Preparation of Spinel Cobalt Oxides and Their Applications to the Catalytic Oxidation of Dibromomethane

Author:Mei Jian

Supervisor:zuo nai qiang


Degree Year:2018





Volatile organic compounds(VOCs)as important precursors play an important role in the formation of composite atmospheric pollution.Catalytic oxidation can degrade VOCs to CO2,H2O and other less harmful compounds at a relatively lower temperature,which is recognized as a promising way for the abatement of VOCs.However,for brominated volatile organic compounds(Br-VOCs),the existing catalytic systems have the problems of low catalytic activity,low product selectivity and low resistance to Br-poisoning.Therefore,developing catalysts that can efficiently degrade Br-VOCs is a priority.As a cheap and environment-friendly transition metal oxide material,spinel cobalt oxides have high catalytic activity for the oxidation of dibromomethane(CH2Br2)as a model molecule of Br-VOCs.However,during the catalytic oxidation of CH2Br2 with spinel cobalt oxides,the spinel cobalt oxides generally have the problems of easy particle agglomeration,low selectivity to CO2 and low resistance to Br-poisoning.To solve these problems,a series of different types of spinel cobalt oxides is designed and synthesized,the catalysts for efficient degradation of CH2Br2 from the perspective of material design are developed,the catalytic performances of different materials for CH2Br2 oxidation are studied,and the mechanism for CH2Br2 oxidation over spinel cobalt oxides is also explored.The major conclusions are summarized as follows:1.P25 TiO2 supported Co3O4 is synthesized for CH2Br2 oxidation,and the effect of Co content on their catalytic activity is studied.The experimental results show that Co3O4/TiO2 has the highest activity when the Co content is 5wt%.The kinetic results show that the reaction order of CH2Br2 is pseudo first-order and the reaction order of O2 is approximately zero-order.The formate species and methoxy species are the main intermediate products generated on the surface of Co3O4/TiO2.The formate species are the precursors of CO and CO2,while the methoxy species are the precursors of CH3Br.2.To solve the problems of low activity and low resistance to Br poisoning of Co3O4/TiO2,Mn-promoted Co3O4/TiO2 is synthesized.The experimental results show that Mn-Co/TiO2 has the highest activity when the molar ratio of Mn/Co is 1.The addition of Mn can enhance the stability of Co3O4/TiO2,and Mn(1)-Co/TiO2 remains stable activity within 30 hours.The formate species are the main intermediate products generated on the surface of Mn-Co/TiO2,so the final products are COx,HBr and Br2.There is a redox cycle on the surface of Mn-Co/TiO2:Co2++Mn4+?Co3++Mn3+,the redox cycle reduces the electron transfer energy between Mn and Co,promoting the adsorption of CH2Br2 molecules.3.Co3O4/CeO2 with different CeO2 morphologies(rod,plate,cube)are synthesized for CH2Br2 oxidation,and the effect of CeO2 morphology on the catalytic performance is investigated.Co3O4/CeO2-rod has a rod-like morphology,Co3O4/CeO2-plate has a plate-like morphology,and Co3O4/CeO2-cube has a cube-like morphology.The experimental results show that CeO2 morphology has an obvious effect on CH2Br2 oxidation.Co3O4/CeO2-rod has a better catalytic activity than Co3O4/CeO2-plate,while Co3O4/CeO2-cube has the lowest activity.The{100}and{110}planes are mainly exposed on the surface of Co3O4/CeO2-rod,which promotes to form high content of Co3+,more surface-adsorbed oxygen and more oxygen vacancies.In addition,Co3O4 has a stronger interaction with CeO2-rod,which improves the redox ability.The formate species and methoxy species are the main intermediate products generated on the surface of Co3O4/CeO2,and the presence of Co3O4 does not change the reaction pathway of CH2Br2,but accelerates the degration of the intermediate products.4.A series of Ti-Co3O4 binary composite oxides is prepared for CH2Br2 oxidation,the effect of Ti on the structure and catalytic performance of Co3O4 is discussed.The experimental results show that Co4Ti1(Co/Ti=4)has the highest catalytic activity.Ti benefits to disperse Co3O4 particles and increases the specific surface areas.Ti can enter the spinel structure of Co3O4 to form Co-O-Ti solid solutions,distorting the spinel structure,and resulting in more defects.The doping of Ti can also effectively improve the surface acidity of the materials and promote the adsorption of CH2Br2 molecules.5.To solve the problems of low activity and low selectivity to CO2 of Co3O4,Co-Mn-Ti ternary composite oxides are synthesized.The experimental results show that Co-Mn-Ti has the highest catalytic activity.Ti and Mn can enter into the Co3O4 spinel structure,distorting the crystal structure to form Co-O-Mn-O-Ti solid solution.The addition of Mn can further improve the specific surface area,surface acidity and redox ability of the materials.6.Ordered mesoporous Co3O4 with special pore structures and conventional Co3O4 nanoparticles are synthesized by hard template method and co-precipitation method,respectively.These materials are tested for CH2Br2 oxidation,and the mechanism is also discussed.Co3O4-B has a disordered mesoporous pore structure,Co3O4-S has a two-dimensional ordered mesoporous pore structure,and Co3O4-K has a three-dimensional ordered mesoporous pore structure.The results show that the pore structure of Co3O4 has an obvious effect on CH2Br2 oxidation.Co3O4-K and Co3O4-S have better catalytic performance than Co3O4-B.The degradation products of CH2Br2 over Co3O4 are CO,CO2,HBr,Br2 and H2O,and no other brominated organic by-products,and Co3O4-K and Co3O4-S have higher selectivity to CO2 and Br2.Co3O4 shows good stability,and the CH2Br2 conversion remains unchanged within 30 hours.Formate species are the main intermediate products during the reaction,and the pore structure of Co3O4 does not change the reaction pathway of CH2Br2.The mechanism for CH2Br oxidation over Co3O4:the adsorption of CH2Br2 and the breakage of C-Br bonds on the surface of Co3O4,and the desorption of Br species and the deep oxidation of intermediate products.In this study,supported cobalt oxides(Co3O4/TiO2,Mn-Co/TiO2,Co3O4/CeO2),composite cobalt oxides(Ti-Co3O4,Co-Mn-Ti)and cobalt oxides with special pore structures(ordered mesoporous Co3O4)are prepared for CH2Br2 oxidation.It provides new ideas for design and preparation of spinel cobalt oxides,and lays theoretical basis of Br-VOCs control.