The Role of Precursor States in Adsorption, Surface Reactions and Catalysis

Among the many concepts which have been developed in surface science and catalysis to explain the nature of adsorption, the role of weakly-held states, so-called precursor states, is one of the most important. The kinetics of precursor-mediated adsorption is described, together with examples showing how significant such effects can be, not just in adsorption itself, but also in surface reactions on crystals and on model supported catalysts. These examples include CO adsorption on Pd single crystals and catalysts and implications for CO oxidation, and extreme examples of the precursor effect, where adsorbate molecules can appear to 'seek out' active sites which are in very low concentration on the surface. The decomposition and oxidation of formic acid is just such an example. Such reactivity patterns then also lead to concepts such as the collection zone' or 'diffusion circle' describing how much of an area of surface is 'swept' by incoming weakly-held molecules during their brief sojourn there.