Studies On Preparation And Catalytic Properties Of Cobalt-and Cerium-based Catalysts For Catalytic Combustion Of Toluene

Nowadays,the severe air pollution caused serious impact on human health and the environment,which has aroused great social concern.Volatile organic compounds（VOCs）are a kind of air pollutant that give rise to photochemical pollution and ozone and other severe environmental problems.Catalytic combustion is considered to be one of the most efficient methods to remove VOCs because of its high removal efficiency,low energy consumption,and no secondary pollution.The key for improving catalytic combustion technology is the development of high efficient catalysts.Co₃O₄,a typical spinel oxides,exhibits excellent catalytic performance in oxidation of CO,CH₄ and VOCs due to its relatively weak Co-O bond.Ce O₂ possesses abundant Ce³⁺/Ce⁴⁺pairs,and the presence of large amount of Ce³⁺increases the inherent oxygen vacancies,which greatly improve the oxygen storage capacity and internal lattice oxygen mobility in ceria.Hence,Ce O₂ has been extensively employed as active component and additive in the catalytic combustion of VOCs.Additionally,it is reported that Ce O₂ with different morphologies have obvious differences in specific area,partic le size,pore distribution and other physicochemical properties,which in turn affects their catalytic performance.Therefore,the synthesis of Ce O₂ with various morphology as well as composite oxides via introduction of a second active metal,has become important methods to improve the catalytic performance of cerium-based catalysts.In this work,Ce O₂-Co O_x aerogels with different composition were synthesized via sol-gel-supercritical drying method and evaluated in the catalytic combustion of toluene.Various characterizations were carried out to reveal the physicochemical properties and catalytic performance of Ce O₂-Co O_x aerogels.The influence of Ce addition on the catalytic performance of cobalt-based aerogels was discussed.In addition,Ce O₂nanorod,Ce O₂ nanoflake and Ce O₂ aerogel were synthes ized using different methods and applied for toluene combustion.The effect of surface morphology on the catalytic performance of Ce O₂ was discussed.The experimental results of this paper were listed as follows:（1）Co O_x and Ce O₂-Co O_x aerogels with different Ce/Co ratios were prepared by sol-gel-supercritical drying method.Experimental results revealed that the addition of Ce had a great influence on the microstructure and catalytic performance of cobalt-based aerogels.The sequence of catalytic activities was Co O_x<Ce O₂-Co O_x-5<Ce O₂-Co O_x-50<Ce O₂-Co O_x-20<Ce O₂-Co O_x-10,and the complete conversion temperature(T₉₀)of toluene oxidation over Ce O₂-Co O_x-10was only 248°C.Moreover,the catalytic activity of Ce O₂-Co O_x-10 had no obvious change in the 36 h stability test.（2）The strong interaction between cobalt and cerium species in the Ce O₂-Co O_x resulted in a significant change in the physicochemical properties of the catalysts.The introduction of Ce into Co O_x inhibited the growth of aerogel particles,which was benefic ial to the formation of continuous porous structure and further improved the dispersion of surface-active species.In addition,the presence of Ce modified the redox properties and increas ing the surface Co³⁺/Co²⁺,relative amount of absorbed surface oxygen species as well as the oxygen mobility,which were recognized as major factors for the superior catalytic performance of Ce O₂-Co O_x-10.（3）Ce O₂ aerogel,Ce O₂ nanoflake and Ce O₂ nanorod were prepared via sol-gel,prec ipitation as well as hydrothermal methods,respectively,and evaluated for toluene oxidation.The experimental results showed that the catalytic performance of the catalysts followed the order:Ce O₂aerogel>Ce O₂ rod>Ce O₂ flake.The T₅₀ and T₉₀ values of Ce O₂ aerogel were 223 and 238°C,respectively,and the Ce O₂ aerogel maintained good catalytic activity during 50 h stability test.（4）Compared with Ce O₂ nanorod and Ce O₂ nanoflake catalysts,the Ce O₂ aerogel had a larger specific surface area,smaller partic le size and higher Ce³⁺/Ce⁴⁺,O_sur/O_latt and more oxygen vacancies.The presence of abundant surface oxygen vacancies contributed to the adsorption and dissociation of gaseous oxygen,and further improved its catalytic activity.