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
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