On the mechanism of the hydrogen-oxygen reaction on Pt(111)

The reaction between hydrogen and oxygen on Pt(lll) is investigated with molecular beam relaxation spectroscopy in the temperature range between 300 and 1200 K. Three procedures were used for preparing the surface, resulting in different reactivities for the hydrogen-oxygen reaction. The observations are not compatible with a reaction mechanism which occurs homogeneously on the surface. Therefore we interpret them in terms of a non-homogeneous surface reaction model in which water formation is catalyzed by special reactive sites (reactive site model). Oxygen 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 temperatures 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 consistent explanation of all our results can be given with the reactive site model when it is assumed that two O atoms take part in the formation of one water molecule. We estimate that the density of reactive sites is about 10(-5) on a well-prepared surface. Experimental results that have been reported previously are discussed. It is shown that the reactive site model can explain also those results quite well. (C) 1998 Elsevier Science B.V. All rights reserved.