Isotopic transient analysis of oxygen exchange over oxides

We investigated the model of oxygen isotopic exchange on solid oxides in flow reactor using numerical and analytical approaches. The study concerns the effect of the exchange mechanism (mono- or diatomic) and of the rate of exchange as well as the oxygen diffusion rate in the bulk on the isotopic response after ¹⁶O/¹⁸O switch. Ratio of ¹⁶O¹⁸O molecular fraction to ¹⁸O atomic fraction is shown to depend on the kinetics of gas-surface exchange, and does not depend on the oxygen diffusion in the oxide bulk. Algorithm of isotopic transient analysis without use of numerical methods was developed. Criterion to discriminate between the types of isotopic exchanges was proposed. Formulas for estimation of exchange rate and diffusion coefficient of oxygen in oxides were derived. The estimates of the surface exchange rate and of the bulk diffusion coefficient of the complex oxide La_0.3Bi_0.7MnO₃ were calculated applying proposed algorithm. The comparison with the results of numerical modelling verified the reliability and sufficiently high accuracy of obtained estimates.