Selective Catalytic Reduction Of No_X By NH₃ Over Mn/ZSM-5 Based Catalysts And Reation Mechanism Study

Selective catalytic reduction （SCR） with ammonia is used widely to remove the NOx currently. However, catalyst with high activity, wide temperature window, good resistance to SO₂and H2O and non-toxicity is still not found. Reaction meachnism of selective catalytic reduction of NO with NH₃ is in debate. NH₃-SCR system is studies by using the catalysts based on Mn/ZSM-5 .Impregnation method and ion-exchanged method were adopted to prepare Mn/ZSM-5 catalysts and Fe modified Mn/ZSM-5 catalysts. Catalytic activity measurement and characterization were carried out. Reaction process on catalysts surface was discussed by DRIFT analysis.It was found that Mn/ZSM-5 catalysts prepared by impregnation method were of good activity and Fe-Mn/ZSM-5 catalysts showed higher activity. NO conversion reached above 90% in the temperature range of 200⁴50°C and nearly 100% in the temperature range of 220³50°C. Mn-ZSM-5 catalysts prepared by ion-exchanged method were of good activity similarly. NO conversion reached above 90% in the temperature range of 220⁵00°C and nearly 100% in the temperature range of 250⁴00°C over Fe-Mn-ZSM-5 catalysts.Metal oxides in catalysts were of good dispersion in ZSM-5 zeolite whether prepared by impregnation method or ion-exchanged method. The specific surface area of catalysts deacreased compared with that unsurpported. More manganese loading or Fe/Mn molar ratio led to co-exitance of Mn³⁺ and Mn⁴⁺ from Mn⁴⁺. Fe³⁺ form was found in catalysts of high Fe/Mn molar ratio. Molecule adsorbed oxygen O₂^2- was found in Mn/ZSM-5 catalysts. Only O₂^2- was found in Fe-Mn/ZSM-5 catalysts of low Fe/Mn molar ratio. Both O^2- and O₂^2- existed in catalysts of high Fe/Mn molar ratio. Only Mn⁴⁺ and molecule adsorbed oxygen O₂^2- existed in Mn-ZSM-5catalysts prepared by ion-exchanged method. Muti-valence manganese existed in Fe-Mn-ZSM-5 catalysts of high Fe/Mn molar ratio. Fe³⁺ form and co-exitance of O^2- and O₂^2- were found in Fe-Mn-ZSM-5 catalysts.DRIFT analysis found that amide NH2 from coordinated NH₃ oxidation took part in reaction over Mn/ZSM-5 catalysts prepared by impregnation method. NH4+ formed on Bronsted acid cites would react with NO₂ from NO oxidation over Fe-Mn/ZSM-5 catalysts. The reaction was more quickly over Fe-Mn/ZSM-5 catalysts than Mn/ZSM-5 catalysts. Large amounts of NH4+formed over NO and O₂ preadsorbed Mn-ZSM-5 catalysts by ion-exchanged method. This made another reaction routes in Mn-ZSM-5 catalysts system. There were muti-routes in Fe-Mn/ZSM-5 catalysts system. Compared with Mn/ZSM-5 catalysts, introduction of Fe3+ to Mn/ZSM-5 catalysts enhanced oxidation of NO to NO₂ and improved SCR reation activity.