High-efficient Cobalt-based Catalysts For Soot Elimination

Diesel exhaust is one of the main sources of urban air pollution.It contains notonly a large amount of harmful gases such as nitrogen oxides and hydrocarbons,but also a large amount of particulate matter(PM2.5).The particulate matter in the exhaust of diesel engine is mainly the soot particles generated during the combustion of diesel.This kind of particulate matter will adsorb SO2 and other carcinogens in the air,which will cause great harm to the environment and human health.Diesel Particulate Filter(DPF)is currently the most efficient technique for removing soot particles,and the key to this technology is the development of high performance DPF regenerated catalysts.Cobalt trioxide has good redox properties and abundant surface oxygen vacancies,and is widely used in VOC oxidation,photocatalysis and other reactions.In this paper,the activity of the cobalt trioxide in the catalytic reaction of soot was studied by using cobalt trioxide single crystal oxides with different crystal faces and cobalt-cerium mixed oxides with three-dimensional ordered macroporous structure as catalysts.The main research contents are as follows:1.A series of single crystal Co3O4 catalysts with exposed different crystal facetswere synthesized by hydrothermal method using cobalt nitrate hexahydrate and sodium hydroxide as precursors.This method obtained tricobalt oxide with different exposed crystal faces by adjusting the proportion of precursors.The physicochemical properties of the catalysts were characterized by XRD,BET,FESEM,TEM,Raman spectra,FTIR,H2-TPR and O2-TPD,and the activity of the catalyst was evaluated by temperature programmed oxidation(TPO)method.The results showed that the morphology of three cobalt tetraoxide single crystals were nanocubes,nano-truncated-octahedrons and nano-octahedrons,wherein the exposed crystal face of the nanocube was(001),the crystal faces exposed by the truncated octahedron were(001)and(111),and the exposed facet of the nanooctahedron was(111).In the activity test,the order of activity of the three catalysts were:nanocube>truncated-octahedron>nanooctahedron.The superior activity of the nanocube catalyst was attributed to the fact it’s exposed(001)crystal plane had more surface oxygen vacancies and trivalent cobalt ions.The surface oxygen vacancies were beneficial to the adsorption and activation of oxygen to produced O2~-,O~-.And the active oxygen species had strong oxidizing properties,while the trivalent cobalt ion had a stronger oxidizing power than the divalent cobalt ion.At the same time,the results of the reaction kinetics study demonstrated that the difference in activity of the cobalt tetraoxide catalyst was related to its exposed crystal plane rather than the specific surface area.The results of different reaction atmospheres showed that both NO and water vapor had a certain promoting effect on the catalytic elimination of soot.Finally,the nanocube catalyst was found to have good stability in stability testing.2.A series of Co3O4-Ce O2 catalysts with three-dimensional ordered macroporeswere synthesized using the colloidal template method.The comparative catalysts with mesoporous structure were prepared by sol-gel method.The physicochemical properties of the catalysts were characterized by XRD,FESEM,TEM,BET,XPS,H2-TPR,O2-TPD,NO-TPD,NO2-TPD,NO-TPO and NO-TPSR,and the activity of the catalysts were evaluated by temperature programmed oxidation(TPO)method.By comparison,the Co50Ce50 catalyst has the best activity,its Tm was 406~oC,and the selectivity to CO2 was nearly close to 100%.The better activity of Co50Ce50 catalyst was attributed to the strong synergistic effects between Co3O4 and Ce O2.The synergistic effects were mainly realized by increasing the redox capacity of the catalyst,increasing the surface oxygen vacancies and the content of reactive oxygen species,enhancing the ability of the catalyst to adsorb and activate O2 and NO in the reaction atmosphere and improving the stability of the three-dimensional ordered structure.At the same time,the activity of the comparative catalyst Co50Ce50B(Tm:434~oC)was much worse than that of the three ordered macroporous structure Co50Ce50(Tm:406~oC),which indicated that the three-dimensional ordered macroporous structures have great promotion to soot oxidation.The regular three-dimensional ordered pore structure improved the mass transfer process of the soot in the catalysts,so that the soot particles can contact more active sites on the catalyst surface,thereby greatly improving the activity of the catalyst.In addition,we investigated the stability of the catalysts by repeating cycles of activity tests,the catalyst still maintained a regular three-dimensional order structure,which meaned that the catalyst with three-dimensional ordered structure had potential application value.