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.