Study Of Catalytic Combustion Of VOCs Over Chromium-based Catalysts

Among technologies for elimination of volatile organic compounds (VOCs) emissions, catalytic combustion has been paid much attention because of its low energy consumption, high efficiency and simple equipment. Moreover, there is no associated pollution such as nitrogen oxides (NOX) production, as it is operated at low temperature. A catalyst with high activity is the key to catalytic combustion. Among the catalysts for chlorinated volatile organic compounds (CVOCs) catalytic combustion, most reports focus on the chromium-based catalysts. However, the Cr-based catalysts usually need high operation temperature and have short service life, so catalysts with high activity and thermal stability are therefore desirable. The chromium-based catalysts are more active for CVOCs abatement than for hydrocarbon. Since the industrial exhaust gases usually contain various kinds of VOCs, such as CVOCs, aromatic, ester, et al, a multi-functional catalyst was developed with the chromium-based catalyst supported Pd, which is effective for CVOCs, oxygenated and aromatic compounds. The detailed contents of this thesis are as follows:1. A series of CrOx/ZrO2 catalysts were prepared by a deposition-precipitation method, and tested for CH4 catalytic oxidation. The XRD results indicated that the chromium species were in form of Cr2O3, while the support ZrO2 was monoclinic, and the crystallite sizes of Cr2O3 and ZrO2 increased with increasing calcination temperature. The cooperation of Cr2O3 and ZrO2 resulted in an increase of the surface area of CrOx/ZrO2 catalysts. It was also found that enhanced activity was obtained on the catalyst with high Cr content, but further increasing Cr content resulted in a suppressed activity. This was due to the increasing amount of active sites with increasing Cr content, but higher Cr content led to larger crystallite of Cr2O3.2. CrOx/Al2O3 samples were characterized using XRD and Raman techniques. It was found that most of the chromium species were in form of crystalline Cr2O3, and some was in form of high oxidation state. The activities of different catalysts for catalytic combustion of dichloromethane were tested, and the results indicated that the catalyst with 20% Cr loading gave the highest reactivity and high thermal stability, with a complete dichloromethane oxidation temperature of 350℃. The NH3-TPD results showed that the increasing Cr loadings enhanced the strength of the acid sites, which reached a maximum on the catalyst with 20% Cr loading. All the results suggested the high oxidation state Cr species may be the active phase, and the acid sites were in favor of the reaction.3. CrOx-ZrO2, Pd/ZrO2 and Pd/CrOx-ZrO2 samples were tested for catalytic combustion of dichloromethane, ethyl acetate and toluene. It was found that the CrOx-ZrO2 catalyst exhibited better activity for catalytic combustion of ethyl acetate and dichloromethane than for toluene, while the Pd/ZrO2 catalyst gave good activity in catalytic combustion of toluene. By adding Pd to the CrOx-ZrO2 catalysts, the reactivity of the catalysts was enhanced. The dual-component catalyst Pd/CrOx-ZrO2 exhibited fairly good activity for catalytic combustion of all the three reactants. The BET and NH3-TPD results indicated that the addition of Pd and Cr increased the specific surface area of the catalyst and the strength of the acid sites, which may improve the activity of the CrOx-ZrO2 catalysts. Also the Pd/CrOx-ZrO2 catalyst showed high thermal stability in the reaction.