Investigations Into The Crystal Facet Activity And Alkali Metal Poisoning Mechanism Of CrMn_1.5O₄ Spinel Catalyst

Nitrogen oxides（NO_x）are the main pollutants in the process of air pollution and prevention,which can lead to a series of environmental problems（such as photochemical smog,acid rain,ozone hole and greenhouse effect）and then endanger the life and health of animals and plants.At present,selective catalytic reduction（NH₃-SCR）technology with NH₃ is considered as the most effective way to remove the fixed source NO_x,which has the advantages of low temperature,high efficiency and low operating cost.The Cr-Mn spinel as SCR catalyst is deemed to be a promising catalyst owing to abundant surface acidity and excellent redox properties.However,Cr-Mn spinel still show the narrow operation window and insufficient SCR activity（especially below 200℃）.In addition,the poisoning mechanism of Cr-Mn spinel by alkali metals still unclear.These problems seriously hinder the further application of the promising Cr-Mn spinel in the field of denitration.This paper mainly carries out research from the following two points:1.Study on crystal plane activity.The physicochemical properties and catalytic activity of catalysts are directly determined by the exposed crystal facet due to its unique atomic arrangement and spatial structure.Therefore,they can be improved by regulating the crystal facet content based on the understanding of crystal facet activity.Herein,Three CrMn_1.5O₄ catalysts with exposed（100）,（110）and（111）facets were synthesized by solid phase method,CTAB method and coprecipitation method,respectively.The activity of crystal facets was firstly determined by experiments（NH₃-SCR activity evaluation,XRD,BET,TEM,XPS,NH₃-TPD,H₂-TPR,O₂-TPD and in situ DRIFTS）,and the effect of different facets on reactants,oxygen vacancy,charge density difference and the energy barrier of rate determining step was further investigated by DFT calculations.The results show that the crystal facet activity obeys（100）facet>（111）facet>（110）facet.The exposed（100）facet of CrMn_1.5O₄-CP catalyst can increase BET surface area,concentration of Mn⁴⁺and O_α,acidity,reducibility,adsorption and activation capacity for reactants.DFT calculations uncover that Mn site on（100）facet shown strongest the adsorption and activation capacity for NH₃ and NO due to its 3O_3c-Mn_5c-2O_4ccoordination bond and abundant charges accumulating near the active adsorption sites.Compared with other facets,oxygen vacancies are most easily formed on（100）facet,and（100）facet shows lowest reaction energy barrier for the rate determining step of H₂O formation.Therefore,preferentially exposed（100）plane of CrMn_1.5O₄ catalyst exhibits best denitration activity.Ultimately,SCR performance of CrMn_1.5O₄catalyst is further improved by regulating（100）facet(reaction rate is 3.20×10^-3mol/（g·s）from 160℃to 240℃and NO conversion is above 90%at 140-300℃).2.Study on alkali metal poison.Alkali metals（Na Cl and KCl）in flue gas can lead to severe poisoning and inactivation of SCR catalyst,but the poisoning mechanism of Cr-Mn spinel are still not reported,and the poisoning effects of Na Cl,KCl and individual atoms are unclear.Herein,the effect of Na Cl and KCl on the physicochemical properties and reaction mechanism of CrMn_1.5O₄ spinel was investigated by a series of characterizations.The results of activity tests show that CrMn_1.5O₄ catalyst is seriously deactivated by Na Cl and KCl,and the poisoning effect of KCl is more severe than Na Cl.Characterization results show that Na Cl and KCl can result in a decrease of BET surface area,pore volume,the concentration of Mn⁴⁺and O_α,surface acidity and redox properties.Moreover,Na Cl and KCl have a strong inhibition effect on NH₃/NO_x species adsorption and activation,and reduce their stability,thereby decreasing the NH₃-SCR activity.Though KCl can deactivate Cr-Mn spinel,not change reaction mechanism of CrMn_1.5O₄ catalyst,but Na Cl cut off E-R mechanism by inactivating L acid and B acid.DFT calculations reveal that Na and K not only damage the Mn-O bond,but also affect the adsorption of NO.in addition,they also hinder the formation of NO_x species.Cl weakened the formation of L acid through competitive adsorption with NH₃ and hindered the formation of B acid and oxygen vacancy.Though the interaction between Na/K and Cl weakens the poisoning effect of individual Na,K and Cl atoms,but still weakens the Mn-O bond and hinders the adsorption activation of NH₃,NH₄⁺and NO and the formation of oxygen vacancies on the surface.In addition,H₂O desorption was the rate-determining step of NH₃-SCR reactions following E-R and L-H mechanism.Na Cl/KCl in E-R mechanism increased reaction heat of rate-determining step and KCl in L-H mechanism elevated its energy barrier.In conclusion,Na Cl and KCl can seriously poison CrMn_1.5O₄ catalyst.