Synthesis Of CeO₂@MnO_x Core-shell Catalyst And The Catalytic Performance For Low Temperature NH₃–SCR

Selective catalytic reduction?SCR?technology is a widely used flue gas denitrification technology at home and abroad,and it is a kind of technology with high stability and efficiency.The good performance of the catalyst in flue gas denitrification is the key to the SCR technology.At present,the mainstream commercial SCR catalyst is V2O5-WO3?MoO3?/TiO₂ series catalyst,which has a high activity temperature window 300⁴00 ?.In order to obtain higher denitrification efficiency,the SCR reactor is usually placed before the dust removal and desulfurization unit,which makes the catalyst susceptible to the dust,plugging and poisoning,and then reduces the service life of the catalyst.Therefore,to solve the above problems,it is necessary to develop environmental-friendly low-temperature SCR catalyst.But due to the low-temperature SCR catalyst easily get deactivated in the atmosphere of SO₂ and H2 O,it is not yet practical in business.Therefore,focus on the development of catalysts with high resistance to SO₂ poisoning and H2 O capability is the key of realizing the commercial application of low-temperature SCR technology.MnO_x has a wealth of variable valence and redox ability,and rich in the reactive oxygen species required SCR reaction cycle,showing low SCR activity excellent,this is because the MnO_x contains a variety of lattice oxygen an important role in the SCR reaction cycle.But manganese-based catalysts are poor in sulfur resistance at low temperatures,and this weakness can be improved by metal doping.Among them,the Ce doped Mn based composite oxide catalyst exhibited good catalytic performance at low temperature and good resistance to sulfur poisoning in NH₃-SCR.The morphology and structure of Mn-Ce composite metal oxides is an important direction to improve and improve the low temperature catalytic properties and sulfur resistance of the catalysts.As a novel catalyst,core-shell structure catalysts exhibit good research prospects in enhancing catalytic activity and protecting core,and have attracted more and more researchers' attention in recent years.The preparation method of the main content of this thesis is to investigate the core-shell structure catalyst and its application in catalytic reduction reaction,structure and properties of CeO₂@MnO_x catalyst concentration,in order to improve the low-temperature activity of the catalyst and the performance of anti SO₂ and H2 O poisoning,were studied and the mechanism of the catalyst was NH₃-SCR.First of all,MnO_x spherical nanoparticles were prepared by chemical precipitation method.MnO₂ nanotubes and MnO₂ nanosheets were prepared by hydrothermal method.CeO₂@MnO_x was prepared by chemical precipitation method with the above three morphologies of MnO_x as the cores,the evaluation results show that MnO_x spherical particles of low temperature catalytic performance is the best.The results of HR-TEM and XRD show that the crystalline structure of nanomaterials and the surface type of active surface determined the low temperature SCR properties of the catalysts.Then,MnO_x spherical nanoparticles were used as the research object,and the catalytic performance of MnO_x nanoparticles catalyst was optimized by adjusting the preparation conditions.When the preparation conditions are as follows: Mn?CH₃COOH?2 as precursor,NH₃.H2 O as precipitant,PEG?6000?as the surfactant and PEG?6000?: Mn 0.03 depending on the conditions of preparation,we can get small spherical nano MnO_x particles with a narrow particle size distribution and uniform size,good dispersion.The catalytic properties of MnO_x based spherical nanoparticles depend on the size distribution and morphology of particles.Reducing the diameter of materials to nanometer scale can greatly improve their catalytic performance.Then,the CeO₂@MnO_x catalyst with CeO₂ as shell was synthesized by chemical precipitation method using MnO_x spherical nanoparticles as the core.The catalytic performance and sulfur resistance of CeO₂@MnO_x have been greatly improved.Furthermore,the preparation conditions of the synthesized core-shell structure were studied.When Ce O₂: MnO_x was 0.6,the catalyst was the best.The XPS and BET characterization results show that the CeO₂ shell with proper thickness is beneficial to form a close coating relationship with MnO_x,which then increases the contact surface of the two oxides,and also could provide a sufficient diffusion channel for the reaction gas.Secondly,by comparing the catalytic performance of CeO₂@MnO_x and Mn-Ce-Ox two catalysts in SCR test,we found that the CeO₂@MnO_x not only has better catalytic performance,but also has better sulfur resistance.And by HR-TEM,XRD,XPS,H2-TPR and other characterization analysises,it could be found that the excellent catalytic performance of CeO₂@MnO_x is attributed to a high proportion of Mn4+,Ce3+ elements,high crystallinity of alpha-MnO₂,smaller particle size and other factors.The key for improving the ability of sulfur tolerant is that the proper CeO₂ shell can protect the active site of MnO_x core and then not easy to be poisoned.Finally,the reaction mechanism of CeO₂@MnO_x core-shell structure catalyst was investigated by Fourier transform infrared spectroscopy?In-situ FT-IR?which including adsorption reaction and transient reaction experiments.The results show that the catalyst surface and the existence of Bronsted acid and Lewis acid.NH₃ exist in Bronsted acid sites is in the form of NH4+ species and coordinated NH₃ exist in Lewis acid sites in the state of-NH2 species form,they are active species in the NH₃-SCR reaction process.Among them,absorbed NH₃ species and-NH2,absorbed NO₂ species ? mononitrate and N2O4 are the main intermediates,and the reactions follow the L-H mechanism and E-R mechanism.