Effect of the A-site cation over spinel AMn2O4 (A = Cu2+, Ni2+, Zn2+) for toluene combustion: Enhancement of the synergy and the oxygen activation ability

To investigate the underlying mechanism of the tetrahedral A2+ cations and the octahedral Mn3+ cations in AMn2O4 spinel catalysts for toluene oxidation, a series of structure-controlled AMn2O4 spinel catalysts were prepared through substituting A2+ and Mn3+ with Cu2+, Ni2+, Zn2+, and Fe3+, respectively. The evaluation results demonstrated that the catalytic activities arrange in the sequence of CuMn2O4 > NiMn2O4 > ZnMn2O4 > CuFe2O4, and the temperature required for 90% conversion of toluene is 205 °C for the CuMn2O4 catalyst. The physical properties of these samples were well characterized by XRD, TEM, N2 adsorption–desorption isotherms. The influences of crystal type, micro structure and specific surface area were successfully eliminated by using KIT-6 as the hard template method. The influence mechanism of A-site metal was further explored by analyzing the results of XPS, H2-TPR and O2-TPD. Results showed that the catalytic activity of the samples was highly affected by the substitution of highly electronegative A-site metal due to the enhancement of low-temperature reducibility and chemisorption oxygen activity. In particular, it was found that the incorporation of A-site metal can improve the ability of the catalyst to activate molecular state O2– to O-, which was highly related to the catalytic activity of the catalyst. The results hold guidance for the design of manganese spinel catalysts on the removal of VOCs.