États d'adsorption, vitesses d'adsorption, de désorption de l'oxygène sur rhénium et mécanismes d'atomisatton, d'oxydation à haute température et basse pression

Abstract

We have studied interactions of oxygen with rhenium at low pressures (10 −4 to 10 −6 torr) in an ultra high vacuum apparatus where all desorption products, including instable and condensible species are analyzed by mass spectrometry. The chemical reactions depend on the surface coverage which is fixed by an equilibrium between the rate of adsorption and the rates of desorption steps, varying with temperature and pressure. We have measured these rates, independently, or directly (pressure variations after a flash, thermal desorption). When the coverage is lower than 0.7, the main reaction is oxygen atomization. For higher values, desorption of atomic oxygen is progressively replaced by desorption of rhenium trioxide, ReO 3, main product of reaction. The following mechanism, based on the kinetics of the elementary steps, experimentally isolated, gives a quantitative picture of all the phenomena, for pressures between 10 −4 to 10 −6 torr, and sample temperatures varying from 1500 to 2300°K: (a) dissociative adsorption of oxygen; (b) desorption of atomic oxygen; (c) desorption of trioxide, only formed by surface reaction. The different rates depend on coverage, frequency factors and binding or activation energies. Optimization of the results in stationary conditions give the values of these parameters. The agreement with the values obtained directly is excellent.