Synthesis Of Dual-Modified N-Doped Ceria And The NH₃-SCR Mechanism Investigation

Nitrogen oxides?NO_x?is one of the main pollutants in atmosphere,while diesel vehicles are the main source of NO_x.Reduction of NO_x emissions from diesel vehicles of great importance.Selective catalytic reduction of NO_x to N2 with NH3?NH3-SCR?has been a general method to purify removal of NO_x from diesel vehicles,considering the properties and complex consitions of mobile devices,the development of cheap,non-toxic,high SCR activity,wide-operation-widow NH3-SCR catalysts with good water,sulfur resistance and good stability have become a hot spot in the DeNO_x technic of diesel vehicles.China has the greatest content of rare earth metals,while ceria is the most abundant rare-earth metal,and it exists mainly in the form ceria dioxide in nature.A large number of studies have shown that CeO₂ with different morphologies have different specific surface areas,crystal face types,defect types?including atomic edges,atomic steps,oxygen vacancies,etc.?,and the promotion of these properties to a certain extent could improvethe catalytic activity behavior of CeO₂,therefore,the synthesis of CeO₂ with specific morphology and its application to NH3-SCR reaction has important theoretical and practical significance.In this research,we first synthesized CeO₂ catalysts with different morphologies,including truncated octahedron,nanorods,nanosheets,and flower-like CeO₂,then tested the NH3-SCR activity of them.The results indicated that the flower-like CeO₂ was the most active,reaching the highest NO_x conversion of 78%and the selectivity of above 90%at 250?,but this could not meet the practical requirements.Thus,to meet practical requirements,modifications should be made based on this flower-like CeO₂.Metal-doping is common to modify SCR catalysts because of the synergistic effect between polymetallics,which can increase the active sites beneficial for SCR reaction,including acid sites,surface-active oxygen species,oxygen vacancies,etc.However,few people have reported the non-metal modification of SCR catalysts.Considering the characteristics of CeO₂,combined with previous studies that the doping of nitrogen can change the electronic state of some metal ions,thereby affecting the catalytic activity,and considering the abundant nitrogen on earth.Therefore,N-doped CeO₂ has potentials in NH3-SCR.According to source of nitrogen atoms in actual application and the actual situation of the factory,we chose the NH3 annealing method to dope nitrogen.We have annealed the flower-like CeO₂ at different temperatures,it is found that the specific surface area of the CeO₂ catalyst decreases with the increase of annealing temperature,and the activity at low temperature?100-200??decreases to a certain extent.However,the activity at high temperature?above 250??has been improved to some extent.The sample annealed at 900? for 2 h showed the highest SCR catalytic activity.By ontroling variates,we found that the simple reduction in specific surface area?annealing in N2 at 900? for 2 h?leads to a decrease in the activity of all temperature ranges of the catalyst,and N-doping?annealing in NH3 at 900? for 2 h?promotes SCR activity,indicating N-doping is effective to promote NH3-SCR activity of CeO₂.At the same time,that the modified CeO₂ exhibited good water and sulfur resistance and stability.To investigate how N-doping promotes the SCR activity of CeO₂,we analyzed the surface species of modified CeO₂.XPS analysis found that nitrogen was doped into the CeO₂ lattice,the content of surface active oxygen and Ce³⁺-have been improved,which are conducive to the SCR reaction.Further X-ray absorption?XAS?techniques showed that the doping of N increases the O vacancies on the surface of CeO₂.Electron spin resonance?ESR?indicates that O2 tend to be absorbed in oxygen vacancies forming various active superoxide radicals?O₂^-?species which were beneficial to SCR reaction.The in situ diffuse reflectance Fourier transform spectroscopy?DRIFT?technique was used to investigate the reaction mechanism.The results showed that the adsorbed NH3 on the surface of the N-doped coral-like CeO₂ was easily "preserved" at high temperature,while the counterpart on the flower-like CeO₂ was easily oxidized to nitrate,which cannot reduce NO_x and reduce SCR activity.Further mechanistic studies indicated that the surface reaction of N-doped coral-like CeO₂ followed the ER and LH mechanisms,and the adsorbed NO_x is the highly active nitrite NO₂^-,while the surface reaction of the flower-like CeO₂ mainly followed the ER mechanism,and the adsorbed NO_x is inert nitrate.Finally,theoretical calculations?based on density functional theory,DFT?revealed that the doping of nitrogen atoms into the substitutional site of CeO₂ lattice reduceed the O vacancy formation energy on the?111?surface of CeO₂,while the formation energy of O2 in the surface of O2 with oxygen vacancies is low,which was conducive to the formation of O₂^-.Furthermore,it was revealed the adsorbed NO_x dominates the surface of N-doped coral-like CeO₂,while the surface of the flower-like CeO₂ is dominated by bidentate nitrate,insistent with XAS,ESR and in situ DRIFT results.