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In order to establish the formation patterns of the Co–Mg oxide system, samples with different Co:Mg ratios and heat treatment temperatures were synthesized and studied. A study of the samples confirmed the phase transition of MgxCo2–xO4 spinels into the corresponding solid solutions at 800–900 °C. The similarity of the formation patterns for different compositions is shown. The rocksalt oxide in low-temperature samples is an anion-modified paracrystalline phase that forms a "true" solid solution only upon spinel decomposition. The TPR profiles of the decomposed Co3O4 spinel show surface Co3O4 peaks and a wide peak corresponding to the well-crystallized CoO, while partial Co3O4 TPR up to 380 °C results in dispersed and amorphous CoO. The high-temperature non-stoichiometric samples are poorly reduced, indicating their low oxygen reactivity. Spinel reoxidation after heat treatment to 1100 °C by calcination at 750 °C showed complete regeneration for MgCo2O4–Co3O4 samples and its absence in case of an excess of MgO relative to stoichiometry. [ABSTRACT FROM AUTHOR]

Co–Mg catalysts for methane combustion were synthesized and studied, revealing the transformation of MgCo2O4 spinel into a CoO–MgO solid solution with oxygen release from the spinel lattice as the calcination temperature increased. Repeated heat treatment of the calcined solid solution at lower temperatures led to spinel regeneration with segregation of the solid solution phase. A TPR of the samples showed the presence of two characteristic peaks, the first of which relates to the transition of Co3+Oh spinel to the Co2+Oh structure of CoO, and the second to the reduction of CoO to Co°. The second peak was observed at 540–620 °C for samples calcined at temperatures below spinel decomposition, and for high-temperature samples at 900–1100 °C. Taking into account the identity of the structure of phases obtained in both cases, the formation of not a true CoO–MgO solid solution, but rather a mixture of ordered oxides ("pseudo-solid solution") in the low-temperature region, was postulated. A study of the activity of the samples showed the high activity of the spinel systems and a linear relationship between the activation energy of methane oxidation and the heat treatment temperature. [ABSTRACT FROM AUTHOR]