摘要:
含氯挥发性有机化合物(CVOCs)是对人体健康和环境危害极大的有机化合物. 常见的有二氯乙烷、三氯乙烯、三氯甲烷、四氯化碳和氯苯等. 有些CVOCs是"三致"(致畸、致癌、致突变)物质, 有些少量进入大气就能破坏臭氧层, 亦或与臭氧等形成光化学烟雾, 引起全球变暖. 因此, 对于其消除迫在眉睫. CVOCs通常采用高温直接燃烧、吸附、光催化氧化和催化燃烧等方法降解. 其中, 催化燃烧是非常有效的. 我们选择氯苯作为CVOCs探针分子是因其不仅存在于农药和化工产品中, 在室内环境中也大量存在, 而负载贵金属和非贵金属型催化剂可用于其催化降解. 贵金属催化剂价格高而且易氯中毒,但过渡金属催化剂价格低且抗失活, 因而是不错的选择. 通常过渡金属催化剂使用V2O5, Cr2O3, MnO2, Co3O4及NiO等活性组分, 而Mn和Co氧化物有较好活性且没有环境污染, 常用作活性组分. 另外, MCM-41是有序介孔硅酸盐和硅铝酸盐家族中M41S的一员, 具有高的比表面积和较窄的孔径分布, 常用作催化剂的载体. 我们利用具有大比表面积、大孔径的MCM-41作为载体, 采用浸渍法负载MnOx, CoOx, MnOx-CoOx等活性组分, 制备系列的催化剂用于低浓度氯苯的催化燃烧,研究催化剂的催化活性、选择性及稳定性. 并利用XRD、N2吸脱附、高分辨电镜-能谱分析、H2-TPR和CB-TPD等手段对MCM-41及催化剂的织构-结构、表面形貌、活性组分分散状态、氧化还原性能及吸附性能等做了系统的研究.采用表面活性剂软模板技术合成了具有大比表面积、大孔径、耐热稳定性高的MCM-41介孔分子筛, 负载不同比例的Mn/Co (摩尔比是3:1、6:1及9:1, 其中总负载量为10%), 以氯苯催化燃烧为探针反应, 筛选出活性最佳时的Mn/Co比例. 活性评价实验结果表明, 各催化剂的活性以下列顺序依次降低: MnCo(6:1)/MCM-41 > MnCo (9:1)/MCM-41 > MnCo (3:1)/MCM-41 > Mn/MCM-41 > Co/MCM-41, 其中MnCo(6:1)/MCM-41活性最佳, 在270 oC即可完全催化燃烧氯苯. 耐久性实验结果显示, MnCo (6:1)/MCM-41在连续反应1000 h后, 其活性没有降低, 表明其具有非常良好的稳定性. XRD实验结果表明, 在Mn/MCM-41及Co/MCM-41催化剂上分别检测到MnO2及Co3O4的特征衍射峰. 在MnCo (6:1)/MCM-41催化剂上, MnO2及Co3O4的特征衍射峰消失, 同时出现了MnCoOx的特征衍射峰, 这是由于MnO2及Co3O4的强相互作用经过焙烧后形成的, 且MnCoOx的特征衍射峰较小, 表明双金属活性组分的分散比单金属催化剂好. N2吸脱附结果显示, MCM-41的比表面积达到805.9 m2/g, 孔体积达到0.795 cm3/g. 负载活性组分后其比表面积及孔体积均有不同程度的减小, 这是由于活性组分进入了MCM-41的孔隙. 高分辨电镜结果表明, MCM-41具有均匀的介孔孔道结构, MnCoOx在MCM-41表面的颗粒小, 分散好. 能谱扫描出Mn, Co, O等元素, 表明活性组分成功地负载在MCM-41载体上. H2-TPR表明, 双金属催化剂的还原峰温较单金属催化剂低, 表明其具有更好的氧化性能. CB-TPD结果表明, MnCo双金属催化剂脱附氯苯的温度高于单金属催化剂, 说明氯苯与催化剂之间的相互作用更强, 即双金属催化剂对氯苯的吸附能力更强, 使得氯苯催化燃烧更加充分, 因此其催化性能更好. 同时, 深入探讨了MnOx, CoOx, MnCoOx和MCM-41之间的相互作用及对催化燃烧性能的影响.%MCM-41 was synthesized by a soft template technique. The specific surface area and pore volume of the MCM-41 were 805.9 m2/g and 0.795 cm3/g, respectively. MCM-41-supported manganese and cobalt oxide catalysts were prepared by an impregnation method. The energy dispersive X-ray spectroscopy clearly confirmed the existence of Mn, Co, and O, which indicated the successful loading of the active components on the surface of MCM-41. The structure and function of the catalysts were changed by mod-ulating the molar ratio of manganese to cobalt. The 10%MnCo(6:1)/MCM-41 (Mn/Co molar ratio is 6:1) catalyst displayed the best catalytic activity according to the activity evaluation experiments, and chloro-benzene (1000 ppm) was totally decomposed at 270 °C. The high activity correlated with a high disper-sion of the oxides and was attributed to the exposure of more active sites, which was demonstrated by X-ray diffraction and high-resolution transmission electron microscopy. The strong interactions between MnO2, Co3O4, MnCoOx, and MCM-41 indicated that cobalt promoted the redox cycles of the manganese system. The bimetal-oxide-based catalyst showed better catalytic activity than that of the single metal oxide catalysts, which was further confirmed by H2temperature-programmed reduction. Chlorobenzene temperature-programmed desorption results showed that 10%MnCo(6:1)/MCM-41 had higher adsorption strength for chlorobenzene than that of single metal catalysts. And stronger adsorption was beneficial for combustion of chlorobenzene. Furthermore, 10%MnCo(6:1)/MCM-41 was not deactivated during a con-tinuous reaction for 1000 h at 260 °C and displayed good resistance to water and benzene, which indicated that the catalyst could be used in a wide range of applications.