Study On Catalytic Performance And Mechanism Of SCR Denitration Catalyst

In recent years,with the development of industry and various industries,air pollution has gradually become a major factor affecting people’s happiness.Among many atmospheric pollutants,nitrogen oxides（NO_x）have become one of the main pollution sources.Therefore,nitrogen oxides have become the top priority of atmospheric governance.In order to effectively remove NO_x,scientific researchers have done a lot of research on this,and selective catalytic reduction（SCR）technology is considered to be one of the most effective technologies.NH₃,hydrocarbons,CO,H₂ and other reducing agents are usually used in SCR process.At present,most industries use NH₃ as reducing agent,V₂O₅-WO₃/TiO₂ catalyst and Mn-based catalyst,but the reaction temperature range of V₂O₅-WO₃/TiO₂ catalyst is narrow（300-400℃）,and vanadium is easy to produce secondary pollution during use.Mn-based catalysts have become a research hotspot because of their rich multivalent states and strong low-temperature redox energy.Secondly,using H₂ as a reducing agent is the most environmentally friendly process for removing NO_x,which can not only remove NO_x from the flue gas,but also reduce NO_x to chemicals with high added value through H₂.In this paper,the low-temperature denitrification performance of NH₃-SCR was studied,especially the catalytic activity,NO conversion rate and reaction mechanism of MnSiO₃ catalyst.The photoassisted denitrification performance of NH₃-SCR over MnTiO_x catalyst was explored,and the application of Fe₂O₃/TiO₂ catalyst in H₂-SCR was developed to improve the selectivity of NH₃.NH₃ was prepared by making NO"waste into treasure".The details are as follows:（1）Vermiculite based MnSiO₃ catalysts for NH₃-SCR low temperature performance and mechanism studyA new manganese silicate（MnSiO₃）catalyst was synthesized by hydrothermal method using silica from the layered silicate clay mineral vermiculite as the ligand and silicon source,and it was used in the NH₃-SCR NO removal reaction.The experimental results showed that MnSiO₃ has higher Mn⁴⁺content,surface adsorbed oxygen（Oα）acidic sites,and reducing substances compared with MnO_x/SiO₂.The MnSiO₃ catalyst has a good physical structure with a BET surface area of up to 211.0 m²/g,an average pore volume of 0.30cm³/g and an average pore size of 4.8 nm.Therefore,MnSiO₃ exhibited 100%NO conversion and more than70%N₂ selectivity at 150°C during the NH₃-SCR reaction.The in situ diffuse reflectance infrared Fourier transform spectroscopy（DRIFTS）study showed that MnSiO₃has a strong NO adsorption capacity and NH₃activation capacity,which is beneficial to the NH₃-SCR activity.（2）Performance and mechanism of MnTiO_x catalyst for photo-assisted NH₃-SCRBased on the previous studies,the use of NH₃-SCR technology has excellent removal of NO,but the reaction temperature is high.Photocatalytic technology has mild reaction conditions and can react at room temperature.Therefore light was introduced into the NH₃-SCR reaction to investigate its low temperature performance.MnTiO_x composite catalyst was prepared by co-precipitation method for the photo-assisted SCR（PSCR）denitration reaction.Combined with experiments and characterization studies,MnTiO_x showed excellent performance as a PSCR catalyst.When exposed to light for 120 min at room temperature,the NO conversion rate was 100%,and the N₂ selectivity reached more than 85%in the whole range.Electron paramagnetic resonance（EPR）analysis showed that there were a large number of oxygen vacancies on MnTiO_x.The computation of optical carrier density functional theory（DFT）shows that the 3d orbital hybridization of Mnand Tiis significantly enhanced under illumination.MnTiO_x catalyst has good electron-hole separation ability,which is beneficial to adsorbing NH₃ and dissociating to form NH₂ fragments and H atoms.In situ DRIFTS showed enhanced adsorption of NH₃ by MnTiO_x under light conditions,resulting in excellent PSCR activity.（3）Performance study of Fe₂O₃/TiO₂ catalyst for H₂-SCR denitrification and synthesis of NH₃Using H₂ as a reducing agent to remove NO can not only avoid to apply NH₃,but also"make waste profitable"to prepare NH₃.Therefore,a combination of hydrothermal and impregnation methods was used to prepare Fe₂O₃/TiO₂-MOF catalysts for the H₂ reduction of NO for ammonia synthesis.The experimental results show that the removal rate of NO at 450℃is 100%,and the selectivity of NH₃ is more than 95%.The catalytic process of Fe₂O₃/TiO₂ as a potential NO reduction catalyst for synthetic ammonia was studied by DFT.The mechanism of NO activation and the reason of NH₃ adsorption/desorption on Fe₂O₃/TiO₂surface were revealed by the state density and charge difference density of NO and NH₃ adsorbed.By calculating the reaction path of NO synthesis ammonia catalyzed by Fe₂O₃/TiO₂,it can be seen that in path 1,the potential determining step is*NO+H⁺+e^-→*NOH,and in path 2 or path 3,the potential determining step is*NO+（H⁺+e^-）→*HNO.The thermodynamic energy barrier of NO dissociation and hydrogenation at the surface of Fe₂O₃/TiO₂ has been calculated.The results show that NO is more easily hydrogenated than dissociated.（4）Performance study of Cu Fe/TiO₂ catalysts for H₂-SCR denitrification and synthesis of NH₃Through the previous research,although NH₃ was successfully synthesized by NO+H₂,the reaction temperature was high,so a series of Cu-doped catalysts were constructed by simple and effective impregnation method.The optimized 8Cu-10Fe/TiO₂ has good catalytic performance for ammonia synthesis using NO+H₂ as raw material（at atmospheric pressure 350℃,NO conversion rate is more than 80%,NH₃selectivity is more than 97%）,which is significantly better than the corresponding 10Fe/TiO₂（at atmospheric pressure 350℃,NO conversion rate is 8%,NH₃ selectivity is 0）.In situ DRIFTS and DFT calculation were used to detect the conversion mechanism（dissociation or association）of intermediate and adsorbed NO.Cu doping promoted the rate-limiting hydrogenation step and reduced the desorption energy of NH₃,thus obtaining higher NO conversion and ammonia selectivity.