The Synthesis Of Shaped CeO₂-MnO_x Mixed Oxides And The Oxidation Performance For CVOCs

With the development of society, human behavior has increasingly affected the natural environment. As a significant fraction of toxic air containments, chlorinated volatile organic compounds (CVOCs) not only have extremely serious effect on the environment, but also damage human body a lot. Because of their good solubility and stability, CVOCs have been widely used in the chemical production process, and the environmental problems have attracted more and more attention. There are many kinds of destruction methods for the recovery of CVOCs, among which catalytic oxidation is considered to be the most energy-efficient and economically feasible method due to its advantages of low combustion temperature, low energy consumption and low secondary pollution. In recent years, so many researchers have focused on the relevant catalytic materials.In this paper, a series of CeO2-MnOx mixed oxide catalysts with different morphologies were prepared by the hydrothermal coprecipitation method, and then evaluated the catalytic performance of CB over CeO2-MnOx (Ce/Mn molar ratio 2:1) catalysts. The effects of different ratios over rod-shaped CeO2-MnOx mixed oxides for the oxidation of CB were also studied. The characterization techniques of XRD, BET, XPS, UV-Raman, TEM, H2-TPR and O2-TPD were used to explore the texture and phase composition of the catalysts, some specific conclusions from this work are drawn as follows:1. The CeO2-MnOx mixed oxides materials with different morphologies synthesized by the simple hydrothermal method have different exposed crystal facets. For rod-CeO2-MnOx catalyst, it exposed{100} and {110} facets, but for plate-CeO2-MnOx and polyhedra-CeO2-MnOx, they all exposed{111} and {100} crystal facets, while the cube-CeO2-MnOx only exposed{100} crystal facets. And we have also investigated the catalytic performance of CB oxidation, the results showed that the rod-CeO2-MnOx catalysts exhibited the best catalytic performance, generating only small amounts of chlorinated by-products during the decomposition of CB. This might be related to the enhanced oxygen vacancies and the increased mobility of the surface-active oxygen species. From the Rietveld refinement results of XRD, it can be seen that this binary oxide system is difficult to form CeMnOx solid solution, the manganese species form new structures on different exposed crystal planes, and the special tunnel-like structure of todorokite (S.G.P2/M:b) and vernadite (S.G.I4/m) is favorable for the effective flow of active oxygen on the surface of the catalyst, so as to improve the catalytic oxidation performance.2. Since heterogeneous catalytic reactions take place on the catalyst surface, the morphology significantly alters the oxygen storage capacity and the distribution of doped metals, then affects the catalytic performance. Apart from the special structure of the manganese species, the existence of surface-active Mn4+/Mn3+ or Mn4+/Mn2+ couples, the enhanced structure defects of CeO2 shown for the higher oxygen vacancies and better electron mobility are likely to be key factors for the high catalytic efficiency of rod-shaped Ce/Mn-1/8 catalyst.3. Influence of the presence of H2O or C6H6 on CB degradation performance were also further studied.The results show that the catalytic activity for CB elimination is obviously decreased in the presence of H2O or C6H6, which may be related to the competitive adsorption of H2O or C6H6 with CB on active sites. Especially, C6H6 shows stronger competitive adsorption on active sites, resulting in the depth of oxidation ability declined and easier to form carbon deposition.4. Based on the results of O2-TPD, the existence of reactive oxygen species in redox process was summarized. Both superoxide (O2-) and peroxides (O-) were all characterized in the general process of O2 dissociations.In this paper, we summarize the recyclic flow of oxygen species in rod-shaped CeO2-MnOx mixed oxide catalysts by combining various characterization techniques:the mutual flow and transformation of O2-, O- and O2-, which enable the catalytic oxidation reaction to proceed rapidly and effectively. At the same time, we also summarize the possible reaction mechanism of the catalytic oxidation of CB over the rod-shaped CeO2-MnOx mixed oxide catalysts:CB adsorbes on the different reactive sites over the catalysts, and then the redox reaction occurs with different states of oxygen species distributed on the surface. Thus, the distribution of Mn ions (Mn4+, Mn3+ and Mn2+), the flow and migration of active oxygen species and the interaction between the components are the key factors influencing the mechanism of CB catalytic oxidation.