| Persistent organic pollutants(POPs), due to their acute toxicity, strong odor and potential bioaccumulation are not only hazardous air pollutants, but are also highly carcinogenic, teratogenic and mutagenic in nature. Therefore, it is essential to develop practical and cost-effective methods to eliminate POPs from gases. Of the available techniques, the catalytic combustion is one of the most effective technologies for the removal of POPs emissions due to its low energy consumption, low processing temperature and high destructive efficiency. Among the catalysts used for the catalytic combustion of POPs, manganese oxides have been reported as the most active ones on account of their high oxygen storage ability and redox properties. Furthermore, when these Mn-based catalysts are modified by rare earth elements, such as Mn-Ce-La-O catalysts, they have been found to display a higher catalytic performance in the oxidation of POPs. However, these oxides are susceptible to rapid catalytic deactivation due to strong adsorption of dissociative Cl species that are mainly generated from the combustion processes. Undoubtedly, any improvement in these catalysts’ resistance to deactivation will extend the application range of manganese oxides in industry. Aiming at this problem, the following work has been done:1. A series of MnCeLa catalysts with different Mn content was prepared by sol-gel method and the catalyst with appropriate Mn/(Mn+Ce+La) ratio was chosen as the target catalyst to study the resistance to chlorine poisoning of the Mn-containing catalyst.5 samples with different Mn content were prepared and activity was conducted for catalytic combustion of chlorobenzene. Sample Mn-CeLa(6:1) exhibited the better activity, whose T50% and T90% were 149℃ and 210℃, respectively. From the XRD results, when the ratio of Mn/(Mn+Ce+La) was less than 6:1, Mn species entered into CeO2 lattice generating MnCeOx solid solution. However, when the ratio was more than 6:1, we can see the reflection of Mn species in the form of Mn2O3. Thus, we can also concluded that Mn/(Mn+Ce+La)=6:1 was the threshold value. Combined with the two aspects, we selected sample Mn-CeLa(6:1) as the target catalyst to study the resistance to chlorine poisoning.2. A series of Sn-MnCeLa catalysts with different Sn content was prepared by the citrate complex method and coprecipitation method, and appropriate amount of Sn was determined. The resistance to chlorine poisoning of MnCeLa and Sn-MnCeLa catalysts at different temperatures was studied.When the Sn content are lower than 0.08, the activity of catalysts are barely changed;However, when Sn content are more than 0.08, the activity of catalysts are decreased drastically. Then the Sn(0.08)-MnCeLa catalyst was chosen as the object, and the stability of MnCeLa and Sn-MnCeLa catalysts at different temperatures, namely the resistance to chlorine poisoning in this article, was studied. The results indicated that the activity of catalysts Sn-MnCeLa are superior than that of MnCeLa catalysts under same conditions. Moreover, the temperatures which were needed to keep stable activity for Sn-MnCeLa and MnCeLa catalysts are 250℃ and 300℃, respectively. Based on the analyses above, we can conclude that the addition of Sn can enhance the resistance to chlorine poisoning of MnCeLa catalysts. A series of characterization that XRD, BET, SEM, Raman, XPS and H2-TPR were conducted to explore the reason why Sn modified MnCeLa catalyst had better resistance to chlorine poisoning: 1) The addition of Sn can significantly increase the concentration of surface adsorbed oxygen and therefore the adsorbed Cl species can remove from the surface of catalyst more easily. 2) Sn doping inhibited the generation of manganese oxygen chlorine species(MnOxCly) indirectly, avoiding the deactivation of MnCeLa catalyst. |
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