负载型La_(0.8)Sr_(0.2)MnO_3燃烧催化剂的载体效应

The Effects Of Support on Loaded Combustion Catalyst of La_(0.8)Sr_(0.2)MnO_3

用浸渍法制备负载型Ｌａ_（０．８）Ｓｒ_（０．２）Ｍｎｏ_３燃烧催化剂，比较了γ－ＡＩ_２Ｏ_３、α－ＡＩ_２Ｏ_３、堇青石（２ＭｇＯ·２Ａｌ_２Ｏ_３、５ＳｉＯ_２）和ＺｒＯ_2四种载体的负载效果．用ＸＲＤ，ＤＰＲ着重研究了γ－Ａｌ_２Ｏ_３载体上由于负载量不同、焙烧温度不同引起的催化剂物相变化和二甲苯完全氧化的催化活性变化．

Combustion catalysts La_(0.8)Sr_(0.2)MnO_3 supported on γ-Al_2O_3, α-Al_2O_3, cordierite (2MgO·2Al_2O_3·5SiO_2) and ZrO_2 were compared. Fruther inevestigation was focused on LSM/ γ-Al_2O_3 catalyst. It was observed that LSM/γ－Al_2O_3 catalyst loaded with 20% (mass fraction) LSM (La_(0.8)Sr_(0.2)MnO_3 or corresponding oxides), heated at 750℃ or above, perovskite-type oxides were found by XRD examination, whereas, the same catalyst loaded with 10% or less LSM, perovskite ox- ides were absent, calcination temperature about 750℃ is necessary for the formation of perovskite structure in LSM/γ-Al_2O_3 catalysts. High activity of complete oxidation of xylen will be obtained when perovskite-type oxides. Investigation of TPR showed that neat LSM or LSM/γ-Al_2O_3(20%) was reduced by H_2-N_2 mixed gas. Two degradation processes took place. In the first, reduced temperature peak was about 350 - 450℃. If reduction ended at 400℃, perovskite structure was retained, which may be due to the reduction of Mn^(3+)to Mn^(2+) on the surface of LSM only. In the second process, perovskite structure was destroied, and La_2O_3, Mn_2O_3, Mn - Sr - O oxides could be obtained, which took place in the temperature range 685 - 750℃ and ended at 800℃. This was proved by TPR experiments (Fig. 3, 5) and XRD patterns (Fig. 4) Catalysts LSM/γ-Al)2O_3(10% or 20%) heated at 500℃ have only one TPR peak, i. e. lower temperature peak. This is due to the absence of perovskite-type oxides in the catalysts. However, neat LSM or LSM/γ-Al_2O_3(20%) heated 750℃ or above, not only the first low temperatUre TPR peak but also the second peak, which is contributed by the perovskite-type oxides in these catalysts ap- peared. Therefore, the second TPR peak, i. e. the higher temperatue peak is a characteristic peak for perovskite-type oxides in the reduced process. When LSM/ γ-Al_2O_3 (10%) catalys is heated at 750℃, no perovskite-type oxides were detected by XRD, and the second reduction peak was absent also in TPR process. The order of the second reduction peak temperature(characterisitic peak of perovskite - type ox- ides) is: neat LSM(750℃）> LSM/γ-Al_2O_3 20% (685-698℃) -deposited LSM/γ-Al_2O_3 (698℃) > LSM/γ-Al_2O_3 15% (677 - 680℃) >(LSM/γ-AL_2O_3 10% 620 - 630℃, for Mn - Al - O medium oxides on surface). It is correleted with the increasing of the effect of support sequential- ly. When LSM/γ-Al_2O_3 catalysts were heated at 900℃, more stable phase,spinel MnAl_2O_4 ap- peared, which could be proved by TPR of model catalyst MnAl_2O_4/γ-Al_2O_3.