The Preparation Of α-MnO₂ Materials With Tetragonal Morphology And Their Catalytic Activities For Catalytic Oxidation Of O-xylene

Atmospheric environment has attracted more and more attention. So The primary task of air pollution control is the governance and control of VOCs(Volatile Organic come Volatile Organic pollutants). Catalytic combustion is an effective way which can oxidize VOCs into carbon dioxide and water. The key of catalytic combustion is the preparation of catalysts with efficient catalytic oxidation performengce.In this paper, potassium permanganate and various alcohol were used as precursors to prepare α-MnO2 with different morphologies at room temperature. The catalytic activity of the as-prepared catalysts was evaluated by the catalytic combustion of the target pollutants o-xylene. The formation mechanism of α-MnO2 with tetragonal morphology, which had high catalytic performance was investigated. The Mn O2 catalysts were analyzed by using XRD(X-ray diffraction), BET(Brunauer-Emmett-Teller), H2-TPR(Temperature programmed reduction), SEM / EDX(Scanning electron microscopy/energy-dispersive X-ray technique),FT-IR(Fourier transmission infrared spectrum). Detailed contents are as follows:(1) α-MnO2 catalyst with various morphologies were successfully synthesized using KMn O4 and a series of alcohol(ethanol, propanol, n-butyl alcohol, 1-amyl alcohol, octyl alcohol, isopropyl alcohol, isobutanol, secondary butanol, isoamyl alcohol, benzyl alcohol,and cyclohexanol) to react at room temperature by changing the calcination temperature. The as-prepared catalyst were donated as Y-Mn O2-400℃、B-Mn O2-400℃、D-Mn O2-400℃、W-Mn O2-400℃、X-Mn O2-400℃、YB-Mn O2-400℃、YD-Mn O2-450℃、ZD-Mn O2-400℃、YW-Mn O2-450℃ 、 BJ-Mn O2-400℃ 、 HJ-Mn O2-450℃. SEM results showed that the morphology of the as-prepared α-MnO2 could be classified into four groups: nanoparticles,nanorods, tetragonal morphology and nanowire. The morphology of catalyst Y-Mn O2-400℃、B-Mn O2-400℃and YB-Mn O2-400℃was nanoparticles; the morphology of catalyst YD-Mn O2-450℃、ZD-Mn O2-400℃ and YW-Mn O2-450℃ was nanorods; the morphology of catalyst D-Mn O2-400℃and W-Mn O2-400℃was mixed state of nanoparticles and nanorods;the morphology of catalyst BJ-Mn O2-400℃ and X-Mn O2-400℃ was tetragonal morphology.The difference of these tetragonal structure was that tetragonal size of BJ-Mn O2-400℃ wasuniform. Whereas catalyst X-Mn O2-400℃ was together into Sphere. The morphology of catalyst HJ-Mn O2-450℃ was nanowire.The reduction capability confirmed by H2-TPR of the as-prepared α-MnO2 catalyst with different morphologies was in the following ascending order : nanowires-Mn O2 < nanorods-Mn O2 < nanoparticles-Mn O2 < tetragonal morphology-Mn O2. The results of catalytic activity test showed that the BJ-Mn O2-400℃catalyst with tetragonal morphology could complete catalytic oxidize o-xylene into carbon dioxide and water at 180℃. Moreover, the catalysts with tetragonal morphology had good catalytic property at lower temperatures. Had the same order as the H2-TPR results.(2) Since BJ-Mn O2-400℃catalyst had the best catalytic activity for o-xylene catalytic combustion, the experimental conditions in the potassium permanganate / benzyl alcohol system were explored about the formation reasons of tetragonal morphology. 1. Optimal reaction molar ratio, α-MnO2 catalyst with tetragonal morphology was obtained with potassium permanganate and benzyl alcohol molar ratio of 1:1.The reason that using overdosed benzyl alcohol(theoretical molar ratio of 4:3) was to ensure that it can act as a template besides as reductant. 2. Suitable reaction temperature: Room temperature or ice-water bath conditions were in favor of the formation of tetragonal structure. 3.Suitable calcination temperature: Tetragonal morphology α-MnO2 catalyst could form at the calcination temperature of 400℃.(3) The template agent ingredient that form tetragonal morphology α-MnO2 was investigated preliminary. In the potassium permanganate / butanol system, the morphology ofα-MnO2 changed from nanorods to tetragonal structure by altering the reaction molar ratio and improving calcination temperature. It was proved that butanol was not a template agent, while butyrate could play the role of template agent for the formation of the tetragonal morphology.However, potassium permanganate /2-butyl alcohol system are still nanorod morphology after changing the experimental conditions. The results showed that the alcohol itself was not template agent. Organic acid, the oxidation products of alcohol, instead of alcohol was template agent. Then, in KMn O4-2-butyl alcohol system, it was observed that morphology was changed from nanorods to similar spherical shape gathered tetragonal structure by addingcertain amount of potassium benzoate. The catalytic activity on o-xylene over tetragonal structure has been significantly improved. On the other hand, benzylamine or its oxidation products can not be used as template agent for the formation of tetragonal morphology.This dissertation was supported by the National Natural Science Founadation of China(No.21147004), the National Natural Science Founadation of Hebei(No. B2013205100) and the Science Foundation of Hebei Normal University(No. L2010Z06).