span the transition series. Thermally induced reductive eliminations to form dihydrogen (or dihydrogen complexes) from dihydride complexes can also be rapid, but this reaction occurs less frequently because the oxidative addition of dihydrogen is typically favored thermodynamically. Reductive elimination to form a C-H bond is the last step of many catalytic reactions, such as the hydrogenation and hydroformylation of olefins. 

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