Physical Considerations and Kinetics

As discussed in Chapter 4 to 10 experiment has shown that electrochemically promoted kinetics very frequently follow the following simple expression  

The parameter a in Equation (11.6) is positive for electrophobic reactions (5r/5O 0, A 1) and negative for electrophilic ones (3r/0O O, A -1). As already discussed in Chapters 4 and 6, more complex electrochemical promotion behaviour is frequently encountered, leading to volcano-type or inverted volcano-type behaviour. However, even then equation (11.6) is satisfied over relatively wide (0.2-0.3 eV) AO regions, so we limit the present analysis to this type of promotional kinetics. It should be remembered thatEq. (11.6), originally found as an experimental observation, can be rationalized by rigorous mathematical models which account explicitly for the electrostatic dipole interactions between the adsorbates and the backspillover-formed effective double layer, as discussed in Chapter 6. 

The electrochemically induced change in work function AO (eq. (1 1.6)) is related to the coverage 0j of the promoting species (e.g. O5 ) on the catalyst surface via the Helmholz equation  

For typical experimental parameter values (a =0.5, NM-1019 atom/m2, P,=l D=3.3-10 30 C-m, T=673) the dimensionless parameter IT equals 32 which implies, in view of equation (11.12), dramatic rate enhancement ratio p values (e.g. p =120) even for moderate (-15%) changes in the coverage 0j of the promoting backspillover species, as experimentally observed. 

The promotional kinetics described by equation (11.6) or by its equivalent equation (11.12) imply uniform distribution of the backspillover promoting species on the catalyst surface. This requires fast ion backspillover relative to its desorption or surface reaction. 

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