Interaction Mechanism Between The Heavy Metal Pollutants And The Vanadium-titanium-Based Selective Catalytic Reduction Catalyst

With the continuous development of the economy and society,nitrogen oxides and heavy metal pollutants are still critical issues for flue gas treatment in thermal power generation,waste incineration,metal smelting,cement industry and glass industry.Selective catalytic reduction(SCR)technology is currently the most widely used stationary source NOx control method and the catalyst is the crux of this technology.Within the SCR system,long-term contact and interaction occurs between heavy metal pollutants and the commercial vanadium-titanium-based SCR catalysts.On the one hand,the deposition of heavy metals on the catalyst surface significantly affects the physicochemical properties and catalytic activity of the catalyst;on the other hand,the SCR catalyst in turn influences the migration and transformation of heavy metals.Therefore,to ensure the safe and stable operation of SCR systems,and to enhance the synergistic control of heavy metal pollutants by further modifying and regulating the migration,transformation and release behavior of heavy metals in the SCR system,it is urgent to investigate the interaction mechanism for heavy metals and the catalysts.However,at this stage,studies on typical heavy metal pollutants such as Hg,As,Se and Pb have mainly focused on experimental studies of heavy metal poisoning mechanisms.In this paper,the interaction mechanisms between heavy metal pollutants and the vanadium-titanium-based SCR denitrification catalysts are investigated in detail based on the density functional theory in quantum chemistry,with respect to the different chemical properties of Hg,As,Se and Pb,the different research stages and the different effects on the catalysts.To address the effect of additive doping on the catalytic oxidation of Hg0 by vanadium-titanium-based SCR catalysts,firstly,based on the review of the theoretical models for commercial vanadium-titanium-based SCR catalyst,the most widely used and practical V2O5/TiO2(001)periodic theoretical model was adopted,and the V2O5-WO3/TiO2 model was obtained considering the effect of additive doping by atom substitution.The doping of WO3 can promote the charge transfer on the catalyst surface,thus enhancing the reactivity.In addition,the adsorption mechanisms of different Hg species on the V2O5/TiO2(001)and V2O5-WO3/TiO2(001)surfaces were explored,the adsorption of Hg0 belongs to physisorption and the adsorption of HCl,HgCl and HgCl2 is chemisorption.Finally,the Hg0 oxidation pathways and the energy barriers on the V2O5/TiO2(001)and V2O5-WO3/TiO2(001)surfaces were calculated.The rate-determining step for the catalytic oxidation of Hg0 on the catalytic surface is the process of HgCl2 generation,on V2O5WO3/TiO2(001)surface the doping of WO3 lowered the energy barrier for Hg0 oxidation and enhanced the catalytic ability of the vanadium-titanium-based SCR catalyst.A comparative analysis of the reaction mechanism on the two models revealed the facilitative effect of the additive WO3 in commercial SCR catalysts on the removal of Hg0 in coalfired flue gas.For the effect of As2O3 on the vanadium-titanium-based catalyst surface during migration conversion,the adsorption of As2O3 and the migration and conversion mechanism to As2O5 were calculated.The adsorption of As2O3 is chemisorption,with charge transfer to the catalyst surface and chemical bonding between As,V and O.After As2O3 adsorption,the adsorption strength of NH3 on the Lewis and Br?nsted acid sites decreases and the acidity of Lewis acid site is more strongly inhibited.As2O3 destroys the Lewis acid sites(V=O)on the catalyst surface during oxidation and reduces the valence of the vanadium species,leading to the catalyst deactivation.Analysis of the conversion pathways of As2O3 on the catalyst shows that As2O3 converts to As2O5 on the catalyst surface,while V2O5 prefers to become V2O4.Considering the doping effect of different additives,two periodic models,V2O5-WO3/TiO2 and V2O5-MoO3/TiO2,were further constructed to study the influence mechanism of the additives doping.MoO3 doping resulted in stronger charge transfer with As2O3,thus enhancing the redox activity of the catalyst to a certain extent.On the V2O5-WO3/TiO2 catalyst surface,the reaction activity between WO3 and As2O3 is lower than that of V2O5,while on the V2O5-MoO3/TiO2 catalyst surface As2O3 reacts preferentially with the more active MoO3,thus protecting the active component V2O5.Therefore,the doping of MoO3 can provide better anti-arsenic poisoning effect.In response to the poor understanding of the influence mechanism of Se species on the SCR catalyst’s electronic structure at this stage,the adsorption mechanism of Se species on the vanadium-titanium based SCR catalysts was investigated,the influence of Se species on the electronic structure of the catalyst was investigated from charge transfer,bond formation,and V=O site activity.The adsorption of elementary Se belongs to chemisorption,while the adsorption of SeO2 can be both chemisorption and physisorption.The adsorption of Se species results in significant charge transfer and the formation of SeO bonds on the catalyst,triggers the reaction of Se species with the catalyst components to form VxSeyOz,WxSeyOz and MoxSeyOz compounds,thus destroying the active sites on the catalyst surface and reducing the SCR activity,reducing the SCR activity of the catalyst.By investigating the correlation between the adsorption energies of Se species and the dband center.the charge transfer amount and the Se-O bonding strength.a preliminary descriptor was obtained to describe the adsorption process of Se species,which can satisfactorily reveal the relationship between the electronic structure and the adsorption energy.Finally,the effect of temperature on the Se species adsorption was investigated by thermodynamic analysis,the adsorption of Se species could proceed spontaneously only below 500 K,and the adsorption was inhibited at high temperatures.In terms of the poor understanding of the Pb species influence on the electronic structure of SCR catalysts and the adsorption and activation process of NH3 species,the adsorption mechanism of Pb0,PbO and PbCl2 on the vanadium-titanium based SCR catalysts was investigated,and the interaction between Pb species and the SCR catalysts was deeply investigated by examining the electronic structure.The adsorption of Pb species on vanadium-titanium-based catalysts is chemisorption,with significant charge transfer occurring between the Pb species and the catalyst surface,the greater the charge transfer indicates the Pb adsorption is more stable.The adsorption of Pb species can only occur spontaneously below 500 K and is inhibited at high temperatures.In addition,by studying the adsorption and activation of NH3 species on the catalyst surface,it was found that Pb species significantly inhibited the activation process of NH3 species on the surface of vanadium-titanium based SCR catalysts,thus affecting the following SCR process.