Performance And Mechanism Of Toluene Degradation By Metal Oxides/SBA-15-assisted Non-thermal Plasma

Volatile organic compounds (VOCs) are one of the major contributors to the atmosphericpollution, how to reduce it has become the urgent problem to be solved. Recently,Non-thermal plasma (NTP)-assisted catalysis technology, which combines the advantage ofreaction at room temperature and atmospheric pressure for NTP and high selectivity ofcatalysis, has emerged as a promising method for decomposition of VOCs. In this system, thereaction probability of high energy active species with VOCs can be increased due torelatively stronger adsorption of the catalyst, consequently inhancing the VOCs removalefficiency. Furthermore, the gas phase oxygen plays an important role for the decompositionof VOCs as an oxidizing agent, and the bulk phase oxygen in metal oxide catalyst is alsoinvolved in the process of VOCs oxidation. Additionally, the exchanging and transferringprocess between gas phase oxygen and bulk phase oxygen on the surface of catalyst arecritical to reveal the oxidation mechanisms of toluene in plasma condition.In this dissertation, three series of catalysts (FeOx/SBA-15, MnxOy/SBA-15, andFeOx-AgOx/SBA-15) were synthesized successfully by wet impregnation method, and theoxidation performance of the toluene was investigated respectively in NTP. In order toexamine the contribution of the gas phase oxygen and bulk oxygen to the toluene oxidation,and the process of exchanging and transferring of them on the catalyst surface, someexperiments were carried out on identifying the intermediates and comparing the difference offresh and used catalyst. Addition, the competitive adsorption process between toluene andoxygen on the catalyst, and the influence of this competitive adsorption to the tolueneremoval in NTP-assisted catalysis condition were studied by the adsorption breakthroughcurves, and the O2-TPD，H2-TPR，XPS，in situ FT-IR of the catalyst adsorbed toluene in thedifferent O2contents. In addition, the role of O3on the toluene removal was investigatedthrough changing the catalyst position in NTP reactor and the toluene concentration.Some conclusions were drawn as follows:1FeOx/SBA-15or MnxOy/SBA-15catalysts increased the removal efficiency of tolueneand COxselectivity significant in the different O2contents, and reduced the organicintermediates. Among the evaluated catalysts, the catalysts with3%Fe and5%Mn loadingexhibited the best toluene removal performance, which could be ascribed to the highdispersion and reducibility of active components. Meanwhile, the bulk oxygen in FeOxorMnxOywas involved in the toluene oxidation in N2plasma. During the toluene oxidation, the catalyst suffered slight deactivation with longer reaction time. However, the2D-hexagonalstructure of SBA-15was not destroyed, and the active compositions were not lost.2Both SBA-15and active components played important role in the process of toluenedecomposition, and the role was different depend on different valencies of active components.For the FeOx/SBA-15catalysts, Fe2+were oxidized firstly by O2to Fe3+, and then theFe3+oxidized the toluene and organic intermediates which adsorbed on the catalyst surface.However, for MnxOy/SBA-15catalysts, the active oxygen were transferred from Mn2O3toMn3O4, then oxidized the toluene over the catalyst surface, subsequently, gas phase O2filledup the vacancies created on the Mn2O3, to maintain the oxidation reaction continued.3The toluene and gas phase O2existed the phenomena of competitive adsorption over theFeOx, but the competitive adsorption did not impact the toluene adsorption state and sitesseriously over the catalyst. The results also demonstrated that the oxygen adsorbed on thecatalyst surface could provide oxygen source and promote the toluene oxidation.4The results of toluene removal by the double metal oxide catalyst (FeOx-AgOx/SBA-15)in plasma (frequency:2k Hz) suggested that the organic intermediates were greatly reduced,and the toluene removal efficiency, CO2selectivity, COxselectivity were greatly increased byFeOx-AgOx/SBA-15. Compared to FeOx/SBA-15, FeOx-AgOxcatalyst had the higher CO2selectivity and better catalyst stability in the plasma condition, which probably due to thesmaller particle sizes, higher dispersion, and higher reducibility of FeOxafter theincorporation of Ag. In addition, for the FeOx-AgOx/SBA-15catalyst, Ag active sites couldalso adsorb toluene, increasing the toluene concentration on the catalyst surface, andimproving the ramification formation of interacting hydroxyl groups. The results alsoindicated that FeOx/SBA-15and FeOx–AgOx/SBA-15catalysts could enhance utilizationefficiency of the active oxygen species by toluene in NTP, but the formed O3had limitedcontribution to toluene oxidation reaction.