MEASUREMENTS OF REACTIVE O2 STICKING COEFFICIENTS AND OXIDATION RATES AT NIOBIUM OXIDE SURFACES

In situ X-ray photoelectron spectroscopy (XPS) was used to determine the reactive sticking coefficient of molecular oxygen on Nb2O5NbO2NbO mixed oxide surfaces in the temperature range 773-973 K and at a pressure of 6.7×10-2 Pa. During these measurements, the competing processes of oxidation (due to a fraction of the surface collisions reacting) and reduction (due to surface oxygen dissolving into the bulk) established a steady state balance. Once at steady state, the sticking coefficients were calculated by dividing the rate of oxygen collisions with the surface (i.e. pressure) into the previously determined rate of oxygen dissolution. This technique proved capable of measuring reactive sticking coefficients on the order of 10-4. The activation energy for oxygen reaction at the niobium oxide surface was found to be 184±7 kJ mol-1. Knowledge of the activation energy for O2 reaction at the surface, and the previously determined values for dissolution into the bulk, permitted estimation of the niobium oxidation rate as a function of surface temperature and O2 pressure. © 1990.