The Preparation Of MnO₂Materials With Various Morphologies And Their Catalytic Activities For Deep Catalytic Oxidation Of O-xylene

Volatile organic compounds (VOCs) such are harmful to the environment. Completecatalytic oxidation techniques that convert an organic compound contaminant into CO2andH2O are an effective pathway to remove VOCs. The most important part of catalyticcombustion is catalyst.In this paper, a series of MnO2with different morphology have been prepared foro-xylene catalytic combusion using KMnO4as oxidant. The methods such as XRD (X-raydiffraction), BET (Brunauer-Emmett-Teller), SEM (Scanning electron microscope), TG/DTA(Thermogravimetric and differential thermal analyses), H2-TPR (Temperature programmedreduction), XPS (X-ray photoelectron spectroscopy) and HETEM (High resolutiontransmission electron microscopy) were applied to research the microstructure of the catalysts.The main reslts are as follows:(1)A series of α-MnO2were successfully synthesized using KMnO4as oxidant, aldehyde(RCHO=acetaldehyde, propionaldehyde, n-butyraldehyde, isobutyraldehyde andbenzaldehyde) as reducing agent (the prouducts donated as MnO2-XY, X represents reducingagent, Y represents the calcination temperature, the unit℃) at room temperature.MnO2-propionaldehyde400, MnO2-n-butyraldehyde aldehyde450, MnO2-isobutyraldehyde500and MnO2-benzaldehyde500have similar morphology with tetragonal structure. BETdata show that these catalyst materials are mesoporous material, o-xylene catalytic resultdisplayed that these catalyst materials have relatively good catalytic activity. The effects suchas instillment reagent order and reaction conditions on the structure and activity of thecatalysts were investigated at the potassium permanganate/propionaldehyde reactant system.We found that only in reaction conditions was propionaldehyde added dropwise to a solutionof potassium permanganate, we can obtain tetragonal structure morphology α-MnO2; only inthe reaction conditions of room temperature, in order to obtain tetragonal structuremorphology α-MnO2.（2）For the (1) catalytic activity of MnO2-benzaldehyde400is relative worst. Followedby calcination temperature increase, MnO2-benzaldehyde500is appeared tetragonal structure,and its temperature of o-xylene complete oxidation decreased from250℃to200℃. TG/DTAdata showing MnO2-benzaldehyde500contains more oxygen vacancies. XPS data confirmedthat MnO2-benzaldehyde500contained almost100%Mn4+ion on their surface and presentedgood mobility of lattice oxygen. H2-TPR data results, MnO2-benzaldehyde500has excellent restore performance. HRTEM test results show that the [200] plane is exposed at thetetragonal structure of MnO2-benzaldehyde500.（3）Mesoporous MnO2were successfully synthesized by hydrothermal method basedon the redox reactions of KMnO4and H2C2O4·2H2O(the prouducts donated as MnO2-X-Y-Z,X represents the hydrothermal reaction time, in units of h; Y represents the reaction molarratio; Z represents the hydrothermal reaction temperature in℃).By controlling the differenthydrothermal reaction time, products have appeared transformation in the morphology andcrystalline. In the hydrothermal reaction time is less than or equal to2h, these initial productsare α-MnO2of microcrystalline micron spherical. The hydrothermal reaction time is greaterthan equal to4h, products are nanowires MnO(OH), MnO(OH) is transformed nanowiresβ-MnO2after calcination at400℃.BET results showed these products are mesoporousmaterial. When the molar ratio of the reactants and the reaction temperature is changed, thecrystalline of products were changed. In this reaction system, the change of crystallineproduct is due to CO which is generated by the decomposition of oxalic acid. Mn4+is reducedto Mn3+by CO, the product of the α-MnO2is converted into MnO(OH), β-MnO2is theproduct which MnO(OH) after calcination. The catalytic activity test results show that micronspherical α-MnO2product MnO2-2-1.8-180could converse o-xylene into carbon dioxide andwater at210oC.This dissertation was supported by the National Natural Science Founadation of China(No.21147004), the National Natural Science Founadation of Hebei (No. B2013205100) andthe Science Foundation of Hebei Normal University (No. L2010Z06).