Study On Preparation And Catalytic Performance Of Manganese-based Catalyst For Catalytic Oxidation Of Styrene

In recent years,O₃ and PM_2.5 pollution problems have become increasingly prominent.As an important precursor of them,volatile organic compounds（VOCs）was listed as the key control contaminate.Thereinto,styrene（C₈H₈）has been widely used in daily production and life,but its foul odor,toxicity,carcinogenicity and atmospheric chemical activity make its harmless treatment imminent.As one of the most widely used technologies in actual industry,catalytic oxidation technology has the advantages of low energy consumption,high safety,low secondary pollution and so on.Among them,transition metal oxides（TMO）have become a research hotspot due to their merits such as cheap and easy to obtain,excellent anti-poisoning performance,and outstanding catalytic activity at low temperature.Due to excellent oxygen storage/release capacity,rich crystal phase structure and non-stoichiometric composition,manganese based catalysts were widely used in the catalytic oxidation of VOCs.The catalytic oxidation of VOCs by Cu-Mn composite oxides has been extensively studied,but there is little understanding of the adsorption and activation of VOCs over these compounds.In addition,previous researches focused on the catalytic degradation of VOCs by oxygen vacancies（OVs）in manganese oxides,while the role of metal vacancies was less studied.Secondly,it was found that low temperature H₂ treatment adjusted the electron distribution on the catalyst surface and even generated OVs,which is conducive to catalytic performance.This paper focused on the regulation of structural defects in manganese based catalysts by different preparation methods,metal doping,H₂ post-treatment,etc.,which was used in the study of low temperature catalytic degradation of C₈H₈.The concrete research content and results are as follows:（1）A series of amorphous Cu-Mn binary oxides with different Cu/Mn ratios were prepared by redox coprecipitation,and the catalytic degradation performance of these materials on C₈H₈ was investigated.The results showed that during co-precipitation,copper replaced K⁺and inserted lattice to introduce more structural defects in amorphous manganese oxides,thus affecting physicochemical properties of materials,including Mn-O bond coordination environment and strength,redox properties,oxygen species migration ability and activity.Among the four catalysts,Cu₁Mn₅O_x maintained high activity over a wide temperature range（50-275℃）,excellent stability（56 h）and water resistance（5%）.However,excessive copper doping led to phase separation of copper oxides,which weakened the interaction between copper and manganese,thereby reducing the activity of the catalyst.（2）In situ programmed temperature experiments and in situ Raman experiments revealed that adsorbed oxygen,O₂,lattice oxygen and copper species all played a positive role in the low temperature adsorption and activation of C₈H₈.In addition,in situ DRIFTS revealed the degradation route of C₈H₈.Firstly,C₈H₈ formed benzyl alcohol and formaldehyde by breaking the double bond of olefin,and the both were rapidly oxidized to benzoic acid and formate.At 150℃,formate totally decayed,whereas benzoic acid degraded at 190℃.After the ring was opened,maleic anhydride developed and then completely degraded into CO₂ and H₂O.（3）Manganese oxides containing manganese defects was prepared by solvothermal method and the catalytic degradation performance of C₈H₈ was studied.The results showed that manganese defects effectively enhanced the redox performance,oxygen migration ability,adsorption and activation of C₈H₈ and O₂,and thus showed excellent catalytic performance.In addition,the catalyst treated with H₂ at low temperature for 1 h effectively adjusted the electronic structure of the catalyst surface,so that electrons transfered from manganese to oxygen,and further improved the redox performance,the migration ability of oxygen species,the capacity of O₂ activation of the catalyst.Furthermore,V_Mn-100-1 had better activity at a wide temperature range（100-280℃）,stability（1900 min）and water resistance（15%）.However,excessive H₂treatment inhibited the redox performance and migration ability of oxygen species,and thus reduced the catalytic performance of the catalyst.（4）The synergistic effect of manganese vacancies and moderate low temperature H₂ post-treatment regulated the adsorption and activation of C₈H₈ and O₂,and weakened the inhibition effect of water vapor on catalytic activity with excessive O₂.Finally,in situ DRIFTS revealed the catalytic degradation mechanism of C₈H₈.Formate completely degraded at 175℃,while benzoic acid achieved ring opening in the whole process,forming maleic anhydride and monodentate carbonate,and finally completely degraded into CO₂ and H₂O.However,excessive H₂ treatment had obvious inhibition on the adsorption and activation of C₈H₈,resulting in the degradation temperature lag of intermediate products,and then affected the catalytic activity.