Study On The Preparation Of Pt-based And Co-based Nano-Catalysts And Their Catalytic Performance For The Oxidative Removal Of Diesel Soot Particles

One of the main causes of PM 2.5 in city area is the formation of soot particles emitted from diesel exhausts and their accumulations,which seriously endangers human health and the environment.The combination of a filter and an oxidation catalyst is an effective way to eliminate diesel vehicle soot particles.The catalytic combustion reaction of soot particles is a typical solid（soot particle）–solid（solid catalyst）-gas（O₂）a three-phase complicated and heterogeneous catalytic reaction.In the process of catalytic soot combustion,high performance catalysts should have the following two advantages:First,the catalyst can provide the condition for contacting well with soot particles;Second,the catalysts should have high intrinsic activity.Therefore,in this thesis work a series of nano core-shell structure catalysts were designed and prepared by using noble metals and rare earth metal oxides as raw materials,and they are supported on a three-dimensionally ordered macroporous（3DOM）support.The core-shell nanoparticles enhance the interfacial effect between noble metals and oxides,and strong interfacial effects can increase the concentration of activated oxygen.We changed the reaction conditions to adjust the thickness of the shell in the core-shell structure,and explored the effect of shell thickness on the activity of the catalyst in the core-shell structure.At the same time,due to the high price of precious metals,we also studied the performance of non-precious metal Co-based nanocatalysts with strong oxidation properties to eliminate soot particles.The surface oxidation mechanism of NO on Co-based catalysts was studied and the changes of surface oxygen vacancies on the surface of Co-based catalyst in the reaction atmosphere were characterized by in-situ Raman spectroscopy.The mechanism of the catalytic process was also discussed during the research.The catalysts were characterized by means of XRD,BET,SEM,TEM,Raman,IR,UV-Vis,XPS,H₂-TPR,O₂-TPD,NO-TPO and DRIFT（infrared spectroscopy）.This thesis introduces the basic experimental methods and analytical characterization methods,which can guide the laboratory study and subsequent pilot research and industrial production.The main research results of this thesis are as follows:（1）The 3DOM Zr O₂ support was successfully prepared by colloidal crystal template method by using polymethyl methacrylate as template.The 3DOM Zr O₂ carrier has three-dimensional ordered macroporous structures with an average diameter of 260±20 nm.The Pt@Ce O_2-x nanoparticles were loaded onto the 3DOM Zr O₂ support by precipitation method.The whole process should be kept stirring during the preparation process by precipitation.The Pt/3DOM Zr O₂ catalyst has good catalytic performance through Pt@Ce O_2-x.The loading of the nanoparticles further enhances the catalytic activity of the catalyst,especially for the Pt@Ce O_2-x/3DOM Zr O₂-1 catalyst,and the light-off temperature of the soot particles decreased,meanwhile the T₅₀ and T₉₀ also decreased.For example,under loose contact conditions,the T₁₀,T₅₀ and T₉₀ of the soot particles on Pt/3DOM Zr O₂ catalyst were 293,363,and 400°C,and the T₁₀,T₅₀ and T₉₀ on the Pt@Ce O_2-x/3DOM Zr O₂-1 catalyst were 275,328,365°C,respectively.The combustion temperatures of the soot particles were further lowered.Based on XPS,H₂-TPR,O₂-TPD,NO-TPO and other characterization results,it was found that Pt@Ce O_2-x/3DOM Zr O₂-1catalyst has the strongest ability to oxidize NO,which can catalyze the combustion of soot particles more effectively and its redox property is the best.The Pt@Ce O_2-x/3DOM Zr O₂-1 catalyst surface has higher content of unsaturated coordination Pt ions.The results among this series of catalysts show that Pt@Ce O_2-x/3DOM Zr O₂-1 catalyst has the best catalytic performance.At the same time,it was found that the thickness of Pt@Ce O_2-xshell on Pt@Ce O_2-x/3DOM Zr O₂ catalyst also has an important effect on the activity of catalytic combustion of soot particles.When the Ce/Pt molar ratio is 1:1（shell thickness is around 2 nm）,the catalyst exhibits the highest catalytic activity.（2）3DOM Zr O₂ was prepared by colloidal crystal template method,and Cu@Pt,Pt,Cu and Cu Pt nanoparticles with uniform particle size were prepared by micro-solution method,and then the nanoparticles were loaded on 3DOM Zr O₂ by deposition method.Under loose contact conditions,the T₁₀,T₅₀,T₉₀of soot particles in the oxidation process over Cu₁@Pt₁/3DOM Zr O₂ catalyst were 265,339,376°C,respectively,and the CO₂selectivity was as high as 99.8%.The T₁₀,T₅₀ and T₉₀ of the soot particles on the Pt/3DOM Zr O₂ catalyst were 293,363,400°C,respectively.i.e.,the combustion temperature of the soot particles was further reduced,which improved the utilization efficiency of precious metal elements.The in-situ infrared characterization method was used to analyze the conversion process of NO on the catalyst surface.During the temperature-programmed process,NO first adsorbed on the surface of the catalyst to form nitrite.As the temperature increased,the nitrite was oxidized to nitrate on the surface of the catalyst.The nitrate is decomposed at high temperture to form NO₂,and the NO₂ is partially adsorbed on the surface of the catalyst to form a nitro group.（3）Three kinds of Co₃O₄ catalysts with different exposed crystal planes were prepared by hydrothermal method.TEM results show that the main exposed crystal plane of Co₃O₄ nanorods is{110},the main exposed crystal plane of Co₃O₄ nanosheets is{112}and the main exposed facet of the Co₃O₄ nanocube is{100}.The order of activity of three different exposed crystal plane catalysts for soot combustion is as follows:{110}>{112}>{100}.When the concentration of NO was changed and other conditions were kept unchanged,it was found that the catalytic performance for soot combustion become worse with the decreasing of NO concentration.These results demonstrated that the catalytic combustion of soot of the catalysts rely on NO in some extent.H₂O and SO₂ molecules will adsorb on the surface of the catalysts when they were added to the reaction atmosphere.In order to study the mechanism of oxidation of NO on Co₃O₄ catalyst,we performed DFT calculation on the oxidation process of NO with three different morphologies.The energy changes from the adsorption of NO molecules on the catalyst surface,the oxidation of NO to NO₂,and desorption of NO₂ from the catalysts were calculated by DFT calculation.The calculation results demonstrated that for the NO oxidation over the Co₃O₄ catalysts with main exposed face of{100},the energy barrier is the highest for the reaction step NO+O_lattice→NO₂（the energy barrier is 1.03 e V）and this reaction step is the rate-determing step.On the contrary,the energy barrier for the oxidation of NO on the surface of facets{110}and{112}are 0.60 e V and 0.65 e V,respectively,which are much smaller than that on the facet{100}.Thus,NO is much earily oxidized on them.The{110}and{112}crystal faces have higher NO catalytic oxidation activity than the{100}crystal plane.（4）The sheet-like Co₃O₄,Ce O₂ and Co₃O₄-Ce O₂ catalysts were prepared by hydrothermal method.The SEM and TEM characterization results proved the formation of sheet-like Co₃O₄,Ce O₂ and Co₃O₄-Ce O₂ catalysts.The Ce O₂ nanoparticles were uniformly dispersed on the surface of Co₄Ce₁O_x catalyst.But the Ce O₂ nanoparticles on the surface of the catalyst agglomerated as the Ce O₂ content increased,the Ce O₂nanoparticles covered on the surface of the Co₃O₄ catalyst.When further enhance the content of Ce O₂,the catalytic performance of the Co₃O₄,Ce O₂ and Co₃O₄-Ce O₂ catalysts was evaluated by temperature-programmed oxidation.The effects of different reaction atmosphere compositions on the TPO results of Co₄Ce₁O_x catalysts were also tested.Among the activity test results,Co₄Ce₁O_x catalyst had the highest activity.The T₅₀ and T₉₀ of Co₄Ce₁O_x catalyst are 340°C and 373°C,respectively.The characterization results of sheet Co₃O₄,Ce O₂ and Co₃O₄-Ce O₂ catalysts show that the Co₄Ce₁O_x catalyst has the best redox performance,and the strongest NO oxidation ability.