硫的掺杂对Co-N-C催化剂转移氢化性能的影响

Influence of Doped Sulfur on the Catalytic Transfer Hydrogeneration Performance over Co-N-C Catalyst

在M-N-C型催化剂中引入非金属元素对其进行改性,是一种卓有成效的催化剂优化方法.针对Co-N-C催化剂在硝基苯转移氢化反应中催化活性的不足,以多孔有机聚合物作为有机前驱体,引入不同的硫源,通过高温热解法设计合成了两种硫掺杂修饰的Co-N-C催化剂,并将其应用于硝基苯的转移氢化反应中,研究硫的引入方式对催化剂构效关系的影响.通过SEM、PXRD、TEM、元素含量、氮气吸脱附分析和XPS等系列表征方法对催化剂进行表征.结果表明,所有Co-N-C催化剂在热解过程中均形成石墨碳层和多壁碳纳米管,钴纳米颗粒形成了相同的物相和碳层包覆结构.硫的掺杂影响了催化剂的形貌和钴纳米颗粒的粒径.硫元素与氮元素之间具有协同掺杂的作用,硫的掺杂影响了氮的掺杂形态,提高了吡啶氮的含量.钴硫相互作用仅存在于由分子硫源制备得到的催化剂Co-NPC-S中.催化转移氢化反应结果表明,硫改性催化剂Co-NPC-S和Co-NSPC的催化活性远高于未改性的Co-NPC,这是因为硫元素的掺杂促进了氮元素的掺杂,强化了碳层与钴纳米颗粒之间的电子转移作用.另外,由骨架硫源制备得到的催化剂Co-NSPC具有最高的催化活性,这可能是由于骨架硫源的使用避免了钴硫相互作用的产生,从而减少了在热解过程中生成可覆盖催化剂活性位点的低价态硫物种,有利于催化剂活性的提高.

Nonmetallic element doping to the M-N-C-type catalyst is an effective catalyst optimization method.In this study,two S-doped Co-N-C catalysts were prepared through high-temperature pyrolysis using porous organic polymers with different sulfur sources to further improve the catalytic activity of the Co-N-C catalyst for the transfer hydrogenation reaction of nitrobenzene.These catalysts were evaluated in the catalytic transfer hydrogenation reaction of nitrobenzene to study the influence of sulfur doping techniques on the catalytic performance.The characterization results of scanning electron microscopy,powder X-ray diffraction,transmission electron microscopy,element analysis,N2 adsorptiondesorption,and X-ray photoelectron spectroscopy show that all Co-N-C catalysts formed graphitic carbon layers and multiwalled carbon nanotubes,whereas cobalt nanoparticles formed the same phase and carbon layer coating structure during the pyrolysis process.Sulfur doping affected the catalyst morphology and cobalt nanoparticle size.A synergistic doping effect was found between sulfur and nitrogen.Moreover,sulfur doping affected the nitrogen doping form,thereby increasing the pyridine nitrogen content.The cobalt-sulfur interaction only existed in the Co-NPC-S prepared from the precursor with molecular sulfur.The catalytic evaluation results depict that the catalytic activities of Co-NPC-S and Co-NSPC were much higher than that of Co-NPC because the doped sulfur facilitated the electron transfer between the carbon layer and cobalt nanoparticles.Moreover,Co-NSPC yielded the best catalytic activity among the catalysts,which might be due to the precursor with skeleton sulfur avoiding the generation of the cobalt-sulfur interaction,thereby consequently reducing the formation of low-valence sulfur-covering active sites during pyrolysis.