1 – x Ca x CoO 3 – δ Perovskites ( х = 0?1) Prepared by the Pechini Method in the Reaction of Deep Methane Oxidation]]>

摘要

The catalytic activity of a series of the perovskite-like oxides La~(1– x )Ca~( x )CoO~(3–δ)( x = 0–1) prepared by the Pechini method (from polymer–salt compositions) in the reaction of methane oxidation was studied. The dependence of the activity and stability of samples on their composition and reaction temperature was revealed. It was found that an increase in the calcium content of the oxides initially led to an increase in the activity (to the values of x = 0.3) and then to a nonmonotonic decrease with an intermediate maximum at x = 0.6. A decrease in the activity of oxides in the course of testing, which was most pronounced in the samples with x = 0.3, 0.6, and 1, was found. The phase compositions, specific surface areas, and microstructures of the oxides were determined before and after tests. According to X-ray diffraction analysis data, only the samples with x = 0–0.4 were single-phase ones; the samples with x > 0.4 were two-phase samples containing the phases of perovskite and brownmillerite. With the use of high-resolution transmission electron microscopy, it was found that the surface of particles was covered with the nanosized particles of simple calcium and cobalt oxides, and the particles of brownmillerite were present in the samples with x ≥ 0.4. The observed increase in the catalytic activity of the samples in a region to x = 0.3 correlated with an increase in the concentration of weakly bound oxygen in the perovskites, and a decrease at x > 0.3, with the appearance of the less active phase of brownmillerite in the samples and an increase in its concentration. The phase composition and the specific surface area of the samples remained unchanged after tests; however, planar defects were detected in the particles of perovskite and the calcium content on the surface increased. This was caused by the appearance and ordering of cationic and anionic vacancies under the action of a reaction medium, which can explain the observed changes in the activ