Carefully Designed Hollow Mn_xCo_3–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 Mn_xCo_3‑xO₄ were successfully synthesized by optimizing the heating decomposition conditions of Mn@Co-ZIFs precursors to form three types of Mn_xCo_3‑xO₄ catalysts with different morphologies, including the hollow Mn_xCo_3‑xO₄ polyhedron (HW-Mn_xCo_3‑xO₄), ball-in-box Mn_xCo_3‑xO₄ polyhedron (BIB-Mn_xCo_3‑xO₄), and nanoparticle Mn_xCo_3‑xO₄ polyhedron (NP-Mn_xCo_3‑xO₄). Interestingly, the structure effect of the Mn_xCo_3‑xO₄ polyhedron on the catalytic oxidation of toluene was systematically investigated. It could be noted that the HW-Mn_xCo_3‑xO₄ sample exhibited superior catalytic performance, and the complete conversion temperature of toluene (T₁₀₀) was 195 °C. Furthermore, the toluene conversion of the HW-Mn_xCo_3‑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 Mn_xCo_3‑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 Mn_xCo_3‑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.