低温等离子体场内复合不同晶型氧化锰催化降解甲苯性能和机理

作为典型的挥发性有机化合物,甲苯通常来源于建筑涂料、交通运输和各种工业生产过程,是PM2.5、臭氧和光化学烟雾的重要前驱体,对环境和人类健康造成巨大影响.近年来,低温等离子体技术因具有在常温常压下就能通过高能电子、活性氧物种和羟基等活性粒子有效降解挥发性有机物的优点而受到广泛关注.然而,高能耗和大量副产物的产生是等离子体技术工业化应用的巨大障碍.当前最有效的策略之一是将等离体技术与催化技术结合,从而加快反应速率,提高产品的选择性和能源利用率.在所应用的催化剂中,MnO2因具有较好的O3分解效率而成为最有潜力的催化剂之一.但是MnO2具有不同的晶型结构、隧道结构和形貌,这些均会显著影响MnO2的催化活性.本文通过一步水热法制备了α-,β-,γ-和δ-MnO2四种MnO2催化剂,并将其用于等离子体催化降解甲苯研究,在此基础上系统考察了等离子体催化降解性能和MnO2不同晶型之间的关系.结果表明,当能量密度为160 J/L时,等离子体单独降解甲苯去除效率为32.5%.引入催化剂能够显著提高甲苯的降解效率,其中α-MnO2效果最显著,甲苯降解效率能够提升至78.1%,β-,γ-和δ-MnO2能够相应提升至47.4%,66.1%和50.0%.采用X射线衍射、拉曼光谱、扫面电子显微镜、透射电子显微镜、比表面积-孔结构分析、氢气程序升温还原和X射线光电子能谱等手段研究了催化剂的理化特性.结果表明,隧道结构、催化剂在等离子体中的稳定性、Mn–O键能和催化剂表面吸附氧均在等离子体催化降解甲苯中发挥了重要作用.在此基础上,通过GC-MS分析降解产生的气相副产物推断甲苯在等离子体和等离子体催化体系中的降解机理.在等离子体催化体系中,通过Mn4+,Mn3+和Mn2+价态的变化,等离子体产生的O3,O2*和其他活性自由基会被吸附到催化剂表面,随后与催化剂吸附的甲苯或中间副产物发生氧化还原反应,将甲苯氧化为CO2等小分子物质.此外,MnO2作为分解O3最有效的催化剂,可以吸附O3并将其分解为O?或者与H2O生成?OH参与到反应中,从而提高甲苯的降解效率.

In-plasma catalytic degradation of toluene over different MnO2 polymorphs and study of reaction mechanism

α-, β-, γ- and δ-MnO2 catalysts were synthesized by a one-step hydrothermal method, and were utilized for the catalytic oxidation of toluene in a combined plasma-catalytic process. The relation-ship between catalytic performance and MnO2 crystal structures was investigated. It was noted that the toluene removal efficiency was 32.5% at the specific input energy of 160 J/L when non-thermal plasma was used alone. The α-MnO2 catalyst showed the best activity among the investigated cata-lysts, yielding a toluene conversion of 78.1% at the specific input energy of 160 J/L. For β-MnO2,γ-MnO2 and δ-MnO2, removal efficiencies of 47.4%, 66.1% and 50.0%, respectively, were achieved. By powder X-ray diffraction, Raman spectroscopy, transmission electron microscopy, scanning electron microscopy, Brunauer-Emmett-Teller, H2 temperature-programmed reduction and X-ray photoelectron spectroscopy analyses, it was concluded that the tunnel structure, the stability of the crystal in plasma, the Mn–O bond strength of MnO2 and the surface-chemisorbed oxygen species played important roles in the plasma-catalytic degradation of toluene. Additionally, the degradation routes of toluene in non-thermal plasma and in the plasma-catalytic process were also studied. It was concluded that the introduction of MnO2 catalysts enabled O3, O2, electrons and radical species in the gas to be adsorbed on the MnO2 surface via a facile interconversion among the Mn4+, Mn3+and Mn2+states. These four species could then be transported to the toluene or intermediate organic by-products, which greatly improved the toluene removal efficiency and decreased the final output of by-products.