| Chlorinated volatile organic compounds (Cl-VOCs) are a kind of atmospheric contaminants commonly encountered, and they can cause great damage to the ecologic environment and human beings. Among various treatment methods for Cl-VOCs elimination, catalytic combustion has already been proved as one of the most economical and effective techniques. For Cl-VOCs destruction, the Cl species formed in the process may sometimes be strongly adsorbed on the catalyst surface or react with the active species to form volatile metal chloride or metal oxychloride, leading to the deactivation of the catalysts and the formation of some polychlorohydrocarbons byproducts with greater toxicity. Therefore, developing novel catalysts with lower cost and higher catalytic performances is one of the key factors for Cl-VOCs elimination.In this thesis, a serious of MOx doped Ce02-MOx mixed oxides catalysts (M= V, Cr, Mn, Fe, Co, Ni and Cu, respectively) and CeO2-CrOx mixed oxide modified by solid acid (zeolites and metal oxides MOy, M=Ti, V, Nb, Mo, W and La, respectively) catalysts were firstly synthesized. Then, some typical Cl-VOCs (CB, DCE, DCM and TCE) were chosen as the reactants, combined with various techniques, such as X-ray diffraction (XRD), UV-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), N2 adsorption/desorption, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), NH3 temperature-programmed desorption (NH3-TPD), in-situ diffuse reflectance infrared spectra of NH3 adsorption, hydrogen temperature-programmed reduction (H2-TPR) and temperature-programmed surface reaction (TPSR), in order to systematically investigate the structure/texture properties of these catalytic materials, the interaction between the solid acid and the CeO2-CrOx mixed oxide, as well as their synergistic effect on the catalytic performances of the catalysts for Cl-VOCs destruction. Besides, the influence of water or benzene in the reaction system on the catalytic performances of the catalysts, the durability, the reason for the catalyst deactivation, the condition for the catalyst recovery and the kinetics for the reaction were also evaluated. The major conclusions were listed below.1. The catalytic performances of CeO2 and CeO2-MOx mixed oxides for Cl-VOCs decomposition were investigated. Compared to CeO2, the catalytic performances of the CeO2-MOx mixed oxides for TCE destruction were significantly promoted, which could be related to the nature of the MOx and the strong interaction between CeO2 and MOX. The catalytic activities of these catalysts decreased in the order of 4CelCu> 4CelCr>4CelMn>4CelFe>4CelCo>4CelNi>4CelV>CeO2. Though 4CelCu represented the best catalytic activity, much more byproduct C2Cl4 was formed, since CuO was the preferable catalyst for the Deacon reaction. The CeO2-CrOx catalyst exhibited preferable catalytic activity with little byproducts detected, mainly due to the formation of large amount of Cr6+ species with strong oxidizing ability resulted from the strong interaction between CeO2 and CrOx. CeO2-CrOx also represented preferable catalytic performances for other Cl-VOCs with different molecule structures.2. The catalytic performances of the (Ce,Cr)xO2/zeolite catalysts for Cl-VOCs decomposition were investigated. Compared to single component catalyst, the catalytic performances of the (Ce,Cr)xO2/zeolite catalysts for DCE destruction were significantly promoted, in the order of (Ce,Cr)xO2/HZSM-5>(Ce,Cr)xO2/H-BETA> (Ce,Cr)xO2/H-USY> (Ce,Cr)xO2/H-MOR, and the formation of the byproducts was also significantly reduced, which was related to the nature of the zeolite and the synergistic effect between zeolite and (Ce,Cr)xO2. This synergistic effect could be illustrated by a multifunctional "adsorption-cracking-oxidation" catalytic mechanism. The acid sites of the zeolites firstly promoted the adsorption and dehydrochlorination of DCE on the surface of (Ce,Cr)xO2/zeolite, and then the high oxidizing ability of (Ce,Cr)xO2 was in favor of oxidizing the reactants and byproducts quickly to form CO2, while the special pore structures of the zeolites could increase the residence time of the Cl-VOCs, all of which could contribute to improving the catalytic performances of the (Ce,Cr)xO2/zeolite catalysts. Moreover, as to the y(Ce,Cr)xO2/HZSM-5 catalysts, (Ce,Cr)xO2/HZSM-5 (mass ratio of (Ce,Cr)xO2 to HZSM-5 was 1:1) with the proper acid sites and oxidizing sites represented the best catalytic activity for DCE destruction. (Ce,Cr)xO2/HZSM-5 also represented preferable catalytic performances for other Cl-VOCs with different molecule structures.3. The catalytic performances of the (Ce,Cr)xO2/MOy catalysts for Cl-VOCs decomposition were investigated. Compared to the single component catalyst, the catalytic performances of (Ce,Cr)xO2/MOy for DCE destruction were promoted at different levels, in the order of (Ce,Cr)xO2/Nb2O5> (Ce,Cr)xO2/TiO2> (Ce,Cr)xO2/WO3> (Ce,Cr)xO2/MoO3> (Ce,Cr)xO2/La2O3> (Ce,Cr)xO2/V2O5. There existed obvious synergistic effect between MOy (Nb2Os, TiO2 and WO3) with strong acid property and (Ce,Cr)xO2 with strong oxidizing ability, which could contribute to improving the catalytic performances of these catalysts. Besides, proper mass ratio of (Ce,Cr)xO2/Nb2O5 could promote the dispersion of (Ce,Cr)xO2 on the surface of Nb2Os with plate shape, and synergistic effect between the acid and redox property could be played to the greatest extent, thus, obviously improving the destructive efficiency of the catalysts. Among them,0.25(Ce,Cr)xO2/Nb2Os represented the best catalytic activity. Moreover, the CeO2-CrOx-Nb2Os catalyst prepared by coprecipation method represented larger specific surface area and mesopore volume, smaller particle sizes, (Ce,Cr)xO2 could be highly dispersed on the surface of Nb2O5, and the acid and redox property could be improved by the strong interaction between (Ce,Cr)xO2 and Nb2O5, further improving its destructive efficiency.4. The influence of water or benzene on the catalytic performances of the CeO2-CrOx, (Ce,Cr)xO2/HZSM-5 and CeO2-CrOx-Nb2O5 catalysts for Cl-VOCs decomposition, the durability and the reason for the catalyst deactivation were investigated. The presence of water in the reaction system inhibited Cl-VOCs conversion at lower temperature due to the competitive adsorption on active sites, but it could promote Cl-VOCs conversion at higher temperature due to the removing of Cl species adsorbed on the catalyst surface, reduce the formation of byproducts and improve the selectivity to HCl and CO2. The presence of benzene in the reaction system slightly inhibited Cl-VOCs conversion at lower temperature due to the competitive adsorption and oxidation on active sites, and promoted the formation of byproducts. Moreover, during the prolonged reaction time, whether in dry air or in the presence of benzene and water, the slight decrease of the catalytic activity in the initial stage was mainly due to the slight coke and Cl species deposition on the catalyst surface. Interestingly, the catalytic activity could be completely recovered by slightly elevating the reaction temperature. All the catalysts represented good durability and high potential for industrial application. |
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