Porous hollow manganites with robust composite shells for oxidation of CO at low temperature

Yolk- and hollow-type manganite-coated silica (SiO₂) microspheres were synthesized via the thermal hydrolysis of urea using a dissolution and deposition method. Core/shell SiO₂ microspheres were used as templates; the products were then annealed at 700 °C for 10 h under an O₂ atmosphere. Yolk microspheres (Mn₂O₃/SiO₂_1T and Mn₂O₃/SiO₂_2T) were obtained when the dissolution and deposition step was repeated once and twice, respectively, while hollow microspheres (Mn₂O₃/SiO₂_3T) were produced when the method was repeated three times. The microspheres had average diameters of 300-310 nm. The Mn₂O₃/SiO₂_3T product possessed the largest Brunauer-Emmett-Teller (BET) surface area and exhibited the best catalytic performance with regard to CO oxidation. The complete conversion of CO was achieved at approximately 200 °C. Other microspheres, including Mn₃O₄/SiO₂_3T, NiO/SiO₂_3T, and Ni/SiO₂_3T, yielded much lower activities than the Mn₂O₃/SiO₂_3T catalyst. The large surface area of the Mn₂O₃/SiO₂_3T microsphere sample, as well as the presence of Mn³⁺ ions, appears to be responsible for its superior catalytic CO oxidation activity amongst the tested catalysts.