Synthesis Of Ag-based Catalysts And Its Selective Catalytic Reduction Of NO In Excess Oxygen

NO_x is one of the main air pollutants and the main harmful components of automotive exhaust.NO_x is not only a threat to human health but also ause a series of environmental pollution such as photochemical smog and acid rain.Therefore,it is very important about how to effectively reduce NO_x emissions.Hydrocarbon selective catalytic reduction?SCR?of NO_x reductant technology still has a lot of research applications,because it has high reduction activities and no secondary pollutions.Ag/Al₂O₃ catalyst is one of the most potential HC-SCR catalysts,because it has the characteristics of long service life and low cost.However,the shortcoming about the low catalyst activity about Ag/Al₂O₃ catalyst in low and medium temperature is not solved yet,and it is harmful to lean combustion engine.As a result,this paper studies the different of catalyst formulation and synthesis method to improve its catalytic activity,and use modern means of characterization to analysis of the structure-activity relationship of the catalyst in excess oxygen.Specifically include the followings:?1?Firstly,based on Ag/Al₂O₃ catalysts,the preparation conditions of conventional impregnation method?such as: Ag content and calcination temperature?and catalytic degradation of NO technological conditions?such as: the dosage of catalyst,gas flow velocity,the ratio of C₃H₆ and NO,O₂ content?were explored.The results showed that the activity of Ag/Al₂O₃ catalysts were closely related to the load of Ag content,and Ag/Al₂O₃ catalyst has reached the best activity of 78%?NO removal rate?when Ag load was 1.5%.This could be due to the suitable load lead to the best of the dispersion of active components.the catalyst of the integral structure and thermal stability would be affected by roasting temperature,and the optimized roasting temperature was 600?.On the basis of the optimization of catalyst preparation conditions,the optimum reaction process was studied by the single factor variable method,and the results were as follows:dosage of catalyst was 200 mg,gas flow was 100ml/min,the ratio of C₃H₆ and NO was1:1,and the content of O₂ was 4%.?2?Secondly,1.5%Ag/Al₂O₃-Imp catalyst was modified by impregnation method for the preparation of 1.5%Ag/0.5%Ti/1%Zr/Al₂O₃-Imp catalyst mainly through introduction additives and modulation dipping way to improve the catalytic activity of catalyst.Research results showed that 1.5%Ag/0.5%Ti/1%Zr/Al₂O₃-Imp catalyst activity wasexcellently,and the removal rate of NO could reach 62% at 400?,and the removal rate of NO could reach 83% at 450 ?.Compared with 1.5%Ag/Al₂O₃-Imp catalyst,1.5%Ag/0.5%Ti/1%Zr/Al₂O₃-Imp catalyst for the NO removal rate increased by 14% at400 ?.XRD characterization results showed that the doping of Zr could enhance the capacity of the catalyst dispersion of active components and the doping of Ti could assume all of oxide with catalyst on amorphous state and help to stabilize the structure of the catalyst.H2-TPR characterization results showed that the doping of Zr and Ti were good for H2-TPR reduction peaks of the catalyst at low temperature to move to low temperature to increase the catalyst activity at low temperature.NH₃-TPD characterization results showed that the doping of Zr and Ti were conducive to increase weak acid and moderate acid center catalyst.O₂-TPD characterization results showed that the doping of Zr and Ti could increase the catalyst for oxygen storage capacity.?3?Lastly,Ag/Al₂O₃ catalysts were modified by sol-gel method for the preparation of 4%Ag-0.75La-2%Ti/Al₂O₃-Sol with Ag-Ti-Zr ternary catalysts.On the basis of analysis the sol-gel method and impregnation method,the dosage of additives and the relationship between the main active component and carrier were optimized to gain the optimal formula of ternary catalyst by sol-gel method.Compared with1.5%Ag/Al₂O₃-Imp catalyst,4%Ag-0.75%La-2%Ti/Al₂O₃-Sol catalyst for the NO removal rate increased by 20% to reach 59% at 400? and increased by 14% to reach81% at 450?.XRD characterization results showed that the doping of La and Ti existed strong correlation between La and Ti oxides and Al₂O₃,and could assume all of oxide with catalyst on amorphous state and help to stabilize the structure of the catalyst.H2-TPR characterization results showed that the doping of La and Ti were good for the increasing of peak height and peak area with H2-TPR reduction peaks and moving to low temperature to increase the catalyst activity at low temperature.NH₃-TPD characterization results showed that the reducing of the weak acid and strong acid and the increasing of moderate acid center were caused by the doping of La and Ti.O₂-TPD characterization results showed that molecular oxygen oxygen adsorption and chemical adsorption were intensified by the doping of La and Ti.