Measurements of reactive O 2 sticking coefficients and oxidation rates at niobium oxide surfaces

Abstract

In situ X-ray photoelectron spectroscopy (XPS) was used to determine the reactive sticking coefficient of molecular oxygen on Nb 2O 5NbO 2NbO 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 O 2 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 O 2 pressure.