Electrocatalysis and Adsorption Effects

Like other heterogeneous chemical reactions, electrochemical reactions are always multistep reactions. Some intermediate steps may involve the adsorption or chemisorption of reactants, intermediates, or products. Adsorption processes as a rule have decisive influence on the rates of electrochemical processes. 

To illustrate the influence exerted by the energy of adsorption of an intermediate on the rate of an electrocatalytic reaction, consider a very simple two-step reaction of the type A — X — B where X, the intermediate, is reversibly adsorbed on the electrode (with a degree of surface coverage 9x). For the sake of simplicity, the electrode surface will be assumed to be homogeneous (i.e., conditions of Langmuir adsorption hold), while the system lacks adsorbed species other than X. The rate, of the adsorption step (the first step) is then proportional to the bulk concentration of the starting material, c, and to the free surface part (1 - 9x) (the part not taken up by species X), while the rate of further transformation of intermediate X, which is tied to its desorption, will be proportional to the surface fraction, 9x, taken up by it  

In the steady state, the rates of adsorption and desorption are equal to each other and to the overall rate of this reaction v, = For the steady-state value of 

The rate constants of the adsorption and desorption step, and depend on the energy, W, of bonding of the adsorbed species to the electrode surface. Higher bond energies imply that adsorption is facilitated and k increases while k decreases. These functions can be formulated as 

One can see when putting these expressions into Eq. (28.3) that at low values of W when k is small (and the overall reaction rate is hmited by the first step), v, increases with increasing W. At larger values of W, increases more slowly and attains a maximnm, then starts to decrease. At this point the overall reaction rate is then limited by the second step. 

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