Synthesis Of Activated Carbon-supported Manganese-palladium Nanocatalysts For Catalytic Decomposition Of Ozone

Ozone is a common air pollutant, which can cause adverse health effects at highconcentrations. In recent years, the removal of gas-phase ozone has been extensivelystudied, with catalytic decomposition being a promising technique for effective ozoneremoval. In this work, activated carbon-supported manganese oxide （MnO_x/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 scanningelectron microscopy （SEM）, energy-dispersive X-ray spectroscopy （EDS）, inductively coupled plasma-atomic emission spectrometry （ICP-AES）, nitrogenadsorption-desorption, X-ray photoelectron spectroscopy （XPS）, and Ramanspectroscopy, and their activity for catalytic ozone decomposition was tested.The Mn element is mainly distributed near the surface of the MnO_x/AC catalysts,forming lichen-like morphology, which is assembly of curled MnO_xnanosheets lessthan10nm in thickness. Raman spectroscopy shows that the MnO_xhasbirnessite-type layered structure, which provides large surface area, as indicated bynitrogen adsorption-desorption test. The average oxidation state of Mn is determinedto be between+3and+4by XPS. Some of the MnO_x/AC catalysts were heat-treatedat300℃in nitrogen, and the MnO_xphase changed to todorokite-type, with theaverage Mn oxidation state slightly reduced.Desinged Mn loading and drying method determine the amount of Mn ionsavailable for the crystal growth of MnO_xon the surface of AC, and are thus keyfactors influencing the formation of the lichen-like morphology. KMnO₄concentration and pH of the precursor solution have little effect on the morphology ofthe MnO_x.The as-synthesized MnO_x/AC catalysts exhibited high activity for catalyticozone decomposition, at ambient temperature, pressure and60%relative humidity（RH）. Although the Mn loading （typically0.04%-0.05%） is far lower than those ofother catalysts, e.g. MnO_x/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 MnO_xnanosheets provides large quantities of active sites for reaction and contributes to thehigher activity. The as-synthesized MnO_x/AC catalysts also exhibit higher stability,but are still negatively influenced by moisture （RH rather than partial pressure ofwater vapor）.To improve the resistance to moisture, the as-synthesized MnO_x/AC were loadedwith palladium （Pd） nanoparticles （NPs） by impregnation method. Afterheat-treatment at300℃in nitrogen, metallic Pd （Pd0） NPs are formed on MnO_xnanosheets. The Pd0NPs are highly monodisperse, with diameters （mean±standarddeviation） of2.4±0.4nm. After subsequent heat-treatment at150℃in hydrogen, thePd0NPs grow slightly larger, with diameters of2.6±0.5nm. However, the ozoneremoval activity and resistance to moisture of the catalysts are not improved by theloading of Pd0NPs, which are unstable and oxidized by ozone during reaction,according to XPS.