B4a Conventional Size Electrodes

A catalytic reaction can often be made pseudo-first order in Q by using a large excess of the reactant A in the solution. Under this condition, if scan rates (v) can be achieved such that the reaction in under full kinetic control of the homogeneous chemical reaction with rate ku the shape of the LSV response curve is sigmoidal and can be described by the model in Table 11.2. The limiting current (/,) is related to kx by i, = nFACp(DpCAkx)m (11-4)

where n is the number of electrons transferred in the catalytic reduction, F is Faraday's constant, A here denotes electrode area, DP is the diffusion

Box 11.2 Simple reductive catalytic pathway.

coefficient of the catalyst, and CP and CA are the catalyst and reactant concentrations, respectively. If the limiting current is obtained by fitting the data to the model in Table 11.2, then kx can be estimated from eq. (11.4).

The conditions of full kinetic control occur within a given range [1] of k\! v. If these conditions cannot be attained experimentally, a model based on numerical solutions of the relevant differential equations must be employed.

A requirement for pseudo-first-order conditions is that CA & 10 CP. Because of the large excess of reactant A, the foot of its direct irreversible reduction wave may overlap the catalytic current-potential curve. In such cases, including an exponential background with the model in Table 11.2 has been successful for the estimation of ih which was then used to obtain k\. Examples of the use of this approach include determination of the rate constant for the reduction of 4,4'-dichlorobiphenyl by anion radicals of anthracenes (Figure 11.13) [16] and by photoexcited organic anion radicals [17].

Was this article helpful?

0 0

Post a comment