[145]). In the first case, a molecule coming from the gas phase collides and interacts with an adsorbed species, and the resulting reaction product desorbs. Alternatively, the second reactant may also be present as physisorbed species, which is essentially identical to the situation in outer-sphere reactions in electrocatalysis . This mechanism allows a reaction to proceed also on a surface that is completely covered by the chemisorbed species. In the LH mechanism, both species participating in the reaction are adsorbed, which means that there is an optimum adlayer composition where the two reaction partners are adsorbed in a 1:1 ratio. Furthermore, the adsorbed species should be intermixed rather than phase separated. Complete occupation of the surface by one of the partners would result in surface blocking, and thus would stop the reaction. Thus, there is a maximum in the reaction rate, which does not exist in the ER mechanism. In the MvK mechanism, the adsorbed reactant reacts with a surface atom of the solid, mostly with oxygen in oxides, and the resulting surface oxygen vacancy is filled by oxygen from the bulk reservoir. This reservoir is replenished in an independent process by reaction of a surface oxygen vacancy with O2, and subsequent diffusion of a surface oxygen into the bulk reservoir.