Study On Preparation Of La(Co,Cu)Modified Manganese Oxide And Its Catalytic Oxidation Of Benzene Series VOC

Benzene series VOC is a more common constituent of volatile organic compounds（VOCs）,which is harmful to human health,and it is also one of the important precursors for the formation of PM_2.5 and ozone.Therefore,the elimination of VOCs is critical to alleviate air pollution problems.Catalytic oxidation is considered as a superior technology in the field of VOCs elimination due to its high efficiency and low secondary pollution.Manganese is suitable as a catalyst active component for the elimination of benzene VOCs due to a variety of variable valence states and strong electron transfer ability.However,the poor activity stability of single manganese oxide and its easy deactivation under water conditions limit its practical application.How to further enhance its catalytic performance and stability is a key issue to be solved in the field of VOCs elimination.In this paper,manganese oxide catalysts were taken as the research object,and the surface structure and active sites of manganese oxide catalysts are regulated by the modification of lanthanum,cobalt and copper,so as to optimize the catalytic oxidation performance of benzene series VOC;the relationship between the structure and the catalytic oxidation performance is systematically investigated;the generation,consumption and replenishment process of the active oxygen species,the activation of oxygen during the reaction process and the reaction pathway of benzene series VOC are emphatically studied,and the reaction mechanism of manganese-based oxide with spinel structure catalyzed oxidation of benzene series VOCs is also explored.In order to improve the activity and activity stability of manganese oxide catalysts,a series of coral-like La_yMn O_x（y=0.02,0.05,0.08,0.1）spinel-type catalysts were prepared by modifying manganese oxides using the rare earth element lanthanum,so as to study the effect of lanthanum addition on the surface structure and catalytic oxidation efficiency.It is found that the moderate addition amount of lanthanum increased the specific surface area,pore volume and surface roughness,and the specific surface area of the La_0.08Mn O_x with 8%lanthanum content increased to 116 m²·g^-1.The addition of lanthanum weakens the Mn-O bond energy by forming La-O-Mn bonds on the surface,making it easier for oxygen species to desorb on the surface,which is conducive to improving the activity of toluene catalytic oxidation.The La_0.08Mn O_x showed better toluene catalytic activity with a T₉₀ of 217 ^oC,which is 36 ^oC lower compared with Mn O_x.The reaction pathway of toluene oxidation is deduced through in situ DRIFTS by analyzing the changes of intermediate species.Surface adsorbed oxygen species and surface lattice oxygen species are found to be the main reactive oxygen species involved in the toluene oxidation reaction,but their oxidizing abilities to intermediate species are different.In view of the shortcomings of surface lattice oxygen species in manganese oxide such as poor oxygen mobility and weak low-temperature oxidation ability,the mobility of surface lattice oxygen species is modulated by the substitution of cobalt,which is a transition metal with variable valence and similar ion radius with manganese.A series of hollow spherical cobalt-manganese spinel oxides(Co_aMn_3-aO_x a=0.5,1,1.5,2,3)were synthesized,and the effects of different Mn/Co ratios on the catalytic oxidation of benzene series were investigated.Studies have shown that the introduction of cobalt can cause the surface electron transfer,promote the activation of oxygen species and enhance the mobility of surface lattice oxygen species.The desorption temperatures of the surface lattice oxygen species on the Co Mn₂O_x are decreased by 110 and 146 ^oC compared with that of the pure Co₃O₄ and Mn O_x,respectively.The introduction of the appropriate amount of cobalt accelerates the mobility of the gas-phase oxygen and surface-adsorbed oxygen species,thus accelerating the catalytic reaction.Co Mn₂O_x exhibits the best catalytic oxidation activities of toluene(T₉₀=265 ^oC)and o-xylene(T₉₀=297 ^oC),as well as excellent recycling ability and water resistance.In addition,the consumption-migration-supplementation process of surface lattice oxygen species is designed,and the reaction mechanism and oxidation pathway of o-xylene on cobalt-manganese spinel oxides are also revealed.In order to improve the activation of gaseous oxygen and the replenishment ability of reactive oxygen species on manganese oxide,three Cu_1.5Mn_1.5O₄ spinel copper-manganese oxides with different degree defect were prepared by microwave heating,sol-gel and co-precipitation.The sheet Cu_1.5Mn_1.5O₄（CM-MW）prepared by microwave heating method exhibited the best activity of toluene(T₉₀=213 ^oC)and benzene(T₉₀=229 ^oC)oxidation,which is mainly attributed to its abundant defects and highly mobility of oxygen species.Compared with Cu_1.5Mn_1.5O₄（CM-SG and CM-CP）prepared by sol-gel method and co-precipitation method,the toluene complete oxidation temperature of CM-MW is reduced by 22 and 70 ^oC,respectively.It is also found that CM-MW exhibits excellent catalytic performance under cycling,high eight hourly space velocity,high temperature and long-term conditions.In order to investigate the effects of surface defect concentration and oxygen mobility on the catalytic oxidation of VOCs,the consumption-activation-replenishment process of oxygen species is proposed by designing in situ DRIFTS and oxygen pulse experiments.It is confirmed that more oxygen vacancies were conducive to the activation of gas-phase oxygen species into reactive oxygen species to participate in the oxidation of VOCs,and accelerated the generation-consumption-generation cycle of reactive oxygen species.In addition,the alone adsorption behaviors of toluene and benzene are confirmed by in situ DRIFTS and mass spectrometry,and the changes of the intermediate are also revealed when both are present.