Electron transfer coefficients estimates

The rate constant of electron transfer (ks) and anodic and cathodic electron transfer coefficients (aa and ac) of the SODs at various pH values were estimated with Laviron s equation and summarized in Table 6.5. Interestingly, the fastest electron transfer of the SODs was essentially achieved in a neutral solution, probably in agreement with the biological conditions for the inherent catalytic mechanisms of the SODs for 02" dismutation, although the electrode processes of the SODs follow a different mechanism. 

The exact values of the critical kinetic parameter depend on the electron-transfer coefficient and the amplitnde. These values are listed in Table 2.3. If the electron-transfer coefficient is not known, an average value of the critical kinetic parameter (fflbax)avr e nsed. The values of (fflbax)avr fo different amplitudes are given in Table 2.4. The error in the estimation of sur by using bax is close to 10%. 

In addition, the split peaks can be used for estimation of electron-transfer coefficient as well as for precise determination of the formal potential of the surface electrode reaction. The potential separation between split peaks is insensitive to the electron-transfer coefficient. However, the relative ratio of the heights of the split peaks depends on the electron-transfer coefficient according to the following function ... 

Here, A max is the voltage loss tolerance due to finite electronic conductivity, b = RgT/ aeffF) is the Tafel parameter with the effective electronic transfer coefficient Ueff of the ORR, and Jq is the operating current density. For instance, at Icl= 10 qm, Jo = 1 A cm , Ueff = I, T = 333 K, and Ai max = 1 mV, the electronic conductivity requirement of the CL is Oei > 0.01 S cm In an ultrathin catalyst layer (UTCL) with thickness L = 100 nm, this bound on Oei is lower, namely, uei > lO- Scm-i. This estimate explains why, in CLs fabricated with the NSTF of the company 3M, a thin film of sputter-deposited Pt provides sufficient electronic conductivity. UTCLs are much less sensitive to the support conductivity. 

If the adsorption coupled reaction (2.144) is totally irreversible, the voltammet-ric complexity is significantly reduced [111, 115]. For the totally irreversible case, the real net peak current is a linear function of the frequency, whereas the peak potentials depends linearly on log(/) with a slope of = 2-3 This slope enables estimation of the electron transfer coefficient, provided the number of exchanged electrons is known. Similarly, the same parameter can be inferred from the half-peak width, which is defined as A p/2 = (63.5 0.5) /% mV. 

Rate laws and coefficients were determined for the oxidation of all intermediate compounds by MnOJ and were compatible with the scheme as presented, i.e. including a route for direct oxidation to pyruvaldehyde. An estimate of k2 ( 5x10 l.mole sec ) suggests an initial electron-transfer to give CH3 COCH2 . This can then be rapidly oxidised in two ways... 

More recently it has been found15 that a correlation exists between spectroscopic parameters of the divalent aqua ions of the metals Cr to Ni, and the polarographic y2. A linear relationship was found between A0 and crystal field splitting parameter, ot the transfer coefficient, n the number of electrons transferred in the reduction, EVl the polarographic half-wave potential and E° the standard electrode potential. The use of the crystal field splitting parameter would seem to be a more sensible parameter to use than the position of Amax for the main absorption band as the measured Amax may not be a true estimate of the relevant electronic transition. This arises because the symmetry of the complex is less than octahedral so that the main absorption band in octahedral symmetry is split into at least two components with the result that... 

While experiments involving solution-phase reactants have provided deep insights into the dynamics of heterogeneous electron transfer, the magnitude of the diffusion-controlled currents over short timescales ultimately limits the maximum rate constant that can be measured. For diffusive species, the thickness of the diffusion layer, S, is defined as S = (nDt)1/2, where D is the solution-phase diffusion coefficient and t is the polarization time. Therefore, the depletion layer thickness is proportional to the square root of the polarization time. One can estimate that the diffusion layer thickness is approximately 50 A if the diffusion coefficient is 1 x 10-5 cm2 s-1 and the polarization time is 10 ns. Given a typical bulk concentration of the electroactive species of 1 mM, this analysis reveals that only 10 000 molecules or so would be oxidized or reduced at a 1 pm radius microdisk under these conditions The average current for this experiment is only 170 nA, which is too small to be detected with high temporal resolution. 

Electron transfer properties of polyhalogenated biphenyls were investigated by cyclic voltammetry. The primary reduction peak of 4,4 -dichlorobiphenyl, involving replacement of halide with hydrogen in an irreversible ECE- type reaction, are under kinetic control of the initial ET step. Electrochemical transfer coefficients, standard potentials and standard heterogeneous rate constants were also estimated from the voltammetric data230. 

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