Controlled synthesis, characterization, and catalytic properties of Mn2O3 and Mn3O4 nanoparticles supported on mesoporous silica SBA-15

A method established in the present study has proven to be effective in the synthesis of Mn2O3 nanocrystals by the thermolysis of manganese(III) acetyl acetonate ([CH3COCH=C(O)CH3](3)-Mn) and Mn3O4 nanocrystals by the thermolysis of manganese(II) acetyl acetonate ([CH3COCH=C(O)-CH3](2)Mn) on a mesoporous silica, SBA-15. In particular, Mn2O3 nanocrystals are the first to be reported to be synthesized on SBA-15. The structure, texture, and electronic properties of nanocomposites were studied using various characterization techniques such as N-2 physisorption, X-ray diffraction (XRD), laser Raman spectroscopy (LRS), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results of powder XRD at low angles show that the framework of SBA-15 remains unaffected after generation of the manganese oxide (MnOx) nanoparticles, whereas the pore volume and the surface area of SBA-15 dramatically decreased as indicated by N-2 adsorption-desorption. TEM images reveal that the pores of SBA-15 are progressively blocked with MnOx nanoparticles. The formation of the hausmannite Mn3O4 and bixbyite Mn2O3 structures was clearly confirmed by XRD. The surface structures of MnOx were also determined by LRS, XPS, and TPR. The crystalline phases of MnOx were identified by LRS with corresponding out-of-plane bending and symmetric stretching vibrations of bridging oxygen species (M-O-M) of both MnOx nanoparticles and bulk MnOx. We also observed the terminal Mn=O bonds corresponding to vibrations at 940 and 974 cm(-1) for Mn3O4/SBA-15 and Mn2O3/SBA-15, respectively. These results show that the MnOx species to be highly dispersed inside the channels of SBA-15. The nanostructure of the particles was further identified by the TPR profiles. Furthermore, the chemical states of the surface manganese (Mn) determined by XPS agreed well with the findings of LRS and XRD. These results suggest that the method developed in the present study resulted in the production of MnOx nanoparticles on mesoporous silica SBA-15 by controlling the crystalline phases precisely. The thus-prepared nanocomposites of MnOx showed significant catalytic activity toward CO oxidation below 523 K. In particular, the MnOx prepared from manganese acetyl acetonate showed a higher catalytic reactivity than that prepared from Mn(NO3)(2).