Research On Catalytic Oxidation Of Lean Benzene At Low Temperature Over Catalysts Assisted By Electric Field

Volatile Organic Compounds(VOCs)are considered to be the fourth largest pollutant in the atmosphere after particulate matter,sulfur dioxide and nitrogen oxide.The emission of VOCs has aroused widespread concern in society due to its wide range of hazards.Catalytic combustion is considered as a promising method for the elimination of volatile organic compounds,but high temperature and noble-metal content in catalsyts is demanded in lean VOCs oxidation process.In this paper,benzene is selected as the representative of lean VOCs gas,and a new technology combining electric field and traditional catalysis was applied for the oxidation of benzene.Catalysts with high activity and water resistance under electric field was synthesized and the physical and chemical properties of the catalyst in the electric field are explored by characterization technology,revealing the synergistic effect between the catalyst support and the electric field.The mechanism of catalytic oxidation of benzene was studied by the in-situ DRIFT technology,and the reaction paths of intermediate species in the process were explored.The contribution of the classical reaction model to the catalytic reaction of benzene was quantified by kinetic calculation.Finally,through the combination of carrier formulation improvement and the addition of intermediate coatings,a monolithic catalyst with the best overall performance under electric field was developed.The main results and conclusions in this work are summarized as follows:Catalysts with Pd supported on Ce_0.75Zr_0.25O₂ and Co₃O₄ were tested on the oxidation of benzene.It is found that the synergistic effect between Ce element in the support and the electric field contributes to the improvement of catalyst activity,and the change of Co element in the electric field affects the water resistance of the catalyst.Combining the characteristics of the two catalysts,Ce-Co was selected as the support and the synthesized Pd/Ce_mCo_nO₂catalysts exhibit both high activity and water resistance.The results show that when Ce/Co ratio in catalysts is 1:3,the complete oxidation of benzene can be achieved at a low temperature of 138?under the effect of electric field.Through catalyst characterization technology,it can be found that CeO₂ in the catalyst has a greater oxygen storage capacity than Co₃O₄,and easier to be reduced in the electric field,releasing more lattice oxygen that facilitates the formation of active oxygen and Pd with high valent on the catalyst surface,providing more active sites for low temperature benzene oxidation.Based on deep understanding of synergistic effect between catalyst support and the electric field,in order to reduce the cost of the catalyst,y%Cu Ce_mZr_nO₂ series catalysts were developed for elimination of benzene.As a result,it was found that the prepared non-precious metal catalyst exhibits low activity,but the water resistance is high.Under electric field,the activity of the catalyst is significantly enhanced.With Ce/Zr ratio of 55:45 and Cu loading of4%,the complete oxidation of benzene can be achieved at 131?.Through characterization experiments,it can be found that Cu ions incorperated in the support lattice weakened the Ce-O bond and improve the oxygen mobility,while Cu_xO species dispersed on the catalyst surface promote the regeneration process of the lattice oxygen and inhibit the adsorption process of benzene on the catalyst surface.The adsorption capacity of the catalyst,mobility of lattice oxygen and regeneration capacity of oxygen impose joint influence of oxidation process of benzene on the catalyst surface.In traditional catalytic reactions,compared with adsorption capacity and oxygen regeneration capacity,the oxygen mobility in the catalyst is low,resulting in poor catalyst activity.The introduction of electric field promotes the release of lattice oxygen in the support,greatly improved the oxygen mobility,resulting in accelerated oxidation rate of benzene.In order to investigate the effect of electric field on catalytic reaction mechanism of benzene,the in-situ technology was applied to investigate the adsorption and oxidation process of benzene over Pd/Ce_mCo_nO₂ and Cu/Ce_mZr_nO₂ catalysts with and without electric field.It is considered that in the absence of electric field,the adsorption and oxidation of benzene on the surface of precious metal based catalysts occured through the L-H mechanism,indicating that benzene molecules and oxygen molecules will first adsorb together at the active sites and then react with each other,while benzene oxidation on the surface of non-precious metals follows the Mvk mechanism and benzene molecules reacts with oxygen species from the catalyst.Under electric field,the catalytic reaction of benzene on the surface of each catalyst at low temperature mainly obeys Mvk mechanism due to enhanced oxygen mobility in the catalyst.In addition,the synergistic effect between Co element and the electric field effectively promotes the water resistance of the precious metal catalyst.Through kinetic calculations,the contribution of different reaction mechanisms to the catalytic reaction of methane was quantified,and the conclusions obtained in the in-situ experiment were further verified.Based on above fundamental research works,the monolithic catalyst suitable for the electric field assisted catalytic system was developed.Through coating amount increasement,support formula optimization and addition of intermediate coating,the synergetic effect between the electric field and monolithic catalyst was enhanced.Based on comprehensively evaluation of activity,stability and adaptability of conditions,monolithic catalyst with best comprehensive performance was obtained,which meets the needs of practical applications.