The product of the fourth step is an adsorbed oxygen molecule. Finally, the fifth step is desorption of the oxygen molecule, which closes the catalytic cycle.
Activation energies of each step depend on the catalysts under study but for most catalysts the following pattern is observed.
The molecular adsorption steps (1) and (3) proceed without activation energy and the heat of adsorption is not too high.
For most catalysts the reaction (2) has a lower activation energy than reactions (4) and (5), although this depends on the metal ion and its oxidation state.
Usually both (4) and (5) have comparable rate constants but in some cases (e.g., Fe-ZSM-5) the step (4) is rate limiting.
An alternative mechanism includes the steps (1) and (2) and the recombination of surface oxygen atoms: