Performance And Reaction Mechanism Of Methanol Oxidation Over Ce-based Catalysts Prepared Using Template Method In Non-thermal Plasma

Volatile organic compounds（VOCs）are the precursors of secondary pollutants such as PM_2.5 and ground-level ozone.In order to improve the quality of the atmospheric environment,VOCs pollution control is imperative.Non-thermal plasma technology has unique advantage of rapid oxidation of VOCs at room temperature and atmospheric pressure.However,this technology has shortcomings such as high energy consumption,low reaction selectivity,and by-product O₃ emissions.The key to improve system performance is to develop high-efficiency catalysts.Manganese oxide and cerium oxide are widely applied in plasma catalysis,and exhibit significant differences in VOCs conversion and reaction selectivity.Exploring the effects and their internal factors of the two catalysts in the plasma will help to understand the mechanism of plasma catalysis and guide the selection and design of catalysts with high activity.In this paper,α-MnO₂ and CeO₂ nanorod catalysts were prepared by hydrothermal method to investigate the performance in the plasma-catalytic oxidation of methanol and the nature of their effects.Subsequently,the Ce-based catalysts was prepared by template method,and the structure and surface state of CeO₂ catalyst were regulated by adjusting calcination temperature to explore its structure-activity relationship in plasma catalysis.The reaction mechanism of CeO₂ synergistic plasma oxidation of methanol were discussed by designing methanol-TPD and in-situ DRIFTS experiments.The main contents and conclusions are as follows:（1）Theα-MnO₂ and CeO₂ nanorod were synthesized by hydrothermal method and applied to the plasma-catalytic oxidation of methanol.At the same input power,CeO₂ demonstrated higher methanol conversion and CO₂ selectivity thanα-MnO₂.The introduction of CeO₂ hardly affected the plasma discharge characteristics,and the improvement of its catalytic activity was main ascribed to the effect of surface catalytic reaction.By characterizing the catalysts with temperature-programmed desorption of oxygen（O₂-TPD）and X-ray photoelectron spectroscopy（XPS）,and performing catalytic methanol oxidation,O₃ decomposition and methanol ozonation experiments at ambient temperature,we revealed that CeO₂,as compared toα-MnO₂,possessed more surface oxygen species that are highly beneficial for activating methanol molecules.Besides,CeO₂ might utilize more effectively the active oxygen species generated by ozone decomposition for complete methanol degradation.In-situ DRIFTS was performed to track the formation of intermediates in catalytic methanol oxidation by ozone over the two catalysts,and suggests that,while a large amount of carbonate by-products accumulate on the surface ofα-MnO₂,CeO₂ generated less surface by-products.（2）Ce-based catalysts（CeO₂-250,CeO₂-300,CeO₂-400 and CeO₂-500）were prepared by template method using metal organic framework materials as precursors and controlling the calcination temperature,and characterized.Results showed that all the catalysts formed CeO₂cubic fluorite crystal structure,the organic ligands in the metal-organic framework material were basically completely decomposed when the calcination temperature exceeds 300°C.CeO₂-300 maintained a complete nanorod structure with the largest specific surface area and the highest surface oxygen vacancies content.Plasma catalytic oxidation of methanol experiments demonstrated that CeO₂-300 exhibited the highest methanol conversion（100.0%）and CO₂ selectivity（90.1%）at an energy density of 464 J/L.The activity of each catalyst corresponded to its surface oxygen vacancies concentration.（3）Methanol-TPD and O₃ decomposition experiments were implemented to explore the effects of methanol adsorption and activation and O₃ decomposition ability of the above CeO₂catalyst on the catalytic performance.It was found that the more exposed external surface of the catalyst,the greater the number of active sites,which were more conducive to adsorb and activate methanol and formed the active oxygen species produced by ozone decomposition on the catalyst surface,thereby facilitating the oxidation of methanol.In-situ DRIFTS experimentswas conducted to study the reaction pathways of the best performing CeO₂-300synergistic plasma oxidation of methanol.In the discharge zone,methanol was oxidized to intermediate products such as formic acid,and then further oxidized to CO,CO₂,and H₂O.On the surface of the catalyst,methanol was initially adsorbed to form the methoxy species,which reacted with the active oxygen species produced by O₃ decomposition and transformed into formate species,finally oxidized to CO₂ and H₂O.