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近年来,过渡金属氮碳材料由于其廉价、高效与持久耐用的性质得到广泛研究,被视为钯基催化剂的良好替代品.除了可应用于电催化领域,过渡金属氮碳材料还可作为有机反应催化剂,并显示出良好的催化性能.金属卟啉化合物因其高效模拟自然酶的仿生催化功能而闻名,然而在均相催化体系中其难回收、易自我氧化失活的缺点大大阻碍了其实际应用.对金属卟啉进行热处理是提高其催化性能与稳定性的有效方法.此外,作为内部含有金属-氮配合键的含碳大环化合物,金属卟啉是一步合成金属氮碳材料的良好前驱体.本课题组已证明以金属钴卟啉作为前驱体制得的金属氮碳催化剂具有良好的催化乙苯氧化性能.在此基础上,本文采用含有不同过渡金属中心的四苯基金属卟啉(四苯基钴卟啉、四苯基铁卟啉和四苯基钴卟啉)为前驱体,通过无模板法热处理制备了过渡金属氮碳催化剂M-N-C(M=Co,Fe,Mn),考察不同过渡金属中心对催化剂性能的影响.所得催化剂采用N_2吸附-脱附、热重(TG)、透射电子显微镜(TEM)、高分辨透射电子显微镜(HRTEM)、拉曼光谱(Raman)和X射线光电子能谱进行了表征.N_2吸附-脱附结果表明,所得M-N-C材料具有不同的比表面积与孔道结构,其中Co-N-C催化剂比表面积最大.TG显示,不同金属卟啉的失重情况不同,四苯基钴卟啉失重最多,四苯基铁卟啉次之,四苯基锰卟啉失重最少.从TEM和Raman结果可见,所得不同金属氮碳材料具有不同的石墨化程度,其中Co-N-C材料具有明显的石墨化层状碳结构,石墨化程度最高,Fe-N-C材料次之,而Mn-N-C材料中的碳主要呈片状无定形状态,表明其石墨化程度最低.这可能是不同过渡金属中心在加热过程中对卟啉结构碳化过程催化效果不同所致,其中钴中心对卟啉结构碳化过程的催化效果最佳.另外,考察了该M-N-C催化剂在无溶剂条件下催化分子氧选择性氧化乙苯的性能.结果发现,不同金属中心的M-N-C催化剂表现出不同的催化性能.这可能归因于金属种类的不同、所得催化剂碳氮结构的差别以及金属中心与氮碳结构的协同效应.此外,这些M-N-C材料作为多相催化剂在以氧气为氧源的无溶剂选择性氧化乙苯反应中表现出良好的催化性能,且多次使用后没有明显的活性损失,具有良好的回收使用性能.
In recent years, due to its low cost, high efficiency and long-lasting properties, transition metal carbonitride materials have been widely studied as a good substitute for palladium-based catalysts.In addition to the field of electrocatalysis, transition metal carbonitride materials can also be used as organic Reaction catalyst, and shows good catalytic performance.Metorphoporphyrins are well-known for their efficient mimicry of biomimetic catalytic functions of natural enzymes, however, the drawbacks of their difficult recovery and easy self-oxidative inactivation in a homogeneous catalytic system greatly hinder their Practical application: Metalloporphyrins heat treatment is to improve its catalytic performance and stability of an effective method.In addition, as a carbon-containing macrocycle containing metal-nitrogen coordination compound, metalloporphyrins is a one-step synthesis of metal nitrogen and carbon materials is good The precursor group has been proved by our group that the metallic nitrogen-carbon catalyst prepared by using metal cobalt porphyrin as precursor has a good catalytic activity for the oxidation of ethylbenzene.On the basis of this, the tetraphenylmetalloporphyrin containing different transition metal centers (Tetraphenyl cobalt porphyrin, tetraphenyl iron porphyrin and tetraphenyl cobalt porphyrin) as precursors, by the template-free heat treatment to prepare a transition metal Carbon catalyst MNC (M = Co, Fe, Mn) was used to investigate the effect of different transition metal centers on the performance of the catalyst.The N 2 adsorption-desorption, TG, TEM, HRTEM, Raman and X-ray photoelectron spectroscopy.The results of N2 adsorption-desorption show that the obtained MNC materials have different specific surface area and pore structure, and the Co-NC catalyst has the largest specific surface area. TG showed that the weight loss of different metalloporphyrins was different, tetraphenylporphyrin was the heaviest, tetraphenylporphyrin was the second, and tetraphenyl manganese porphyrin was the least .From TEM and Raman results, we can see that the different metal nitrogen The carbon materials have different degree of graphitization. Among them, Co-NC material has obvious graphitized layered carbon structure with the highest graphitization degree and the second is Fe-NC material, while the carbon in Mn-NC material is mainly flaky amorphous State, indicating that the degree of graphitization is the lowest, which may be due to different transition metal centers in the heating process of the porphyrin carbonization process due to different catalytic effect, in which the cobalt center of the porphyrin structure carbonation process the best catalytic effect.In addition, The M -NC catalyst can catalyze the selective oxidation of ethylbenzene by molecular oxygen under the condition of no solvent.The results show that MNC catalysts with different metal centers show different catalytic properties.This may be attributed to the different metal species, , As well as the synergetic effect of the metal center and the carbon-carbon structure.In addition, these MNC materials exhibit good catalytic performance as heterogeneous catalysts in the solventless selective oxidation of ethylbenzene with oxygen as the oxygen source, and after repeated use No significant loss of activity, with good recycling performance.