Ozone Catalytic Oxidation Removal Of Benzene And Ammonia From Simulated Air Over AgMn/HZSM-5 Catalyst

Benzene (C6H6) and ammonia (NH3) are commonly-found air pollutants, which are posing a great threat to human health and environmental development. Therefore, it is of great importance and urgency to remove C6H6 and NH3 from polluted air. So far, catalytic oxidation has been regarded as the most promising method for C6H6 and NH3 removal among various techniques. Particularly, Ozone Catalytic Oxidation (OZCO), using O3 as oxidant, has been attracting extensive attention during recent years due to its low reaction temperature, high reaction rate and high removal efficiency.In this work, HZSM-5 (HZ) supported Ag-MnOx catalyst (AgMn/HZ), which was prepared via incipient wetness co-impregnation method, was used for OZCO removal of C6H6 and NH3 from simulated air for the first time, and was compared with HZ, Ag/HZ and Mn/HZ catalysts. The catalysts were characterized by BET, XPS, H2-TPR and UV-Vis DRS techniques. Besides, preliminary research on the cycled "adsorption-air electric discharge" plasma catalytic removal of C6H6 over the AgMn/HZ catalyst was also carried out in this work. The main results presented in this dissertation have been summarized as follows:1. During OZCO removal of C6H6, compared with HZ, Ag/HZ and Mn/HZ catalysts, AgMn/HZ catalyst possessed not only the highest C6H6 and O3 conversions but also the highest CO2 selectivity and lowest CO selectivity. Mn3+ is the main active site for O3 decomposition, and a positive correlation between O3 decomposition and Mn3+ content was observed over AgMn/HZ catalysts since Mn3+ is easy to transfer its electron to O3 for O3 decomposition. When the Mn loading was relatively low (< 2.4 wt.%), the initial activities of AgMn/HZ catalysts increased with increasing Mn loading; when the Mn loading was relatively high (> 2.4 wt.%), the initial activities of AgMn/HZ catalysts were kept almost constant. Besides, the deactivation rate of AgMn/HZ catalysts first reduced and then increased with increasing Mn loading, and when the Mn loading was 2.4 wt.%, AgMn/HZ catalyst showed the best stability, which could be attributed to the least amount of carbonaceous accumulation over the used AgMn/HZ catalyst with 2.4 wt.% of Mn, as indicated by the temperature programmed oxidation (TPO) results of the used catalysts. Then the effects of humidity on OZCO of C6H6 over the AgMn/HZ catalyst were investigated. Compared with dry gas, water vapor greatly enhanced the initial activity and stability of the AgMn/HZ catalyst, and when the absolute humidity was 0.1-0.2 vol.%, AgMn/HZ catalyst showed the best initial activity and stability. Additionally, raising reaction temperature appropriately could increase not only the activity and stability of the AgMn/HZ catalyst, but also its CO2 selectivity. When the initial concentration ratio of O3 to C6H6 was 9.5,?100% of C6H6 conversion could be achieved over the AgMn/HZ catalyst at 80? and a space velocity of ?283000 mL·h-1·g-1, and O3 conversion was kept above 90%; CO2 selectivity was?80%.2. In order to explore the reaction mechanism for OZCO of C6H6 over the AgMn/HZ catalyst, OZCO of pre-adsorbed C6H6 was performed at room temperature. As shown by the temperature programmed desorption (TPD) and In situ DRIFTs studies, C6H6 preferentially adsorbs on Ag site over the AgMn/HZ catalyst till Ag is near saturation, and then adsorbs on HZ support. The roles of Ag and MnOx in OZCO of pre-adsorbed C6H6 over the AgMn/HZ catalyst were then clarified based on the comparative studies of gaseous products over HZ, Mn/HZ, Ag/HZ and AgMn/HZ catalysts and the TPO & TPD profiles after OZCO of pre-adsorbed C6H6. Ag promotes the complete oxidation of adsorbed C6H6 to CO2, and MnOx speeds up O3 decomposition rate and OZCO rate of C6H6 adsorbed on Ag sites. Compared with dry gas, moisture is beneficial to the oxidation of pre-adsorbed C6H6 to COx (including CO2 and CO), and lowers the accumulation of the unreacted pre-adsorbed C6H6, and by-products on catalyst surface.3. During OZCO removal of NH3, compared with HZ, Ag/HZ and Mn/HZ catalysts, AgMn/HZ catalyst exhibited the highest activity, good stability and high N2 selectivity. When the initial concentration ratio of O3 to NH3 was 1.73,99%of NH3 conversion with ?94% of N2 selectivity was obtained over the AgMn/HZ catalyst at room temperature and a space velocity of ?150000 mL·h-1·g-1. Then the effects of reaction temperature on OZCO of NH3 over the AgMn/HZ catalyst were investigated. Finally, the possible pathway for OZCO of NH3 was proposed for the AgMn/HZ catalyst based on the comparative studies of OZCO of pre-adsorbed NH3 with OZCO of NH3 in gaseous products and surface species studied by the TPD and attenuated total reflectance (ATR) characterization of the used catalysts.4. Preliminary investigation on the cycled "adsorption-air electric discharge" plasma catalytic removal of C6H6 over the AgMn/HZ catalyst has also been conducted. Compared with HZ, Mn/HZ and Ag/HZ catalysts, AgMn/HZ catalyst possessed the highest breakthrough capacity of C6H6,, which is almost two times more than that of the unsupported HZ zeolite. Then the effects of discharge parameters on plasma oxidation of adsorbed C6H6 over the AgMn/HZ catalyst were discussed. During the cycled "adsorption-air electric discharge" process, under the conditions of ?20000 mL-h-1·g-1 of space velocity,6 W of input power,0.4 vol.% of absolute humidity and 25 min of discharge, the adsorbed C6H6 conversion increased fast with cycle number during the first three cycles, and nearly all adsorbed C6H6 could be oxidized into CO2 afterwards. Additionally, CO2 selectivity was kept around 100% during all the five cycles.