Study On Mechanism Of Denitrification Of NH₃-SCR Based On Rare Earth-based Catalyst

The main atmospheric pollutant NO_X emitted by thermal power plants is the major cause of many pollution phenomena such as acid rain and photochemical smog.The control and treatment of nitrogen oxide pollution has received increasing attention from society.At present,due to high denitration efficiency and high selectivity,selective catalytic reduction with NH₃ as reductant is widely used in many denitrification processes.In the whole NH₃-SCR process,the development and development of highly efficient catalysts is at the heart of this technology.At present,commercial catalysts are mainly vanadium-based catalysts,among which vanadium in vanadium-based catalysts is toxic,and there are some problems such as high cost.Therefore,the development of new catalysts to replace vanadium-based catalysts has increasingly become the focus of SCR technology.In recent years,people have used the unique properties of rare earth elements to improve conventional catalysts?Rare earth oxides have the mobility of lattice oxygen and the variable valence state and surface acidity and alkalinity of cations?.In particular,a catalyst prepared by doping rare earth elements into a transition metal has been widely concerned because of its good denitration performance and sulfur resistance.In this paper,Baiyun Ebo rare earth concentrate is used as the material source of the catalyst.The valuable mineral is leached by the reaction vessel with nitric acid,and the filtrate is filtered and dried to obtain a rare earth-based denitration catalyst.Study on denitration catalysts prepared under different acid concentrations and different calcination temperatures.The elemental composition of the catalysts was determined by XRF.The activity of NH₃-SCR was detected by activity test.The physicochemical properties were investigated by means of XRD,SEM,BET,TPD and TPR.The adsorption reaction characteristics of the gas on the catalyst surface were studied by in situ FTIR technology,and the reaction mechanism and path of the catalyst were presumed.The main results obtained are as follows:1.The denitration activity of the rare earth-based catalyst was detected by an NH₃-SCR activity evaluation device.The results show that the catalyst prepared by treatment with 15mol/L nitric acid has the best denitrification effect.When the acid concentration was 15mol/L and the calcination temperature was 600?,the catalytic activity increased first and then decreased with the increase of reaction temperature.When the reaction temperature is 350?,the denitration rate of the catalyst can reach 60%or more.2.The XRF method was used to understand the elemental composition of the catalyst.The elemental composition and element content indicate that nitric acid will substantially dissolve the valuable minerals in the rare earth concentrate,and the acid concentration was15mol/L,which was the highest.There are three main categories of elemental composition,namely rare earth metal elements and transition metal elements and alkaline earth metal elements.The TG-DSC method was used to understand the thermal decomposition properties of the catalyst,which indicated that the decomposition was complete at 700?,so the calcination temperature was selected from 500?,600?,700?.The crystal structure of the catalyst was understood by XRD,The results show that as the calcination temperature increased,the major phase peak of catalyst became sharper,and the crystallinity was enhanced.At the same time,an obvious shift peak of the CeO₂ diffraction peak was detected,indicating that other elements were doped,and there was synergy between the elements.The physicochemical properties of the catalyst were revealed by means of characterization such as BET,SEM,TPD and TPR.The results show that the catalyst obtained at the calcination temperature of 600?has better NH₃/NO adsorption characteristics and strong redox properties.3.The adsorption reaction characteristics of the gas on the catalyst surface were studied by in situ FTIR technology.The adsorption of NH₃ on the surface of the catalyst mainly exists in the form of NH₃ coordinated by L acid sites and NH₄⁺adsorbed by B acid sites.The adsorption of NO_X on the catalyst surface mainly in the form of bidentate nitrate,monodentate nitrate and bridge nitrate.The presence of O₂ promotes the adsorption of NO on the catalyst surface,a certain concentration of O₂ promotes NH₃ dehydrogenation to NH₂,but the NH₂ transition oxidation will cause NH₃ to be oxidized to NO,N₂,N₂O.Therefore,it is necessary to control the concentration of O₂ to prevent the transition oxidation of NH₃.The catalyst has two denitration reaction mechanisms under low temperature conditions.One is the E-R mechanism.It is found that NH₃ is more easily adsorbed and activated on the surface of the catalyst,and the amount of nitrates is extremely small.It is considered that the reaction between the adsorbed NH₃ species and the gas phase NO_X is mainly carried out.The other is the L-H mechanism,the reaction between the adsorbed species of NO_X?nitrates?and the adsorbed species of NH₃?amino species?.