Study On Denitration Performance And Mechanism Of NH₃-SCR Catalyst For Modification Of Rare Earth Tailings By Ce Combined With Mn/W/Co

At present,NH₃-SCR is the most widely used and mature denitration technology.But among the widely used V₂O₅-WO₃（Mo O₃）/Ti O₂catalysts,V₂O₅is expensive and toxic.Therefore,the preparation of green and inexpensive denitration catalysts has become the focus of this technology research.Bayan Obo rare earth tailings are natural minerals,which are environmentally friendly and low cost.At the same time,it contains Fe,Ce and other active elements that are conducive to catalytic denitration,and there is a complex co-association relationship between the elements.Hence rare earth tailings are natural raw materials for denitration catalysts,but its temperature window is narrow（350-450℃）.In this thesis,in order to widen the temperature window of the rare earth tailings catalysts,the original active elements（Ce,Mn）and transition metals（W,Co）are further added.Therefore,Ce modified rare earth tailings catalyst and Ce-M（Mn,W,Co）modified rare earth tailings catalyst were prepared.The denitration activity of catalyst is improved by enhancing the original synergistic relationship between elements.The effects of Ce addition（2.5%,5%,7.5%,10%）,the type of combined element M and the ratio of Ce to M on the denitration performance of catalysts were explored.The catalysts were characterized by XRD,SEM,EDS,BET,XPS,NH₃-TPD,H₂-TPR and DRIFTS.The experimental results showed that:The optimal Ce modification amount of rare earth tailings is 7.5%.After Ce modification,the temperature window is widened to 250-400℃,in this temperature range,the denitration rate is increased by 70%,and the N₂selectivity is above 80%.Adding an appropriate amount of Ce makes the active components such as CeO₂and Fe₂O₃evenly dispersed on the catalyst surface,increasing the specific surface area and exposing more active sites.However,exceeding the optimal modification amount makes the active species agglomerate and inhibits the increase of denitration efficiency.In addition,the oxygen vacancies and surface acid amount are increased by the strong electronic interaction between Fe-Ce.It is conducive to the adsorption reaction.The addition of Mn,W,Co broadened the temperature window and denitration efficiency of the catalyst.In the Ce-Mn modified rare earth tailings catalyst,the optimal Ce:Mn is 1:1,the temperature window shifts to low temperature,and the denitration efficiency can reach 92%at150℃.The interaction between Mn O_xand Ce or Fe in rare earth tailings improves the dispersion of active components and redox cycle,and increases the content of Fe³⁺and Mn⁴⁺.This makes the redox capacity too strong,resulting in poor N₂selectivity.Among Ce-W modified rare earth tailings catalyst,the optimal Ce:W is 3:1,and the denitration efficiency at350℃is 85%.The addition of W further promoted the induction of Ce on Fe and increased the Fe³⁺content.At the same time,W inhibits the medium temperature and low temperature redox reaction,and the N₂selectivity is more than 95%in the reaction temperature range,which has a significant advantage over the Ce-Mn modified rare earth tailings catalyst.The Ce-Co modified rare earth tailings catalysts has the best overall performance,the optimal Ce:Co is 2:1,the denitration efficiency at 350℃can reach 90%,and the N₂selectivity is greater than 90%in the reaction temperature range.The addition of Co improves the redox ability of the catalyst,so it accelerates the oxidation of Co²⁺to Co³⁺by Ce⁴⁺and Fe³⁺.This resulted in a significant increase in B acid sites,which in turn enhances NH₃capacity.Both B acid adsorption and L acid adsorption exist on the Ce and Ce-M（Mn、Co）modified rare earth tailings catalyst surface.NO participates in SCR reaction in the form of NO₂（ad）,monodentate nitrate(Mⁿ⁺-ONO₂),bridging nitrate([2Mⁿ⁺]=O₂NO)and nitrite(Mⁿ⁺-ONO).In Ce modified rare earth tailings catalyst,the Br（?）nsted acid site is more favorable than Lewis acid site,NH₄⁺forms-NH₂at the active center of Fe³⁺and Ce⁴⁺,reacts with NO,and the E-R mechanism is dominant.NO mainly reacts with NH₃adsorbed species in the form of monodentate nitrate.Bidentate nitrate is not reduced by NH₃and occupies active sites,which is not conducive to SCR reaction.After the addition of Mn,NO₂adsorbed on the surface of rare earth tailings participates in the reaction and promotes the"fast SCR"reaction.Intermediate products such as NH₂NO and NH₄NO₃were formed,and the activity was improved at low temperature.In addition,coordination NH₃appears at the Lewis acid site of Mn⁴⁺,which reacts with NO to produce N₂O after excessive oxidation,reducing the selectivity of N₂.After the addition of Co,the adsorption strength of Br（?）nsted acid site,the type of nitrate and the stability of monodentate nitrate increase,and new NH⁴⁺and-NH₂are formed at the Co³⁺acid site,which can preferentially react with NO,enhance the E-R mechanism and facilitate the NH₃-SCR reaction.Compared with Ce-Mn modified rare earth tailings catalyst,the intermediate product NH₄NO₃of Ce-Co modified rare earth tailings catalyst can continue to react with no to produce NO₂,N₂and H₂O,which improves the selectivity of N₂.