Preparation Of Spinel Cobalt Oxides And Their Applications To The Catalytic Oxidation Of Dibromomethane

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 CO_x,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.