铈基复合氧化物催化剂在SiO_2表面的失活机制

Deactivation Mechanism of CeO_2-Based Mixed Oxide Catalysts Supported on SiO_2

以甲苯催化燃烧为模型反应,通过调节不同Cu-Mn-Ce(CMC)复合氧化物在多孔SiO_2(KIT-6)上的负载量,研究了SiO_2表面与CMC作用对催化剂物理化学性能的影响。发现低负载量下CMC氧化物出现明显失活现象,与SiO_2接触会抑制氧化物活性相的形成,SiO_2量的减少可使CMC复合氧化物活性得到逐步恢复。X射线衍射(XRD)、程序升温还原(H2-TPR)、N2吸附(BET)和透射电镜(HRTEM)等表征表明,SiO_2不对CMC晶相结构产生影响,这种失活机制是由于SiO_2表面的丰富羟基作用,导致表面氧化物高度分散,活性氧物种从晶格氧转变为表面氧。复合氧化物的晶格氧对催化燃烧起到关键性作用,通过焙烧去除SiO_2表面羟基和减少SiO_2用量,可使复合氧化物晶格氧的数量增加,恢复复合氧化物催化剂活性。

Here we reported the effect of the Cu-Mn-Ce-SiO2 （CMC-SiO2） interaction on the physical and chemical aspects of the catalytic combustion of toluene by adjusting the loading amount of the CMC mixed oxide on SiO2. Notably, the CMC/KIT-6 catalyst with low CMC loading performed poorly with an obvious deactivation, owing to the inhibition of the metal oxides active sites, while the activity recovered after washing away some SiO2. The catalysts were characterized by X-ray diffraction （XRD）, H2 temperature-programmed reduction （H2-TPR）, N2 adsorption, and high-resolution transmission electron microscopy （HRTEM）. Although there is no change in crystal structure after loading on SiO2, active oxygen species immigrate from lattice to surface for SiO2 surface rich in hydroxyl groups and having high dispersion of CMC, leading to deactivation of the CMC catalyst. However, it is worth mentioning that the lattice oxygen played a key role in catalytic combustion. The activity of the CMC catalyst recovered when the quantity of lattice oxygen increased upon removing surface --OH groups by calcination or removing some SiO2 by alkali washing.