Electron-transfer number apparent

Zn UPD on Pt Underpotential deposition of zinc was observed on Pt(l 11) in alkaline solution as a sharp cyclic voltammetric (CV) peak, in contrast to the behavior on polycrystalline Pt, when several broad UPD peaks were observed [193]. The changes of the peak potential with concentration of Zn02 were equal to 60 mV/log [Zn02 ] and led to the apparent electron transfer number ria = 1. 

This is only an approximate representation, in which a is the transfer coefficient and na is the apparent electron number involved in the pseudo-elementary reaction. This na is different from the total electron transfer number of the reaction ( ). 

ORR Apparent Electron-Transfer Number and the Formed Percentage of Peroxide Measured by RRDE Technique 221... 

Due to the production of H2O2 or HO2 through a 2-electron-transfer pathway, the overall electron-transfer number of the ORR process is always less than 4. This electron-transfer number is normally called the apparent number of electrons transferred per O2 molecule. Actually, this apparent number of electron transfer can be measured by the RRDE technique, from which the percentage of H2O2 formation in the ORR can also be calculated. Generally to say, the apparent number of electron transfer and the percentage of peroxide produced in the ORR process are two important pieces of information in evaluating the ORR catalyst s catalytic activity. 

As discussed in Chapter 5 for RDE technique, this apparent electron-transfer number of ORR or the overall ORR electron-transfer number can be obtained using the slope of the... 

Koutecky—Levich plot, measured at different electrode rotating rates, if the O2 concentration, the O2 diffusion coefficient, and the solution kinetic viscosity are known. Here, we give the expression of apparent electron-transfer number of ORR as functions of both disk and ring currents, measured using the RRDE technique. 

Substituting Eqn (6.49) into Eqn (6.48), the expression of apparent electron-transfer number of ORR can be obtained ... 

Assuming the exchange current density and apparent electron transfer number for Pt oxidation and O2 reduction are the same, the value of E can be obtained through Equation 2.45, and is 1.06 V. However, the value of this rest potential can be changed by altering the extent to which RO covers the electrode surface. 

On the basis of theoretical calculations Chance et al. [203] have interpreted electrochemical measurements using a scheme similar to that of MacDiarmid et al. [181] and Wnek [169] in which the first oxidation peak seen in cyclic voltammetry (at approx. + 0.2 V vs. SCE) represents the oxidation of the leucoemeraldine (1 A)x form of the polymer to produce an increasing number of quinoid repeat units, with the eventual formation of the (1 A-2S")x/2 polyemeraldine form by the end of the first cyclic voltammetric peak. The second peak (attributed by Kobayashi to degradation of the material) is attributed to the conversion of the (1 A-2S")x/2 form to the pernigraniline form (2A)X and the cathodic peaks to the reverse processes. The first process involves only electron transfer, whereas the second also involves the loss of protons and thus might be expected to show pH dependence (whereas the first should not), and this is apparently the case. Thus the second peak would represent the production of the diprotonated (2S )X form at low pH and the (2A)X form at higher pH with these two forms effectively in equilibrium mediated by the H+ concentration. This model is in conflict with the results of Kobayashi et al. [196] who found pH dependence of the position of the first peak. 

The UPD of Zn + on Au(lll), Au(lOO), and Au(llO) was studied in phosphate buffer with addition of NaCl04 and NaCl [200]. The apparent number of electrons transferred in the UPD process was nearly one, irrespective of the single-crystal face. Although UPD shift potential ATp was independent of the kind of solutions, the CV characteristics were altered in different solutions. In NaCl solutions, two UPD peaks appeared in contrast to the case of phosphate and perchlorate solutions. 

A photoinduced electron relay system at solid-liquid interface is constructed also by utilizing polymer pendant Ru(bpy)2 +. The irradiation of a mixture of EDTA and water-insoluble polymer complex (Ru(PSt-bpy)(bpy) +, prepared by Eq. (15)) deposited as solid phase in methanol containing MV2+ induced MV 7 formation in the liquid phase 9). The rate of MV formation was 4 pM min-1. As shown in Fig. 14, photoinduced electron transfer occurs from EDTA in the solid to MV2+ in the liquid via Ru(bpy)2 +. The protons and Pt catalyst in the liquid phase brought about H2 evolution. One hour s irradiation of the system gave 9.32 pi H2 after standing 12 h and the turnover number of the Ru complex was 7.6 under this condition. The apparent rate constant of the electron transfer from Ru(bpy)2+ in the solid phase to MV2 + in the liquid was estimated to be higher than that of the entire solution system. The photochemical reduction and oxidation products, i.e., H2 and EDTAox were thus formed separately in different phases. Photoinduced electron relay did not occur in the system where a film of polymer pendant Ru complex separates two aqueous phases of EDTA and MV2 9) (see Fig. 15c). 

The role of the iron-sulfur clusters in many of the proteins that we have just considered is primarily one of single-electron transfer. The Fe-S cluster is a place for an electron to rest while waiting for a chance to react. There may sometimes be an associated proton pumping action. In a second group of enzymes, exemplified by aconitase (Fig. 13-4), an iron atom of a cluster functions as a Lewis acid in facilitating removal of an -OF group in an a,P dehydration of a carboxylic acid (Chapter 13). A substantial number of other bacterial dehydratases as well as an important plant dihydroxyacid dehydratase also apparently use Fe-S clusters in a catalytic fashion.317 Fumarases A and B from E. coli,317 L-serine dehydratase of a Pepto-streptococcus species,317-319 and the dihydroxyacid... 

Cyclic voltammetry can (i) determine the electrochemical reversibility of the primary oxidation (or reduction) step (ii) allow the formal potential, E°, of the reversible process to be estimated (iii) provide information on the number of electrons, n, involved in the primary process and (iv) allow the rate constant for the decomposition of the M"+ species to be measured. Additional information can often be obtained if intermediates or products derived from M"+ are themselves electroactive, since peaks associated with their formation may be apparent in the cyclic voltam-mogram. The idealized behaviour illustrated by Scheme 1 is a relatively simple process more complicated processes such as those which involve further electron transfer following the chemical step, pre-equilibria, adsorption of reactants or products on the electrode surface, or the attack of an electrogenerated product on the starting material, are also amenable to analysis. 

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