摘要
在多相催化体系中,开发具有高催化活性的多相催化剂是实现小分子绿色转化的核心.过渡金属碳化物,特别是碳化钼(Mo_(2)C),因其活性与贵金属相当且成本低廉,在小分子转化方面具有应用潜力.二维(2D)过渡金属碳化物因其平面结构两侧具有可暴露的活性位点,在小分子转化和能量储存方面受到越来越多的关注.然而,在催化剂或电极材料的批量制备过程中,二维层的堆积会导致表面活性位点损失,严重影响其性能.此外,层状材料的堆积还可能阻碍有机分子的有效传递,形成传质屏障,进一步影响催化过程.特别是,二维过渡金属碳化物表面的氧化会导致活性位点的大量损失,这已成为当前亟待解决的关键问题.因此,开发一种能确保层状Mo_(2)C材料的机械和化学稳定性的有效合成方法,对于推动其实际应用至关重要.本文利用可控氧扩散蚀刻法合成了一种富氮碳鞘包覆的“阶梯状”2D-Mo_(2)C异质结材料(2D-Mo_(2)C@NC).该材料因具有独特的阶梯层状结构和整流界面,可以显著促进CO_(2)与邻苯二胺的羰基化反应,并对CO_(2)与多种二胺衍生物的羰基化反应均表现出较好的催化活性.通过密度泛函理论计算和紫外光电子能谱分析发现,Mo_(2)C与NC壳层的接触界面形成了整流接触,导致电子从NC壳层流向Mo_(2)C,进而形成富电子的层状Mo_(2)C.随着NC壳层氮含量的增加,Mo_(2)C的电子富集程度逐渐增加.进一步的理论计算和CO_(2)程序升温脱附实验表明,CO_(2)吸附依赖于2D-Mo_(2)C的电子富集程度,并随2D-Mo_(2)C的电子富集程度增加而增强.这种电子富集特性使得2D-Mo_(2)C表面能够活化CO_(2)分子,形成高度扭曲的预吸附结构(键角为118.8°),有利于后续与二胺的羰基化反应.邻苯二胺与CO_(2)羰基化反应的吉布斯自由能计算结果表明,电子的富集程度对反应决速步的能垒影响较小.因此,推测CO_(2)分子在富电子Mo_(2)C表面的预吸附增强是促进CO_(2)羰基化反应的直接因素.实验结果也表明,2D-Mo_(2)C@NC催化剂催化邻苯二胺与CO_(2)羰基化反应的催化活性随2D-Mo_(2)C的电子密度增强而提高.优化后的阶梯状异质结2D-Mo_(2)C@NC样品的CO_(2)吸附量高于核壳状异质结Mo_(2)C@NC样品,且催化性能和CO_(2)吸附量存在明显的正相关关系,证实了阶梯状异质结结构所暴露出的更多富电子Mo_(2)C活性位点可以促进CO_(2)的预吸附及随后的活化过程.此外,X射线光电子能谱结果表明,富电子的2D-Mo_(2)C表现出优于Mo_(2)C的化学稳定性,不易被氧化.在优化条件下制得的2D-Mo_(2)C@NC催化剂表现出了最佳催化性能,在较低的反应温度下,其催化邻苯二胺与CO_(2)羰基化反应生成2-苯并咪唑啉酮的TOF值为6.1‒10.2 h^(‒1),比文献报道的最佳结果高4.2倍.综上,本文成功地开发了一种有效的可控氧扩散蚀刻策略,合成出具有高机械和化学稳定性的“阶梯状”2D-Mo_(2)C异质结材料,并用于高效催化CO_(2)和邻苯二胺羰基化生成2-苯并咪唑啉酮.该可控氧扩散蚀刻方法可以扩展到其他二维碳化物的制备,并进一步扩展其在光催化和电催化系统中的应用,为二维异质结材料的合成和应用提供了参考和新思路.
The development of two-dimensional heterogeneous catalysts with highly exposed active site areas for heterogeneous catalytic systems is highly promising for achieving the green transformation of small molecules.2D transition metal carbides(TMCs)show great promise for catalysis and carbon capture but suffer from heavy aggregation and autooxidation.Mass transfer barrier is also a standard problem of lamellar TMCs caused by slight aggregation of layers.Here,we successfully developed a method to prepare“ladder-like”heterojunctions of Mo_(2)C layers confined inside nitrogen-rich carbon sheaths(2D-Mo_(2)C@NC)via an effective oxygen-diffusion etching strategy,acting as efficient catalysts for CO_(2)fixation.The enhanced electron enrichment of the as-integrated 2D-Mo_(2)C subunits induced by the Schottky barrier could further keep the exposed Mo_(2)C surface from possible autooxidation and aggregation confronted by conventional 2D TMCs.The experimental results and density functional theory(DFT)calculation further demonstrate that electron-rich Mo_(2)C could boost the adsorption and activation of CO_(2)for universal carbonylation of various diamines,providing a turnover frequency value(TOF)of 10.2 h^(–1) to produce benzimidazolone,which is 5.2 times of that of the state-of-the-art catalyst in the literature under even critical conditions.
作者
许玉帅
汪红辉
李奇远
张仕楠
夏思源
许冬
类伟巍
陈接胜
李新昊
Yu-Shuai Xu;Hong-Hui Wang;Qi-Yuan Li;Shi-Nan Zhang;Si-Yuan Xia;Dong Xu;Wei-Wei Lei;Jie-Sheng Chen;Xin-Hao Li(School of Chemistry and Chemical Engineering,Frontiers Science Center for Transformative Molecules,Shanghai Jiao Tong University,Shanghai 200240,China;Shanghai Research Institute of Chemical Industry Co.Ltd.,Shanghai 200062,China;Institute for Frontier Materials,Deakin University,Waurn Ponds Campus,Locked Bag 20000,Geelong Vic.3220,Australia)
基金
国家自然科学基金(22071146,21931005)
上海市科学技术委员会(20520711600).
