Complex pathways in dissociative adsorption of oxygen on platinum

Gas adsorption on solid surfaces is the basis of heterogeneous catalysis. Gas-surface interactions may be complex and in many cases the fundamental mechanisms of the chemisorption process are hard to discern. The macroscopic kinetics of a heterogeneous catalytic reaction are usually modelled within the Langmuir model(1), which assumes that free adsorption sites are occupied at random. The adsorption of oxygen on a platinum (111) surface has been studied extensively as a model system for surface chemical processes generally(2-15), owing to its significance in platinum catalysed oxidation reactions such as that of CO and NO. Here we show that even for this web studied system the chemisorption process may be much more complicated than the Langmuir model implies. Our observations with the scanning tunnelling microscope show that the dissociation probability for an oxygen molecule becomes affected by chemisorbed species in the vicinity that have dissociated already. This introduces a dynamic heterogeneity in the adsorption mechanism which leads to kinetically limited ordering of the adsorbate. This effect is likely to be quite general and to affect the bulk kinetics of catalytic reactions conducted at the high temperatures and pressures of most industrial heterogeneous catalysis.