Regular Article
Ultrafine Pd nanoparticles immobilized on N-doped hollow carbon nanospheres with superior catalytic performance for the selective oxidation of 5-hydroxymethylfurfural and hydrogenation of nitroarenes

https://doi.org/10.1016/j.jcis.2019.06.062Get rights and content

Abstract

The fabrication of ultrafine noble metal nanoparticle (NMNP)-based catalysts is significant for heterogeneous catalysis due to their excellent performance for organic transformations. In this study, N-doped micro-mesoporous hollow carbon nanospheres (HCN) with a nitrogen content of ∼3.5 wt% are easily prepared by simple carbonization. Then, ultrafine Pd NPs are immobilized on HCN, affording Pd/HCN as a dual-function catalyst which exhibits superior catalytic activity for the selective oxidation of 5-hydroxymethylfurfural (HMF) and hydrogenation of nitroarenes, under mild reaction conditions. The doping of HCN with nitrogen is beneficial for the high dispersion, anchoring, and particle size control of ultrafine Pd NPs, leading to the maximum utilization of Pd atoms and further enhancing the catalytic activity of Pd/HCN. In addition, the obtained Pd/HCN catalyst exhibits excellent reusability and stability. Hence, this study demonstrates the prospect of developing ultrafine NMNP-supported catalysts with high performance for organic transformations.

Introduction

Supported noble metal catalysts with highly dispersed active sites have been extensively investigated and applied in modern energy conversion and heterogeneous catalysis [1], [2], [3], [4]. Several studies have confirmed that an effective strategy for optimizing the activity of metal-NP-supported nanocatalysts involves the regulation of the morphology of the metal NPs (e.g., size, shape, and crystal orientation) [5], [6], [7]. Especially, ultrafine noble metal NPs (NMNPs) exhibit superior catalytic performance compared to traditional bulk metals due to their higher surface-to-volume ratio and abundant active sites provided by the large fraction of surface atoms [8], [9], [10]. Although some success has been reported in this field, ultrafine NMNPs cannot be used for practical applications because of a number of limitations. Owing to the Oswald ripening or weak van der Waals forces [11], [12], [13], [14], [15], ultrafine NMNPs anchored on supports may suffer from severe agglomeration and leaching, leading to the limited reusability. The design of ultrafine NMNPs with a controllable size and high stability is the main strategy for rendering the best catalytic activity. Thus far, some synthetic routes to prepare ultrafine NMNP-based catalysts have been developed by the utilization of a spatial confinement effect of materials to limit the growth of NMNPs and prevent their agglomeration or by the addition of suitable organic end-capping ligands and/or polymers to increase the interaction between ultrafine NMNPs and supports [16], [17], [18]. For example, well-dispersed Pd NPs have been deposited on magnetic materials by tripyridine ligands and applied for selective hydrogenation [19]. Xu’s group has employed the double solvent method for successfully immobilizing ultrafine AuNi alloy NPs on metal organic frameworks, which exhibits excellent activity for the production of hydrogen using ammonia borane [18]. Hockneyk et al. have successfully supported monodisperse Pd NPs on a silica support and applied the Pd NPs for the catalytic decomposition of methanol [20]. However, from the catalytic viewpoint, the presence of surfactants can block the active site surface, thereby decreasing the catalytic activity and selectivity. For stabilizing ultrafine NMNPs, the use of organic capping ligands is often limited in high-temperature catalytic reactions. Hence, it is still a challenge to design a low-cost, facile strategy to stabilize and control the growth of ultrafine NMNPs.

As an ideal catalyst support, functional porous nanomaterials (e.g., N-doped carbon and P-doped carbon) exhibit a large specific surface area, pore structure, and strong metal–support interactions [21], [22], [23], [24], [25]. These characteristics are beneficial for the uniform dispersion of ultrafine NMNPs on the support surface, immobilization, particle size reduction, increase of surface-active sites, and participation in the transport of reactants, thereby promoting the catalytic performance. N-doped hollow carbon (HCN), exhibiting a unique shape, low density, good electricity, thermal conductivity, and abundant micro-mesoporous pores, has already been successfully applied for the adsorption of pollutants [26]. In addition, it is an ideal supporting material for confining and anchoring ultrafine NMNPs.

In this study, HCN material was successfully synthesized by the facile pyrolysis of hollow poly(aniline-to-pyrrole) (PACP) nanospheres and used for the high immobilization and stabilization of ultrafine Pd NPs. The as-obtained Pd/HCN exhibited excellent catalytic activity and stability for the selective oxidation of 5-hydroxymethylfurfural (HMF) and hydrogenation of nitroarenes. The catalytic oxidation of HMF affords 2,5-dimethylfuran (DFF), which is widely used in the dyestuff, medicine, and pesticide industries [27], and the hydrogenation of nitroarenes affords aromatic amines, exhibiting considerable value for the production of dyes, medicine, and pesticides [28]. The superior catalytic performance of Pd/HCN is mainly related to the synergistic effects of the following factors: (1) the unique morphology and N doping of HCN are beneficial to the dispersion, anchoring, and size control of ultrafine Pd NPs, (2) the multiscale hierarchical pore structure of HCN is beneficial for the mass transfer during catalysis. Hence, this study should provide a useful strategy that not only bridges the field of HCN-based materials but also open new avenues for the fabrication of ultrafine NMNP-based catalysts.

Section snippets

Materials

Aniline and pyrrole were purchased from Kefeng Industry & Commerce Co., Ltd. (Shanghai). Octylphenol polyoxyethylene ether (OP-10) and APS ((NH4)2S2O8, >98%) were purchased from Shandong Shuangshuang Chemical Co., Ltd. Pd(OAc)2, nitroarenes, and HMF were purchased from Tianjin Heowns Biochem. All solvents were purchased from Lianlong Bohua Medical Chemical Co., Ltd.

Synthesis of HCN

HCN was synthesized by the calcination of hollow PACP nanospheres. Typically, aniline (0.38 mL), pyrrole (0.29 mL), OP-10 (0.06 g),

Characterization

HCN was fabricated by the direct pyrolysis of hollow PACP spheres (Scheme 1). As rich N atoms contain lone pairs of electrons in the HCN skeleton, Pd2+ can be uniformly adsorbed. During the rapid reduction by NaBH4, ultrafine Pd NPs were precisely anchored on the HCN support to form a Pd/HCN nanocatalyst.

Fig. 1a shows the TGA profiles of the PACP spheres to examine the thermal stability of the prepared copolymer. The weight of the PACP spheres slightly decreased, mainly related to the release

Conclusions

In conclusion, HCN with abundant N atoms and numerous mesopores was prepared by the simple carbonization of PACP hollow spheres, followed by the modification with well-defined ultrafine Pd NPs, affording a Pd/HCN nanocatalyst. The results revealed that hierarchical mesoporous Pd/HCN material is a bifunctional catalyst with high activity and selectivity for the selective oxidation of HMF and hydrogenation of nitroarenes. The excellent reusability benefitted from the porous shell HCN structure,

Acknowledgements

This work was supported by the Natural Science Foundation of Gansu (No. 18JR3RA274).

References (44)

  • N. Wang et al.

    In situ confinement of ultrasmall Pd clusters within nanosized silicalite-1 zeolite for highly efficient catalysis of hydrogen generation

    J. Am. Chem. Soc.

    (2016)
  • Y. Yu et al.

    Architectural design of heterogeneous metallic nanocrystals–principles and processes

    Acc. Chem. Res.

    (2014)
  • L. Shang et al.

    Graphene-supported ultrafine metal nanoparticles encapsulated by mesoporous silica: robust catalysts for oxidation and reduction reactions

    Angew. Chem. Int. Ed.

    (2014)
  • Y. Chen et al.

    Immobilizing highly catalytically active noble metal nanoparticles on reduced graphene oxide: a non-noble metal sacrificial approach

    J. Am. Chem. Soc.

    (2015)
  • J. Li et al.

    Hollowing Sn-doped TiO2 nanospheres via ostwald ripening

    J. Am. Chem. Soc.

    (2007)
  • J. Huo et al.

    Hollow ferrocenyl coordination polymer microspheres with micropores in shells prepared by Ostwald ripening

    Angew. Chem. Int. Ed.

    (2010)
  • N. Tian et al.

    Direct electrodeposition of tetrahexahedral Pd nanocrystals with high-index facets and high catalytic activity for ethanol electrooxidation

    J. Am. Chem. Soc.

    (2010)
  • H.-C. Ma et al.

    Pd NPs-loaded homochiral covalent organic framework for heterogeneous asymmetric catalysis

    Chem. Mater.

    (2017)
  • H. Zhong et al.

    Tailor-made porosities of fluorene-based porous organic frameworks for the pre-designable fabrication of palladium nanoparticles with size, location and distribution control

    Chem. Sci.

    (2016)
  • Daniel Esken et al.

    Au@ZIFs: stabilization and encapsulation of cavity-size matching gold clusters inside functionalized zeolite imidazolate frameworks, ZIFs

    Chem. Mater.

    (2010)
  • Q.L. Zhu et al.

    Immobilizing metal nanoparticles to metal-organic frameworks with size and location control for optimizing catalytic performance

    J. Am. Chem. Soc.

    (2013)
  • M. Guerrero et al.

    Taking advantage of a terpyridine ligand for the deposition of Pd nanoparticles onto a magnetic material for selective hydrogenation reactions

    J. Mater. Chem. A

    (2013)
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