Carefully Designed Hollow MnxCo3–xO4 Polyhedron Derived from in Situ Pyrolysis of Metal–Organic Frameworks for Outstanding Low-Temperature Catalytic Oxidation Performance

In this paper, three different structures of MnxCo3-xO4 were successfully synthesized by optimizing the heating decomposition conditions of Mn@Co-ZIFs precursors to form three types of MnxCo3-xO4 catalysts with different morphologies, including the hollow MnxCo3-xO4 polyhedron (HW-MnxCo3-xO4), ball-in-box MnxCo3-xO4 polyhedron (BIB-MnxCo3-xO4), and nanoparticle MnxCo3-xO4 polyhedron (NP-MnxCo3-xO4). Interestingly, the structure effect of the MnxCo3-xO4 polyhedron on the catalytic oxidation of toluene was systematically investigated. It could be noted that the HW-MnxCo3-xO4 sample exhibited superior catalytic performance, and the complete conversion temperature of toluene (T100) was 195 °C. Furthermore, the toluene conversion of the HW-MnxCo3-xO4 sample had no significant decrease at 188 °C for 30 h, indicating that it exhibited excellent stability for the toluene oxidation reaction. Through a series of characterizations, it was concluded that the morphology and structures of MnxCo3-xO4 catalysts could evidently alter the surface atomic ratio of Co2+/(Co3+ + Co2+), the Brunauer–Emmett–Teller (BET) surface area, the number of surface adsorbed oxygen, the interaction between Mn and Co3O4, and so on. In particular, we discovered that the catalytic activity of MnxCo3-xO4 polyhedron was obviously improved with the increase of the surface atomic ratio of Co2+/(Co3+ + Co2+). In addition, the large BET surface area, lots of surface adsorbed oxygen, strong interaction between Mn and Co3O4 would speed up the catalytic oxidation of toluene.