Cooperative effect of the Co–Mn mixed oxides for the catalytic oxidation of VOCs: Influence of the synthesis method

Abstract

Mixed oxides of manganese, cobalt and their mixture were synthesized by auto-combustion and co-precipitation methodology maintaining a constant M2+/Al3+ ratio of 3.0, which is characteristic of the oxides obtained from the thermal decomposition of hydrotalcite-type precursors (in manganese oxides M2+=Mg+Mn, in cobalt oxides M2+=Mg+Co and in manganese-cobalt oxides M2+=Mg+Mn+Co). The catalysts were characterized by the following techniques: X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption, temperature-programmed reduction (H2-TPR), temperature programmed desorption of oxygen (O2-TPD) and 18O isotope exchange. All of the materials were evaluated for the catalytic oxidation of two organic compounds of different reactivities: toluene and 2-propanol. It was observed that the joint participation of the absorbed oxygen species on the surface and the lattice oxygen atoms are responsible for the catalytic activity of the materials. However, the redox properties and the oxygen mobility play a determining role in the oxidation of the two volatile organic compounds (VOCs), with the oxygen mobility playing a more significant role in the cobalt oxides, whereas the redox properties are fundamental in the manganese oxides and in the Co and Mn mixture. The existence of a cooperative effect between the Co and Mn oxides is demonstrated when the co-precipitation method is used for the synthesis of the mixed oxide. This effect is not observed when auto-combustion is used for the synthesis; therefore, the autocombustion manganese oxide is the most active catalyst in the oxidation of the two VOCs by this methodology.