Preparation Of Activated Carbon-supported Manganese Oxides For Catalytic Decomposition Of Ozone

Ozone is a hazardous air pollutant even at the ppb level. Recently, researchers haveput great emphasis on the removal of gas phase ozone. Compared to other methods,catalytic decomposition is a promising technique for effective ozone removal due to itslow cost and requirement for no extra energy.Manganese oxide catalysts supported on activated carbon (AC, MnOx/AC) forozone decomposition were prepared by in situ reduction of the permanganate on thesurface of AC. To study the in situ growth of MnOx, the effects of Mn loading andhydrogen treatment on the structure, ozone decomposition activity of MnOxsupportedon activated carbon, the morphology, oxidation state, and crystal phase of the supportedmanganese oxide were characterized by scanning electron microscopy (SEM), X-rayphotoelectron spectroscopy (XPS), X-ray diffraction (XRD), electron spin resonance(ESR), Raman spectroscopy (Raman), temperature-programmed reduction (TPR),energy-dispersive X-ray spectroscopy (EDS) and inductively coupled plasma-atomicemission spectrometry (ICP-AES) and so on.The morphology, oxidation state, and crystal phase of the supported MnOxprepared at different loading time (10,20,40min,2h,3h,12h) were studied toillustrate the in situ growth of manganese oxide on the surface of AC. The loading ofmanganese oxides can be divided into two stages: deposition by reaction in the earlystage and deposition of aqueous MnO2in the latter stage. SEM results showed that themorphology of supported MnOxlayer changed from a porous lichen–like structure tostacked nanospheres when the loading time increased. The activity for thedecomposition of low concentration ozone at room temperature was related closely tothe morphology. The10min MnOx/AC showed the best performance, which was due toits porous lichen-like structure while the12h MnOx/AC with the thickest MnOxlayerhad the lowest activity due to its compact morphology.When the Mn loading was increased from0.44%to11%, the supported MnOxchanged from a porous lichen-like structure to stacked nanospheres and thicknessincreased from180nm to710nm The crystal phase changed from poorly crystallineβ-MnOOH to δ-MnO2with the oxidation state of Mn increasing from+2.9–+3.1to+3.7–+3.8. The activity for the ozone decomposition was closely dependent on themorphology and the loading amout of the supported MnOx. The1.1%MnOx/AC showed the best performance due to its porous lichen-like structure and relatively high Mnloading, while the11%MnOx/AC with the thickest MnOxlayer had the lowest activityowning to its compact morphology.After the catalysts with different manganese loadings were reduced with hydrogenat300℃, the morphology of0.44%and1.1%MnOx/AC did not show obvious changeand the ozone decomposition activity remained at70%~80%. However, themorphology of5.5%and11%MnOx/AC became denser and its ozone decompositionactivity increased from~50%to~90%.XPS analysis showed that the valence states of MnOxof MnOx/AC decreased from+4to+3, while the valence states of manganese oxide on the surface of MnOx-H/ACremained about+2–+3. As indicated by O1s spectrum, part of the absorbed H-O-Hand Mn-O-Mn of MnOx/AC turned into Mn-O-H during the ozone decompositionprocess, while the spectrum of MnOx-H/AC did not show significant change. Besides,the amount of H-O-H of MnOx/AC remained at20%-30%and that of MnOx-H/ACremained almost none, which could also affect the ozone decomposition activities.