Theoretical Study On Denitration Mechanism Of Cu-SSZ-13 Catalyst

With the development of the economy and the improvement of people’s living standards,the number of vehicles has increased year by year,and the nitrogen oxides（NOX）emitted by them has become the main source of air pollution,causing great harm to the human and the environment.Therefore,the removal of NOX has become more and more important.Many scholars have studied the removal mechanism.However,the mechanism of denitrification process including the formation of intermediate species and rate control steps is still unclear.Therefore,the systematic theoretical study of the denitrification mechanism is conducted.Based on density functional theory,the mechanism of NH3-SCR reaction denitrification on Cu-SSZ-13 catalyst is studied.On the 42T cluster model,the relationship between Si/Al ratio and Br?nsted acid strength is discussed.The model of zeolite cluster with Cu as active site is constructed.And the mechanism of denitration is studied in this model.In addition,try to modify the Cu-SSZ-13 zeolite with Fe as active species,the main conclusions are as follows:（1）Determination of Cu position:As the number of Al atoms increased,Si atoms are not easily substituted by Al atoms.And the structural stability of the zeolite gradually deteriorates.The deprotonation energies of the more stable H protons on the O atom connected to the same site is slightly increased,and the acid strength is slightly decreased.That is to say,the influence of the number of Al atoms on the acid strength is not obvious,which provides a theoretical basis for obtaining zeolites of specific acid strength in experiments.When Si/Al=41,the different positions of active center Cu in the zeolite model are studied.By comparing the interaction energies,Mulliken charge and Projected DOS analysis,it is determined that Cu is more stable locating in the six-membered ring containing Al atoms.The zeolite of Cu-SSZ-13 is used as a basis for subsequent denitration studies.（2）Study on the mechanism of NH3 oxidation and denitration in Cu-SSZ-13 zeolite:By studying the oxidation mechanism of NH3 with O and OH,it is found that NH3preferentially reacted with OH to form NH2 and H2O,and the energy barrier is 81.78k J·mol^-1.For NH3 dissociation,both NH3 and NH2 are difficult to dehydrogenate;Since the reaction of NH with O and OH mainly form N or NO,the reaction of NH with O and OH is a side reaction for denitration.Therefore,the main route for determining oxidation is the reaction of NH3 with O and OH and the reaction of NH2 with O and OH.The main oxidation products are NH2 and NH.During the process of denitration,NO easily forms[NH2NO]and[NHNO]with NH2and NH.It is found that[NHNO]is easier to generate than[NH2NO],in which the boat type of[NHNO]needs an exotherm of 116.79 k J·mol^-1,and the chair type of[NHNO]needs an exotherm of 94.80 k J·mol^-1.The reaction energy barriers of the boat type of[NHNO]generating N2 and OH is 218.95 k J·mol^-1,the chair type of[NHNO]generating N2and OH reaction energy barriers is 186.48 k J·mol^-1.And the reaction energy barriers of[NH2NO]generating N2 and H2O is 361.05 k J·mol^-1.In comparison,NO is more susceptible to denitrification with NH.（3）Study on denitration mechanism of Fe-modified Cu-SSZ-13 zeoliteStudying the different positions of the active species Fe on the zeolite,it is determined by comparing interaction energies that Fe is relatively stable in the six-membered ring containing Al atoms.On this basis,the denitration reaction of NO with NH2 and NH is discussed.During the denitration process,NO easily forms[NH2NO]and[NHNO]with NH2 and NH.It is found that[NHNO]is easier to generate.Among them,the boat type of[NHNO]needs an exotherm of 120.01 k J·mol^-1,and the chair type of[NHNO]needs needs an exotherm of 126.25 k J·mol^-1.The reaction energy barriers of the boat type of[NHNO]generating N2 and OH is 103.51 k J·mol^-1,the chair type of[NHNO]generating N2 and OH reaction energy barriers is 216.83 k J·mol^-1.And the reaction energy barriers of[NH2NO]generating N2 and H2O is 357.55 k J·mol^-1.In comparison,NO is more susceptible to denitrification with NH.Compared with denitration of Cu-SSZ-13 zeolite,[NH2NO]generating N2 and H2O reaction energy barriers is lower than denitration of Cu-SSZ-13 zeolite by 3.5 k J·mol^-1;The N2 and OH reaction energy barriers of the chair type of[NHNO]is 30.35 k J·mol^-1 higher than that of the Cu-SSZ-13 zeolite,while the N2 and OH reaction energy barriers of the boat type of[NHNO]is lower than that of the Cu-SSZ-13 zeolite by 115.44 k J·mol^-1.In summary,Fe as an active component is more advantageous for the denitration reaction.