Theoretical Study On Modification,Sulfur Resistance And Alkali Metal Poisoning Of α-Fe₂O₃ Denitration Catalyst

As a common air pollutant,nitrogen oxides are seriously harmful to human health and ecological environment,which has been paid more and more attention by the Ministry of environmental protection.NH3-SCR control technology is the mainstream technology for flue gas denitration in coal-fired power plants;and its core technology is the catalyst.Currently,the most widely used commercial vanadium-titanium catalysts have disadvantages of high reaction temperature,secondary pollution,and high cost.Therefore,looking for environmentally friendly,low-cost,and excellent denitrification catalysts has become a research hotspot.Iron-based catalysts have received widespread attention due to their low cost.In this paper,density functional theory is used to explore the modification mechanism of the first-row transition metals(Mn,Ti,Cr and Ni)on the α-Fe2O3 catalyst from the molecular scale;Based on the optimized doping model of Mn,the elements of Co and Ce are introduced to study the mechanism of sulfur resistance of α-Fe2O3 catalyst when they are doped alone or in combination with Mn;the poisoning mechanism of alkali metals on α-Fe2O3 catalyst is studied.The main conclusions are as follows:(1)Four dopants can be doped into the top layer of the Fe2O3 supercell,and can change the surface structure and properties.The oxygen vacancy formation energies of Mn,Cr and Ni doped Fe2O3 models decrease,especially in Mn doped system.The Evac of Mn doped system is-0.687 eV.Compared with the Cr and Ni doped systems,the Mn and Ti doped systems can enhance the adsorption of NH3 and O2.In other words,the surface acidity of Fe2O3 increases and NO is more easily oxidized by oxygen on the Fe2O3 surface to form NO2,which is beneficial to the rapid NH3-SCR reaction.Among them,the Mn atom has the best enhancement effect,and its adsorption energies are 3.226 eV and 2.650 eV for NH3 and O2,respectively.For increasing the NH3-SCR reactivity of the Fe2O3 catalyst,the Cr and Ni atoms are not suitable as the dopants,the Ti atom is still to be further studied and the Mn atom is the best choice.(2)SO2 tends to adsorb on the Fe site of α-Fe2O3(001)surface with o-terminal,and the adsorption energy is 0.686 eV.On the surface of Mn-Fe2O3,the adsorption energies of SO2 on Mn and Fe sites increase to 2.836 and 3.002 eV,respectively.The results show that the sulfur resistance of α-Fe2O3 catalyst decreases sharply with the introduction of Mn;on the surface of Co-Fe2O3,SO2 can not be adsorbed on the Co site,but the sulfur resistance near the Co site is almost unchanged;on the surface of Ce-Fe2O3,SO2 tends to be adsorbed on the Ce site,thus protecting the adjacent active site.However,the adsorption energies of NH3 at Ce and Co sites are only 0.505 eV and 0.754 eV,respectively,which means that the surface acidity of the catalyst can not be improved by introducing Co or Ce alone.On the surface of the catalyst modified by composite dopants such as Ce/Mn-Fe2O3 and Co/Mn-Fe2O3,the anti-sulfur mechanism of Ce or Co dopant remains unchanged and the anti-sulfur effect is slightly enhanced,and the acid strength of active sites is significantly improved.Among them,the adsorption energy of NH3 at the Co site increased to 2.864 eV.Therefore,the Mn/Co and Mn/Ce modified catalysts have excellent sulfur resistance and denitrification activity.(3)The alkali metals easily adsorb on the non-atomic center site of α-Fe2O3 to form ionic bonds with adsorption energies of 3.34 and 3.72 eV,respectively.After the addition of K and Na,the strength of Lewis acid sites on the surface are not affected,while the strength of Br(?)nsted acid sites is significantly decreased.Both the K and Na atom can enhance the NO and NO2 adsorption capacity of the α-Fe2O3 catalyst and the promoting effect of K is stronger.The adsorption energy of NO(1.656 eV)in the K-Fe2O3 system is significantly higher than that of NH3(1.149 eV),which causes the competitive adsorption with NH3 to deactivate the α-Fe2O3 catalyst.The O2 adsorption capacity of the α-Fe2O3 catalyst is inhibited,which hinder the conversion of NO to NO2.In addition,the formation of H2O is strongly restrained,which is another reason for the decrease of de-NOx efficiency.It can be seen that in the application of α-Fe2O3 catalyst,avoiding alkali metals poisoning is particularly important to ensure its high efficiency and long-life.