Preparation,Properties And Structural Characterizations Of Meso-Porous La-Co-Ce Complex Oxide Catalysts

1,A series of La-Co-Ce-O complex oxide catalysts with various atomic ratios (La+Co/La+Co+Ce) were prepared by the citric acid complex-organic template decomposition method. During the synthesis, citric acid was used as the coordination agent and cetyltrimethylammoniumbromide (CTABr) as the organic template. It gave rise to a clear solution of soluble homogeneous mixed metal oxides containing a cationic surfactant in acidic medium, which was precipitated in basic medium, and followed by hydrothermal treatment. The catalysts were characterized by using BET, XRD, XPS, H₂-TPR, O₂-TPD(O) and Co K-edge EXAFS techniques. The results of physisorb of N₂ show that the samples possess high BET surface areas and uniform mesoporous sizes (pore diameter is between 37 and 39A) after calcined at 500oC. H₂-TPR and O₂-TPD(O) results reveal that these catalysts exhibit very good redox properties, especially for the adsorption,activation and transferring of oxygen species. The structural characterization results of XRD, XPS and Co K-edge EXAFS indicate that the main active phase is highly dispersed Co3O4 crystallinity, and that there exist strong interaction between the components and cobalt phases in the complex oxides. Compared with those prepared by conventional impregnation method, the samples obtained by the citric acid complex-organic template decomposition method possess much higher component dispersion, much smaller crystal sizes (12⁴1nm) and much lower light-off temperatures (T50 is decreased ⁴0oC) for CO oxidation. However, the oxidation activity is not totally depended on the BET surface area of the sample, the content and crystal size of Co₃O₄ crystallinity. While the oxygen-transferring ability of CeO₂ in the samples and the surface atomic percentages of La, Co and Ce are major factors, which are relevant to the high activity. When the surface atomic percentages of La, Co and Ce are close to each other, they show the best catalytic synergetic effect. For C3H8 oxidation, the mesoporous materials also show very high activity, which is depended on the BET surface area of the sample. Additionally, the samples prepared by the citric acid complex-organic template decomposition method exhibit strong ability to resist the sintering and SO₂ poisoning. 2,A series of La-Co-Ce-Al-O catalysts were prepared by wetness impregnation method using La-Ce-Al-O as support and Co(NO₃)2.6H₂O as precursor. Two kinds of La-Ce-Al-O complex oxides were obtained by the Sol-Gel method and by the modification of γ -Al₂O₃ with La and Ce, respectively. The catalysts were characterized by means of BET, XRD, XPS, H2-TPR and Co K-edge EXAFS. The catalytic activity of the samples for CO oxidation were evaluated, the effect of the content of La2O3 and the preparation method on the properties of the samples was discussed. The results of characterization work reveal that the catalysts prepared by Sol-Gel method possess much higher BET surface area and the addition of La2O3 prominently increases the dispersion of CeO2 and Co3O4, the crystal sizes of CeO2 are about half of those prepared by impregnation method. The results of activity evaluation indicate that the samples by Sol-Gel method are more active than those by impregnation method. The catalytic activity of the samples was enhanced by the addition of La2O3 during support modification. The optimum content of La2O3 is about 5wt% of La2O3/Al2O3. When the ratio of La2O3/Al2O3 reaches 10wt%, the catalytic activity of the samples for CO oxidation decreases obviously.