Mn/Ce/La/Al_2O_3催化剂中锰物种的精细结构研究  

Fine Structures of Manganese Species in Highly Dispersed Mn/Ce/La/Al_2O_3 Catalysts

作　　者：李想[1] 孟明[1] 姚金松[1] 谢亚宁[2] 胡天斗[2] 刘涛[2] 

机构地区：[1]天津大学化工学院催化科学与工程系,天津300072 [2]中国科学院高能物理研究所,北京100049

出　　处：《无机化学学报》2007年第2期211-216,共6页Chinese Journal of Inorganic Chemistry

基　　金：高等学校博士学科点专项科研基金资助项目(No.20040056028);天津市自然科学基金资助项目(No.05YFJMJC09700)

摘　　要：采用浸渍法制备了La、Ce助剂改性的γ-Al2O3负载锰氧化物催化剂,运用XANES、EXAFS、XRD、XPS和H2-TPR等方法对催化剂的结构进行了表征,探讨了助剂对于催化剂中高分散Mn物种的精细结构、分散状态和存在形式的影响,并与样品的CO氧化活性相关联。XANES和EXAFS结果表明,500℃焙烧的样品中Mn物种主要以超细Mn2O3微晶形式存在,该物种由于高度分散使其配位对称性显著降低,无长程有序结构。TPR结果表明,样品中存在3种不同分散状态的表面Mn物种,即较难还原的Mn3+-O-Al3+相互作用物种,尺寸相对较大的三维分散的Mn2O3微晶,以及二维高度分散的Mn物种,后者是CO氧化反应的主要活性相。虽然Ce的加入使Mn物种的分散度有所降低,但Ce与Mn物种间的相互作用弱化了Mn-O键,加速了反应过程中活性氧物种的传递,提高了氧化还原循环的效率。同时,La的加入进一步促进了Ce物种在载体表面的分散,加强了Ce物种与Mn物种间的相互作用及催化协同性。The manganese oxides catalysts supported on γ-Al2O3 and promoted by La and/or Ce were prepared by impregnation method. The structures of the catalysts were characterized by XANES, EXAFS, XRD, XPS and H2-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. The results of XANES and EXAFS demonstrate that the manganese species in the samples calcined at 500 ℃ mainly exist as superfine crystallites of Mn2O3, which have no long-range crystal structures, and show very high dispersion and very low coordination symmetry. TPR results show that there are three kinds of surface dispersed manganese species in samples, namely the strongly interacted dimensional Mn2O3 crystallites with larger size and the two-dimensional species Mn^3＋-O-Al^3＋, the three- 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 mobility 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.

关 键 词：Mn/Ce/La/Al2O3催化剂 XANES EXAFS CO氧化 

分 类 号：O614[理学—无机化学]