A distinctive feature of multi-step reactions is the emergence of intermediate species whose activities cannot be directly controlled externally. For the sake of simplicity, we will consider these intermediate species to be bound to the same surface site throughout the reaction process (Eq. 5, not displaying non-intermediate reactants), although migration between sites or desorption into the solution cannot be excluded a priori. We denote a free site as * , and a surface bound specie with the superscript *, otherwise the species (capital letters) are in solution.