First-principles Study On The Catalytic Reduction Of NO By Fe And Ni Doped ZnAl₂O₄

Nitric oxide（NO）is one of the main gas molecules that cause air pollution.Selective catalytic reduction technology（SCR）is an effective means to remove NOx.Its main mechanism is that the reducing agent NH₃ or hydrocarbon（HC）catalytically reduces NOx under the action of a catalyst.For N₂,H₂O,etc.Among them,spinel-type oxides can be used as good HC-SCR catalysts,especially ZnAl₂O₄,which has good stability and catalytic properties.Transition metal doping will improve its application in the field of catalysis,but the catalytic mechanism needs to be further studied.In this paper,density functional theory is used to study the catalytic mechanism of Fe and Ni doped ZnAl₂O₄.First,a Fe,Ni-doped ZnAl₂O₄ model was established,and the adsorption properties of NO,H₂O,CH₄,C₂H₄ and other molecules on the（100）plane were mainly studied.And through molecular co-adsorption,molecular dynamics,transition state theory,etc.,its catalytic mechanism and the role of catalysts have been studied.On the Fe-doped ZnAl₂O₄（100）surface,neither CH₄ nor C₂H₄ has good adsorption properties.Hydrocarbon molecules do not interact with the surface of the spinel catalyst,especially the metal sites,which is different from the conventional SCR reaction.The adsorption of methane and ethylene on Ni-doped ZnAl₂O₄ is smaller,and the amount of charge transfer is also small.It shows that their adsorption is not the first step of the reaction.O₂ is easier to form chemical bonds on the Fe-doped ZnAl₂O₄（100）surface,and the strongest adsorption energy is-2.23 eV;while the adsorption energy on Ni-doped ZnAl₂O₄（100）is smaller,generally less than-1 eV.Both ends of O₂ bond with the base metal atom to have stronger adsorption energy.H₂O will chemically adsorb on Fe/ZnAl₂O₄,and the strongest adsorption site is Zn site.It has a stronger effect on Ni/ZnAl₂O₄,and it will produce super chemical adsorption on the Ni-doped surface,and the adsorption energy can reach about-9eV,which may affect and occupy the catalytic active site and cause a poisoning reaction.By calculating the structure of the entire reaction,it is found that when CH₄ is a reducing agent,the reaction enthalpy of Fe/ZnAl₂O₄ with the entire reaction molecule is about-7 eV.Negative values represent exothermic heat,indicating that this reaction can occur thermodynamically.When C₂H₄ is used as the reducing agent,only half of the reaction enthalpy is-3.45eV.The enthalpy of the CH₄-SCR reaction of Ni/ZnAl₂O₄is also about-7.25 eV,but the enthalpy of the reaction of C₂H₄-SCR can reach-14.32eV.It shows that Fe-doped zinc-aluminum spinel is more suitable for the reaction of CH₄ as a reducing agent.The Ni-doped substrate is more suitable to use C₂H₄ as the reducing agent.When O₂ and NO are co-adsorbed,it can absorb the electrons of NO,making it easier to oxidize.At a certain angular distance,it can directly generate nitrate-like structures with NO.Cracking into O atom state can also oxidize NO to NO₂^-or NO₃^-.This nitrate-like structure will subsequently oxidize hydrocarbons and gradually decompose into N₂,H₂O and other products.CH₄ is difficult to adsorb on the Fe/ZnAl₂O₄（100）surface,and it is also difficult to split and dehydrogenate.However,the presence of O atoms can greatly reduce the barrier to dehydrogenation and increase the adsorption capacity of CH₃ on the surface.The activation of methane molecules can make it easier for subsequent reactions with nitrate-like salts.This paper discusses the catalytic reduction of NO by CH₄ and C₂H₄ on Fe,Ni-doped zinc aluminum spinel,understands the interaction between reactive molecules and the surface,and discovers the different mechanisms of hydrocarbons and NH₃ as reducing agents,and explores.The promotion effect of O₂ on the reaction.It has certain guiding significance for understanding the catalytic reduction process of hydrocarbons on spinel.