Charge coverage coefficient

The q-r-E relation was first introduced by Schmidt [3.54] as the charge coverage coefficient, and later described as the electrosorption valency by Vetter and Schultze [3.226, 3.227] ... 

Since the electron transfer of the interfacial redox reaction, + cm = H.a> on electrodes takes place between the iimer Helmholtz plane (adsorption plane at distance d ) and the electrode metal, the ratio of adsorption coverages 0h,j/ in electron transfer equilibrium (hence, the charge transfer coefficient, 6z) is given in Eqn. 5-58 as a function of the potential vid /diOMn across the inner Helmholtz layer ... 

The Tafel slopes are given in Table 9. The first slope (bx) is the lower one and it occurs at more positive potentials the second (b2) has higher values and occurs at slightly positive potentials, i.e., at lower coverages (8). Two slopes were not observed at all temperatures. Table 9 also contains corresponding coverages and the charge transfer coefficient, a ... 

Table 9 Experimental Tafel Slopes, Charge Transfer Coefficients and Surface Coverages for Halogen-Active Carbon Electrodes at Various Temperatures and Coverages... 

For a typical monomolecular coverage, T — 10 10 mol/cm2, an electrode roughness factor r = 1000 and an extinction coefficient ads = 107 cm2/mol, the light-harvesting efficiency is, in comparison to the preceding case, very high, intimate contact with the semiconductor surface, hence the conditions for charge injection from S into the semiconductor are almost ideal (q9j—>100 per cent). 

Adsorption at a charged sur u% where both electrostatic and specific chemical interactions are involved can be discussed in terms of the Langmuir adsorption isotherm, where the distribution coefficient b is given by the exponential of a sum of the electrochemical and electrostatic forces. In this treatment the fractional surface coverage, 6, is given by [43]... 

It is interesting to note that the symmetry factor P did not appear in any of these equations. This is because the rate-determining step assumed here does not involve charge transfer. The current depends indirectly on potential, through the potential dependence of the fractional coverage 0. The transfer coefficient is = 2, as can be seen in Eq. 43F, corresponding to a Tafel slope of b = - 30 mV at room temperature. 

The Faradaic current is proportional to the rate of electron production, (mol cm s ), which is equal to the sum of Vi and Dj [Eqs. (50) and (51)]. Likewise dd/dt is proportional to r, the rate of production of MH [Eq. (50)]. Here Qi is again the charge required per square centimeter for complete monolayer coverage by the intermediate. Following Gileadi and Conway (130), V3 is to be defined as the rate of hydrogen production in step 3, or half the rate of consumption of adsorbed H in that step, with the consequence that a coefficient two appears in Eq. (54) ... 

Adams, et al. [89] could find no evidence of differing efficiency of entry between charged and uncharged radicals or of any effect of the extent of surface coverage with emulsifier or of ionic strength collisional or diffusive entry models predicted entry rate coefficients which were too large. Subsequently Maxwell, et al. [60] concluded that the rate-determining process was the time required for an initiator radical to add two styrene residues in the aqueous phase and thereby acquire sufficient hydrophobic character to adsorb onto a latex particle. 

Here the coefficient, B, is called attraction constant, in analogy with the corresponding factor in the Frumkin adsorption isotherm. However, it has got the opposite sign, that is, it reflects the repulsion between the adsorbed species, the amplitude of which is diminished owing to the relaxation of the adsorbed ion ensemble (factor k). The entropy term for the adsorption state is taken without the saturation term , compare Eqs. (71) and (72), since the charging degree for s,p-metals is mostly very low with respect to the complete coverage. 

In this equation a is the lateral attraction coefficient, Bq a constant related to the free energy of adsorption at the potential of zero charge (p.z.c.) for the pure supporting electrolyte solution, and 0 the fractional monolayer surface coverage. With higher surfactant concentrations the parameter a may reach the value a > 2. The adsorption isotherm has then an S-shaped course. In the region of electrode potentials U at which a > 2 the strong attraction between surfactant molecules leads to their two-dimensional condensation. The function... 

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