The Study On The Ce-based Catalyst For Low-temperature Catalytic Combustion Of Chlorobenzene

Volatile chlorinated organics have found extensive applications in the industrial production, but their residues are considered as pollutants leading to long-term contamination of the environment, destroying the ozone layer in the atmosphere, causing green-house effect and thus menacing human existence. Among various available technologies treating the CVOCs as pollutant, catalytic combustion as one of the effective methods has the characteristics such as treating low concentration of CVOCs at low temperature with low energy consumption and good efficiency in simple process installation. Nevertheless, there are still problems to be solved, such as short life of the employed catalysts and the formation of polychlorinated compound during this technological process. Research on the catalysts of high-performance has become necessary in the catalytic combustion of CVOCs, a promising technology in realization of environmental protection.In this paper, transition metal oxides as well as the rare earth oxide CeO2 were adopted as catalysts in study of the catalytic combustion of CVOCs. By using catalytic combustion of chlorobenzene (CB) as the reaction model, catalysts of Mn-Ce/Al2O3, Mn-Ce/Mg-Al2O3 and CeO2 blended with transition metal were studied in respect of performance and mechanism of low-temperature combustion. Some achieved results make contribution to the scientific fundamentals for development of the catalysts of high-activity and long life span available for low-temperature catalytic combustion of CVOCs. The results achieved in this paper are listed as follows:1. Low-temperature Combustion of CB over Mn-Ce/Al2O3Catalysts of Mn-Ce/Al2O3 with various ratios of Me/Ce were prepared by the once-through impregnation method of equal volume in the solution mixture of manganese nitrate and cerium nitrate. Characterization of the catalysts by means of XRD, H2-TPR, XPS and BET measurement indicates that the inter-action between Ce and Mn reduces the crystal size of MnOx, increases dispersion degree and therefore, improves redox performance. Catalysts of Mn-Ce/Al2O3 were tested in the catalytic combustion of CB by the feeding stream of 1000ppm CB,10% O2 with the balance of N2 Results indicate that all the catalysts in the test exhibited good activity in conversing completely CB at the temperature 400℃. Of these catalysts, MngCe2/Al2O3 was the best in terms of conversion 90% of CB only at the temperature 342℃. Main products from oxidation were CO2, H2O and HC1 but a little of Cl2 and polychlorobenzene (below lOppm) were formed at higher temperature. CeO2, if too much doped in the catalyst, eg., MnsCes/Al2O3, Mn2Ce8/Al2O3, will reduce the share of the main active component Mn and thus degrade the redox performance of the catalyst, and hence low activity. In addition, when the reaction temperature becomes below 350℃, the catalysts Mn-Ce/Al2O3 are readily to deactivate, perhaps owing to strong adsorption of the Cl species formed during reaction onto the surface of the catalyst so as to reduce mobility of the oxygen species in the catalyst. But, when temperature rises over 350℃, Cl species on the surface will through Deacon reaction with oxygen in the gas phase leave the surface in the form of Cl2 or HCl. For this reason, catalysts can remain active for quite long time in the reaction.2. Effect of doping Mg on the Catalytic Performance of Mn-Ce/Al2O3Catalysts Mn-Ce/Mg-Al2O3 of various ratios of Mn/Ce were prepared through modifying Al2O3 by impregnation method of equal volume in the solution of magnesium nitrate. Results from catalyst characterization indicate that the support Al2O3 and the doped Mg form MgAl2O3 so that interaction between Mn and Al2O3 becomes weak; activity structure Mn4+-Mn3+ is formed as some of the Mn species turn from MnO2 to Mn2O3. As a result, redox performance of MnOx is improved. Mn2O3 increases its share in the catalyst with the increased content of Mg and overdose of Mg can possibly turn all MnO2 to Mn2O3 and instead, redox performance becomes poor. In a catalyst containing Ce and Mn, doping Mg helps more of the Mn3" species migrate into the fluorite-like structure of CeO2 to form the Mn-Ce-Ox solid solution that is of high concentration of oxygen vacancies. Therefore, dispersion degree of both Ce species and Mn species is greatly improved and, as a consequence, catalysts Mn-Ce/Mg-Al2O3 have improved redox performance.In the test of catalytic combustion of CB, catalysts Mn-Ce/Mg-Al2O3 all exhibited higher activities as compared with catalysts Mn-Ce/Al2O3, of which the one with Mg 3%(wt.) and Mn/Ce=8/2 was the most active and conversion remained 75% at the temperature 350℃. Main products from oxidation were CO2. H2O and HCI but a little of Cl2 and polychlorobenzene (below 5ppm) were formed at higher temperature. Decrease in polychlorobenzene content is related with the improvement of redox performance. Nevertheless, deactivation still occurred when the reaction temperature was below 350℃, for doping Mg can neither improve activity of the Deacon reaction nor reduce the adsorption strength of Cl.3. Low-temperature Combustion of CB over CeO2 doped with Transition MetalsCatalysts TM-Ce were prepared by the sol-gel method to blend various transition metals(V, Mn, Fe, Co, Ni) with CeO2 at the ratio TM/Ce=1/9(mol). Results from catalyst characterization indicate that the constituents blended in the catalysts remain in the state of high degree of dispersion and in addition, cations of the other metals except vanadium migrate into the fluorite-like structure of CeO2 to form the solid solutions. Compared with the pure CeO2, redox performance of the catalysts TM-Ce has been much improved.In the test of catalytic combustion of CB, activities of the catalysts TM-Ce were found better than that of the pure CeO2. Catalyst VCe in particular exhibited the highest activity, which converted 90% of CB at the temperature 395℃. Main products from oxidation were CO2. H2O and HCl. The test of activity stability at the temperature 350℃informed that all the TM-Ce catalysts but VCe deactivated with various degrees in the catalytic combustion of CB. The reason is probably that the weak adsorption of Cl over the surface of VCe makes Cl desorption easy and thus mobility of oxygen is promoted.Further study on the VCe was carried out to investigate the effect of the content of the doped V on the structure and performance of the catalyst. Characterization by means of XRD, H2-TPR, XPS and Raman indicates that V species highly disperse in the form of vanadium oxides over the surface of CeO2 and CeVCO4 is therefore formed if the high dose of V. Of catalysts VCe of various ratios of V/Ce, V1Ce9 exhibited the highest activity. The vanadium oxides, which highly disperse over the surface of CeCO2. can effectively isolate the Cl species formed during the combustion from CeCO2 on the surface and consequently prevent the catalyst from deactivation.