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Preparation, Characterization Of Rare Earth-Porous Materials And Its Catalytic Performance For CVOCs And NVOCs Decomposition

Posted on:2012-11-18Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q Q HuangFull Text:PDF
GTID:1111330371955361Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Volatile organic compounds (VOCs) are a series of common air pollutants. They are hazardous to both the environment and human health. Catalytic combustion has been identified as one of the most effective techniques for VOCs elimination. Halogenated VOCs is a kind of air pollutants significantly harmful to atmosphere, especially for chlorinated VOCs (CVOCs), which not only destroy the ozone layer, but also cause persistent and cumulative influence on human health and ecological environment. The breaking of C-C1 bond, which is more easily based on thermal analysis, replaces the breaking of C-H bond during CVOCs catalytic oxidation. However, the produced Cl species will strongly adsorb on catalysts surface or react with active species in the form of volatile metal chloride resulting in the loss of active phases. On one hand, it causes the deactivation of the catalysts. On the other hand, Cl2 reacting with hydrocarbons produces the polychlorinated hydrocarbons, leading to the secondary pollution. Nitrogen-containing VOCs (NVOCs) is another kind of air pollutants. The crucial point for NVOCs catalytic oxidation lies in the control of NOx production, avoiding the secondary pollution. Recently, develop the catalysts with low cost, high activity, high selectivity and good stability for VOCs catalytic oxidation has been one of the most important issues in environmental catalysis research field.In the present thesis, Y type zeolite and pillared interlayered clays with porous structure were chosen as the supports. Systematic investigation on the preparation of CeO2 modified and MOx-CeO2 composite oxides modified porous catalytic materials was done. Then, typical CVOCs (DCE, DCM and TCE) and NVOCs (n-butylamine, ethylenediamine and acetonitrile) were chosen as the reactants. The influence of the structure and surface characteristics of the supports, the synergy between active phases and the supports, as well as the interaction between transition metal and Ce on the catalytic performance and durability of the catalysts for CVOCs and NVOCs catalytic decomposition was studied. The structure-texture, surface acidity distribution and redox property of the catalysts were characterized by a combination techniques of N2 adsorption-desorption, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), diffuse reflectance infrared spectra of pyridine adsorption (DRIFT), hydrogen temperature-programmed reduction (H2-TPR), temperature-programmed surface reaction (TPSR). The main conclusions are as follows:1. The influence of the structure and surface characteristics of the supports, and the synergetic effect between active phases and the supports on the catalytic performance for VOCs decomposition was investigated. For CVOCs catalytic decomposition, zeolites show higher catalytic activity for CVOCs destruction than Al2O3 and SiO2. The acidity strength and ratio of strong acidity to weak acidity of the catalysts are the critical factors to the dehydrochlorination of DCE. The interaction between CeO2 and USY zeolite results in the increment of strong Lewis acidity concentration and the improved mobility of the active oxygen species, which evidently enhance the dehydrochlorination and deeper oxidation of DCE over the catalysts. The enhanced catalytic performance for DCE decomposition over 12.5CeCeO2-USY-IM prepared by impregnation method can be due to stronger interaction between CeO2 and USY zeolite, which brings in the improved dispersion of CeO2 on USY zeolites, optimized ratio of strong acidity to weak acidity, the better migration of active oxygen species from bulk to surface, and the enhanced synergy between CeO2 and USY zeolite. For NOVCs catalytic decomposition, the pillaring modification results in the increase in surface area and the pore volume of M-PILC (M=Ti, Zr and Al). Al-PILC(2,60) preserves the regular two-dimensional layer structure with enlarged interlayer distance. However, the layer structure collapses, forming so-called "house of cards" structure for Zr-PILC(2,60) and Ti-PILC. Ti-PILC(12,20) prepared under the following condition:Ti pillaring agent aged for 12 h and the pillaring process operated at 20℃exhibits the highest surface area and the most abundant mesoporous structure. It favors the dispersion of CrOx-CeO2, the interaction between Cr and Ce, the mobility of surface oxygen species and n-butylamine adsorption on catalysts surface, which further improves the catalytic performance and control quality of NOx for n-butylamine oxidation.2. The effect of the interaction between transition metal and Ce on the catalytic performance for VOCs destruction was investigated. The results reveal that the interaction between transition metal and Ce enhances the catalytic performance for VOCs destruction. For CVOCs catalytic destruction, such interaction is beneficial to the conversion and deeper oxidation of C2H3Cl, especially for the interaction between Cr and Ce, which obviously promotes the metal oxides dispersion on USY zeolite, the mobility of active oxygen species, and optimizes the ratio of strong acidity to weak acidity, thus, further enhances the dehydrochlorination and deeper oxidation of DCE. For NVOCs catalytic destruction, such interaction improves the catalytic performance for n-butylamine oxidation over 8MCe(6:1)/Ti-PILC(12,20) catalysts compared with that over 8MCe/Ti-PILC(12,20) catalysts. Especially for the interaction between Cr and Ce, which evidently enhances the mobility of active oxygen species over the catalysts, therefore, Ti-PILC(12,20) supported CrOx-CeO2 catalysts exhibit higher catalytic performance and better control quality of NOx for n-butylamine oxidation.3. The influence between Cr and Ce on the catalytic performance and durability of the catalysts for VOCs decomposition was investigated. For CVOCs catalytic decomposition, proper doping amount of Cr on CeO2-USY catalysts enhances the interaction between Cr and Ce,the dispersion of active phases on USY zeolite and the mobility of active oxygen species, which favor the direct oxidation of intermediates and by-products, thus, inhibit the production of chlorinated by-products during CVOCs decomposition. The sequence of catalytic performance for various CVOCs decomposition over 12Cr-12.5CeO2-USY follows the order:DCE> DCM> TCE. In addition, the durability of 12Cr-12.5CeO2-USY is improved due to the slight coke deposition, better stability of active phases and enhanced persistence to HC1 brought by the interaction between Cr and Ce. For NVOCs catalytic decomposition, proper amount of CeO2 addition shows a better enhancement to the interaction between Cr and Ce. 8CrCe(6:1)/Ti-PILC(12,20) exhibits better catalytic performance and control quality of NOx for NOVCs oxidation. The catalytic performance for various CVOCs decomposition over 8CrCe(6:1)/Ti-PILC(12,20) decreases in the order of n-butylamine> ethylenediamine> acetonitrile.8CrCe(6:1)/Ti-PILC(12,20) shows good durability during long term test for n-butvlamine oxidation.
Keywords/Search Tags:USY zeolite, Pillared interlater clays, Rare earth, Transition metals, VOCs catalytic decomposition
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