Microstructure evolution and oxidation states of Co in perovskite-type oxide Ba1.0Co0.7Fe0.2Nb0.1O3-δ annealed in CO2 atmosphere

Ba1.0Co0.7Fe0.2Nb0.1O3-delta(BCFN) oxide with perovskite cubic structure was synthesized by solid state reaction method. CO2 corrosion of BCFN membrane was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFT) and X-ray absorption fine structure spectroscopy (XAFS). Cobalt (Co) K-edge absorption spectra of BCFN annealed in CO2 reveal that the oxidation states of Co in all the samples were larger than +3 and they decreased with the increase of calcination time. At 800 degrees C, 1% CO2 introduced into He could speed up the reduction of Co cations in comparison with pure He. In addition, sulfate ions in the bulk of BCFN membrane preferred to migrate to the surface under CO2 calcination and form monoclinic Ba(CO3)(0.9)(SO4)(0.1) besides orthorhombic witherite. Moreover, SEM results indicate that the nucleation and growth of carbonates grains started at the grain boundary of the membrane.