物理化学学报 >> 2018, Vol. 34 >> Issue (6): 719-730.doi: 10.3866/PKU.WHXB201712011

论文 上一篇    

以金属有机骨架为牺牲模板制备MnOx-CeO2及其催化氧化

林雪婷1,付名利1,2,3,*(),贺辉1,吴军良1,2,3,陈礼敏1,2,3,叶代启1,2,3,胡芸1,2,3,王逸凡1,WENWilliam4   

  1. 1 华南理工大学，环境与能源学院，广州 510006
    2 广东省大气环境与污染控制重点实验室，广州 510006
    3 挥发性有机物污染治理技术与装备国家工程实验室，广州 510006
    4 Centre for Clean Environment and Energy, Environmental Futures Centre, School of Environment, Griffith University, Gold Coast, QLD4222, Australia
  • 收稿日期:2017-10-27 发布日期:2018-03-20
  • 通讯作者: 付名利 E-mail:mlfu@scut.edu.cn
  • 基金资助:
    国家自然科学基金(51578245);国家自然科学基金(51378213);国家自然科学基金(51108187);国家自然科学基金(21777047); ( )

Synthesis of MnOx-CeO2 Using Metal-Organic Framework as Sacrificial Template and Its Performance in the Toluene Catalytic Oxidation Reaction

Xueting LIN1,Mingli FU1,2,3,*(),Hui HE1,Junliang WU1,2,3,Limin CHEN1,2,3,Daiqi YE1,2,3,Yun HU1,2,3,Yifan WANG1,William WEN4   

  1. 1 School of Environment and Energy, South China University of Technology, Guangzhou 510006, P. R. China
    2 Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control (SCUT), Guangzhou 510006, P. R.China
    3 National Engineering Laboratory for VOCs Pollution Control Technology and Equipment, Guangzhou 510006, P. R. China
    4 Centre for Clean Environment and Energy, Environmental Futures Centre, School of Environment, Griffith University, Gold Coast, QLD4222, Australia
  • Received:2017-10-27 Published:2018-03-20
  • Contact: Mingli FU E-mail:mlfu@scut.edu.cn
  • Supported by:
    The project was supported by the National Natural Science Foundation of China(51578245);The project was supported by the National Natural Science Foundation of China(51378213);The project was supported by the National Natural Science Foundation of China(51108187);The project was supported by the National Natural Science Foundation of China(21777047);the Guangdong Natural Science Foundation, China( )

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摘要：

以均苯三甲酸合铈-金属有机骨架(CeBTC-MOF)作为模板制备系列不同Mn含量的MnOx-CeO2催化剂，用于甲苯催化氧化。应用X射线衍射(XRD)、N2物理吸附-脱附、热重分析(TG)、元素分析(EA)、电感耦合等离子体发射光谱法(ICP-OES)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、程序升温还原(H2-TPR)、X射线光电子能谱(XPS)、拉曼光谱(Raman)和紫外可见漫反射(UV-vis)等手段对催化剂进行了表征。结果表明，通过MOF模板法制备的复合氧化物具备棒状形貌、高度分散、高比表面积和纳米晶体颗粒等特征。Mn在引入MOF的过程中，一部分进入CeO2晶格形成固溶体，另一部分则分散在CeO2表面，且分散的Mn分为单层分散态和晶相态。其中，CeO2载体表面和Mn分散物之间的强相互作用是影响活性的重要因素。当表面分散的Mn低于单层分散阈值6.2%时，Mn以嵌入模型的形式与表面CeO2发生强相互作用，有效促进催化剂的还原从而提高活性；当表面分散的Mn超过单层分散阈值6.2%时，载体表面形成Mn3O4晶相结构，对活性无明显促进作用。

关键词: 金属有机骨架, MnOx-CeO2复合氧化物, 甲苯, 单层分散阈值, 氧空位

Abstract:

A series of MnOx-CeO2 with different Mn contents was prepared using CeBTC-MOF as the sacrificial template. These constituted a new kind of porous crystalline materials assembled by cerium as metal ions and 1, 3, 5-benzenetricarboxylic acid as organic ligands. The composite oxides exhibited good redox properties and were tested as catalysts in the oxidation of toluene. To obtain insight into the structure-activity relationship of the catalysts, the samples were characterized using powder X-ray diffraction (XRD), nitrogen adsorption-desorption, thermogravimetric analysis (TG), elemental analysis (EA), inductively coupled plasma-optical emission spectrometry (ICP-OES), scanning electron microscopy (SEM), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman), and UV-Vis diffuse reflectance spectroscopy. Studies of the CeBTC-MOF template showed that the metal-organic framework could be completely decomposed at a calcination temperature of 300 ℃. Therefore, CeBTC-MOF decomposed and generated CO2 and H2O during the calcination process. The gas molecule spilled out from the structure to form the interior void space. The spilling out could be controlled by varying the calcination temperature. This regulated the quantity and size of the interior void, which in turn made the surface area controllable. The secondary building unit of CeBTC-MOF was oxidized to nano-sized crystalline particles, which exhibited outstanding interfacial contact. SEM and TEM results showed that the composite oxides prepared by pyrolysis of the CeBTC-MOF template exhibited rod-shaped nanocrystalline particles. While introducing Mn into MOF, part of Mn entered the ceria lattice to form solid solution and the remaining Mn was dispersed on CeO2 surface. The elemental mappings revealed a well-proportioned distribution of Mn, which confirmed the successful formation of bimetallic metal oxides using the MOF-template method. All the samples exhibited sizes and shapes similar to their parent MOFs. As for catalytic activity, all the composite oxides showed better performances than pure CeO2 for catalytic oxidation of toluene. This could be attributed to higher concentration of oxygen vacancies, which was characterized by Raman spectroscopy. In addition, the XPS results indicated that Mn4+/(Mn2++Mn3+), Ce4+/Ce3+, Olatt (lattice oxygen), and Osur(surface oxygen) all participated in the redox process during catalytic oxidation of toluene, which helped elucidate the mechanism at a micro level.

Interestingly, the catalytic activity did not improve further when the Mn content of the composite oxides reached 5%. This could be ascribed to two different states of the dispersed Mn: monolayer dispersion state and crystalline phase. The strong interaction between ceria oxides and dispersed Mn species played an important role in affecting catalytic activity. The results showed the presence of a monolayer dispersion threshold (6.2%), confirmed by XPS characterization, which was in accordance with all the characterization results; it was proved that this threshold had a significant impact on the catalytic activity. When the dispersed Mn content was lower than the monolayer dispersion threshold, Mn reacted with the surface CeO2 in the form of an incorporation model, leading to charge transfer and higher concentration of oxygen vacancies, which in turn effectively promoted the catalytic performance. When the dispersed Mn content exceeded the monolayer dispersion threshold, Mn3O4 was formed on the CeO2 surface; this disrupted the promotion of catalytic activity, which explains the same catalytic activity of all the samples (5% MnOx-CeO2, 8% MnOx-CeO2, and 10% MnOx-CeO2).

This successful formation of bimetallic metal oxides using CeBTC-MOF template indicated that composite oxide synthesis was feasible using the MOF template method. To obtain high catalyst performance of these composite oxides, it was important to control the metal content at the level of the monolayer dispersion threshold.

Key words: Metal-Organic Framework, MnOx-CeO2 composite oxides, Toluene, Monolayer dispersion threshold, Oxygen vacanc