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空气中的甲醛主要来源于化工、建材、涂料、装潢材料以及机动车尾气.甲醛具有光化学活性,对人体具有致癌致畸作用.高浓度甲醛对人体健康和空气环境危害极大,室内低浓度甲醛对人体也有很大伤害.因此,消除室内、机动车尾气以及工业生产过程中的甲醛非常必要.目前,去除甲醛的方法主要有吸附法、光催化法和催化燃烧法.其中,催化燃烧法具有去除效率高、起燃温度低、适用范围广、设备操作简单以及无二次污染等优点,因而非常适用于去除高浓度和低浓度甲醛.该方法的核心是催化剂的制备和筛选.近年来,用于甲醛催化燃烧的催化剂主要是负载型贵金属和金属氧化物.由于贵金属催化剂成本较高,所以金属氧化物催化剂备受关注.MnO_2种类繁多,既包括人工合成的棒状、线状、管状、球状和孔状等形貌,还包括自然界存在的α,β,γ和δ等类型.其中,介孔MnO_2因具有较大的比表面积和特殊的孔道而应用于乙醇、甲苯、苯等挥发性有机物的催化氧化反应.目前,尚未见三维(3D)有序介孔MnO_2催化氧化甲醛的报道.本文以合成的3D有序介孔KIT-6分子筛为硬模板剂,采用纳米浇筑法制备出3D有序介孔MnO_2材料.为了比较,采用水热法合成了α-MnO_2和β-MnO_2纳米棒.采用X射线粉末衍射、N_2吸附-脱附、透射电子显微镜和X射线能谱(XPS)等方法对催化剂进行了表征.在微型固定床石英管反应器上评价了催化剂催化甲醛氧化活性,采用气相色谱(GC)联接热导检测器(TCD)和质谱检测器(MSD)检测产物和反应物的含量.表征结果表明,3D-MnO_2复制了KIT-6硬模板的三维有序立方对称介孔结构(ia3d),且具有金红石型β-MnO_2晶相,属软锰矿,具有较大的比表面积和双孔分布介孔结构,最大孔径分别位于3.7和11.4nm处.3D-MnO_2样品具有清晰的孔道结构,而α-MnO_2和β-MnO_2纳米棒为无孔的一维纳米单晶材料.另外,3D-MnO_2表面暴露了较多的(110)晶面,有利于增加表面Mn~(4+)离子.XPS结果证实3D-MnO_2表面存在较多的Mn~(4+)离子,这些Mn~(4+)离子为甲醛催化反应提供了丰富的活性位,有利于提高甲醛氧化活性.评价结果表明,3D-MnO_2具有良好的低温催化性能,于130℃即可将甲醛完全转化成CO_2和H_2O;而在同样条件下,α-MnO_2纳米棒和β-MnO_2纳米棒分别在140和180℃才能完全转化甲醛.3D-MnO_2具有良好的甲醛催化性能主要归因于特殊的介孔结构、较大的比表面积和较多的表面Mn~(4+)离子.
Formaldehyde in the air comes mainly from chemicals, building materials, coatings, decorating materials and motor vehicle exhaust formaldehyde has photochemical activity and has carcinogenic and teratogenic effects on human body.High concentration of formaldehyde is very harmful to human health and air environment, indoor low concentration of formaldehyde Therefore, to eliminate indoor, motor vehicle exhaust and industrial production of formaldehyde is very necessary.At present, the removal of formaldehyde are mainly adsorption, photocatalysis and catalytic combustion.Among them, catalytic combustion has Removal of high efficiency, low light-off temperature, wide range of applications, simple equipment operation and no secondary pollution, which is very suitable for the removal of high concentration and low concentration of formaldehyde.The core of this method is the preparation and screening of catalyst.In recent years, Catalysts for the catalytic combustion of formaldehyde are mainly supported noble metals and metal oxides, and metal oxide catalysts are of interest due to the high cost of precious metal catalysts, including a wide range of synthetic, rod-shaped, linear, tubular, spherical And pore morphology, but also include the natural existence of α, β, γ and δ and other types. Among them, the mesoporous MnO_2 Has a large specific surface area and special channels for the catalytic oxidation of volatile organic compounds such as ethanol, toluene, benzene, etc. At present, no three-dimensional (3D) ordered mesoporous MnO 2 has been reported for the catalytic oxidation of formaldehyde.In this paper, 3D ordered mesoporous KIT-6 molecular sieve was used as hard template to prepare 3D ordered mesoporous MnO 2 by nano-pouring method.For the purpose of comparison, α-MnO 2 and β-MnO 2 nanorods were synthesized by hydrothermal method.Using X-ray The catalysts were characterized by X-ray powder diffraction, powder X-ray diffraction, N 2 adsorption-desorption, transmission electron microscopy and X-ray photoelectron spectroscopy.The catalytic activity of the catalyst for the catalytic formaldehyde oxidation was evaluated on a miniature fixed bed quartz tube reactor. (TCD) and mass spectrometry (MSD) were used to detect the contents of the products and reactants.The characterization results showed that 3D-MnO 2 replicated the three-dimensional ordered cubic symmetric mesoporous structure (ia3d) of KIT- , And has a rutile β-MnO 2 crystal phase, which belongs to pyrolusite, has a large specific surface area and a mesopore structure with a maximum pore diameter of 3.7 and 11.4 nm, respectively, and has a clear pore structure in the 3D-MnO 2 sample α-MnO 2 and β-MnO 2 nano In addition, more (110) crystal planes are exposed on the surface of 3D-MnO 2, which is helpful to increase the surface Mn 4+ ions. XPS results confirm that the surface of 3D-MnO 2 is more (4+) ions, which provide a rich active site for the catalytic reaction of formaldehyde and help to improve the formaldehyde oxidation activity. The evaluation results show that the 3D-MnO 2 has good low-temperature catalytic properties, Formaldehyde was completely converted to CO_2 and H_2O at 130 ℃, while under the same conditions, α-MnO_2 and β-MnO_2 nanorods were completely converted to formaldehyde at 140 and 180 ℃, respectively. The properties are mainly attributed to the special mesoporous structure, larger specific surface area and more surface Mn 4+ ions.