| The control of nitrogen oxides(NOx)in coal-fired flue gas is of great significance to improve the environment.Selective catalytic reduction with NH3(NH3-SCR)is regarded as one of the most effective NOx control technologies,and its core is de-NOx catalyst.Compared with commercial vanadium-based SCR catalysts,iron-based catalysts have the advantages of cheap,efficient and environmentally friendly,and have become a hot research topic.However,the previous reports mainly focus on the experimental aspect,and the mechanism research at the molecular level is relatively scarce.Therefore,based on density functional theory,this thesis takes γ-Fe2O3 as the research object to study the doping modification,anti-As2O3 poisoning performance and Hg0 oxidation mechanism of γ-Fe2O3 catalyst,which provides theoretical support for the development of γ-Fe2O3 catalyst.The main research contents and results are as follows:(1)The mechanism of the effect of transition metal Ti doping modification on de-NOx performance of γ-Fe2O3 is studied.Various doping models of single Ti and double Ti at different Fe sites on the γ-Fe2O3(001)surface are constructed.The adsorption characteristics of O2,NO and NH3 molecules on γ-Fe2O3(001)surface before and after Ti doping are discussed,and the reaction mechanism is analyzed.The results illustrate that Ti atom tends to be doped at octahedral Feoct site.The adsorption of O2 onto the catalyst surface can be enhanced through the Ti doping,and the adsorption performance increases with the increase of Ti doping amount.Both single Ti and double Ti doping inhibit the N-terminal adsorption of NO on the surface of the catalyst.Ti can enhance the strength of Lewis acid sites and promote the adsorption of NH3,thus promoting the SCR reaction.The doping of Ti increases the energy barrier of NO2 formation.Ti doping inhibits the formation of NH and N,avoids the excessive oxidation of NH3,improves utilization of NH3 and NH2,inhibits the N2O produced by E-R mechanism,and enhances the selectivity to N2.Therefore,the doping of Ti can improve catalytic performance of γ-Fe2O3 in the NH3-SCR of NO.(2)The adsorption of As2O3 on the surface of γ-Fe2O3 catalyst and the mechanism of doping modification to improve the anti-As2O3 poisoning performance are studied.The adsorption properties of As2O3 on the intact and O-deficient γ-Fe2O3(001)surfaces are calculated.At the same time,the catalyst model of γ-Fe2O3 doped with Mo and Ti is established,and the mechanism of the arsenic poisoning ability of the additive doping is discussed,and the influence of the doping amount is considered.The results show that the As2O3 tends to be chemically adsorbed at Feoct sites on the γ-Fe2O3(001)surface with the O-terminus,the adsorption energy is-0.75 eV,and strong interaction and electron transfer occur during the adsorption process.When there are O defects on the surface,the adsorption energy of As2O3 molecules is increased.Mo and Ti tend to be doped at Feoct sites,which enhances the adsorption capacity for As2O3,and the adsorption energies increase to-1.30 eV and-1.13 eV,respectively.Increasing the doping amount of Mo can promote the adsorption of As2O3.As2O3 tends to react with the more active Mo and Ti,thus protecting the active Fe sites from arsenic poisoning.The doping of Ti also inhibits the adsorption of As2O3 at adjacent Fe sites.The doping of Mo and Ti also promotes the adsorption of NH3 on the surface of the catalyst and enhances the acidity of the surface,which is beneficial to the SCR reaction.The doping of Mo and Ti can improve the resistance of γ-Fe2O3 catalyst to arsenic poisoning.(3)The mechanism of adsorption and oxidation of Hg0 by HCl on γ-Fe2O3 surface is investigated.The adsorption models of Hg0,HCl,HgCl and HgCl2 on γ-Fe2O3(001)surface are constructed,and the reaction mechanism of catalytic oxidation of Hg0 by HCl on γ-Fe2O3 surface is analyzed.The results illustrate that the Hg0 tends to be chemically adsorbed at Feoct site with an adsorption energy of-0.41 eV.HCl can be dissociated and adsorbed on the surface of the catalyst to form adsorbed Cl and hydroxyl groups,thus promoting the adsorption of Hg0.HgCl can be molecularly chemisorbed on γ-Fe2O3(001)surface and act as an intermediate.HgCl2 tends to be adsorbed in parallel on the surface of γ-Fe2O3 with an adsorption energy of0.66 eV.Meanwhile,the oxidation of Hg0 by HCl on γ-Fe2O3 follows the L-H mechanism,that is,the chemisorbed Hg0 reacts with the dissociatively adsorbed HCl.The heterogeneous oxidation process of Hg0 by HCl is carried out through a two-step reaction path,that is,Hg0(ads)→HgCl(ads)→HgCl2(ads). |
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