臭氧是一种常见的空气污染物，空气中臭氧的去除近年来备受关注，其中催化分解法是一种广为研究的臭氧去除技术。本研究用颗粒活性炭原位还原KMnO4法制备了活性炭载锰氧化物（MnOx/AC）催化剂，用扫描电镜（SEM）、X射线能谱（EDS）、电感耦合等离子体原子发射光谱（ICP-AES）、氮气吸附-脱附实验、X射线光电子能谱（XPS）和拉曼（Raman）光谱对催化剂进行了表征，并测试其在常温、常压、高湿度条件下催化分解臭氧的性能。该法制备的MnOx/AC，其Mn元素主要分布于活性炭载体的表面，形成的MnOx为片厚小于10 nm的卷曲纳米薄片，广泛覆盖于载体表面，呈“地衣状”，具有较大比表面积。Mn的平均价态在+3价与+4价的之间，不经热处理的MnOx为水钠锰矿型层状锰氧化物；在N2气氛中300℃加热后，Mn的平均价态稍微降低，且转变为钡镁锰矿型锰氧化物。设计负载量和干燥方式对MnOx形貌形成有重要的影响，KMnO4溶液浓度和pH对MnOx形貌的影响很小。在常温、常压、相对湿度60%的条件下，活性炭原位还原KMnO4法制备的MnOx/AC具有良好的催化分解臭氧性能。其Mn负载量远低于以乙酸锰等为前驱体用浸渍法制备的MnOx/AC等其他催化剂，但相同反应条件下的臭氧去除率显著高于后者。催化剂表面的卷曲纳米薄片形貌为MnOx催化分解臭氧提供了大量反应位点，是其高活性的重要原因。原位还原法制备的MnOx/AC具有更高的抗湿能力和稳定性，但其催化分解臭氧性能仍受相对湿度影响。为进一步提高MnOx/AC的抗湿能力，用浸渍法在MnOx/AC表面负载Pd纳米颗粒，制备了低Pd负载量的Pd/MnOx/AC，并测试其分解臭氧性能。在N2中经300℃加热得到的Pd/MnOx/AC，其表面MnOx卷曲纳米薄片上负载了单分散的零价Pd纳米颗粒，平均粒径（均值±标准差）2.4±0.4 nm。上述样品经H2中150℃加热后，Pd纳米颗粒未发生团聚，但粒径稍微变大（2.6±0.5 nm）。在MnOx/AC表面负载Pd纳米颗粒后，催化剂分解臭氧性能没有提高；在H2中150℃还原后效果变差。XPS结果表明，零价Pd纳米颗粒在反应过程中不稳定，会被臭氧氧化而价态升高。
Ozone is a common air pollutant, which can cause adverse health effects at high concentrations. In recent years, the removal of gas-phase ozone has been extensively studied, with catalytic decomposition being a promising technique for effective ozone removal. In this work, activated carbon-supported manganese oxide (MnOx/AC) catalysts were synthesized by in situ reduction of potassium permanganate (KMnO4) with granular activated carbon (AC), without subsequent high temperature treatment. The as-synthesized catalysts were comprehensively characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), inductively coupled plasma-atomic emission spectrometry (ICP-AES), nitrogen adsorption-desorption, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy, and their activity for catalytic ozone decomposition was tested.The Mn element is mainly distributed near the surface of the MnOx/AC catalysts, forming lichen-like morphology, which is assembly of curled MnOx nanosheets less than 10 nm in thickness. Raman spectroscopy shows that the MnOx has birnessite-type layered structure, which provides large surface area, as indicated by nitrogen adsorption-desorption test. The average oxidation state of Mn is determined to be between +3 and +4 by XPS. Some of the MnOx/AC catalysts were heat-treated at 300℃ in nitrogen, and the MnOx phase changed to todorokite-type, with the average Mn oxidation state slightly reduced.Desinged Mn loading and drying method determine the amount of Mn ions available for the crystal growth of MnOx on the surface of AC, and are thus key factors influencing the formation of the lichen-like morphology. KMnO4 concentration and pH of the precursor solution have little effect on the morphology of the MnOx.The as-synthesized MnOx/AC catalysts exhibited high activity for catalytic ozone decomposition, at ambient temperature, pressure and 60% relative humidity (RH). Although the Mn loading (typically 0.04%-0.05%) is far lower than those of other catalysts, e.g. MnOx/AC prepared by impregnation with manganese acetate as Mn precursor, the ozone removal rate is significantly higher than the latter catalysts, under the same reaction conditions. The high surface area of the curled MnOx nanosheets provides large quantities of active sites for reaction and contributes to the higher activity. The as-synthesized MnOx/AC catalysts also exhibit higher stability, but are still negatively influenced by moisture (RH rather than partial pressure of water vapor). To improve the resistance to moisture, the as-synthesized MnOx/AC were loaded with palladium (Pd) nanoparticles (NPs) by impregnation method. After heat-treatment at 300℃ in nitrogen, metallic Pd (Pd0) NPs are formed on MnOx nanosheets. The Pd0 NPs are highly monodisperse, with diameters (mean±standard deviation) of 2.4±0.4 nm. After subsequent heat-treatment at 150℃ in hydrogen, the Pd0 NPs grow slightly larger, with diameters of 2.6±0.5 nm. However, the ozone removal activity and resistance to moisture of the catalysts are not improved by the loading of Pd0 NPs, which are unstable and oxidized by ozone during reaction, according to XPS.
