| The rapid development of industry not only promotes social progress but also causes serious environmental pollution.Thereinto,the treatment of NOx from flue gas is most crucial part.Among the methods,NH3 selective catalytic reduction of NO(NH3-SCR)has become the mainstream technology for NOx removal,and is widely used for denitrification of stationary sources and diesel engines.Due to its diverse valence state(Ce4+/Ce3+cycle),cerium-based catalysts have attracted the attention of many researchers for their excellent oxygen storage/release capacity and strong redox ability,and have been widely used in NH3-SCR reaction.Simultaneously,hollow structures have been widely and effectively applied in many fields due to their excellent characteristics,such as low density,large specific surface area and independent cavity.Thus,the synthesis of cerium-based oxide catalysts with composite hollow structure and their application in eliminating NOx has important theoretical and practical significance.In this paper,by changing various factors in the preparation conditions,such as elemental ratio,heating rate,reaction time and precursor introduced path,we prepared composite cerium-based oxide hollow catalysts with controllable morphology and structure,and the synthesis mechanism and activity change rule in NH3-SCR reaction were investigated.The specific research contents are as follows:(1)Composite CeO2-MnOx hollow materials were successfully prepared with carbon spheres generated from glucose hydrothermal treatment as hard templates.According to the experimental results,when the ratio of nMn/nce reached above 0.43,the CeO2-MnOx composite showed double-layer structure.In addition,the composite had a more complete hollow structure than those of single component oxides,with changes in crystalline phase,valence state and texture properties.In particular,the presence of Mn was conducive to the diffusion of CeO2 into the carbon sphere to form a multi-layer hollow structure under experimental conditions,indicating that the two elements have certain synergistic effects in the synthesis process.(2)When higher Mn/Ce ratio was selected as the synthesis condition,CeO2-MnOx composite hollow materials with different internal layers were obtained by adjusting the heating rate in the calcination process.With the increase of heating rate,the number of inner layers of the hollow structure increased gradually,so was the activity of NH3-SCR reaction.The results showed that abundant surfactant species,higher redox performance and the special multilayer structure could explain the enhanced activity of multi-shell structured catalysts.(3)On the basis of the first two part,the synthesis of composite CeO2-WOx hollow material was conducted.It was found that the hollow structure could not be well formed by using carbon sphere as template,while could be obtained by one-step hydrothermal synthesis of glucose and metal salts.With the increase of the hydrothermal time in the preparation process,the structure of the product was changed from solid structure to yolk-shell structure and finally to hollow structure.It was worth noting that NH3-SCR catalytic activity of the catalysts with cavity structure was better than that of the solid structure,indicating that the existence of cavity structure could promote the catalytic performance of CeO2-WOx catalyst.The activity results were mainly attributed to different internal structures.In addition,we proposed a synthetic mechanism for the formation of different internal structures at different hydrothermal time.It was believed that in the process of carbon spheres formation from glucose,an internal hydrophobic-external hydrophilic structure was generated.Due to the hydrophilicity of Ce and W elements,the elements diffused from the internal hydrophobic region to the surface hydrophilic layer.Furthermore,different elemental distributions are formed on the carbon spheres through different reaction time,thus resulting in different internal structures after carbon removal.(4)Combining the above parts,we obtained two kinds of hollow CeO2-TiO2 catalysts with different cavity sizes through carbon sphere template method and one-step hydrothermal method of glucose and metal salt.The results showed that,compared with the catalyst with small cavity size,the catalyst with larger cavity size showed slightly higher NOx removal activity,but significantly enhanced SO2 resistance.After an in-depth study of the chemical and physical properties of the catalysts,we found that the difference in the SO2 resistance of the two catalysts was mainly attributable to the difference in physical properties,that is,the different cavity sizes of the catalysts caused different ABS decomposition.In particular,catalysts with larger cavity size had faster ABS decomposition rate and less ABS residual,so as to obtain better SO2 resistance. |
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