The Role and Stability of Adsorbed Intermediates

Intermediates are commonly formed in chemical reactions, as well as in electrode reactions. The preferred mechanism is that which involves the most stable intermediates, since this is the path of lowest Gibbs energy of activation. For reactions taking place in the gas phase or in the bulk of the solution, the stability of different species can be calculated, or at least estimated, from existing thermodynamic data. This is not the case for electrode reactions. For the HER discussed earlier, a hydrogen atom was assumed to be an intermediate. The standard potential for the formation of this species in solution, that is, for the reaction 

The hydrogen atom formed as an intermediate is stabilized by adsorption. To clarify this point, we should perhaps write Eq. (7.2) in explicit form, as follows  

The situation is similar in the case of oxygen evolution, where OHads and Oads are postulated as adsorbed intermediates, even though these radicals are very unstable in the bulk of the solution. The same type of argument has been employed to justify the existence of adsorbed intermediates in some complex organic reactions, such as RCOOads in the Kolbe reaction of decarboxylation of some organic acids and formation of hydrocarbons 

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