表面应力介导的碳基Pt纳米催化剂用于硝基芳烃的高选择性加氢

Enhancing the chemoselective hydrogenation of nitroarenes:Designing a novel surface‐strained carbon‐based Pt nanocatalyst

苯胺衍生物是合成药物、杀虫剂、颜料和染料等化工原料的重要中间体,其主要由硝基芳烃选择加氢制得.该反应通常在较高的温度和压力下进行,尤其是底物中含有其他官能团(如卤素、醛、酮和酯等)会因脱卤和过度加氢造成产物的选择性下降,其原因可能是-C-X和-C=C双键比-NO2基团具有更高反应活性以及它们在催化剂表面的不同吸附强度和构型.因此,含竞争官能团的硝基芳烃定向转化从根本上取决于高选择性加氢催化剂的合理设计与制备.为实现含有可还原性基团的硝基芳烃高选择性加氢,研究人员开发了多种方法来构建Pd、Pt基非均相催化剂,如有机分子修饰金属表面、产生载体氧空位、降低金属颗粒组装体尺寸、构筑核壳结构复合材料以及与第二种金属形成合金/金属间化合物.本文采用一种简便可放大的热还原策略合成具有高选择性和稳定的表面应变铂基纳米催化剂(Pt NPs).通过溶剂热法制备Pt/OCNTs预催化剂,然后在H_(2)气氛下对其进行高温处理获得目标催化剂.在该过程中,Pt NPs尺寸稍微增加且被迁移的碳原子覆盖,碳原子源自CNTs载体.Pt NPs外围保护性碳层可有效抑制其聚集和浸出,有望提高催化剂的选择性和稳定性.结合球差校正透射电子显微镜、X-射线吸收精细结构谱和X-射线光电子能谱分析,在退火过程中发现Pt NPs和CNTs载体之间存在较强的金属-载体相互作用(SMSI),进一步升温后Pt NPs表面的部分碳涂层被蚀刻成碳原子,并在Pt NPs表面诱导产生压缩应变.因此,Pt/OCNTs催化剂的热还原处理成功地诱导了Pt NPs和CNTs载体之间的SMSI效应以及Pt NPs的压缩应变.将制备的催化剂用于硝基芳烃底物的选择性加氢,结果表明,表面应变的Pt/CNTs-800H(在氢气气氛中,800℃热处理)催化剂对苯乙烯和硝基苯混合物及4-硝基苯乙烯加氢的化学选择性几乎完全反转,说明CNTs惰性载体在将Pt NPs从非选择性催化剂转化为对4-硝基苯乙烯高选择性加氢催化剂中起到关键作用.硝基芳烃加氢反应结果表明,经高温H_(2)处理后的Pt/CNTs-800H催化剂的加氢选择性显著提高且对4-溴硝基苯加氢具有较好的循环稳定性.同时,该策略也适用于5wt%Pt/C和Pt/rGO催化剂,证明本文处理方法在碳基催化材料中的良好普适性.综上,本文不仅发现Pt NPs和CNTs之间存在SMSI效应,而且为制备具有压缩应变的Pt NPs提供了一种新途径,为解决Pt基非均相催化剂选择性差和稳定性低等问题提供一定参考.

Supported Pt nanoparticles(NPs)are highly active catalysts for heterogeneous catalytic hydrogenation reactions;however,controlling their selectivity remains the biggest challenge toward their applicability.Herein,we propose the formation of a highly selective and stable,surface‐strained Pt‐based nanocatalyst via a facile and scalable thermal reduction treatment.Spherical‐aberration‐corrected transmission electron microscopy(SACTEM)and various spectral analytic techniques reveal a strong metal‐support interaction between the Pt NPs and carbon nanotubes(CNTs)support during the annealing process.Thereafter,a fraction of carbon atoms is etched from the carbon‐coated Pt NPs,inducing a compressive strain on the surface of the Pt NPs.Notably,the chemoselectivity of the surface‐strained Pt/CNTs‐800H catalyst(where 800 represents the heat‐treatment temperature;H represents a hydrogen atmosphere)is almost completely different compared to that of its pristine counterpart.This catalyst is used for the hydrogenation reactions of a styrene and nitrobenzene mixture as well as 4‐nitrostyrene.Interestingly,similar findings were observed with 5 wt%Pt/C and Pt/rGO catalysts,confirming that this treatment could be generalized.Hence,it has great potential in the design and synthesis of carbon‐based catalytic materials.