Oxidation Mechanism of Chemisorbed Carbonaceous Species

The oxidation state of the adsorbed species is frequently characterized [4, 7, 21, 70] by the ratio (s6h s6h)/6ox- For instance a value of about 1.3 was reported [70] for the O-type species, and of about 4 for the CH-type species. If a linear dependence is assumed between (sGh sCh) the number of sites blocked by chemisorbed species of j different types 

After an adequate washing procedure, the oxidation of C HpOq may be studied separately if the rate of oxidation is adjusted to avoid overlapping with the formation of the oxygen layer. The latter condition is easily fulfilled on electrodes with large real surface. 

It was found [1, 10, 12] that the potential of the initial part of the arrest due to the oxidation of C HpOq (see Fig. 39) did not change with pH if measured at equal currents versus a hydrogen electrode in the same solution. A straight line resulted if the potential of the initial part was plotted as a function of log /. A chemical reaction between C HpOq and was postulated [1,10,12] to be rate-determining on a heterogeneous surface of the Temkin type [71]  

Here f(0) expresses the dependence upon coverage with C HpOq, and /toH is the chemical potential of OH d- It follows from the assumed equilibrium of reaction 26  

The combination of Eq. 35 and Eq. 36 leads to a logarithmic dependence of I upon U. The correctness of the preceding interpretation depends 

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Neither the reactant-pair mechanism [19] nor the water discharge mechanism [25] are in agreement with the pH dependence for the oxidation of type I species. The pH dependence may be explained as suggested by Frumkin and Podlovchenko [4] in their discussion of the mechanism of ethanol oxidation. A chemical reaction between adsorbed carbonaceous species and OH d is supposed rate-determining on a heterogeneous surface of the Temkin type [34]. The discussion is the same as that for the oxidation mechanism of chemisorbed carbonaceous species in section 11 of chapter IX. 

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