Mg and Al substituted cobalt spinels as catalysts for low temperature deN2O—Evidence for octahedral cobalt active sites

A series of cobalt spinel oxides with the chemical composition: Co3O4, MgCo2O4, MgCoAlO4, CoAl2O4, Mg0.5Co0.5Al2O4 and MgAl2O4 was synthesised, characterised and studied for catalytic decomposition of N2O. Selective replacement of cobalt ions in the tetrahedral and octahedral sites was employed to probe the specific activity of both types of centres. The combined experimental XRD, UV–vis, RS, XPS, BET, TPR, Temperature Programmed Catalytic Reaction (TPCatR) methods and DFT modelling allowed to explore the mechanistic role of the coordination (tetrahedral, octahedral) and valence (+2, +3) states of cobalt ions in the Co3O4 spinel matrix. The prime active sites of deN2O reaction were definitely identified as the octahedral Co3+ ions (Ea = 15–17 kcal mol−1), whereas the tetrahedral Co2+ ions were found to be clearly much less active (Ea = 27–28 kcal mol−1).