Early Removal of Weak-Binding Adsorbates by Kinetic Separation

When a surface is exposed to a binary mixture of gases at the same partial pressures, it is expected that the species that binds to it more strongly will occupy a higher percentage of the surface due to its preferential adsorption. However, that is not necessarily the case during the equilibration process because the species with smaller binding adsorbs faster. Here, we demonstrate that a competition based on both binding energies and adsorption rates rules the kinetics of adsorption of a binary mixture on a homogeneous lattice of sites. This generates a selectivity process that has the opposite effect to the one that happens at equilibrium: the early removal of the weaker species by its faster adsorption.