Carefully Designed Hollow Mn_<i>x</i>Co_3–<i>x</i>O₄ 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_{<i><i>x</i></i>}Co_3‑<i>x</i>O₄ were successfully synthesized by optimizing the heating decomposition conditions of Mn@Co-ZIFs precursors to form three types of Mn_{<i><i>x</i></i>}Co_{3‑<i><i>x</i></i>}O₄ catalysts with different morphologies, including the hollow Mn_<i>x</i>Co_3‑xO₄ polyhedron (HW-Mn_<i>x</i>Co_3‑<i>x</i>O₄), ball-in-box Mn_<i>x</i>Co_3‑<i>x</i>O₄ polyhedron (BIB-Mn_<i>x</i>Co_3‑<i>x</i>O₄), and nanoparticle Mn_<i>x</i>Co_3‑<i>x</i>O₄ polyhedron (NP-Mn_<i>x</i>Co_3‑<i>x</i>O₄). Interestingly, the structure effect of the Mn_<i>x</i>Co_3‑xO₄ polyhedron on the catalytic oxidation of toluene was systematically investigated. It could be noted that the HW-Mn_<i>x</i>Co_3‑<i>x</i>O₄ sample exhibited superior catalytic performance, and the complete conversion temperature of toluene (<i>T</i>₁₀₀) was 195 °C. Furthermore, the toluene conversion of the HW-Mn_<i>x</i>Co_3‑<i>x</i>O₄ 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_<i>x</i>Co_3‑<i>x</i>O₄ 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_<i>x</i>Co_3‑<i>x</i>O₄ 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.