Controllable Synthesis of Multi-Heteroatoms Co-Doped Hierarchical

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Controllable Synthesis of Multi-Heteroatoms Co-Doped Hierarchical Porous Carbon Spheres as an Ideal Catalysis Platform Shuliang Yang, Yanan Zhu, Changyan Cao, Li Peng, Wendy L. Queen, and Weiguo Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03283 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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ACS Applied Materials & Interfaces

Controllable Synthesis of Multi-Heteroatoms CoDoped Hierarchical Porous Carbon Spheres as an Ideal Catalysis Platform

Shuliang Yang,ab Yanan Zhu,ac Changyan Cao,*ac Li Peng,b Wendy L. Queen*b and Weiguo Songac

a

Beijing National Laboratory for Molecular Sciences, Laboratory of Molecular Nanostructures

and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P.R. China. b

Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne

(EPFL), EPFL-ISIC-Valais, Sion, 1950, Switzerland. c

University of Chinese Academy of Sciences, Beijing, 100049, P.R. China.

ACS Paragon Plus Environment

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ABSTRACT The synthesis of porous carbon spheres with hierarchical porous structures coupled with the doping of heteroatoms is particularly important for advanced applications. In this research, a new route for efficient and controllable synthesis of hierarchical porous carbon spheres co-doped with nitrogen, phosphorus, and sulfur (denoted as NPS-HPCs) was reported. This new approach combines in situ polymerization of hexachlorocyclophosphazene and 4, 4'-sulfonyldiphenol with the self-assembly of colloidal silica nanoparticles (SiO2 NPs). After pyrolysis and subsequent removing the SiO2 NPs, the resulting NPS-HPCs possess high surface area (960 m2/g) as well as homogeneously distributed N, P and S heteroatoms. The NPS-HPCs are shown to be an ideal support for anchoring highly dispersed and uniformly sized noble metal NPs for heterogeneous catalysis. As a proof of concept, Pd NPs are loaded onto NPS-HPCs using only methanol as a reductant at room temperature. The prepared Pd/NPS-HPCs are shown to exhibit high activity, excellent stability and recyclability for hydrogenation of nitroarenes.

KEYWORDS: hierarchical structure, porous carbon spheres, doping, heterogeneous catalysis, nitroarene reduction

INTRODUCTION Relative to standard microporous carbon materials, porous carbon spheres possess several unique characteristics such as regular geometry, high surface area, and tunable porosity and particle size.1-5 Recently, porous carbon spheres have received tremendous attention for applications related to the adsorption, heterogeneous catalysis, and energy storage and conversion.6-11 Over the past few years, one research hotspot is the synthesis of porous carbon spheres that offer hierarchical porous structures and contain accessible heteroatoms doped at the molecular level.12-


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When compared to standard mesoporous carbon materials, the co-doped, hierarchical carbon

spheres can provide higher surface areas, pore volume and abundant accessible active sites.17-19 These features lend to heterogeneous catalysts that offer optimized adsorption, mass transport, and activity.20-21 Heteroatoms such as nitrogen, phosphorus, sulfur and fluorine are recently shown to enhance their properties for specific applications, including CO2 capture, metal-free carbocatalysis, hydrogen evolution, and oxygen reduction.22-29 The heteroatoms can additionally coordinate metals, and hence, these materials have further been employed as supports for various metal NPs.30-38 Multifarious synthetic approaches have been disclosed for preparing porous carbon spheres with hierarchical porous structures, including soft and hard template methods, solvothermal carbonization, emulsion polymerization and the Stöber method.2, 39-41 In order to simultaneously introduce heteroatoms, an appropriate precursor must be selected and then combined with the aforementioned methods. For example, through the incorporation of phenolic compounds with suitable reactive groups, such as aminophenol, hydroxybenzenesulphonic acid and fluorophenol, porous carbon spheres containing N, S and/or F heteroatoms can be fabricated using the Stöber method.2 In another research, Feng et al. reported the synthesis of N-doped mesoporous carbon nanospheres with tunable pore sizes by taking advantage of the polymerization of aniline and the co-assembly of colloidal SiO2 NPs.15 Although great progress has been made, most reported porous carbon spheres with doped heteroatoms only contain single or bimodal micro- and/or mesopores. It is well known that hierarchical porous structure with micro-, meso-, and macropores is favored for many applications.7, 42-43 For example, Liu et al prepared N, P codoped hierarchical porous carbon microspheres (NPHCMs) using colloidal SiO2 nanospheres as the template. The prepared hierarchical structures with micro-meso-macroporous porosity could


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not only enlarge the electro-active surface area effectively, but also improve the ion diffusion and electron transport obviously, which is beneficial for the improvement of electrochemical performance when NPHCMs were served as the electrodes for supercapacitors.42 In addition, compared to materials containing a single heteroatom, the physicochemical properties of the multi-doped materials can be regulated broadly because of the synergistic effects between the dissimilar atoms.25, 44-49 However, to date, reports related to synthesizing porous carbon spheres with hierarchical micro-meso-macropores containing multi-heteroatoms are very rare. In this study, we develop a new route for efficient and controllable synthesis of nitrogen, phosphorus, and sulfur co-doped hierarchical porous carbon spheres (denoted as NPS-HPCs) by combining polymerization of hexachlorocyclophosphazene and 4, 4'-sulfonyldiphenol with the co-assembly of colloidal SiO2 NPs. After pyrolysis and subsequent removing SiO2 NPs, the resulted NPS-HPCs possess hierarchical micro-meso-macroporous structure with N, P and S homogeneously distributed as well as high surface area (960 m2 g-1). The characteristics of NPSHPCs render it an ideal support for anchoring Pd NPs inside for heterogeneous catalysis. The prepared Pd/NPS-HPCs exhibit high activity, good stability and satisfying recyclability with the hydrogenation of nitroarenes as test reaction. EXPERIMENTAL SECTION Synthesis of SiO2 NPs with 290 nm diameter: In a 250 mL beaker, 4.5 g NH3·H2O (25%) was added into 180 mL ethanol and 30 mL H2O. The solution was stirred for 5 min at room temperature, then 15.9 g tetraethyl orthosilicate (TEOS) was added into the above-mentioned solution. After stirring at room temperature for 1 h, the white precipitates were collected and washed three times with ethanol and dried in the vacuum oven for 12 h.


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Preparation of SiO2@PZS self-assembly: In a 150 mL round bottom flask, 400 mg SiO2-290 NPs were dispersed in 80 mL methanol. After 15 min of sonication, another 40 mL methanol solution with 280 mg hexachlorocyclophosphazene and 630 mg 4, 4'-sulfonyldiphenol dissolved was added dropwise. After continuous agitation for five more minutes, 740 µL triethylamine (TEA) was added drop by drop and the solution was stirred for another 6 h. After that, the white precipitates were centrifugated and washed with ethanol for 3 times and dried in the vacuum oven for 12 h. Preparation of SiO2@NPS-HPCs: In a ceramic combustion boat, 600 mg of the above obtained SiO2@PZS material was pyrolyzed at 900 °C with temperature ramp of 2 °C/min for 3 h with argon as the shielding gas. The obtained product was denoted as SiO2@NPS-HPCs. Preparation of NPS-HPCs: In a 100 mL PTFE autoclave, the obtained SiO2@NPS-HPCs were etched in 4.0 mol/L NaOH aqueous solution for 7 h at 100 °C. When the etching time was reached, the black product was washed with millipore water for five times and ethanol for two times and dried in the vacuum oven for 12 h. The collected product was named as NPS-HPCs. Preparation of Pd/NPS-HPCs: In a 50 mL flask, 10 mg of NPS-HPCs assembly were dispersed in 20 mL methanol and ultrasonicated for 15 minutes. Then, this solution was kept in a 25 oC water bath for 30 min to decrease the temperature deviation during the next reducing process. Then 124 µL 0.0564 mol/L Pd(NO3)2 aqueous solution was added dropwise. After continuous agitation for 5.5 h, the product was centrifuged and washed with ethanol for 3 times and dried in the vacuum oven for 12 h. Typical reaction procedure for the nitroarenes hydrogenation reaction: Nitroarene (0.5 mmol), catalyst Pd/NPS-HPCs (3.87 wt%, 3.0 mg) and n-tridecane (51 µL, 0.21 mmol) were


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mixed with 2 mL ethanol. The atmosphere in the reactor was exchanged with H2 for three times to guarantee the inside air is replaced by H2 completely. Then, the reactor was placed in a 25 oC water bath with stirring for a specific time. When the time was reached, the reaction suspension was centrifuged to remove solid and the liquid was analyzed by a Shimadzu GC-2010 gas chromatograph furnished with a flame ionization detector (FID) and a Rtx-5 capillary column. The identity was confirmed by Shimadzu GCMS-QP2010S gas chromatography-mass spectrometry (GC-MS) instrument. Characterizations: X-ray diffraction (XRD) patterns were conducted on a Rigaku D/max-2500 diffractometer with Cu Kα radiation (λ= 1.5418 Å) using 2o/min as the scanning rate. The voltage and current during the test were 40 kV and 200 mA respectively. Scanning electron microscopy (SEM) images were collected on HITACHI S-4800 SEM, and all the transmission electron microscopy (TEM) characterizations were performed on high-resolution TEM (JEOL, JEM-2100F). X-ray photoelectron spectroscopy (XPS) data was collected on the VG Scientific ESCALab220i-XL spectrometer using Al Kα radiation. Energy-dispersive X-ray spectroscopy (EDS) mappings were carried out on the JEOL 2100F scanning TEM (STEM) microscopy. The Pd content was analyzed by Shimadzu ICPE-9000 inductively coupled plasma atomic emission spectroscopy (ICP-AES). N2 adsorption-desorption isotherms were run on a Quantachrome Autosorb AS-1 instrument at -196 oC. The Brunauer-Emmett-Teller (BET) method was adopted to ascertain the surface area of the materials.


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RESULTS AND DISCUSSION Synthesis and characterizations of NPS-HPCs The procedure for synthesizing NPS-HPCs is displayed in Scheme 1. Firstly, a stable dispersion containing SiO2 NPs and monomers of hexachlorocyclophosphazene and 4, 4'-sulfonyldiphenol is prepared. After that, the polymerization and the co-assembly with SiO2 NPs are launched after adding triethylamine, which allows the formation of super self-assembly of SiO2@PZS composite spheres (PZS: poly(cyclotriphosphazene-co-4,4’-sulfonyldiphenol), Scheme 1).50-52 Finally, NPS-HPCs are obtained by calcination and subsequent etching of the SiO2 NPs with NaOH aqueous solution. The atomic dispersion of N, P and S atoms in PZS polymer ensures the homogeneously incorporating N, P and S heteroatoms throughout the carbon spheres during the carbonization process. In addition, the resulting carbon spheres possess a hierarchical porous structure. The decomposition of the PZS polymer under high temperature generates micro/meso pores in the walls, and the SiO2 etching process produces macropores. All these features are beneficial for adsorption and catalysis.

Scheme 1. Schematic illustration for preparing NPS-HPCs.


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Figure 1 shows the corresponding typical images of samples obtained in each step. Uniform SiO2 NPs with diameter about 290 nm were first prepared through Stöber method (Figure 1a) and dispersed in the ethanol solution, which was then mixed with methanol solution containing hexachlorocyclophosphazene and 4, 4'-sulfonyldiphenol to form a stable dispersion. After addition of triethylamine and stirring for 6 h, the super self-assembly SiO2@PZS composite spheres with size of around 1.7 µm were formed (Figure 1b). Many SiO2 NPs were observed on the surface of the spheres. TEM images together with EDS mapping (Figures S1a-b) also confirmed that SiO2 NPs are assembled and coated with a PZS layer, which allows the formation of a spherical superstructure. To distinguish the surface PZS layer more clearly, a polytetrafluoroethylene-assisted SiO2 removal method was adopted to remove partial SiO2.53-54 From the Figures S1c-d, the superficial doped carbon layer derived from PZS layer could be observed obviously, which further confirmed the successful deposition of PZS layer on the surface of SiO2 NPs. Figure 1c displays a SEM image of the SiO2@PZS composite spheres after calcination at 900 ºC for 3 h under argon atmosphere. It can be clearly seen that more SiO2 NPs appear on the outer surface owing to the shrinkage of the PZS polymer during the calcination treatment process. After etching the SiO2 NPs, thermogravimetric analysis displayed > 99% weight loss under air atmosphere, suggesting SiO2 NPs were indeed completely removed (Figure S2). SEM images revealed that the final NPS-HPCs retained the original spherical morphology without structure collapse, and bowl-like macropores were clearly observed on the surface (Figure 1d-1e). TEM images further indicated the presence of integrated macropores in the inner core and bowl-like macropores on the edge of NPS-HPCs (Figure 1f). The wall thickness between two neighboring pores was determined to be only ca. 2 nm (Figure S3). EDS mapping indicated N, P and S atoms were distributed uniformly throughout the carbon spheres (Figure


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1g). XPS results further showed the existence of C, O, N, P and S elements (Figure S4a). The contents of N, P and S in the NPS-HPCs were 1.89 at%, 1.32 at% and 1.16 at%, respectively. The high-resolution XPS spectrum of N 1s revealed that the N species contained pyrrolic N, graphitic N, pyridinic N, and N-oxide species (Figure S4b).55-56 A P 2p spectrum contained two peaks at 132.6 and 133.9 eV, corresponding to P-C and P-O, respectively (Figure S4c).57-58 High resolution of the S 2p spectrum displayed three peaks at 163.6, 164.8, and 168.0 eV, which correspond to S 2p3/2 and S 2p1/2 of the C-S-C and the oxidized S, respectively (Figure S4d).59-60 All these data confirm that the heteroatoms were successfully doped into the carbon spheres.

Figure 1. SEM images of (a) SiO2 NPs, (b) self-assembly of SiO2@PZS, (c) SiO2@NPS-HPCs, (d-e) NPS-HPCs, (f) TEM image of NPS-HPCs and (g) EDS-mapping images of NPS-HPCs.


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Nitrogen adsorption isotherms were performed to analyze the surface area and pore size distribution of the obtained NPS-HPCs. As exhibited in Figure 2, the isotherms exhibited typeIV with slight hysteresis loop, indicating the presence of major micropores (743 m2 g-1) and minor mesopores in the NPS-HPCs (217 m2 g-1). The micropore size distribution deduced from Density functional theory (DFT) method showed a peak centered at around 0.5 nm and a broad peak around 1.5-4 nm (inset of Figure 2). The BET surface area and total pore volume of NPSHPCs were 960 m2 g-1 and 1.86 cm3 g-1, respectively. The high pore volume also confirmed the prepared NPS-HPCs sample is highly porous.

Figure 2. Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution of NPS-HPCs (inset). SiO2 NPs played a very important role for the formation of the NPS-HPCs. First, the presence of SiO2 NPs significantly affected the morphology of the PZS superstructure. The polymerization of hexachlorocyclophosphazene and 4, 4'-sulfonyldiphenol with SiO2 NPs produced super self-assembled SiO2@PZS spheres, where SiO2 NPs were uniformly incorporated into the PZS matrix (Figure 1b). Without the SiO2 NPs, irregular aggregates with


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dense spheres were obtained (Figure S5), suggesting that the spherical morphology of SiO2@PZS composite stems from the co-assembly of SiO2 NPs and PZS polymer. Second, SiO2 NPs could avoid unwanted carbon conglutination and aggregation during calcination process, resulting in highly porous SiO2@NPS-HPCs spheres. After etching the SiO2 NPs, NPS-HPCs without structure collapse were readily obtained. Third, the macropore size of NPS-HPCs can be easily tuned by varying the SiO2 NPs size. As shown in Figure S6, super self-assembly SiO2@PZS composite spheres could also be obtained with SiO2 diameters of 100 nm, 150 nm and 380 nm. Self-assembly process of the SiO2@PZS superstructure To understand the self-assembly process of the SiO2@PZS spheres, samples obtained from varying reaction times were gathered and characterized by SEM and TEM. At 10 min, a thin layer of PZS was deposited on the surface of SiO2 NPs, and several NPs were connected to form small self-assembled units (Figures S7c-7d and S8). With the reaction time was prolonged to 20 min, more SiO2 NPs attached, forming larger self-assembled units (Figures S7e-7f); however, at this stage there were still many free un-assembled SiO2 NPs in the solution (Figure S9). When the reaction time reached to 45 min, fewer free SiO2 spheres existed (Figures S7g-7h), and the self-assembly process was completed after 90 min (Figures S7i-7j). According to the above-mentioned results, we propose the self-assembly processes should be accomplished in the following manner: (1) firstly PZS monomers are adsorbed on the surface of SiO2 NPs through hydrogen bonding interactions,61-62 (2) after the addition of triethylamine, the PZS polymer is formed and grow on the surface of SiO2 NPs, (3) with continued polymerization of the PZS and accumulation of SiO2 NPs, larger self-assembled structures are formed, which are energetically favoured in solution to reduce the free energy, and (4) with the depletion of PZS


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monomers, the co-assembly of the SiO2 NPs and PZS polymer is terminated. The result is a uniform super self-assembly of SiO2@PZS composite spheres. The proposed self-assembly processes can be further verified by the following controlled experiments. Reducing the ratio of the PZS precursors to SiO2 NPs (the PZS precursors were decreased to 140 mg hexachlorocyclotriphosphazene and 315 mg 4,4’-sulfonyldiphenol with the same 400 mg SiO2 NPs), many SiO2 NPs were left in the reaction mixture without undergoing self-assembly (Figure S10). In contrast, high ratio of PZS precursors vs SiO2 NPs (the PZS precursors were increased to 560 mg hexachlorocyclotriphosphazene and 1260 mg 4,4’-sulfonyldiphenol with the same 400 mg of SiO2 NPs) lead to severe aggregates of SiO2@PZS self-assembly spheres (Figure S11). The driving force for the self-assembly should be from the cross-linked reaction of PZS polymer and the interaction such as hydrogen-bond interaction between PZS precursors and SiO2 assembly units.61-64 Moreover, for negatively charged SiO2 NPs surfaces, the in-situ produced triethylammonium chloride (TEACl) cationic surfactant will also interact with negatively charged SiO2 NPs to promote the dispersity of SiO2 NPs in the reaction suspension, meanwhile this kind of interaction will further assist the deposition of negatively charged PZS polymer precursors on the TEA+-modified SiO2 NPs surface by electrostatic attractive interactions.50, 65-66 Preparation and characterizations of Pd/NPS-HPCs and catalytic performance The presence of high densities of Lewis bases, including N, P, and S, makes the NPS-HPCs also ideal supports for anchoring noble metal NPs inside to produce supported heterogeneous catalysts. Also, the hierarchical porous structure of the NPS-HPCs favored homogenous dispersion of the noble metal NPs and improved mass transport during catalysis. Thus, we further investigated the NPS-HPCs as a support to produce a Pd/NPS-HPCs heterogeneous catalyst. In a typical procedure, Pd/NPS-HPCs were prepared while stirring a methanolic


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solution containing the NPS-HPCs and Pd(NO3)2 at room temperature for 5.5 h. TEM images revealed that Pd NPs with average size of 4.3 nm were distributed uniformly on the NPS-HPCs (Figures 3a-3d and Figure S12). Tomographic focused ion beam scanning electron microscopy (FIB-SEM) showed that Pd NPs were located throughout the inner macropores (Figure S13). ICP-AES analysis revealed that Pd loading on Pd/NPS-HPCs was 3.87 wt%. XPS analysis and HRTEM confirmed that Pd existed mainly as metallic Pd (Figure S14a and Figure 3e). Comparing with the metallic Pd, the Pd binding energy of Pd/NPS-HPCs obviously shifted to higher binding energy position (+0.12 eV) when compare with metallic Pd. The shift confirmed the presence of electronic interactions between Pd and NPS-HPCs because the heteroatoms with different electronegativity will tune the electron density around carbon atoms, which will further result in the enhanced binding energy of Pd on NPS-HPCs.67-72 However, there were no obvious peaks related to metallic Pd in the XRD pattern (Figure S14b). All these results strongly suggested that Pd NPs were very small and also uniformly distributed throughout the NPS-HPCs framework. There was no Pd aggregation during the preparation process. It is noteworthy that the synthesis method for producing Pd/NPS-HPCs is very simple and environmental-benign. No addition of a reductant or ligand is needed. Thus, the surface of Pd NPs is clean, which is also very important for enhanced catalytic activity.73-74


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Figure 3. (a-b) TEM images, (c) Dark-field HRTEM, (d) EDS mapping for Pd element distribution, (e) High-resolution TEM image of Pd/NPS-HPCs. As the essential building blocks, aniline and its derivatives are very important feedstocks in various industrial fields. Hydrogenation of nitroarenes to produce anilines with heterogeneous catalysts is one of the most efficient and atomic economy route.75-76 Therefore, we chose nitroarene reduction to test the catalytic activity of the as-prepared Pd/NPS-HPCs. As shown in Table 1, aniline product was not detected without adding the catalyst (entry 1). NPS-HPCs alone also could not promote this reaction (entry 2). When Pd/NPS-HPCs were used as catalyst, the reaction proceeded very fast with a very high turnover frequency (TOF) value of about 953 h-1 under ambient reaction conditions (entry 3). It is worth noting that such a TOF value is higher than that of commercial Pd/C and those of many catalysts reported in the literatures, indicating


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the superior activity of Pd/NPS-HPCs (Table S1). The nitroarenes with different functional groups were also well tolerant with Pd/NPS-HPCs catalyst (entries 4-11). For Cl-substituted nitrobenzene, the dechloronization reaction occurred, which was also a reflection of high catalytic performance (entry 10). A hot-filtration experiment confirmed the reaction proceeds in a heterogeneous way (Figure S15). Cycling experiments showed the catalyst can be recycled at least five times with no obvious activity loss (Figure S16), indicating excellent stability of the Pd/NPS-HPCs. ICP-AES analysis of the used catalyst also revealed the Pd loading was approximately the same as that in the fresh catalyst (3.93 wt%); this proved that Pd leaching was negligible during the cycling processes. Table 1. Selective reduction of various nitroarenes[a].


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[a] Reaction conditions: catalyst (3.0 mg), n-tridecane (51 µL, internal standard), nitroarenes (0.5 mmol), EtOH (2.0 mL), 25 oC, H2 (0.1 MPa). Because of the different solubility of substrate, [b] 2.5 mL EtOH and [c] 2 mL CH2Cl2 was used, respectively. The excellent performance of Pd/NPS-HPCs can be ascribed to the structural reasons as below: (i) the 3D interconnected large pore structure facilitates mass transport;77 (ii) the high specific internal surface areas result in easy accessibility to a large number of active sites; (iii) the large macropores, which result from the super assemblies of SiO2 NPs, can behave as nanoreactors concentrating the react molecules inside, and (iv) the high densities of well-dispersed heteroatoms help to stabilize large numbers of small Pd NPs on the internal catalyst surface.78-81 Last, the process used for the reduction of Pd2+ is environmentally friendly and benign as it is carried out at room temperature and only MeOH as clean reductant. CONCLUSIONS In summary, nitrogen, phosphorus, and sulfur co-doped hierarchical porous carbon spheres (NPS-HPCs) are synthesized by combining the polymerization of hexachlorocyclophosphazene and 4, 4'-sulfonyldiphenol with co-assembly of colloidal SiO2 NPs, followed by a pyrolysis and etching process. The resulting NPS-HPCs possess hierarchical micro-meso-macroporous structure, high internal surface areas and a homogeneous distribution of N, P and S heteroatoms. NPS-HPCs are shown to be an ideal support for anchoring Pd NPs for heterogeneous catalysis. The prepared Pd/NPS-HPCs exhibit high activity, excellent stability and recyclability for hydrogenation of nitroarenes. ASSOCIATED CONTENT


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Supporting Information Experimental details, details of characterization data including TEM images, EDS mapping, XPS spectra, thermogravimetric analysis, SEM images, SEM-FIB imaging and XRD patterns, hotfiltration experiments, cycling performance and comparison of activity for nitroarenes hydrogenation. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] ACKNOWLEDGMENT This work was financed by the National Natural Science Foundation of China (NSFC 21333009, 21573244, 21573245), Chinese Academy of Sciences-Peking University Pioneer Cooperation Team and the Youth Innovation Promotion Association of CAS (2017049). S.Y. and L.P. were supported by the Swiss National Science Foundation under grant PYAPP2_160581. We thank Dr. Bo Guan and Xiang Li for their help of FIB-SEM imaging. REFERENCES (1) Sun, X.; Li, Y., Colloidal Carbon Spheres and Their Core/Shell Structures with Noble-Metal Nanoparticles. Angew. Chem. Int. Ed. 2004, 43, 597-601. (2) Liu, J.; Wickramaratne, N. P.; Qiao, S. Z.; Jaroniec, M., Molecular-Based Design and Emerging Applications of Nanoporous Carbon Spheres. Nat. Mater. 2015, 14, 763-774.


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(3) Gong, Y.; Xie, L.; Li, H.; Wang, Y., Sustainable and Scalable Production of Monodisperse and Highly Uniform Colloidal Carbonaceous Spheres using Sodium Polyacrylate as Dispersant. Chem. Commun. 2014, 50, 12633-12636. (4) Ai, K.; Liu, Y.; Ruan, C.; Lu, L.; Lu, G. M., Sp2 C-Dominant N-Doped Carbon SubMicrometer Spheres with A Tunable Size: A Versatile Platform for Highly Efficient OxygenReduction Catalysts. Adv. Mater. 2013, 25, 998-1003. (5) Zhao, J.; Niu, W.; Zhang, L.; Cai, H.; Han, M.; Yuan, Y.; Majeed, S.; Anjum, S.; Xu, G., A Template-Free and Surfactant-Free Method for High-Yield Synthesis of Highly Monodisperse 3Aminophenol-Formaldehyde Resin and Carbon Nano/Microspheres. Macromolecules 2013, 46, 140-145. (6) Liu, J.; Yang, T.; Wang, D.-W.; Lu, G. Q.; Zhao, D.; Qiao, S. Z., A Facile Soft-Template Synthesis of Mesoporous Polymeric and Carbonaceous Nanospheres. Nat. Commun. 2013, 4, 2798. (7) Bin, D.-S.; Chi, Z.-X.; Li, Y.; Zhang, K.; Yang, X.; Sun, Y.-G.; Piao, J.-Y.; Cao, A.-M.; Wan, L.-J., Controlling the Compositional Chemistry in Single Nanoparticles for Functional Hollow Carbon Nanospheres. J. Am. Chem. Soc. 2017, 139, 13492-13498. (8) Li, M.; Wang, C.; O'Connell, M. J.; Chan, C. K., Carbon Nanosphere Adsorbents for Removal of Arsenate and Selenate from Water. Environ. Sci.: Nano 2015, 2, 245-250. (9) Chen, M.; Yu, C.; Liu, S.; Fan, X.; Zhao, C.; Zhang, X.; Qiu, J., Micro-Sized Porous Carbon Spheres with Ultra-High Rate Capability for Lithium Storage. Nanoscale 2015, 7, 1791-1795. (10) Lu, A.-H.; Sun, T.; Li, W.-C.; Sun, Q.; Han, F.; Liu, D.-H.; Guo, Y., Synthesis of Discrete and Dispersible Hollow Carbon Nanospheres with High Uniformity by Using Confined Nanospace Pyrolysis. Angew. Chem. Int. Ed. 2011, 50, 11765-11768.


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Controllable Synthesis of Multi-Heteroatoms Co-Doped Hierarchical

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