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在生物难降解有机污染物的处理中,类芬顿高级氧化(advanced oxidation process, AOP)技术受到广泛的关注[1]。其中以过硫酸盐高级氧化技术最为典型[2]。大量研究表明,过硫酸盐通过加热[3-4]、紫外[4]和过渡金属离子[5-6]等形式的活化,可以有效实现对有机物的降解。然而,常见的活化方法在实际应用中存在局限性。其中,热活化和紫外活化需要消耗额外的能量,而过渡金属离子的引入会导致化学污泥的产生以及潜在的二次污染。
近几年,各类型碳材料由于来源广泛、价格低廉、耐用性好、无化学污泥等优点逐渐受到关注,对其催化性能的研究也有许多报道[7-9]。通常,具有完美六角蜂窝拓扑结构的碳催化剂(石墨)常表现出较低的催化活性。值得注意的是,除六角蜂窝拓扑结构以外,碳材料中还存在缺陷结构,包括边缘缺陷、空位/空穴缺陷、拓扑缺陷等[10]。其中,边缘缺陷是指碎片状微小碳面的边界,是含量最丰富的缺陷类型[11-12]。边缘位置的碳原子通常具有高于面内碳原子的电荷密度,具有更高的电子活性。空位/空穴缺陷是指面内一个或多个碳原子的缺失,可认为是位于催化剂内部的边缘缺陷[13]。此外,碳面中常见的非六元环,如五元环、八元环等,是极为重要的拓扑缺陷。各类型拓扑缺陷中不规则的碳原子数通常会导致电子重排,使其具有良好的电子活性[14]。有研究[15-17]表明,富缺陷的碳催化剂比无缺陷碳催化剂具有更好的吸附和催化性能。
由于氮元素的电负性高于碳元素,将氮原子掺杂到碳材料中会显著破坏原有π共轭结构,造成局部电子重排[18-20],通常称为氮杂质缺陷。常见的氮掺杂方法分为原位合成法和后处理法。原位合成法可分为活化法、水热法、化学气相沉积法、模板法、溶胶-凝胶法等;而后处理法是指将氮源在一定条件下引入到已有的碳材料中,属于间接合成法。一般情况下,以氨气、氨水、尿素等氮源和碳源混合,通过高温热处理即可有效实现碳材料的氮掺杂,掺杂态的氮一般以吡咯氮、吡啶氮和石墨氮等形式存在。有研究表明,除了氮掺杂含量,氮杂质缺陷的种类也可能影响催化剂的催化活性[21]。氮缺陷的存在有利于过一硫酸盐(peroxymonosulfate,PMS)的吸附,并可通过与相邻碳原子的协同作用实现对PMS的高效活化。尽管已经开展了大量的研究,但对PMS的催化效率仍有待提高。此外,氮掺杂碳催化剂的催化性能与氮含量或缺陷种类之间的内在催化机制尚不清楚。
基于上述研究结果,本研究在氮气气氛下,使用一步热处理法制备了一系列不同氮含量的碳催化剂,分别通过场发射扫描电镜、高分辨透射电镜、X射线衍射分析、X射线光电子能谱等手段对所制备催化剂的物化性质进行了详细分析。将所制备催化剂与PMS结合,构建了非均相高级氧化体系用于双酚A(bisphenol A,BPA)的降解。对催化剂投加量、氧化剂投加量、初始pH、共存化学组分等条件的影响进行了详细分析,并通过顺磁共振分析、活性物种淬灭实验、电化学分析以及量子化学计算等手段,对可能的降解机理进行了深入分析。
氮掺杂碳催化剂活化过一硫酸盐的活性位点分析及其对双酚A的降解机制
Analysis of active sites in nitrogen-doped carbocatalysts for peroxymonosulfate activation and the degradation mechanism of bisphenol A
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摘要: 为了探究氮掺杂碳催化剂中不同类型缺陷点位在活化过一硫酸盐((PMS))时的反应活性,以碳黑和二氰二胺混合物为前驱体,通过热解得到了一系列不同氮掺杂量的碳催化剂(CNx),并对所制备催化剂的缺陷度、化学组分以及PMS活化性能进行了研究。结果表明,增加碳催化剂中的高活性氮杂质缺陷点位可有效促进催化剂的PMS活化性能;不同本征缺陷点位对PMS活化性能也表现出显著差异。活性物种淬灭实验、顺磁共振分析和电化学分析等结果表明,CNx/PMS体系对双酚A(BPA)的降解过程遵循以单线态氧(1O2)为主导的非自由基途径,催化剂表面的电子传递机制也有一定贡献。以上研究结果可为识别氮掺杂碳催化剂中的活性点位和高活性催化剂的定向合成提供参考。Abstract: In order to understand the reactivity of different defect sites in nitrogen-doped carbocatalysts with peroxymonosulfate (PMS), a series of carbocatalysts (CNx) with different nitrogen contents were prepared by pyrolyzing a mixture precursor consisted of carbon black and dicyandiamide. The defect degree, chemical composition and PMS activation performance of CNx were studied. The results showed that increased nitrogen defect sites with high-reactivity could promote the activation of PMS, and different intrinsic defect sites also exhibited significant differences in PMS activation. The results of reactive oxidation species quenching experiment, electron paramagnetic resonance analysis and electrochemical analysis showed that the bisphenol A (BPA) degradation by CNx / PMS was a non-radical pathway process, which was dominated by singlet oxygen (1O2), and the electron transfer mechanism also contributed to some extent. The above results could shed light on the identification of active sites in nitrogen-doped carbocatalysts and the oriented synthesis of catalysts with high catalytic activity.
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Key words:
- AOPs /
- PMS /
- nitrogen-doped carbocatalysts /
- nitrogen defect /
- intrinsic defect
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表 1 XPS分析中各元素含量以及N1s精细谱的各组分含量
Table 1. Content of each element in XPS analysis and the content of each component in N1s spectrum %
样品 C O N PLN PDN GPN Oxide-N CN600 86.94 3.44 9.63 2.92 5.96 0.54 0.21 CN700 96.02 2.35 1.64 0.31 0.79 0.15 0.39 CN800 97.35 2.65 - - - - - CN900 97.62 2.38 - - - - - CN1000 97.80 2.20 - - - - - -
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