Catalytic Combustion Of Chlorinated Volatile Organic Compounds Over Mn-based Catalysts

With the rapid development of science and technology and the expansion of human activities, awareness concerning natural environment is seriously necessary for sustainability. Chlorinated volatile organic compounds (CVOCs), which are widely used in industries, may become a source of atmospheric pollutants and the solution of the contamination is coming up on the agenda.Technologies dealing with this problem include catalytic hydrodechlorination, catalytic steam reforming, photocatalytic oxidation and catalytic combustion. Among these, catalytic combustion, for low energy consumption, low processing temperature and high efficiency, has been widely used and is considered to be a very promising environmental technology. The common catalysts used in the CVOCs catalytic combustion are mainly the supported noble metal catalysts, transition metal oxide catalysts and solid acid catalysts. In this paper the MnOx catalyst which is cheap and environmental friendly is employed. Through modification by adding Ce and La into MnOx, catalysts of MnCeLaOx were prepared for the catalytic combustion of chlorobenzene(CB) as a model of COVCs. The catalytic behaviors and mechanism of the catalytic combustion of CVOCs over MnCeLaOx were investigated.A series of catalysts of MnCeOx, MnCeLaOx and single metal oxide catalysts, which were prepared by complexation method, were tested in the catalytic combustion of chlorobenzene under the condition of1000ppm CB,10%O2, with the balance of N2at GHSV=15,000h"1. The activities of pure MnOx, CeO2and La2O3are not good enough with the T90of CB combustion above440℃. But the activity is much improved with the addition of Ce into the MnOx, with the result of T90of CB combustion decreasing to246℃. Characterization by XRD, XPS, TPR and Raman shows that the addition of Ce makes a large portion of Mn species enter CeO2fluorite lattice, leading to the formation of MnCeOx solid solution which owns high redox ability. Decomposition of MnCeOx solid solution at the high calcination temperature causes the problem of thermal stability. The addition of La, which leads to form the structure of Mn-O-Ce-O-La, can promote the thermal stability of MnCeOx solid solution even calcined up to750℃, and accordingly the catalyst presents high activity. The catalytic activities in the catalytic combustion of CB over the series of Mn(y)CeLaOx catalysts with various ratios of Mn/(Mn+Ce+La) are investigated. The catalytic activities are improved with the increase in the ratios of Mn/(Mn+Ce+La), and Mn(0.86)CeLaOx exihibits the highest catalytic activity with the temperature of complete combustion for CB as low as225℃. The high activity of Mn(0.86)CeLaOx ascribes to the high mobility of oxygen on the interface between MnCeOx and MnOx. The products from the catalytic combustion of CB were detected, and all catalysts under study have more than99.5%selectivity to carbon oxides. The mainly products are CO2, H2O and HC1, and trace of polychlorobenzene was formed at high temperature for the catalysts with low content of Mn.The stable activities of Mn(y)CeLaOx catalysts were tested by GC-MS. The deactivation ocurred at low temperature during the reaction on all of the catalysts when the temperature below300℃. Combined with the characterizations of the fresh and used catalysts, the possible reason is that the dissociation of CB occurs on the interface between MnCeOx and MnOx and the produced Cl species absorbed strongly on the Ce species around the interface causing deactivation. But activities keep stable above350℃. The treatment in air can recover partly the activity of the used MnCeLaOx catalysts at350℃, and a large amount of Cland HC1were detected in the reaction effluent. This indicates that the Deacon reaction becomes possible on the Mn species around the interface and Ce species, which removes the produced Cl species in the formation of CI2. The addition of H2O or heptane can improve the removal of Cl species from the surface of catalysts. Based on the investigation of kinetics of CB combustion, the rate controlling step of CB combustion over the MnCeLaOx catalysts is the removal of Cl species from the surface of catalysts.KMnCv was employed as one of the precursors for Mn species to the modified theMnCeLaOx catalysts with the content of KMnCo4from0.5%to40%. The characterizations and activity tests show that addition of low content of KMnCu improves the redox ability of MnCeLaOx catalysts. With the increase of the content of KMno4, the activities of CB combustion decreased, due to the destruction of MnCeOx solid solution. XPS result shows that the greatly increased amount of surface absorbed oxygen ascribes to the strong basicity of K element, which retards the absorption of CB on the surface of catalysts.The perovskite-like catalysts with high thermal stability such as LaCoO3, LaMnO3and LaFeCO3were also prepared for investigation. And further these catalysts were modified by the added species of Ce, Sr, Pr, Sm and Cu. The activity results show that the doped species affect the activities of perovskite-like catalysts, and Lao.5Sro.5MnO3and LaogCeoiCoO3catalysts present good activities for CB catalytic combustion with T90of CB combustion about332℃and345℃respectively. Finally,1%of Ru was loaded on the perovskite catalysts to solve the problem of poisoning by Cl species, but the results are not good enough owing to the different catalytic mechanism between perovskite and Ru in the catalyst.