Preparation Of Mn-based Catalyst Enhanced By High Gravity For Catalytic Ozonation Of Nitrobenzene Wastewater

Heterogeneous catalytic ozonation technology utilizes solid catalysts to decompose ozone into hydroxyl radical(·OH)with high oxidation potential to accelerate the degradation of refractory organic pollutants in wastewater.This technology has the advantages of high mineralization efficiency,no secondary pollution,and easy recovery of catalysts,and has become the preferred technology in the field of refractory organic wastewater treatment.Using supported metal oxides as heterogeneous catalysts can effectively improve the catalytic ozonation efficiency of organic pollutants.Researches have shown that the catalytic ozonation activity of?-Al₂O₃-supported manganese oxides on aromatic compounds are greatly enhanced with the addition of transition metals(Cu,Fe)and rare earth metals(Ce)which have unsaturated electrons in 3 d and 4 f orbitals.For heterogeneous catalytic ozonation system,the decomposition and chemical reaction of ozone are interfacial reactions,which are related to the dispersion of active components on the surface of support as well as the number of active sites such as Lewis acid sites,surface hydroxyl groups and oxygen vacancies.How to improve the dispersity of the active components on the surface of support to ensure more catalytic active sites and stability is the key to improve the catalytic performance of the heterogeneous catalyst.Ozone mass transfer is affected not only by the driving force or resistance,but also by the compatibility of catalyst and reactor.However,the conventional gas-liquid contact reactor has low ozone utilization rate and·OH yield due to the low solubility of ozone in water.How to strengthen the mass transfer and decomposition of ozone become the key to improve the utilization rate of ozone and to reduce the production cost.Based on the characteristics of enhanced inter-phase mass transfer and reaction by high gravity,?-Al₂O₃ supported manganese-copper/cerium oxide catalysts were prepared by high gravity-assisted impregnation method with the rotating packed bed(RPB)to improve the surface dispersity.In addition,the heterogeneous catalytic ozonation and RPB were coupled for the degradation of nitrobenzene containing wastewater to further improve the ozone utilization efficiency and·OH radical yield,reduce the production cost and realize the sustainable development of heterogeneous catalytic ozonation.Combined with catalyst characterization and DFT calculation,the relationship between catalyst surface composition,structure properties and catalytic performance was revealed,and the reaction mechanism of heterogeneous catalytic ozonation was elucidated.The main research contents and results are as follows:(1)The Cu-MnO_X/?-Al₂O₃ was prepared by high gravity-assisted impregnation method.RPB accelerated the adsorption equilibrium and promoted the uniform dispersion of the active components on the surface of the support by strengthening the micro-mixing and liquid-solid mass transfer.The Cu-MnO_X/?-Al₂O₃(H)catalyst prepared by RPB in 45 min had abundant surface hydroxyl groups,oxygen vacancies and high surface content of Mn⁴⁺and Cu⁰,which had comparable catalytic activity for nitrobenzene mineralization to that of catalyst prepared by the stirred tank reactor in 6 hours.(2)The RPB coupled with Cu-MnO_X/?-Al₂O₃ catalytic ozonation was used to improve the utilization efficiency of ozone and the degradation efficiency of nitrobenzene.Compared with Glass bead/O₃/RPB system,Cu-MnO_X/?-Al₂O₃/O₃/RPB system increased the?_TOC from 60.3%to 81.7%,and R_u from 62.6%to 82.2%by adjusting?,C_O3,Q_L/Q_G and p H to weak acidity,indicating a synergistic effect between RPB and heterogeneous catalytic ozonation.(3)The Ce-MnO_X/?-Al₂O₃ was prepared by high gravity-assisted impregnation method for catalytic ozonation of nitrobenzene,and a three-level and seven-factor orthogonal test was used to optimize the preparation parameters.The catalyst prepared with 0.24 M solution at the molar ratio Ce:Mn of 3:7,rotational speed N_R of 1000 r/min,liquid flow rate Q_L of 80 L/h,and calcination temperature T of 500?showed the best catalytic activity.The oxygen vacancies,redox couples of Ce(?)/Ce(?)and Mn(?)/Mn(?)and synergistic promotion of Ce and Mn promoted the adsorption and dissociation of electron-deficient ozone molecules into*O₂ and*O,and promoted the formation of·O₂^-and·OH.(4)A quadratic response surface regression equation for catalytic ozonation of nitrobenzene by Ce-MnO_X/?-Al₂O₃/O₃/RPB was established by response surface analysis.The influence order of each factor on?_NB was C_O3>Q_L>p H>?,and there was a significant interaction between?and C_O3.Under the conditions of C_O3=64 mg/L,Q_L=84 L/h,p H=7.50and?=27.6,the?_NB can reach 100%.The direct reaction kinetics and indirect reaction kinetics of nitrobenzene by RPB coupled heterogeneous catalytic ozonation were studied by solute consumption and competitive kinetics method.The heterogeneous catalytic ozonation kinetics model of nitrobenzene was established.Ce-MnO_X/?-Al₂O₃/O₃/RPB system was suitable for the degradation of nitrobenzene in a wide range of p H,while the Cu-MnO_X/?-Al₂O₃/O₃/RPB system was suitable for weak acid environment.For Ce-MnO_X/?-Al₂O₃/O₃/RPB system,k _O3,NB=0.384?0.973 L/(mol·s),and k_·OH,NB=2.63×10⁹?3.78×10⁹ L/(mol·s).The coupling of RPB with heterogeneous reaction system greatly promoted both the indirect and direct reaction of nitrobenzene with O₃.(5)Based on first-principles density functional theory(DFT),the active sites and catalytic mechanism of?-Al₂O₃ supported manganese oxide catalyzed ozone decomposition were revealed.For Cu-MnOx/?-Al₂O₃(110),the loading of Cu O and MnO₂ increased the Lewis acidity of the tri-coordination Al49 and facilitated the formation of oxygen vacancy on?-Al₂O₃(110)surface,which promoted the adsorption and dissociation of ozone at the Lewis acid site and oxygen vacancy into surface atomic oxygen*O and O₂.O₃ formed hydrogen bond with the surface hydroxyl group of Cu-MnOx/?-Al₂O₃(110),which was transformed into HO₃·by electron transfer.For Ce-MnO_X/?-Al₂O₃(110),ozone was decomposed into surface atomic oxygen*O and O₂at the Lewis acid sites of Al49 and Ce³⁺,and the absolute adsorption energy was higher than that of Cu-MnO_X/?-Al₂O₃(110).The presence of Ce O₂ significantly reduced the oxygen vacancy formation energy of?-Al₂O₃(110)surface(E_OV?0.02 e V),which was also lower than that of Cu-MnO_X/?-Al₂O₃(110).Moreover,the synergy between Mn and Ce atoms accelerated the electron transfer,which promoted the adsorption and dissociation of O₃ into*O₂and*O on the oxygen vacancy with the lower reaction energy barrier.