Study On The Macroporous Al₂O₃-supported Au-based Nanoparticle Catalysts For The Oxidative Removal Of Soot

The rapid development of our country's economics has also simultaneously led to serious environmental pollution problems.In recent years,most areas in China have been affected by haze weather sometimes,which can threat people's life and health.Thus,it is urgent to solve the problem of air pollution.It has been proven that the PM from vehicle emissions is one of the main reasons for the formation of urban haze.The Ministry of Environmental Protection has enacted a series of emission regulations to limit the emission of PM.At present,it is generally accepted that one of the most effective methods for reducing the soot emission from the automotive vehicles is the catalytic after-treatment technique.The catalyst is the core of catalytic after-treatment technique.Therefore,the study and development of highly active catalysts for oxidative removal of soot particles is an urgent and challenging research task.Soot combustion is a deep oxidation reaction containing three phases of gas-solid-solid heterogenous reactants.The contact between the solid reactant and catalyst and the redox property of the catalyst are the two key factors to affect the activity of the catalyst.The design and preparation of three-dimensionally ordered macroporous?3DOM?catalysts with large pore size could reduce reaction diffusion resistance and improve the effective contact between soot and catalyst.Noble metal nanoparticles are favorable for the adsorption and activation of molecular oxygen and increase the redox ability of the catalyst,which improve the catalytic activity of the catalyst.In this work,the 3DOM Al₂O₃ was chosen as support and Au is the active component.There are two approaches to further improving the activity and stability of the catalyst involving the design of multilayer CeO₂/3DOM Al₂O₃-supported gold nanoparticles and 3DOM Al₂O₃-supported bimetal Au-M nanoparticles catalyst.Various characterization methods were used to study the relationships among catalyst structure,physicochemical properties and catalytic activity.Moreover,the reaction mechanism for soot combustion was proposed.The main research contents and conclusions are as follows:?1?Polymethyl methacrylate?PMMA?microspheres with uniform particle size and good monodispersity were prepared by the method of soap-free emulsion polymerization,and the colloidal crystal template was assembled through centrifuge sedimentation.3DOM Al-Ce mixed oxide catalysts from metal nitrate,citric acid and ethanol were successfully prepared by using colloidal crystal templating method?CCT?.Based on the results of XRD,Al–Ce mixed oxide catalysts could form solid solutions with suitable ratio of Al to Ce.Compared with disordered macroporous?DM?Al-Ce mixed oxide catalysts prepared by the method of citric acid complex combustion,3DOM samples show higher catalytic activity for soot combustion due to enhanced contact efficiency between soot and catalyst.When the ratio of Al to Ce is 3:7,the3DOM AlCe?3:7?mixed oxide catalyst showed the highest catalytic activity for soot combustion,in which the values of T₁₀,T₅₀ and T₉₀ were 297,384 and 427 ^oC,respectively.3DOM structures of catalyst kept well even after aging at 900 ^oC for 5 h.And the crystallite size of 3DOM Al–Ce mixed oxide catalyst did not change obviously.However,the crystallite size of CeO₂ dramatically increased after aging.In addition,3DOM AlCe?3:7?catalyst still exhibited high activity after aging at high temperature for 5 h and after six-cycle runs from TPO results,while the activity of CeO₂ catalyst significantly decreased.These results demonstrate that the doping of Al can enhance the catalytic and thermal stability of catalyst.?2?3DOM Al₂O₃ support was prepared by CCT method.x-CeO₂/3DOM Al₂O₃catalysts were successfully prepared by the method of micropores-diffused precipitation.Multi-layer carrier x-CeO₂/Al₂O₃-supported Au nanoparticle catalysts were successfully synthesized by reduction-deposition method.There exist the strong metal–support interactions between Au and CeO₂ in these catalysts.The presence of CeO₂ can stabilize Au nanoparticles and reduce the average particle size of Au.Al-Ce-O solid solution in CeO₂/Al₂O₃ catalysts can be formed and create a large amount of oxygen vacancies and increase the redox ability of the catalyst.The catalytic activity evaluation results demonstrate that the loading of Au can enhance the catalytic activity of x-CeO₂/3DOM Al₂O₃ catalysts,especially in reducing the ignition temperature and improving the selectivity of CO₂.In addition,the amount of cerium oxide could influence the activity of the catalyst and improve the stability of the catalyst.Au/20%CeO₂/Al₂O₃ catalyst shows the highest activity and its T₁₀,T₅₀,T₉₀ and S^m_CO2 are 267 ^oC,372 ^oC,426 ^oC and96.7%,respectively.In addition,the T₅₀ value of Au/20%CeO₂/Al₂O₃ catalyst is almost constant after three-cycle runs from TPO results.?3?3DOM Al₂O₃-supported bimetallic Au_n-Pd_m alloy nanopaticles catalysts,i.e.,Au_n-Pd_m/3DOM Al₂O₃ catalysts were successfully prepared by the method of micropores-diffused reduction?MDR?.The results of XPS show that there exist electron transfers between Au and Pd.The presence of Au can inhibit the formation of PdO.In addition,the two metals of Au and Pd exhibited favorable synergistic effect.The activities of supported bimetal catalyst were higher than those of supported sigle metal.When the ratio of Au to Pd is 1:1,the catalyst shows the highest catalytic activity.Its T₁₀,T₅₀,T₉₀ and S^m_CO2 are 290 ^oC,389 ^oC,437 ^oC and 98.2%,respectively.Similarly,Au₂-Pd₂/3DOM Al₂O₃ catalyst was repeatedly tested for three times.Supported bimetal nanoparticle catalyst still kept high activity,demonstrating that the introduction of Pd can improve the stability of noble metal Au nanoparticle catalysts.?4?The sulfur resistance of the catalyst has always received much attention from catalysis researchers.The obtained noble metal nanoparticle catalysts were pretreated at350 ^oC for 2h in the presence of 4000 ppm SO₂.The particle size change of the pretreated bimetallic nanoparticles was much smaller than that of the pretreated single metal nanoparticles.In addition,the surface composition of supported bimetallic nanoparticle catalyst had changed after pretreated by SO₂ and partial PdO was reduced to metallic Pd,the acidity of the catalyst also increased obviously.These changes have important influences on the activity of the catalyst.The activity evaluation results demonstrate that supported bimetallic catalyst has good sulfur resistance.Moreover,SO₂ pretreatment is beneficial to the improved activity of the catalyst.The T₁₀,T₅₀ and T₉₀ for SO₂-Au₂-Pd₂/Al₂O₃ were 300,366 and 412 ^oC,respectively,and its S^m_CO2 was99.1%.?5?In order to improve the stability of Au nanoparticle catalyst and further reduce the cost of bimetallic noble catalyst,Au-M?M:Cu?Co?Ni and Sn?nanoparticles supported on the surface of 3DOM Al₂O₃ were prepared by the method of micropores-diffused reduction.The supported bimetallic catalysts exhibit higher catalytic activity compared with supported single metal nanoparticle catalysts.The catalytic activities follow the order:Au-Co/3DOM Al₂O₃>Au-Cu/3DOM Al₂O₃>Au-Ni/3DOM Al₂O₃>Au-Sn/3DOM Al₂O_3.When the ratio of Au to Co was 1:3,the catalyst showed the highest catalytic activity.Its T₁₀,T₅₀,T₉₀ and S^m_CO2 were 323 ^oC,379 ^oC,414 ^oC and98.8%,respectively.The core-shell structural Au@Cu/3DOM Al₂O₃ catalysts were prepared using ammonia borane as reducing agent and they showed higher activities than those of alloy Au-Cu/3DOM Al₂O₃ catalysts.