This is essentially the Eley–Rideal mechanism [2] in which steps (1) and (2), and (3)–(5) are grouped together. Here X denotes an initial state of the active site, in the case of transition metal catalysts it is a cation exposed to the surface. The first step (1) is the physical adsorption of nitrous oxide on the site. The step (2) is the dissociation of the adsorbed N2O molecule yielding a gas-phase N2 molecule and an active surface oxygen atom. The steps (3) and (4) are the physical adsorption and decomposition of the second N2O molecule, respectively. 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: