Preparation Of MnO_x/CeO₂ Catalysts With Different Morphologies And Their Catalytic Properties For Chlorobenzene Combustion

Chlorinated volatile organic compounds（CVOCs） which are mainly generated from industrial processes are recognized as major contributors to air pollutants as its high toxicity. Therefore, it is essential to develop a practical method to eliminate CVOCs in gases. Of the available techniques, catalytic combustion is one of the most effective technologies for the removal of destruction of CVOCs emissions due to its high removal efficiency and low operating temperature. Among the catalysts used for the catalytic combustion of CVOCs, manganese oxides are known to be active catalysts on account of their redox properties and no toxinity. Furthermore, catalysts based on MnO_x supported on metal oxides, especially MnO_x/CeO₂ catalysts, have been reported to display higher catalytic performance for the oxidation of CVOCs compared with MnO_x catalysts alone. Accordingly, the nature of the support material is critical to the catalytic performance of MnO_x-based catalysts. Reports have shown that the catalytic property is dependent on the morphology of support. Thus, in this work, we present the effect of ceria morphology on a Mn-based catalytic system for the catalytic combustion of chlorobenzene（CB）. Aiming at this problem, the following work has been done:1. Ceria nanorods（CeO₂-NR）, ceria nanocubes（CeO₂-NC）, and ceria nanoparticles（CeO₂-NPs） were synthesized by hydrothermal and co-precipitation methods, respectively. A wet impregnation method was utilized to incorporate manganese oxides over CeO₂ nanomaterials. The catalytic performance of MnO_x/CeO₂ nanomaterials with three different shapes was examined in the catalytic combustion of CB, which was used as a model compound for chlorinated volatile organic compounds（CVOCs）. The catalytic activity tests revealed that all the catalysts exhibit high catalytic activity and demonstrate resistance to chlorine poisoning. The catalytic activity follows the sequence: MnO_x/CeO₂-NC >MnO_x/CeO₂-NPS>MnO_x/CeO₂-NR. Therefore, these results demonstrated that catalytic performance of the MnO_x/CeO₂ catalyst was greatly affected by the CeO₂ morphology. Higher catalytic activity of MnO_x/CeO₂-NC may be attributed to abundant Mn4+ species, oxygen vacancies and surface-adsorbed oxygen, which are associated with exposed（100） crystal planes.2. MnO_x/CeO₂ nanocubes catalysts were chosen as the target catalysts to the further study and then a series of MnO_x/CeO₂ nanocubes catalysts with different Mn content were prepared by a wet impregnation method and were investigated to study the effect of Mn loading on the MnO_x/CeO₂ nanocubes catalysts for the catalytic combustion of chlorobenzene. We found that the MnO_x/CeO₂ nanocubes catalysts with 11 wt% Mn loading exhibited the best catalytic activity. When the contents of Mn is more than 11 wt %, some active sites will be covered, leading to the decreasing of catalytic activity, On the contrary, when the contents of Mn is less than 11 wt %, the active sites are not surfficent.3. The experimental and theoretical methods were used to elucidated the vital effect of MnO_x on the MnO_x/CeO₂ nanocubes. The DFT calculations obviously demonstrated that the MnO_x/CeO₂（100） models was more easily to form the oxygen vacancies and CB moleculars were preferentially adsorbed on MnO_x/CeO₂（100） models than CeO₂（100） models, which resulted in a easier formation of C-O bond between CB and surface oxygen and thus enabled a faster CB decomposition into COx, and can therefore resulted in a higher CB conversion on the MnO_x/CeO₂（100） surface. Raman and H2-TPR results demonstrated that the higher amount of oxygen vacancies and the strong interation between MnO_x and CeO₂ lead to the enhanced catalytic activity of MnO_x/CeO₂（100）. Thus, the combination of experimental and theoretical investigations of CeO₂（100） models and MnO_x/CeO₂（100） models for catalytic oxidation of CB reveals the vital effect of MnO_x on the CeO₂ nanocubes in enhancing the catalytic performance for catalytic oxidation of CB. Thus, MnO_x/CeO₂（100） nanocubes can be used as a potential catalysts for the elimination of CVOCs.