Correlation between structure, surface defect chemistry and 18O/16O exchange for La2Mo2O9 and La2(MoO4)3

Abstract

The La₂Mo₂O₉ and La₂(MoO₄)₃ powders were synthesized using a solid-state reaction method and used to prepare dense ceramics. X-ray photoelectron spectroscopy was used to study the chemical composition and charge numbers of the elements in the subsurface area of dense ceramics of lanthanum molybdates. The spectra were measured under an ultra-high vacuum of 7 × 10⁻¹¹ atm at 30 °C and 600 °C, and under an oxygen atmosphere at 2 × 10⁻³ atm at 600 °C and 825 °C. High resolution spectra for La 3d, Mo 3d and O 1s states were obtained and analyzed. The kinetics of oxygen exchange were considered in the framework of a two-step model including the consecutive steps of dissociative adsorption and the incorporation of oxygen. The oxygen adsorption (r_a) and incorporation (r_i) rates were calculated. Correlations between the oxide surface defect chemistry and the rates of individual oxygen-exchange steps were discussed.