Catalytic Performance Of HZSM-5 Modified By Mn/Fe-Mn For The NH₃-SCR

Nitrogen oxides (NO_x) are responsible for causing acid rain, photochemical smog, the greenhouse effect and other environmental problems, that have been serious harmful to human health. With the motor vehicles increase and industrial development, the emissions will be increasing. That will inevitably cause serious ecological and environmental deterioration. Therefore, it is imminent to eliminate NO_x pollution problem. Currently, the technology of Ammonia selective catalytic reduction (SCR) is the dominant technology used to control the pollutants of NO_x, and the key to this technique is to choose an excellent catalyst. Zeolites and modified zeolites are considered to have practical application prospect because of their excellent activity and selectivity, and ZSM-5 zeolites loaded by transition metal have been paid more attention. HZSM-5 is a derivative of ZSM-5, commonly used for industrial solid catalyst carrier due to its relative right pore structure and acid property. The catalyst carrier can be modified by adding a suitable active component or accessory ingredient in order to enhance their catalytic performance of Ammonia selective catalytic reduction. This paper is focused on the study of such supported catalysts, choosing a suitable catalyst carrier, active component or additive and finding the ideal compounding method of them, with a view to finding an efficient, economical and practical supported catalyst to provide reference for the research and industrial applications of the SCR catalyst.In our experiment, HZSM-5 was modified under different preparation conditions. First, HZSM-5 modified by various Mn loadings (0～10wt%) in calcinations temperature of 550℃condition were prepared by ion-exchange method and the isometric incipient impregnation method. It was found: the catalytic performance of the catalysts for Ammonia selective catalytic reduction of NO could be improved by adding amount of Mn. We compared the denitration effects of catalysts which were prepared by ion-exchange method and the isometric incipient impregnation method, and found that ion-exchange method had great advantages. 5%Mn/HZSM-5(550℃) prepared by ion-exchange method was the best catalyst. Therefore, the catalysts in follow-up experiments were prepared by ion-exchange method. The complex modified catalysts were prepared based on 5%Mn/HZSM-5(550℃) by adding additives Fe, and the Fe/Mn(molar ratio) of 0.10, 0.25, 0.50 and 1.0. The performance of their Selective Catalytic Reduction of NO was examined. The results showed that the performance of composite-modified cataalysts was more superior than that of single-modified. Adding amount of Fe not only improved the catalytic activity of the catalysts, but also greatly enhanced the thermal-stability of the catalysts. The activity of (0.25)Fe- Mn/HZSM-5 (Fe/Mn molar ratio of 0.25) reached 98.31% at 300℃, and remained above 90% when the temperature was 300～500℃. The effect of Mn and Fe composite modification catalysts with different ion-exchange order and different calcinations temperature were also investigated. The results showed that the best ion-exchange order was adding Mn firstly and then adding Fe, and the optimal calcination temperature was 550℃. A combination of technigues such as ESEM, BET surface area and X-ray diffraction were used to characterize the catalysts. And through further analysis, the impact of O2, NH₃ on the catalytic activity of (0.25)Fe-Mn/HZSM-5(550℃) catalyst were studied. We also studied the relationship between the catalytic activity and the reaction time at 300℃.