Role of oxygen vacancies and Mn⁴⁺/Mn³⁺ ratio in oxidation and dry reforming over cobalt-manganese spinel oxides

Spinel type cobalt-manganese oxides Mn_xCo_3-xO₄ (x = 0; 0.05; 0.10; 0.15) were prepared by co-precipitation method, from cobalt and manganese salts in the presence of ammonium carbonate. X-ray diffraction (XRD), texture measurements (BET/BJH), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for their characterization. Their catalytic behavior in ethanol combustion, dry reforming with CO₂ and dehydrogenation was investigated. The results showed that the cubic structure of Co₃O₄, corresponding to spinel network, is not disturbed by adding manganese. Results obtained revealed that Mn_0.15Co_2.85O₄ catalyst, with the highest content of Mn⁴⁺ cations in the octahedral sites and oxygen defects, display superior catalytic activity compared to the others. The apparent activation energies, calculated from Arrhenius plots, are as follows: E_a combustion < E_a non-oxidative dehydrogenation < E_a dry reforming, showing that cobalt-manganese spinel type oxides are good candidates for combustion.