The Adsorption Mechanism Of NO_x Molecules On The Surface Of Ga And Fe Doped ZnAl₂O₄ Spinel

Spinel-type oxide ZnAl₂O₄ with excellent physical and chemical properties,good mechanical properties and a wide range of active temperatures,and it is widely used in various fields such as light-transmitting conductive materials,ceramics,catalysts,catalyst carriers,materials and so on.At present,Transition-metal supported ZnAl₂O₄used as a favorable heterogeneous catalyst,it has wide application in photocatalytic organic synthesis,organic dye,and hydrocarbon selective catalytic reduction and removal of nitrogen oxides（NO_x）.In this paper,the research background of selective catalytic reduction of nitrogen oxides（NO_x）by spinel oxides is reviewed,and the adsorption properties of small molecules on the surface of spinel catalysts and their research status are summarized.The structure and properties of Ga/Fe-doped ZnAl₂O₄（100）surface based on density functional theory calculations.The adsorption properties of NO and NO₂ on Ga/Fe-doped ZnAl₂O₄（100）surface were calculated.The effect of H₂O and O₂coverage on the adsorption properties of NO on ZnGaAlO₄（100）surface was investigated.Effect of coverage on NO₂ adsorbed on Fe-doped ZnAl₂O₄（100）.The effect of O₂ on the reaction enthalpy of reducing NO_x by C₂H₄ was studied.The effect of existing H₂O and O₂ molecules on HC-SCR NO₂ over Fe-doped ZnAl₂O₄（100）surface was calculated.The results of calculations support theoretical guidance for studying of the surface structure of the spinel oxide and design and preparation catalyst.Our main results as follows:Ga/Fe-doped in ZnAl₂O₄（100）surface decrease the surface energy.Doping of Ga and Fe improved adsorption performance of NO and NO₂ over ZnAl₂O₄（100）surface,and doping of Fe was more notable.The adsorption energies of NO and NO₂ on the ZnGaAlO₄（100）and ZnFeAlO₄（100）surface were-1.471eV and-2.666eV,-1.986eV and-2.916eV,respectively.The electronic states of Fe3d/Ga4p and Zn 3d existed synergy effect,hybridizing and overlaying with molecular orbitals of NO and NO₂,provided driving force for NO and NO₂ adsorption.The O₂ molecule is more likely to form a double-coordination structure with Ga and Zn atoms on the ZnGaAlO₄（100）surface,and its adsorption energy is-2.665 eV.H₂O molecules easily adsorbed on the Zn sites which protruding termination layer.H₂O molecules adsorbed on the ZnGaAlO₄（100）surface to generate hydrogen bonds and formed molecule multiple rings greatly enhanced adsorption stability.On the surface of ZnGaAlO₄（100）,the coverage of O₂ has little effect on the adsorption of NO below 1/2ML.After coverage exceeds 1/2ML,the NO is strongly repelled,and adsorption energy of NO on the surface is sharply reduced.H₂O has an activation effect on the surface,and adsorption energy of NO on the surface increases with the increase of H₂O coverage.The effect of O₂ on the catalytic reduction of NO_x on hydrocarbons in the ZnGaAlO₄（100）surface was found that O₂ result in the reactants and products in a higher energy state.In addition,increasing the coverage of O₂ increases the enthalpy of reaction of NO with C₂H₄.When the O₂ coverage is 1/2ML,the enthalpy of reaction△H is-187.65kcal mol-1,and the O₂ coverage is 3/4ML,the enthalpy of reaction△H is-243.36 kcal mol-1,which reduces the activation energy of the reaction.Thus,the selective catalytic reaction temperature of NO removal with C₂H₄ was reduced.On the surface of ZnFeAlO₄（100）,with the increase of NO₂ coverage,the adsorption bond length of NO₂ is longer and the adsorption strength is decreased.The average adsorption energy of NO₂ molecules is also decrease due to the strong repulsion between NO₂ molecules.The ability of the surface to transfer electrons to NO₂ enhanced gradually,but the average net charge of NO₂ is reduced.When existing of O₂ and H₂O on adsorption system of C₂H₄-SCR-NO₂ over ZnFeAlO₄（100）,H₂O and O₂ made the adsorption system more stable,NO₂,C₂H₄,H₂O and O₂ molecules existed synergetic mechanism,charges transformed form ZnFeAlO₄（100）into gas molecules.