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

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.