孔结构对纳米浇注的中孔钙钛矿上甲醇氧化反应动力学的影响（英文）

发布时间：2018-09-11 09:16

【摘要】：限制工业有毒气体(如挥发性有机化合物)的排放是当今社会主要的挑战之一.因此,迫切需要开发出消除污染物、且不造成二次污染的环境友好技术.其中热燃烧技术比较有效,但通常该过程会采用贵金属催化剂以实现低温高活性,由于贵金属催化剂的成本较高,不利于工业应用,因此,人们一直致力于研究和开发新型材料以替代贵金属催化剂.研究发现,在许多部分氧化或完全氧化反应(特别是烃类或挥发性有机物的氧化反应)中,钙钛矿类复合金属氧化物(ABO_3)具有与贵金属类似的催化性能.但是该材料的制备需在高温(700℃)条件下进行,使得其比表面积(30m~2/g)很低,因而限制了其应用.可见,欲使该材料在工业上得到广泛应用,必须在制备技术上实现很大的突破,即制得高比表面积的钙钛矿类材料.已有人通过在200℃下焙烧成功地制备了比表面积为100m~2/g的钙钛矿氧化物,但继续提高焙烧温度,所制样品的比表面积下降.在过去20年中,中孔氧化硅及随后众多中孔材料的成功制备,使得合成具有极高比表面积的非硅基材料(如碳,金属氧化物,和碳化物等)成为可能.在这些材料的制备方法中,纳米浇铸法因其特别适用于制备具有高比表面积的单金属或单金属氧化物而备受青睐.采用纳米浇铸法已经成功制得一系列材料,并用于很多催化反应中.但文献报道大多只局限于温度、催化剂组成或比表面积对其活性的影响.为了能将这些材料成功用于工业应用,需要对其表面反应机理和相关反应动力学进行深入的研究.最近,本课题组采用纳米浇铸法制备了高比表面积的中孔钙钛矿类氧化物,并考察了它们的催化性能.结果表明,在各种气相反应中,所制纳米浇注的钙钛矿类氧化物具有比相应体相氧化物更高的催化效率.基于此,本文以在不同温度老化的SBA-15为硬模板,采用纳米浇铸法制备高比表面积的LaMnO_3材料,运用X-射线衍射、N_2吸附-脱附、透射电镜、程序升温还原和O_2-程序升温脱附等方法分析所制材料的晶相、织构、表面和氧化还原等性质,考察了其孔结构参数对其催化甲醇完全氧化反应性能和动力学的影响,以深入理解该类材料的催化性能.结果表明,以35,100,140℃老化制得的SBA-15为模板剂,成功地制得了La在A位,Mn在B位的一系列LaMnO_3材料,它们具有可调控的比表面积(80-190 m~2/g);同时,材料的比表面积与所用硬模板剂的老化温度存在很好的关联,且比表面积最大的样品具有最高的催化活性.测量了各催化剂在不同空速(19500-78200h~(-1))条件下甲醇氧化反应结果,从而得到了各催化剂的速率常数,发现它们随着催化剂的比表面积而变化.再结合阿伦尼乌斯方程,采用线性回归法测得了所制备的三个催化剂上该方程的指前因子和表观活化能;发现在所考察的反应条件下,所有催化剂上反应的表观活化能较低,且保持不变.另外,指前因子与催化剂比表面积之间存在线性关系,表明尽管各催化剂的比表面积不同,但单位比表面积的甲醇氧化的比活性是相同的.由于在制备过程中很难除去残余的Si物种,因此未来工作中我们将进一步考察残余物种对纳米浇注的钙钛矿类材料性质的影响.
[Abstract]:Limiting the emission of industrial toxic gases (such as volatile organic compounds) is one of the major challenges in today's society. Therefore, there is an urgent need to develop environmentally friendly technologies to eliminate pollutants without causing secondary pollution. The high cost of noble metal catalysts is not conducive to industrial applications. Therefore, people have been committed to research and develop new materials to replace noble metal catalysts. However, the preparation of this material needs to be carried out at high temperature (700 C), so its specific surface area (30m~2/g) is very low, which limits its application. It can be seen that in order to make this material widely used in industry, great breakthroughs must be made in the preparation technology, that is, high specific surface area of perovskite materials. Material. Perovskite oxides with a specific surface area of 100 m~2/g have been successfully prepared by calcination at 200 C, but the specific surface area of the samples decreased with increasing calcination temperature. In the past 20 years, mesoporous silica and many subsequent mesoporous materials have been successfully prepared, resulting in the synthesis of non-silicon-based materials (such as carbon) with very high specific surface area. Among the preparation methods of these materials, nano-casting method is especially suitable for preparing single metal or single metal oxide with high specific surface area. A series of materials have been successfully prepared by nano-casting method and used in many catalytic reactions. In order to successfully use these materials in industrial applications, it is necessary to study their surface reaction mechanism and related reaction kinetics. Recently, our group has prepared mesoporous perovskite oxides with high specific surface area by nano-casting method. The results show that the nano-cast perovskite oxides have higher catalytic efficiency than the corresponding bulk oxides in various gas-phase reactions. Based on this, the nano-cast LaMnO_3 with high specific surface area was prepared by using SBA-15 as hard template at different temperatures and X-ray diffraction. The crystalline phase, texture, surface and oxidation-reduction properties of the prepared materials were analyzed by means of emission, N_2 adsorption-desorption, transmission electron microscopy, temperature-programmed reduction and O_2-temperature-programmed desorption. The effects of pore structure parameters on the catalytic performance and kinetics of methanol complete oxidation were investigated. A series of LaMnO_3 materials with adjustable specific surface area (80-190 m~2/g) were successfully prepared by using SBA-15 as templates aged at 35,100,140 C, and the samples with the largest specific surface area had the highest catalysis. The rate constants of the catalysts were obtained by measuring the results of methanol oxidation at different space velocities (19500-78200h-1). It was found that the rate constants of the catalysts varied with the specific surface area of the catalysts. In addition, there is a linear relationship between the pre-exponential factor and the specific surface area of the catalyst, indicating that although the specific surface area of the catalysts is different, the specific activity of methanol oxidation per unit surface area is the same. Similarly, since it is difficult to remove the residual Si species during the preparation process, we will further investigate the effect of the residual species on the properties of nano-cast perovskite materials in future work.
【作者单位】：拉瓦尔大学化学系Matériaux
【基金】：supported by the the National Science and Engineering Research Council(Canada) the Fonds Québécois de la Recherche sur la Nature et les Technologies(Province of Quebec)
【分类号】：O621.251;O643.36

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