Reaction of molecular hydrogen with the 100 face of MoO 3: II. Kinetics initiated by atomic hydrogen and characterization of the surface electronic state

Abstract

In order to provide a physical background to the model proposed in Part I, the kinetics of the reaction between the 100 face of a MoO 3 single crystal and, first a mixture of 4% atomic hydrogen in H 2, and later molecular hydrogen only, has been studied. The rate processes of the activation step and of the stationary step are entirely comparable with those observed for one single crystal surface loaded with platinum particles. Thus atomic hydrogen in the gas phase as well as atomic hydrogen produced by the dissociation of molecular hydrogen on a Pt particle may prepare the favourable surface state able to dissociatively chemisorb molecular hydrogen, ruling out — once again — the classical model of the hydrogen spillover process. This “favourable” state has a Fermi level which is 0.25 eV lower than that of initial MoO 3, as shown by measuring the work function with a Kelvin probe. This lowering is in good agreement with the variation of the free energy between MoO 3 and H 1.6MoO 3, measured electrochemically by others. This suggests that the protons inserted into the surface layers transform the initial MoO 3 layers into layers with composition H 1.6MoO 3. The starting material is thus transformed into a biphasic system, the diffusion of the reaction boundary between the two types of layers being the overall rate limiting process. The Fermi energy of H 1.6MoO 3 being known, it is possible to show that in transformed surface layers the conduction and the valence bands overlap, in agreement with the approximate profile of this band observed by XPS for a reacted single crystal surface. The d character of this band would explain why molecular hydrogen can be dissociatively chemisorbed when this favourable surface state is obtained. The fast electron delocalization within the Mo-O-Mo bonds yields fast oscillations in the oxidation states of the molybdenum atom in the surface layer, accounting for the presence of “oxidized” and “reduced” sites. The formal equation observed in Part I for the rate of stationary step is therefore explained, the impinging H 2 molecules reducing temporarily the oxidized fraction of the surface.