Morphology Impact Of CeO₂ On Methanol Oxidation In Non-thermal Plasma

Non-thermal plasma（NTP）technology as a promising method for removal of diluted VOCs has received increasing attention because of its unique properties,such as quick response and easy operation at room temperature and atmospheric pressure.Nevertheless,high energy consumption,incomplete degradation and the low CO₂ selectivity have been observed in such a system.The rational design and fabrication of catalysts are a key issue in the plasma-catalytic system to achieve desired performance.Moreover,the reaction mechanism of VOCs oxidation in plasma-catalytic system is not clear,and thus lacks theoretical support for the practical application.Ceria has been widely investigated for the catalytic decomposition of VOCs,on account of its unique properties such as abundant oxygen vacancies.The effect of CeO₂morphology on efficient methanol oxidation was investigated in a plasma-catalytic system at room temperature.The surface structure and chemical properties of the catalysts will be determined using characterization methods.The enhanced performance of the catalysts in plasma will be assessed comprehensively.Then,the correlations of morphology-dependent chemical and structural factors of the catalysts with the corresponding catalytic performances in plasma will be established;Experiments were further carried out to analyze the sources and relative importance of surface active oxygen species over CeO₂ in the plasma;The mechanism of the plasma-CeO₂ will be promoted based on the results of in-situ Raman and in-situ Fourier-transform infrared（FTIR）,etc.The main contents and conclusions are as follows:（1）CeO₂ with different morphologies（i.e.rod,particle,and cube）were prepared by a hydrothermal method and characterized by the Brunauer-Emmett-Teller（BET）,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,Raman,oxygen temperature-programmed desorption（O₂-TPD）and X-ray photoelectron spectroscopy（XPS）.The CeO₂ rod catalyst had the most amount of oxygen vacancy.All the catalysts were tested for methanol（446 ppm）oxidation in plasma,and the results showed that CeO₂rod catalyst exhibited the highest methanol conversion（94.1%）,CO₂ selectivity（90.1%）（0.9 W）.There exists the direct relationship between the performance of the catalysts towards methanol decomposition and the amount of surface oxygen vacancy.（2）The presence of CeO₂ catalyst could hardly change the discharge characteristics in such a plasma-catalytic system.It can be inferred that the better performance of plasma-catalytic systems（compared to plasma alone）should be ascribed to the surface catalysis of CeO₂.Experiments（methanol-TPD,catalytic reaction in different background gas at room temperature,comparison of in-plasma catalysis（IPC）and post-plasma catalysis（PPC）,O₃ catalytic oxidation）were carried out to analyze the the sources and relative importance of surface active oxygen species over CeO₂ in the plasma.The results showed that two sources of surface active oxygen species were involved in surface catalysis.One was the surface oxygen species present in the catalyst itself,the other was the short-lived species and long-lived species in the plasma region.The oxygen species from O₃ decomposition played the most important role in improving catalytic performance,which could reach 90.1%of relative importance.（3）In-situ Raman and ozone catalytic oxidation of CO or methanol experiments were conducted to study the surface catalytic oxidation mechanism.Oxygen vacancy on CeO₂catalysts was the active site of ozone decomposition.The more oxygen vacancies in the CeO₂ rod catalyst showed better ozone decomposition performance and produced more reactive oxygen species,which could deeply oxidize methanol to CO₂,ultimately resulting in high performance in the plasma-catalytic system.（4）The major reaction pathways of methanol oxidation were discussed based on the in situ FTIR experiments.In the discharge zone,the intermediates produced by methanol oxidation were formaldehyde and formic acid.And the final product（CO）was mainly generated via gas-phase plasma degradation of methanol.On the surface of the catalyst,three methoxy species were initially formed,and then transformed into monodentate and bidentate formates prior to mineralization to CO₂ in the plasma-catalytic system.CO₂ was mainly produced by the deep oxidation of methanol by the catalyst.