Accelerated synthesis of MnO₂ nanocomposites by acid-free hydrothermal route for catalytic soot combustion

MnO₂ nanocomposites with porous structure were successfully synthesized by a facile hydrothermal route from KMnO₄ without the addition of any acid. Among CeO₂, CuO and Co₃O₄, an accelerated synthesis of MnO₂ assisted by CeO₂ was observed, which contributed to the redox transformation between Ce³⁺ and Mn⁷⁺. The as-prepared catalysts were characterized by XRD, N₂ adsorption-desorption, ICP, SEM, TEM, H₂-TPR, and XPS. Lattice shrink occurred after the hydrothermal reaction of CeO₂ with KMnO₄, while lattice expansion in CeO₂ was observed after calcination at high temperature. The above phenomena are ascribed to the conversion between small sized Ce⁴⁺ and large sized Ce³⁺ in CeO₂. The concentration of surface Ce³⁺ decreases after the hydrothermal reaction with KMnO₄. The greater proportion of Ce³⁺ after calcination results in more oxygen vacancies that are beneficial for the catalytic oxidation of soot. The best catalytic performance is acquired over the CeMn-600 catalyst and the corresponding T₁₀, T₅₀, and T_m,l are 320 °C, 418 °C and 420 °C, respectively. The strong interaction between the CeO₂ and MnO₂ is also a key factor for outstanding catalytic activity for soot combustion.