ON THE MECHANISM OF THE HYDROGEN-OXYGEN REACTION ON PT(111)

The reaction between hydrogen and oxygen on Pt(lll) is investigated wi th molecular beam relaxation spectroscopy in the temperature range bet ween 300 and 1200 K. Three procedures were used for preparing the surf ace, resulting in different reactivities for the hydrogen-oxygen react ion. The observations are not compatible with a reaction mechanism whi ch occurs homogeneously on the surface. Therefore we interpret them in terms of a non-homogeneous surface reaction model in which water form ation is catalyzed by special reactive sites (reactive site model). Ox ygen diffusion is found to be the rate-limiting process by which water is formed on Pt(lll) at low oxygen coverages (<0.03 ML) and temperatu res below 800 K. The activation energy for diffusion E-a is 11.3+/-0.5 kcal mol(-1). Between 800 and 1200 K we find an effective activation energy for the reaction which is negative (-14.8 kcal mol(-1)). A cons istent explanation of all our results can be given with the reactive s ite model when it is assumed that two O atoms take part in the formati on of one water molecule. We estimate that the density of reactive sit es is about 10(-5) on a well-prepared surface. Experimental results th at have been reported previously are discussed. It is shown that the r eactive site model can explain also those results quite well. (C) 1998 Elsevier Science B.V. All rights reserved.