MULTIPLICITY IN ADSORPTION ON HETEROGENEOUS SURFACES

In adsorption-desorption phenomena, interaction among adatoms can caus e multiple steady states and, consequently, hysteresis loops. Starting from a master equation and applying a mean field approximation to obt ain the kinetic equations, adsorption on a heterogeneous surface with two types of randomly distributed adsorbent sites is characterized by different activation energies. Moreover, interaction among adsorbates contributes to the total activation energy. In general, depending on t he values of the kinetic parameters and the heterogeneity degree, up t o three stable steady states can exist for values of the parameter J, which measures the interaction energy, greater than the critical one, Jc. Once the other parameters have been fixed and considering differen t heterogeneity degrees, the highest Je is obtained for the case of a 50-50% site distribution. Mobility of adatoms on the surface disfavors multistability, and interactions stronger than those corresponding to a case without mobility are required to cause multiplicity of stable steady states. On the other hand, a dimerization reaction catalyzed by a surface like the one above described does not change essentially th e number of stable steady states. Thus, for the case considered, multi plicity is a result due to the adsorption process and dogs not depend on the reaction one. Multistability causes different reaction rates fo r given conditions, depending on the initital states. When three or fo ur types of adsorbent sites are considered, multistability does not ap pear. Also, when different continuous distributions of types of adsorb ent sites are considered, multistability is also destroyed. Therefore, it; can be concluded that an ''excess'' of surface heterogeneity prev ents the occurrence of multistability in adsorption with interacting a dsorbates.