Cyclic voltammetry catalytic mechanism

For further contributions on the dia-stereoselectivity in electropinacolizations, see Ref. [286-295]. Reduction in DMF at a Fig cathode can lead to improved yield and selectivity upon addition of catalytic amounts of tetraalkylammonium salts to the electrolyte. On the basis of preparative scale electrolyses and cyclic voltammetry for that behavior, a mechanism is proposed that involves an initial reduction of the tetraalkylammonium cation with the participation of the electrode material to form a catalyst that favors le reduction routes [296, 297]. Stoichiometric amounts of ytterbium(II), generated by reduction of Yb(III), support the stereospecific coupling of 1,3-dibenzoylpropane to cis-cyclopentane-l,2-diol. However, Yb(III) remains bounded to the pinacol and cannot be released to act as a catalyst. This leads to a loss of stereoselectivity in the course of the reaction [298]. Also, with the addition of a Ce( IV)-complex the stereochemical course of the reduction can be altered [299]. In a weakly acidic solution, the meso/rac ratio in the EHD (electrohy-drodimerization) of acetophenone could be influenced by ultrasonication [300]. Besides phenyl ketone compounds, examples with other aromatic groups have also been published [294, 295, 301, 302]. 

The advantage of using a sacrificial anode has been clearly pointed out. Magnesium was found to be the most convenient, the oxidation of which produces Mg ions which can enter the catalytic cycle to cleave the nickela-cycle intermediate and liberate Ni for further catalytic cycles (Scheme 7). Such a mechanism has been substantiated on the basis of the formation of the nickelacycle and its characterization by cyclic voltammetry. In the absence of Mg (reactions conducted in a divided cell in the presence of ammonium ions) the nickelacycle does not transform and the reaction stops when all the starting nickel compound has been reacted. Upon addition of MgBr2 to an electrochemically prepared solution of the nickelacycle, Ni(II) is recovered [114]. 

In Fig. 6 is shown a series of cyclic voltammograms which demonstrate that the catalytic properties of the the complex are due to chemistry that originates from the second bpy-based reduction wave. Using bulk electrolysis and cyclic voltammetry techniques combined with digital simulation methods, the following mechanism can be proposed for electrocatalytic CO production in CH CN... 

The equilibrium potential for a aingle cell, given by equation (11), for the cathodic and anodic reactions (5) and (8), is -406mV for a process gas containing 2000 ppm HgS and an anode product of pure sulfur vapor. To this must be added the overpotentials needed for both electrode reactions and ohmic loss. The electrode reactions have been studied in free electrolyte on graphite electrodes . Potential-step experiments showed very rapid kinetics, with exchange currents in both cathodic and anodic direction near 40 mA/cm . Cyclic voltammetry verified a catalytic reaction mechanism with disulfide as the electro-active species. At the cathode ... 

Fig. 5.3. Effect of the chemical kinetics on the cyclic voltammetry of the first-order catalytic mechanism at a macroelectrode.

A. Molina, C. Serna, and J. Gonzalez. General analytical solution for a catalytic mechanism in potential step techniques at hemispherical microelectrodes Apphcations to chronoamperometry, cyclic staircase voltammetry and cyclic linear sweep voltammetry, J. Electroanal. Chem. 454, 15-31 (1998). 

Figure 6.1 shows the cyclic voltammetry of the second-order catalytic mechanism for different kinetics with = 10 and the concentrations... 

Fig. 6.1. Cyclic voltammetry of the second-order catalytic mechanism (black lines) under linear diffusion conditions and for a fully reversible electrode reaction (grey line).

The extent to which the electrode surface is covered by chemisorbed hydrogen atoms has been classically demonstrated by cyclic voltammetry and chronopotentiometry, particularly with respect to elucidation of the mechanism of the hydrogen evolution reaction and in the electrochemistry of fuel cells. The involvement of such intermediates is also consistent with the known mechanisms of catalytic hydrogenation, both from the vapor phase and the liquid phase. These results also indicate coadsorption of R species. 

Chemical kinetics at tubular electrodes (systems of circular cross-section) have been considered recently in [77, 78]. Both for catalytic ErevCcat and for E vCirr (follow-up) mechanisms the theory of linear-sweep voltammetry and cyclic voltammetry was elaborated. The effects of the reaction rate constant, of the flow velocity, and of the potential scan rate on the shape of current curves are presented graphically. The deductions derived from both theories were tested on the reduction of Fe(III) in the presence of hydrogen peroxide (catalytic system) and on the oxidation of 1,4-phenylenediamine in alkaline medium (E vCin. mechanism). 

The mechanism of oxidation of D-glucose in alkaline media at gold electrodes has been investigated. Experiments using cyclic voltammetry at a rotating disc electrode indicated that the mass-transport-limited reaction proceeds an enediol intermediate hydrogen-bonded to catalytic hydrous gold oxide. The enediol is... 

Voltammetry is a common electroanalytical technique for characterization of enzyme-modified electrodes. In cyclic voltammetry (CV), a potential window is scaimed in the forward and reverse directions while the resulting current is measured. This technique is useful for determining the reduction potential of the enzyme or coenzyme and for determining the overpotential for the system, which, in turn, corresponds to efficiency. Using this technique, detailed information about the catalytic cycle of the system can be determined including electron transfer kinetics, reaction mechanisms, current densities, and reduction potentials [6,7]. 

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