Date Published: February 07, 2019
Publisher: John Wiley and Sons Inc.
Author(s): Yongjian Ai, Zenan Hu, Lei Liu, Junjie Zhou, Yang Long, Jifan Li, Mingyu Ding, Hong‐Bin Sun, Qionglin Liang.
Fabricating efficient and stable nanocatalysts for chemoselective hydrogenation of nitroaromatics is highly desirable because the amines hold tremendous promise for the synthesis of nitrogen containing chemicals. Here, a highly reactive and stable porous carbon nitride encapsulated magnetically hollow platinum nanocage is developed with subnanometer thick walls (Fe3O4@snPt@PCN) for this transformation. This well‐controlled nanoreactor is prepared via the following procedures: the preparation of core template, the deposition of platinum nanocage with subnanometer thick walls, oxidative etching, and calcination. This highly integrated catalyst demonstrates excellent performance for the catalytic transfer hydrogenation of various nitroaromatics and the reaction can reach >99% conversion and >99% selectivity. With the ultrathin wall structure, the atom utilization of platinum atoms is highly efficient. The X‐ray photoelectron spectroscopy results indicate that partial electrons transfer from the iron oxides to Pt nanowalls, and this increases the electron density of snPt nanoparticles, thus promoting the catalytic activity for the transfer hydrogenation of nitroaromatics. For the reduction of 4‐nitrophenol, the reaction rate constant Kapp is 0.23 min−1 and the turnover frequency (TOF) is up to 3062 h−1. Additional reaction results illustrate that this magnetic nanoreactor can be reused more than eight times and it is a promising catalytic nanoplatform in heterogeneous catalysis.
Fabricating stable and high catalytic activity nanocatalyst is an eternal pursuit for catalysis and material scientists, not only for their fundamental scientific interest, but also for many industry technological applications.1, 2, 3, 4, 5, 6, 7, 8, 9 The precious metals (such as Au, Ag, Ru, Rh, Pd, and Pt) are indispensable component of many heterogeneous catalysts, which are used in chemical, pharmaceutical, petroleum, and energy industry. These catalysts usually own unique properties, such as excellent electronic conduction, numerous of reactive corners, and high‐specific surface area.10, 11, 12, 13, 14, 15 More than atomic scale size, the ultrasmall noble metal nanoparticles (NPs) especially at subnanometer level possess unprecedented properties compared to conventional bulk materials. The exploitation of novel subnanometer materials for heterogeneous catalysis is one of frontier research.16, 17, 18, 19, 20, 21, 22
The detailed formation of the Fe3O4@snPt@PCN was illustrated in Figure1a. First, the NaPdCl4 was used to fabricate the Pd nanocube, which was used as a template in the following. Then, the Pt was carefully deposited to generate the subnanometer wall. After that, the Pd@Pt was encapsulated by the polydopamine (PDA) then the Pd core was etched away through the oxidative etching method with HCl and FeCl3. Finally, the residue was calcined by the tube furnace in N2 atmosphere. The morphologies of intermediates for each stage were well characterized by high‐resolution transmission electron microscopy (HR‐TEM) and its accessories apparatus. Figure 1b and Figure S1 in the Supporting Information show the Pd template nanocubes are highly uniform dispersed with the size of 20 nm. Furthermore, the X‐ray photoelectron spectroscopy (XPS) (Figure S2, Supporting Information) characterization demonstrates that the Pd nanocubes are successfully fabricated with high purity. Figure 1c,d and the scanning transmission electron microscopy (STEM) in Figure S3 in the Supporting Information obviously demonstrate that the Pt atom is deposited on the surface of Pd nanocubes, and the thickness of the wall is subnanometer level. What’s more, the selected area electron diffraction (SAED), energy‐dispersive spectrometer (EDS) mapping, and XPS characterizations in Figures S3–S5 in the Supporting Information clearly confirm that the Pd@snPt is successfully fabricated as the Figure 1a designed. The result of the Figure 1e illustrates that the Pd@snPt is encapsulated by the PDA and the thickness of the PDA layer was about 15 nm. The STEM and EDS mapping in Figure S6 in the Supporting Information highly consent to the HR‐TEM result. The linear scan (Figure S7, Supporting Information) further verified that the Pd@snPt@PDA was synthesized smoothly. The Figure 1f and Figure S8 in the Supporting Information show that the Pd template at the center sites of the Pd@snPt@PDA has been etched away by the oxidative processing with the FeCl3 and HCl solution to obtain the hollow snPt@PDA, where the Fe3+ is chelated in the structure. Finally, the snPt@PDA structure was annealed at 500 °C for 3 h in the high purity N2 atmosphere with a heating rate of 5 °C min−1. Finally, we obtain the Fe3O4@snPt@PCN yolk‐shelled nanocubes catalyst (Figure 1g).
In summary, we have fabricated an interesting magnetically hollow platinum nanocage and it displays high activity when used as a catalyst nanoreactor. The magnetic hollow nanobox was prepared through the template preparation deposition, oxidative etching, and out shell protection procedures. The thickness of the walls was about subnanometer which contains about six platinum atoms. Due to the high atom utilization efficiency and rapid electronic transform during the reaction, this highly integrated nanomaterial demonstrated high effectiveness for the catalytic transfer of the nitroaromatics to the corresponding anilines. Following reaction, the catalyst can be easily recovered and reused for more than eight times.
Fabrication of Magnetic Hollow Subnano Thick Walls Nanocages—Preparation of Pd Template Nanocubes: The protocol for the fabrication of Pd nanocubes was the same as previous literature reported.4 In a typical procedure, 8 mL of deionized (DI) water was first added to the round‐bottomed flask. Successively, 105 mg of polyvinylpyrrolidone (PVP), 60 mg of ascorbic acid, and 600 mg of KBr were added to the flask. The mixture solution was ultrasonicated for 10 min to dissolve all of the substances. Then, this blend solution was moved to an oil bath and heated at 80 °C for 10 min under magnetic stirring. Subsequently, 3 mL Na2PdCl4 (20 mg mL−1) aqueous solution was added into the preheated solution quickly. The reaction solution was refluxed at 80 °C for another 3 h. The final product was washed with DI water for three times and collected by centrifugation. The precipitate (1.68 mg mL−1) was redispersed in 11 mL of ethylene glycol and refrigerated at 4 °C for further use.
The authors declare no conflict of interest.