Kinetics of oxygen exchange with oxides Ba0.5Sr0.5(Co0.8Fe0.2)1−xMexO3−ẟ (Me = Ta, W) in non-equilibrium conditions

Abstract

Oxygen chemical surface exchange coefficient \({k}^{\updelta }\) for Ba_0.5Sr_0.5(Co_0.8Fe_0.2)_1−xMe_xO_3−ẟ (Me = Ta, W) has been measured by oxygen pressure relaxation method in the temperature range 600–800℃ and oxygen pressure 1.3–34.7 mbar. The comparison of the values of the tracer \({k}^{*}\) and chemical \({k}^{\updelta }\) oxygen surface exchange rate constants allowed to evaluate the additional oxygen capacity of the surface layer, which is different from the bulk oxygen capacity, characterized by the thermodynamic factor \({\text{w}}_{\text{O}}=\frac{1}{2}\frac{\partial \text{ln}\left({\text{pO}}_{2}\right)}{\partial \text{ln}\left(3-\updelta \right)}\) calculated from the \(\text{T}-{\text{pO}}_{2}-\left(3-\updelta \right)\)–diagram. The possible reasons were related to the specific phase composition of the surface layers responsible for the oxygen exchange process. The \({\text{pO}}_{2}\) dependence of the chemical oxygen exchange coefficient was discussed in terms of surface coverage with adsorbed oxygen anionic forms. The relationship between the mechanism of surface oxygen exchange, determined either during equilibration of oxygen pressure or gas phase composition (oxygen isotope exchange), was explained in terms of Fleig’s theory (https://doi.org/10.1039/b618765j). The relationship between the chemical composition of the surface and the mechanism of the surface oxygen exchange is discussed.