Structure-control Synthesis Of Zeolite For NO_x Removal With NH₃

The rapid development of social economy has intensified the destruction to ecological environment.The serious air pollution leads to the deterioration of the atmospheric environment and frequent occurrence of haze.Therefore,it is urgent to control the air pollution.Nitrogen oxides(NOx),one of the main air pollutants,are mainly derived from the energy conversion process of fossil fuels.The formation of acid rain will endanger human body and architectural relics.Photochemical smog will accelerate the formation of PM2.5 particles and haze,and the greenhouse effect will also seriously destroy the ozone layer.How to efficiently and economically eliminate nitrogen oxides is a major research in air pollution control.In this thesis,we focus on the zeolite catalysts with excellent selective catalytic reduction with ammonia(NH3-SCR)performance for exhaust gas from floating diesel.Firstly,we researched the different topological structures,and then evaluated the influence of morphologies;finally,core-shell structure catalysts were synthesized to evaluate the NH3-SCR performance.Through in-depth study and analysis of the catalysts'structure and active center revealed the relationship between structure and catalytic performance of zeolite catalysts.The main results are as follows:In this thesis,starting from the different topological structures zeolite,six Cu-modified zeolite catalysts with different topological structures were studied on the NH3-SCR performance.According to the pore structures,these zeolites can be divided into straight channel(MFI,BEA,FAU)and cage types(CHA,AFX,LEV),and the NH3-SCR activity was gradually improved from straight channel to cage.A series of characterizations were confirmed,that the samples with cage structure have more acid sites and ion exchange sites.Moreover,a clear correlation was found between zeolite structure and the nature of exchanged Cu species.Cage type zeolites with stronger acidity and double six-membered ring(d6r)structure led to the stabilization of isolated Cu ion monomers.Therefore,the ultra-fine cage zeolites show excellent low-temperature NH3-SCR performance and wide active temperature window,which has great potential value.After the previous study,it was shown that the topological structure has an effect on the performance of NH3-SCR.In order to ensure the deNOx efficiency,the reductant NH3 is often overused,which will cause the NH3 penetration.Based on the result,the selective catalytic oxidation of NH3(NH3-SCO)through SCR reaction with different topological zeolite catalysts was studied.The selective catalytic oxidation of ammonia is the last mitigation step in the exhaust treatment by a 4-way catalytic converter to convert any excess and unreacted NH3(that was used as a reductant of NOx)to the environmentally benign N2 and H2O.Here,we report a series of highly reactive and selective nano Cu/zeolites for the NH3-SCO reaction and nano Cu/ZSM-5 shows the best activity.Furthermore,small micropore(10-membered ring,10-MR)topologies were found to be crucial in maintaining high N2 yield.This study found that the 3D zeolites with 10-MR structure showed good performance and practical application value for ammonia oxidation,which provided direction for the selection of ammonia oxidation catalyst.In addition to the different topological structures,the special crystal morphology of the zeolite catalysts was also studied.A series of Cu-ZSM-5 catalysts with different crystal morphologies were synthesized(nanoparticles,nanosheets,hollow spheres)and comparatively investigated for their performances in the NH3-SCR.Significant differences in SCR behavior were observed among these zeolite catalysts with diverse morphologies.High-resolution transmission electron microscopy(HR-TEM)reveals that the as-synthesized 2D nanosheet ZSM-5 majorly exposes the(0 1 0)crystal plane.Based on H2-TPR,operando XANES,and EXAFS data,the redox ability of the copper ions under SCR atmosphere was investigated.Accordingly,it is proposed that the rapid switch between Cu2+ and Cu+ species is essentially crucial for an ideal low-temperature SCR activity.In situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS)and density functional theory(DFT)calculations imply that the(0 1 0)crystal surface of zeolite favors the NH3-SCR reaction,accompanying with readily Cu2+(?)Cu+ cycling,which leads to a superior activity of nanosheet Cu-ZSM-5 at low temperature.In the last part,we focus on the core-shell structure zeolite catalyst and evaluate the effect of its special structure on the NH3-SCR performance,especially for its water and sulfur resistance and hydrothermal stability.Cu/(ZSM-5@CeO2)catalyst exhibited the best NH3-SCR performance and SO2 tolerance due to the synergistic effect of redox ability and acidity.A series of characterizations were employed to reveal possible reasons for the high activity and poisoning resistance of Cu/(ZSM-5@CeO2)sample.The CeO2 shell not only can stabilize the isolated Cu2+ ions,but also produce more oxygen vacancies,which can improve the redox properties.Furthermore,the SO2 tolerance experiment shown that the CeO2 shell can effectively restrained the formation of nitrate,showing a high SO2 resistance.Therefore,the construction of core-shell structured catalyst is significantly to promote the practicle application of deNOx catalysts.