Study On Reaction Mechanism Of Iron Based SCR De-NO_x Catalyst At Medium And Low Temperature

Iron based SCR NO_x removal catalyst has become a hot spot in the research and development which is cheap,non-toxic and efficient.More and more Fe-based catalysts come out,which brings new vitality to NH₃-SCR NO_x removal technology.However,the mechanism of Fe-based SCR process is not very clear,which seriously affects the further development and application of this catalyst.On the other hand,the first principle based on quantum chemistry has been deeply applied to various fields of new materials research,especially has become a powerful tool for catalyst research.However,there are few reports on the mechanism of Fe-based catalysts using first principles.Therefore,in this paper,γ-Fe₂O₃ with high SCR activity was selected as the representative of Fe-based catalyst,and the SCR reaction mechanism of dopedγ-Fe₂O₃ was studied by first principles,which laid a foundation for further development and research of Fe-based catalyst.In this paper,the crystal structure ofγ-Fe₂O₃ was constructed and 001 surface was selected as the active surface of SCR reaction.The first principle and density function theory（DFT）was applied to the research.The adsorption configuration and corresponding adsorption energy of NH₃,NO_x,O₂,H₂O,N₂ and other small molecules on the surface ofγ-Fe₂O₃ catalyst were calculated,and the adsorption characteristics of the above small molecules on the catalyst surface were evaluated.The catalyst surface was obtained and the activation energy of elementary reactions of SCR was determined,and the most probable reaction path and reaction rate control step were determined.On this basis,DFT calculation was used to study the mechanism of improving SCR activity and sulfur resistance ofγ-Fe₂O₃ catalyst by doping Ce and Mn elements.Finally,the denitrification activity ofγ-Fe₂O₃ catalyst was tested in a fixed bed reactor,and then,the velocity field,pressure field,temperature field and reactant concentration distribution field in the SCR reactor ofγ-Fe₂O₃ catalyst were simulated by using the activation energy and other relevant parameters calculated by DFT,and the research conclusion was basically consistent with the experimental results.The main research results are as follows:（1）The mechanism of NH₃-SCR reaction on the surface ofγ-Fe₂O₃ catalyst was revealed.Through DFT calculation,it is found that NH₃,NO_x,O₂and other reactants are adsorbed on the catalyst surface firstly during SCR reaction.For NH₃ molecules,NH₃ combine with the L acid site on the surface to form adsorbed NH₃,and then react with lattice oxygen on the surface to generate NH₂ free radicals;then,NH₂ react with gaseous or adsorbed NO to generate NH₂NO,while the intermediate product NH₂NO is unstable and easy to decompose,and finally N₂ and H₂O can be desorbed from the catalyst surface.Under aerobic conditions,the adsorbed NH₃ reacts with the active oxygen generated by oxygen dissociation to form NH₂ and other species,and the activation energy of this process is much lower than the former,and the formation of NH under aerobic condition is also easier.NH₂ react with NO to form NH₂NO and decompose,and NH react with NO to produce NHNO and decompose,and finally produce N₂ and H₂O.Under aerobic conditions,NO is oxidized by oxygen to form NO₂,which participates in the rapid SCR reaction.（2）The mechanism of Mn and Ce doping to improve the SCR denitration activity ofγ-Fe₂O₃catalyst was explained.DFT calculation results show that the adsorption configurations of NH₃ and NO on the surface of Mn and Ce dopedγ-Fe₂O₃ catalyst are basically the same as those of undoped surface,and 2x1 surface is the optimal doping surface,which can promote the adsorption of reactants on the surface.Secondly,on the active sites close to Mn on the surface of Mn doped catalysts,Mn-Fe synergetic catalysis can reduce the activation energy of each step of elementary reaction and effectively promote the SCR reaction.At the far active sites,Mn doping introduces more valence electrons and changes the surface acid sites,which also promote the SCR reaction.However,on the active sites near the Ce atom on the Ce doped catalyst surface,the process of the subsequent dehydrogenation of NH₂ by Ce-Fe synergetic catalysis is more difficult than that of two octahedral iron catalysts.Therefore,Mn doping can improve the SCR activity of the catalyst,but Ce doping can promote SCR reaction under certain conditions,and excessive doping inhibit SCR reaction.（3）The difference mechanism of Ce and Mn doping in enhancing sulfur resistance ofγ-Fe₂O₃catalyst was explored.DFT calculation shows that on Ce doped surface,the adsorption energy of active site of Ce to sulfur oxide is larger,so Ce doping can attract sulfur oxide near Ce,thus protecting other active sites of Fe.Moreover,on Ce doped surface,the activation energy of ammonium sulfate decomposition process is greatly reduced,which promotes the decomposition of ammonium sulfate on the surface of Ce doped catalyst.On the surface of Mn doped catalyst,the adsorption energy of sulfur oxide is not different from that of undoped catalyst,and Mn doping has little effect on the decomposition of ammonium sulfate.Therefore,Ce doping can improve the sulfur resistance ofγ-Fe₂O₃ catalyst,while Mn doping has little effect on sulfur resistance.（4）The macro applicability of the micro mechanism ofγ-Fe₂O₃ catalyst for SCR denitration obtained by the first principle was verified.In this paper,the NO_x removal activity ofγ-Fe₂O₃catalyst was tested,and the activity under different conditions was studied.And then,the velocity field,pressure field,temperature field and reactant concentration distribution field in SCR denitrification reactor ofγ-Fe₂O₃ catalyst were simulated by using the activation energy and other related parameters calculated by first principles.The calculated results are compared with the results of denitrification activity test.It was observed that the concentration of NH₃,NO and O₂decreased gradually with the increase of the height of reaction gas flowing through the SCR reactor,while the concentration of H₂O and N₂ increased gradually,which made the NO content in the outflow bed gas tend to zero,which proved that theγ-Fe₂O₃ catalyst has high SCR NO_x removal efficiency.Moreover,with the decrease of flow rate,the increase of ammonia nitrogen ratio,the increase of reaction temperature and the increase of oxygen content,the NO_x removal efficiency increases.These conclusions are consistent with the experimental results.Therefore,it is verified that the micro mechanism of SCR denitration ofγ-Fe₂O₃ catalyst revealed by the first principle can guide Fe-based catalyst very well.