Microstructure Regulation Of Manganese Based Catalysts For Toluene Catalytic Degradation Performance

Volatile organic compounds?VOCs?are one of the main pollutants that cause photochemical smog,low-altitude ozone,and haze to damage the ecological environment and endanger human health.At present,the method of catalytic oxidation to eliminate VOCs has attracted wide attention because of its lower reaction temperature,higher removal efficiency and lower secondary pollutant emissions.In order to achieve the efficient removal of VOCs,the design and control of the catalyst is the key to this process.Noble metal-based catalysts have a high catalytic activity in the catalytic oxidation of VOCs.However,the economic benefits of this catalyst need to be improved,and they face the problems of poor stability and poor resistance in actual working conditions.The manganese oxide catalyst has the prospect of application in the catalytic oxidation of VOCs due to its low price,abundant reserves,and good redox and oxygen storage capacity.However,the current research is mainly focused on the optimization of preparation methods,multimetal composites,and the control of morphology and structure.The structure-activity relationship between the surface of manganese oxide catalyst and the catalytic oxidation performance of VOCs,and the degradation mechanism of VOCs are still lacking.In this thesis,the design of MnO₂catalyst is based on the structure design such as the modulating of K ions,the control of surface Mn coordination number and the strategy of stabilizing oxygen vacancies via Cu loading,and the structure-activity relationship between the catalyst and VOCs catalytic oxidation and the corresponding toluene degradation mechanism are studied.The main contents of this article are:?1?By regulating the K ions,the performance of MnO₂ catalysts in the catalytic oxidation of toluene in the presence of excess Mn and K ions are explored.Among them,when the molar ratio of KMnO₄ to?NH₄?₂C₂O₄ is 5:1,the obtained MnO₂ catalyst has a sheet-rod structure intersecting interface.The catalyst possess low Mn average valence state and rich surface oxygen vacancies.Moreover,the excessive K ions are precipitated on the surface,which can further increase the surface oxygen vacancy content and activity.In addition,because the catalyst contains abundant active sites,toluene is more easily oxidized to benzene anions on its surface,which reduces the coverage of the active sites by the intermediate products,thereby improving the stability of the catalyst.?2?Through the adjustment of the synthesis method,it is found that the difference in MnO₂ performance of different exposed crystal planes is due to the different coordination number of Mn on the surface.Among them,MnO₂ containing more 4-coordinates has the best activity and stability,because 4-coordinated MnO₂ has more active sites and more oxygen vacancies.At the same time,it has the best adsorption performance for toluene,which can be oxidized to benzoic acid and other species by lattice oxygen at room temperature.Through in-situ DRIFTS and gas chromatography-mass spectrometry,the mechanism of toluene degradation can be explored,that is,toluene is gradually oxidized to benzyl alcohol,benzaldehyde,and benzoic acid,followed by decarboxylation to produce benzene species,and then gradually oxidized to benzoquinone,phenol,acetic acid,etc.The final products are carbon dioxide and water.?3?The MnO₂ catalyst was treated by H₂ reduction,and it was found that the performance of the MnO₂ catalyst without CuO was reduced due to the lack of surface-active oxygen after reduction.However,while the MnO₂ catalyst loaded with a small amount of CuO and reduced at 200?,the elemental Cu emerged while the structure of MnO₂ remains unchanged,the performance of the catalyst is significantly improved,and has good water resistance and stability.The study found that due to the interaction between Cu and the carrier MnO₂,the active oxygen at the MnO₂ interface can be stabilized after reduction treatment,and more oxygen vacancies can be created due to the active Cu-Mn interface.The catalyst shows good catalytic performance only under oxygen-rich conditions.In the absence of gas-phase oxygen,it can only adsorb more toluene rather than oxidize the toluene into benzyl acid because the lack of active lattice oxygen.