24450
J. Phys. Chem. B 2006, 110, 24450-24456
Controlled Synthesis, Characterization, and Catalytic Properties of Mn2O3 and Mn3O4 Nanoparticles Supported on Mesoporous Silica SBA-15 Yi-Fan Han,* Fengxi Chen, Ziyi Zhong, Kanaparthi Ramesh, Luwei Chen, and Effendi Widjaja Institute of Chemical and Engineering Sciences, 1, Pesek Road, Jurong Island, Singapore 627833 ReceiVed: August 2, 2006; In Final Form: August 29, 2006
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 ([CH3COCHdC(O)CH3]3-Mn) and Mn3O4 nanocrystals by the thermolysis of manganese(II) acetyl acetonate ([CH3COCHdC(O)sCH3]2Mn) 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 N2 physisorption, X-ray diffraction (XRD), laser Raman spectroscopy (LRS), temperatureprogrammed 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 N2 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 (MO-M) of both MnOx nanoparticles and bulk MnOx. We also observed the terminal MndO 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 SBA15 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.
Introduction Manganese oxides (MnOx) are of great importance in catalysis, electrochemistry, ion-exchange materials, magnetite, and batteries, among other areas.1-4 In the recent past, controlled synthesis of MnOx nanomaterials has attracted considerable attention from both academia and industry.5-7 Among various preparation methods using porous materials as solid nanoreactors, MnOx nanocrystals with various morphologies, e.g., nanoarrays, nanowires, nanospheres, and clusters, have been obtained by a method called “nanocasting”.8,9 Generally, inorganic precursors are first introduced into channels of host materials, such as mesoporous silicas. Then, MnOx nanocrystals are produced with a replica of the confined space by a subsequent calcination. Because of the facile phase transformation of MnOx, during preparation, only multivalence mixtures of manganese oxides (MnO2, Mn2O3, and Mn3O4) are usually obtained. To date, direct synthesis of manganese oxide with a defined crystalline structure in nanoreactors, especially silicabased materials, is still far from easy. Mn2O3 nanoparticles can be produced through a conventional method, namely, thermal * To whom correspondence should be addressed. E-mail: han_yi_fan@ ices.a-star.edu.sg.
treatment of MnO2 above 823 K or reduction in H2 above 523 K.10-13 Actually, both high-temperature treatment and H2 reduction can usually give rise to the aggregation and phase transformation of MnOx nanoparticles. Although the generation of Mn3O4 nanoparticles from Mn-containing organic clusters12 and the synthesis of MnO2 nanoparticles from Mn(NO3)213 inside SBA-15 have been reported recently, the fabrication of Mn2O3 nanoparticles on mesoporous materials has rarely been addressed so far. Presently, there is a growing interest in precisely controlling the crystalline phase of manganese oxide at the nanoscale in order to understand the mechanistic aspects of CO oxidation at low temperatures. Furthermore, MnO exhibits unique antiferromagnetic character at temperatures below TN ) 118 K, and it becomes an ideal material for the study of electronic and magnetic properties of rock salt oxides;14 MnO2 and Mn3O4 are widely used as catalysts and electrode materials.15 In addition, we recently observed that bulk Mn2O3 could potentially be a highly efficient combustion catalyst at low temperatures (
