Fine structures of manganese species in highly dispersed Mn/Ce/La/Al2O3 catalysts

The manganese oxides catalysts supported on gamma-Al2O3 and promoted by La and/or Ce were prepared by Impregnation method. The sLruetures of the catalysts were characterized by XANES, EXAFS, XRD, XPS and H-2-TPR. The effect of the promoters on the fine structures, the dispersion and the state of the manganese species was investigated and correlated to the CO oxidation performance of the samples. Tile results of XANES and EXAFS demonstrate that the manganese species in the samples calcined at 500 degrees C mainly exist as superfine crystallites of M2O3, which have no long-range crystal structures, and show verN high dispersion and very low coordination symmetry. TPR results show that there are three kinds of surface dispersed manganese species In samples, naniely the strongly interacted species Mn2O3, the three-dimensional Mn2O3 cryslallites with larger size and the two-dimensional dispersed Mn oxides, which is regarded as the main active phases for CO oxidation. Although the addition of Ce decreases the dispersion of Mn species, the interaction between Ce and Mn species weakens the Mn-O bond, which in turn facilitates the inobilltv of active oxygen species and increases the efficiency of redox cycle during the reaction. Meanwhile, the presence of La further improves the dispersion of Ce species on the surface of the support, and therefore strengthens the interaction and the catalytic synergism between Ce and Mn species.