A MOF-Templated Fabrication of Hollow Co4N@N-doped Carbon

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A MOF-Templated Fabrication of Hollow Co4N@N-doped Carbon Porous Nanocages with Superior Catalytic Activity Jianping Sheng, Liqiang Wang, Liu Deng, Min Zhang, Haichuan He, Ke Zeng, Feiying Tang, and You-Nian Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00573 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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ACS Applied Materials & Interfaces

A MOF-Templated Fabrication of Hollow Co4N@N-doped Carbon Porous Nanocages with Superior Catalytic Activity

Jianping Sheng†, Liqiang Wang†, Liu Deng†,‡,*, Min Zhang‡, Haichuan He†, Ke Zeng†, Feiying Tang†, You-Nian Liu†,§,*∗ †

College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan

410083, P. R. China ‡

School of Material Science and Energy Engineering, Foshan University, Foshan, Guangdong

528000, P. R. China §

State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan

410083, P. R. China

∗

Corresponding author. Phone/Fax: +86-731-8887-9616 E-mail Addresses: [email protected] (L. Deng); [email protected] (Y.-N. Liu).

ACS Paragon Plus Environment

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ABSTRACT: Metallic Co4N catalysts have been considered as one of the most promising nonnoble material for heterogeneous catalysis due to its high electrical conductivity, great magnetic property and intrinsic high activity. However, the metastable properties seriously limit their applications for heterogeneous water phase catalysis. In this work, a novel Co-MOF derived hollow porous nanocages (PNCs) composed of metallic Co4N and N-doped carbon (NC) were reported for the first time. This hollow 3D PNCs catalyst was synthesized by taking advantage of Co-MOF as a precursor for fabricating 3D hollow Co3O4@C PNCs, along with the NH3 treatment of Co-oxides frames to promote the in-situ conversion of Co-MOF to Co4N@NC PNCs. Benefiting from the intrinsic high activity and electron conductivity of metallic Co4N phase and the good permeability of hollow porous nanostructure as well as the efficient doping of N into the carbon layer. Besides, the covalent bridge between active Co4N surface and PNCs shells also provides facile pathways for electron and mass transport. The obtained Co4N@NC PNCs exhibits excellent catalytic activity and stability for 4-nitrophenol reduction in terms of low activation energy (Ea = 23.53 kJ mol−1), high turnover frequency (TOF = 52.01 × 1020 molecule g−1 min−1) and fast apparent rate constant (kapp = 2.106 min−1). Furthermore, the magnetic property and the stable configuration make the catalyst excellent recyclability. It is hoped that our finding could pave a way for construction of other hollow transition metal-based nitride@NC PNCs catalysts for wide applications. KEYWORDS: Co4N, nanocages, MOF-derived, nanocatalyst, water phase catalysis


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1. INTRODUCTION The heterogeneous catalysis in aqueous solutions is a promising strategy for oxidation and hydrogenating processes, especially for various waste water treatment.1 Thus developing active and durable heterogeneous catalysts have been extensively studied. However, up till now, most reported heterogeneous catalysts were usually composed of noble metals, which limits their applications.2,3 Consequently, great efforts have been devoted to developing heterogeneous catalysts based on earth abundant and inexpensive elements, for example, transition metal chalcogenide, carbide, nitride and phosphide.4,5 Among them, metallic cobalt-based catalysts have been widely investigated.6,7 For example, Co4N holds great promise as a new material for catalysis, due to its incorporating N atoms into the interstices of cobalt unit cells. It exhibits high electrical conductivities, great magnetic property and intrinsic high activity and so on.6,8 Xie and coworkers reported a Co4N porous nanowire array, which offered excellent catalytic activity towards oxygen evolution reaction (OER), owing to the intrinsically superior electrical conductivity.6 Wang et al. prepared a Co4N nanosheet-based catalyst which exhibited high CO2 hydrogenation efficiency. Its TOF was 64 times than Co nanosheets.8 However, the practical applications of Co4N-based nanocatalysts are limited by the metastable properties of cobalt nitride.9 To overcome undesirable instability and aggregation, carbon-based materials are usually employed as a matrix to immobilize nanocatalysts.10-12 N-doped carbon (NC) matrices have recently drawn much attention due to their outstanding features of facile preparation, high conductivity and environment friendly.13 The C atoms in the carbon matrix can be substituted by the doped N atoms, and the charge transfer occurs between the doped N and adjacent C atoms, which can undermine the electroneutrality of adjacent C atoms and create more active sites for electrophilic and nucleophilic attack.14 Shen et al. fabricated a cobalt-iron double sulfide which


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covalently entrapped in N-doped porous graphitic carbon basement, the obtained nanostructure revealed enhanced ORR and OER performance.15 However, in the immobilization process, a partial of the active sites of Co4N-based nanocatalyst inevitably tends to be buried under the covered carbon layer, resulting in the decrease of the catalytic activity. To preserve the activity and stability of the nanocatalyst is still a challenge to face. Recently, the well-defined 3D hollow porous nanocages (PNCs) have been established for high-efficiency nanocatalysts configuration because of their unique features of low density, high surface-to-volume, high penetrability and low coefficients of thermal expansion and refractivity.16,17 For instance, Xu et al. reported Pd nanoparticles (NPs) supported on 3D mesoporous nanocages, their high catalytic performance was attributed to the satisfactory dispersion and stabilization of nanocage matrices toward Pd NPs.18 Consequently, the directly insitu growth of metallic Co4N with PNCs structures could be a promising strategy for enhance the stability and catalytic activity of Co4N@NC heterogeneous catalysts. However, controllable assembly of PNCs structures, as well as Co4N and N-doped compositions remains great challenging. Metal−organic frameworks (MOFs) have attracted considerable attention due to the superior properties, such as high porosity, flexible tunability and well-defined architecture. Many works have demonstrated that MOFs hold great promise in catalysis, gas storage and separation, energy storage, sensing and drug delivery.19,20 However, their applications are hindered by the limited mass transfer and modest catalytic activities because of their lengthy diffusion path and less exposed active sites. Besides, the weak coordination between metal ions and organic ligand is apt to a structural instability.21 On the other hand, using MOFs as precursors to construct hollow MOF-derivatives is not only a solution to tackle the above deficiencies, but also can obtain many


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novel nanostructures.22 For instance, Hu et al. prepared a hollow Fe2O3 nanostructure through thermal decomposition of Prussian blue coordination polymers. By tuning the calcination temperatures and selecting the Prussian blue NPs with different hollow cavities, crystalline αFe2O3 and γ-Fe2O3 can be selectively formed.23 Su et al. synthesized a yolk-shell CdS microcube from Cd-Fe Prussian blue analogues through microwave-assisted hydrothermal method, and the yolk-shell CdS microcube showed high photocatalytic performance.24 Therefore, we try to employ MOFs as a precursor to synthesize metal nitride@NC PNCs. To the best of our knowledge, there are few reports about preparation of metal nitride@NC PNCs hybrids using MOFs as a precursor. Herein, as a proof-of-concept study, we first propose and demonstrate a novel kind of 3D PNCs composed of metallic Co4N and NC matrix by direct nitridation of Co-MOF derived Co3O4@C PNCs. Taking advantage of Co-MOF as a precursor for fabricating 3D hollow structure of Co3O4@C PNCs, along with the NH3 treatment of Co-oxides frames to promote the in-situ conversion of Co-MOF to Co4N@NC PNCs, a preciously engineered 3D Co4N@NC PNCs catalyst was successfully obtained. More importantly, during the nitridation, partial regions of the carbon matrix can be burned off to form a porosity NC with the help of caustic NH3 gas.13 The combination of metallic Co4N and NC matrix with preciously engineering 3D hollow nanocage configuration makes the Co4N@NC PNCs catalyst exhibit high catalytic activity and superior long-term stability. The fantastic magnetic properties of Co4N@NC PNCs also endow the excellent recyclability.


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2. MATERIALS AND METHODS 2.1. Synthesis of Co-MOF Nanocubes and Co3O4@C PNCs. Co-MOF was synthesized according to a reported procedure with a slight modification.25 Typically, Co(CH3COO)2·4H2O (0.75 mmol, 187 mg) was dissolved in 100 mL deionized water under vigorous stirring for 10 min to form solution I. Then K3[Co(CN)6]2 (0.4 mmol, 133 mg) and polyvinylpyrrolidone (PVP, 3.0 g) were dissolved in 100 mL water to form solution II. A pink turbid solution formed when solution I was injected to solution II slowly and regularly using a syringe. The mixture was left in ambient for 24 h reaction. After that, the pink precipitate was collected and washed with water for several times, followed by drying in vacuum. Finally, the as-prepared Co-MOF precursor was placed in porcelain boat and heating at 450 oC in a tube furnace under N2 for 1 h with a ramping rate of 10 oC min−1, Co3O4@C PNCs were obtained. 2.2. Synthesis of Co4N@NC PNCs. To produce Co4N@NC PNCs, the nitridation was carried out at 500 oC under NH3 atmosphere. In detail, 1.0 g of as-prepared Co3O4@C PNCs was placed in a porcelain boat and a 100 mL min−1 of N2 flow keep constant pass through the tube after the air displacement by N2 for 3 times. The reaction temperature was increased from room temperature to final nitridation temperature through two stages which with different heating rates, i.e., 5 oC min−1 of heating rate at the first stage and then switch to 1 oC min−1 when there was only 100 oC left before the final nitridation temperature. Take the nitridation under 500 oC one as an example, at first, with a heating rate of 5 oC min−1 to reach 400 oC, and then switched to 1 oC min−1 from 400 oC to 500 oC. The atmosphere was switched to a 150 ml min−1 of NH3 flow when the temperature reached 500 oC and the Co3O4@C PNCs was left in NH3 atmosphere for 2 h reaction. After the nitridation reaction, the reactor was cooled to room temperature at cooling rate of 5 oC min−1 under N2 atmosphere. The samples with different nitridation


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temperatures of 200, 300, 400, 500 and 600 oC were also obtained (label as Co3O4@NC PNCs200, CoO@NC PNCs-300, Co4N@NC PNCs-400, Co4N@NC PNCs-500 and Co4N@NC PNCs600, respectively) through the abovementioned procedure except for the different final nitridation temperatures. 2.3. Catalytic Reduction of 4-Nitrophenol and Methylene Blue. The reduction of 4nitrophenol (4-NP) by NaBH4 was chosen as a model reaction to evaluated the catalytic hydrogenation activity of the catalyst. Typically, the reaction was carried out in a quartz cuvette, and the reaction process was monitored using UV-vis spectroscopy at room temperature. First, 3.0 mL of aqueous 4-NP solution (0.1 mM) and 10.0 mg NaBH4 (0.25 mmol) was mixed in quartz cuvette, the obtained mixture solution showing a deep yellow. Subsequently, 10 µL of Co4N@NC PNCs dispersion liquid (5 mg mL−1) was added into the above yellow solution. UVvis absorption spectroscopy was utilized to determine the reaction progress. In recycling, the Co4N@NC PNCs catalyst was separated from the reaction system by magnetism separation and washed three times with deionized water, then subjected to the next reaction run. The conversion of 4-NP can be calculated by the following equation: Conversion (%) = (C0−Ct) / C0 × 100% (1) Where, C0 is the initial concentration of 4-NP, Ct is the concentration of 4-NP at the time t. The catalytic reduction of methylene blue (MB) by NaBH4 in the presence of Co4N@NC PNCs was also executed similar to the procedure of 4-NP. 2.4. Characterization of Catalysts. The morphology and structure of the samples were characterized by transmission electron microscopy (TEM; FEI Titan G2 60-300 with spherical aberration correction, USA) and X-ray powder diffraction (XRD; D/max 2550 X-ray


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diffractometer, Rigaku, Japan). X-ray photoelectron spectroscopy (XPS) was carried out using an ESCALAB 250Xi (Thermo Fisher Scientific, USA) with Al Kα excitation (150 W, Mono 500 µm), and the energy step size and pass energy were 0.05 and 30 eV, respectively. Binding energy calibration was based on C 1s at 284.8 eV. Nitrogen adsorption-desorption isotherms were determined by the Brunauer-Emmett-Teller (BET) test using an ASAP 2020 surface area and pore analyzer (Micromeritics Instruments, USA). Magnetic measurements were recorded using a vibrating sample magnetometer model EV9 (MicroSense, MA, USA) at room temperature. The absorbance of solution was monitored on a UV-2450 spectrophotometer equipped with a quartz cuvette (Shimadzu, Japan).

3. RESULTS AND DISCUSSION 3.1. The Characterization of Co4N@NC PNCs. The synthetic strategy of Co4N@NC PNCs is schematically illustrated in Scheme 1, which includes the growth of Co-MOF nanocube, the thermal treatment under N2 atmosphere and final nitridation under NH3 atmosphere. The morphology and structure evolution are characterized by transmission electron microscopy (TEM). Solid Co-MOF nanocubes with an average diameter 150 nm were formed with smooth surface (see Figure 1a). The solid nanocubes were successfully converted into Co3O4@C PNCs without collapse after the oxidation process. Besides, the smooth surface turned into rougher surface, and several pores are found in the walls of the obtained Co3O4@C PNCs which attributed to deterioration and carbonization of the organic linkers and surfactant PVP molecules (Figure 1b). The high-resolution TEM (HRTEM) shows a distinct lattice fringe of 0.286 nm, which is consistent with the (220) lattice plane of Co3O4. After nitridation, not only the 3D PNCs


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structure of Co3O4@C PNCs, but also porous network in the nanocage walls are retained well. As shown in HRTEM, the distinct lattice fringe of 0.207 nm was observed, which is consistent with the (111) lattice plane of Co4N, confirming the successful transformation of Co3O4 to Co4N (see Figure 1c). It should be noted that the Co-MOF derived PNCs consist of multiple Co4N nanoparticles covered by the thin NC layers, which originated from the carbonation and nitridation of the PVP and organic linker (see Figure S1c). The HAADF-mapping images and EDS results are also confirmed that the as-obtained Co4N@NC PNCs-500 is composed of Co, N, O and C. And these elements are homogenous spatial distributed in Co4N@NC PNCs-500 (Figure 1f and 1g). Comparison of the magnified TEM images of Co3O4@C PNCs and Co4N@NC PNCs-500, a slight size expansion and more porous structure of Co4N@NC PNCs500 are observed. And higher surface roughness and shorter crystalline domain length of Co4N@NC PNCs-500 are also found in comparison with the Co3O4@C PNCs (Figure S1b and c). Besides, slight lattice disorder and dislocations can be seen in the HRTEM image (red areas in inset of Figure S1c) of Co4N@NC PNCs-500. These changes could be attributed to the surface defects forming via the released gas and the corrosion of NH3 in the nitridation treatment.13 As reported in the previous literature,26 the surface defects can act as active adsorption sites and/or catalytic activity sites, which is beneficial to the efficient adsorption and catalytic conversion on these sites. Considering the increased amount of surface defects, the Co4N@NC PNCs-500 catalyst is expected to possess excellent enhanced catalytically property. The transformation of crystal structure was then investigated by XRD. As shown in Figure 1d, various diffraction peaks of the obtained Co-MOF are perfect matched to cubic phase of Co3[Co(CN)6]2·H2O (JCPDS No.77-1161) and no other impurity was found. After annealing treatment, the diffraction peaks at 2θ = 31.2, 36.8, 44.7, 59.3 and 65.2 o are observed, which are


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ascribed to a spinel Co3O4 (JCPDS No. 42-1467). Finally, the peaks of the nitrated-products are at 2θ = 44.2, 51.4 and 75.7 o respectively, which are consistent with the (111), (200) and (220) plane of Co4N phase (JCPDS No.41-0943). The XRD results also confirm the successful conversion of Co3[Co(CN)6]2 to Co4N. To further verify the formation of Co4N, XPS was carried out to characterize the products before and after nitridation. An exclusive N 1s peak was observed in the nitridation product (see Figure 1e). The high resolution spectra of Co 2p, N 1s and C 1s for the final products are shown in Figure 1h−j. The peaks at 796.5 and 781.0 eV are ascribed to Co 2p1/2 and Co 2p3/2 for Co4N, respectively. While the peaks at 794.9 and 779.7 eV can be assigned to 2p1/2 and Co 2p3/2 for Co3O4 residual, no Co0 (778.0 and 793.0 eV) was found, indicating that the Co3O4 precursor was converted to Co4N rather than Co0 after the nitridation.6, 27

The peak at 778.8 eV corresponding to Co 2p3/2 spectra could be ascribed to carbon-bonded

cobalt atoms (Co−C).28 As shown in N 1s spectra, the binding energy of 397.7 eV is consistent with the Co−N in the Co4N phase.29 Whereas, the peaks at 398.4, 399.6 and 400.8 eV can be attributed to pyridinic-N, pyrrolic-N and graphitic-N, respectively.13 The existence of the Co−N, along with abundant pyridinic-N, pyrrolic-N and graphitic-N indicate that the successful transformation of Co oxides to Co4N, as well as the doping N atoms into the carbon layers. Benefiting from the doping of abundant N atoms into the carbon layers which covered on the surface of Co4N, the adsorption and catalytic performance of metallic Co4N could be further improved. Furthermore, the binding energy of 284.7, 285.6 and 288.6 eV in C 1s spectra are consistent with the C−C, C−N and C−O/C=O in the carbon matrix.30 A peak located at 283.9 eV which belongs to cobalt-bonded carbon atoms (C−Co) is also observed, these two peaks, which located at 283.9 (C−Co) and 778.8 eV (Co−C), indicate the existence of covalent bond between Co4N and NC layer. The covalent bridge between active Co4N surface and porous NC shells


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would provide facile pathways for electron and mass transport as well as maintain Co4N stable during the catalytic process.15, 28 As illustrated in Figure S2, the nitrogen adsorption/desorption isotherm and pore size distribution curves show the hierarchically porous features of Co4N@NC PNCs-500, with the BET surface area of 11.734 m2 g−1. These hierarchically porous structures could significantly facilitate the transportation of the reactant and the products in/out the Co4N@NC PNCs-500. Furthermore, the magnetic hysteresis of Co4N@NC PNCs-500 was measured at room temperature in the applied magnetic field sweeping from −20 to 20 kOe. The magnetic hysteresis loop was plotted in Figure 2, indicating the presence of ferromagnetic.31 The magnetization saturation (Ms) value, the coercivity (Hc) and retentivity (Mr) of Co4N@NC PNCs-500 are 88.90 emu g−1, 380.86 Oe and 15.72 emu g−1, respectively, similar to the previous report.32 The magnetic properties of Co4N@NC PNCs-500 should facilitate the magnetic separation of the catalyst from water by an external magnet, leading to the facile operation for the recycle. Annealing temperature plays a crucial role in promoting the transformation from Co3O4 to Co4N phase and crystallinity of Co4N@NC PNCs during the nitridation process. The transformation of structures and components for the samples under different temperatures (200, 300, 400, 500 and 600 oC) was investigated by TEM, XRD and XPS. As shown in the TEM images (Figure 3a−e), the hollow nanostructure can be retained when the annealing temperature was lower than 600 oC. An obvious lattice fringe transformation upon the temperature increasing can be observed in HRTEM images of the samples prepared under different temperatures. The lattice fringe of 0.286 nm was found in Co3O4@NC PNCs-200 which was ascribed to the (220) plane of Co3O4 at 200 oC. Whereas, the lattice fringe of 0.246 nm, which corresponds to (111) plane of CoO was obtained when the annealing temperature was lower than 300 oC. When the


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pyrolysis temperature exceeded 400 oC, the typical lattice fringe of 0.207 nm corresponding to the characteristic (111) lattice plane of Co4N was observed. An obvious growth of sub-units of the nanocages (small solid Co3O4 NPs in the walls of nanocage grew to bigger porous Co4N NPs) with temperature rising can be observed. Large aggregates can be obtained when annealing temperature reached to 600 oC. The lattice fringe of 0.207 nm can also match well with the characteristic lattice plane of Co4N, suggesting that the product is also a pure Co4N except for the collapsed structure. XRD was also used to monitor the changes of crystal structure. As shown in Figure 3f, the crystal phase conversion pattern of products is also found. The typical diffraction peaks of Co3O4 (JCPDS No. 42-1467) at 2θ =31.2, 36.8, 44.7, 59.4 and 65.2 o are maintained at a low annealing temperature (200 oC). When the temperature was raised to 300 oC, the characteristic diffraction peaks of Co3O4 disappeared and the diffraction peaks at 2θ = 36.5, 42.4, 61.5 and 73.7 o, corresponding to characteristic plane of CoO appeared (JCPDS No. 43-1004).33 This transformation from Co3O4 to CoO may attribute to the reducibility of NH3 or carbothermal reduction.34 When the temperature reached to 400 oC, the diffraction peaks of metallic Co4N at 2θ = 44.2 o emerged. However, the weak peaks indicate a poor crystallization of Co4N (JCPDS No.41-0943). The products obtained at 500 and 600 oC reveal clear characteristic diffraction peaks at 2θ = 44.2, 51.4 and 75.7 o, which matches well with the (111), (200) and (220) lattice plane of Co4N (JCPDS No.41-0943). A higher crystallization of the Co4N is also observed. Besides, an obvious shift towards the lower angle side can be observed from (111) lattice plane, indicating the lattice expansion due to the incorporation of N atoms into the Co lattice (see Figure S3).


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The surface chemical states under different nitridation temperature were also monitored by XPS (Figure 4). In the Co spectra, both the two dominating peaks, Co 2p1/2 and Co 2p3/2, show a positive chemical shift along with the nitridation temperature increasing, meaning a modulation of the surface electronic band bending after NH3 nitridation process.29 The residual Co−O peaks mean the slight exist of cobalt oxide layer on surface of Co4N@NC PNCs-500. The percentage of Co−N increases obviously with the increase of annealing temperature, suggesting the higher temperature are more likely to facilitate the transformation of Co3O4 to Co4N. The breakdown of stoichiometric Co3O4@C PNCs after NH3 nitridation can also be detected from the O 1s spectra (see Figure S4). The intensity of O2− (529.8 eV), which located in the crystal lattice of Co3O4 or CoO, decreases significantly with the increase of nitridation temperature, implying the efficient transformation of Co3O4 to Co4N. While there is no obvious change to the intensity of surface hydroxy and/or adsorbed oxygen species (531.6 eV).29 3.2 Catalytic Performance of 4-NP Reduction. The catalytic performance of Co4N@NC PNCs-500 was evaluated using model reaction of the reduction of 4-nitrophenol (4-NP) to 4aminophenol (4-AP) in the presence of NaBH4. 4-NP exhibits a characteristic absorption peak at 400 nm in NaBH4 solution due to the formation of 4-nitrophenolate ions under the increasing alkalinity, which corresponds to a color change of light yellow to dark yellow (Figure 5a). The absorbance peak at 400 nm can remain for 30 min in the absence of any catalysts, indicating that 4-NP was not hydrogenated by aqueous NaBH4 (Figure S5a). After adding a small amount (5 × 10−5 g) of Co4N@NC PNCs-500, the hydrogenation commences and the time-dependent absorption spectra intensity show a rapid decrease at 400 nm. The evolution of UV-vis spectra along with reaction time for the hydrogenation of 4-NP to 4-AP was also recorded. Meanwhile, the absorption peak at around 300 nm was observed with quickly decolorization of the solution


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from dark yellow to colorless within 90 s, signaling the formation of the hydrogenated products 4-AP (see Figure 5b). The catalytic performance of the Co-MOF precursor and Co3O4@C PNCs were also evaluated (Figure S5b−c). The plot of Ct/C0 against the reaction time is shown in Figure 5c, where Ct and C0 represents the concentration of 4-NP at the time of t and 0 min, respectively. As seen from Figure 5c, a 90% of decolorization rate is achieved in the presence of Co-MOF within 5 min. However, the reactivity of the Co-MOF nanocube was significantly decreased after 5 cycles, which ascribed to the structural instability of Co-MOF resulted from the weak coordination between Co2+ and [Co(CN)6]3− (Figure S8). In comparison, the decrease of 4NP with Co3O4@C PNCs is negligible, and the reduction of 4-NP with the Co3O4@C PNCs as a catalyst only resulted in less than 10% decrease in absorbance after 30 min reaction (Figure 5c and S5c). This slightly decrease in absorbance intensity can be attributed to the adsorption of 4NP on the Co3O4@C PNCs, rather than the catalytic reduction of Co3O4@C PNCs towards 4NP. This unsatisfactory performance of Co3O4@C PNCs could be ascribed to the intrinsically inferior electrical conducting properties of Co3O4 phase. PNCs configuration is another essential factor to the excellent performance of as-prepared Co4N@NC PNCs-500. Figure 5c and S5d show the catalytic performance of the Co4N@NC NPs which obtained through adequately grinding Co4N@NC PNCs-500 for 1 h. An obviously decline of hydrogenation activity was observed, whereas the TEM and XRD characterizations reveal that the grinding operation cannot destroy the nanoscale morphology and crystal structure of the Co4N@NC PNCs-500 except for the collapse of PNCs structure (Figure S6). The results display that the hollow nanocage structure with porous walls is a crucial to the high catalytic performance of Co4N@NC PNCs500.


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The plots of ln(Ct/C0) versus reaction time for the reduction of 4-NP are shown in Figure 5d. In the reaction, the concentration of NaBH4 can be considered as a constant since it is in great excess. Thus, the reduction of 4-NP can be fitted by the pseudo-first-order kinetics. The linear fit with a coefficient of determination very close to unity also supports the assumption of pseudofirst-order kinetics. The apparent rate constant kapp, calculated using equation 2, is 0.531, 0.019 and 2.106 min−1 for Co-MOF, Co3O4@C PNCs and Co4N@NC PNCs-500 catalysts, respectively. −dCt / dt = kapp Ct

(2)

Where, Ct is the concentration of 4-NP at time t and kapp is the apparent rate constant. The detailed results are listed in Table 1, and the as-prepared Co4N@NC PNCs-500 shows the highest kapp than Co-MOF precursor and Co3O4@C PNCs. As displayed in Figure 6b, the activation energy (Ea) and pre-exponential factor (A) of the reaction using Co4N@NC PNCs-500 as the catalyst were 23.53 kJ mol−1 and 2.712 × 104, respectively (listed in Table 2). The asprepared Co4N@NC PNCs-500 can significantly reduce the activation energy and elevate the apparent rate constants of 4-NP catalytic reduction. The TOF values were calculated and the results were shown in Figure 5f, Figure 7d and Table 1. First, the TOF values are 14.54 × 1020, 1.008 × 1020 and 52.01 × 1020 molecule g−1 min−1 for Co-MOF, Co3O4@C PNCs and Co4N@NC PNCs-500, respectively. Co4N@NC PNCs-500 exhibits the highest catalytic activities. Second, the products obtained at higher nitridation temperature are of higher TOF than those obtained at lower temperature. This is because the component transformation, as evidenced by TEM, XRD and XPS measurements (see Figure 3 & 4). The sample obtained at lower nitridation temperature (200 oC) was composed mostly of


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Co3O4 phase with small part of Co4N phase. The unsatisfied performance may be caused by the intrinsic inferior activity of Co3O4 phase, the shortage of defects and the dense carbon layer covered on the metal particles’ surface (see Figure S7a). When the nitridation temperature was elevated to 300 oC, the obtained CoO catalyst can get a 90% of convert ratio within 5 min to 4NP (Figure S7b). When temperature further elevated to 400 oC, the content of Co4N was increased significantly. Although the sample obtained at 400 oC has poor crystallinity, its activity has been greatly improved (Figure S7c). The catalytic activity reaches a maximum when nitridation temperature raised to 500 oC (Figure S7d). The excellent catalytic performance of the Co4N@NC PNCs-500 may attribute to the intrinsic high activity of metallic Co4N phase as well as the good permeability of hollow porous structure and the doping of N into the carbon layers. The decreased TOF over 600 oC can be ascribed to the deterioration of the pores and partial blocking of the diffusion routes of the reactant molecules to the active sites in the nanocages (Figure S7e). These results suggest that increasing the degree of crystallization of the products does not benefit the catalytic activity of the nanocatalyts. On the other hand, because of the complex nature of the interactions among Co4N, NC and the reactant molecules, a complete understanding of the different catalytic performances of the microspheres remains challenging. However, our facile approach for the preparation of Co4N@NC PNCs will be of great potential in discovering high-performance catalysts for various industrially important reactions. The obtained results of kapp and Ea over Co4N@NC PNCs-500 are also compared with other catalysts for this reaction, and the results were presented in Table S1. For example, kapp is 0.448 min−1 for hollow porous Au NPs, 0.5298 min−1 for Pd/C NPs and 1.8 min−1 for Ag nanocages, which is lower than 2.106 of Co4N@NC PNCs-500. The Co4N@NC PNCs-500 catalyst exhibits the highest catalytic activity for 4-NP reduction. The high efficiency of 3D Co4N@NC PNCs-


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500 towards 4-NP can be ascribed to the following proposal mechanism (see Figure S8): Firstly, the faster penetration and diffusion of 4-NP into the inner space of Co4N@NC PNCs were obtained via the abundant mesoporous pores. Then, more adsorption of 4-NP are onto the surface of the exposed active sites of Co4N@NC PNCs-500 NPs in hollow frames. Subsequently, 4-AP formed efficiently through the hydrogenation reduction of 4-NP, benefiting from the high electrical conductivity of Co4N complex, which can facilitate the electron transfer from BH4− to 4-NP, and reduces the activation energy. Besides, the porous NC layer covalent bridged with the surface of Co4N NPs, not only can act as an expressway between Co4N particles to transport electrons from the oxidation sites to reduction sites, but also can support abundant activity react sites attributed to the existence of N-doped sites and surface defects. At last, the produced 4-AP desorbed from the surface of Co4N@NC PNCs and diffused out conveniently from the hollow nanoporous structure via the abundant mesopores. 3.3 Recyclability of the Prepared Co4N@NC PNCs-500. The recyclability of catalyst is very important for heterogeneous catalytic process. Due to its magnetic property, the as-prepared Co4N@NC PNCs-500 catalyst can be easily separated from the solution by a magnet (Figure 5b). To test the recyclability of the catalysts, ten successive cycles of catalytic reduction were carried out with Co4N@NC PNCs-500. As revealed in Figure 8, in the first 6 cycles, completely reduction of 4-NP was completed within 3 min, but in the seventh run, the decolorization of 4NP have to spend over 3 min. For comparison, the recyclability of Co-MOF was also carried out. As shown in Figure S9, during the first 4 cycles, the conversion of 4-NP was more than 83% in 5 min reaction, but start from the fifth run, an obvious decrease was observed. These results indicate that the excellent stability and recyclability of the as-prepared Co4N@NC PNCs-500 catalyst, owing to the stable nanocage-like structure and the protection of covered NC layer.


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3.4 Catalytic Performance for MB. The catalytic reduction of MB in the presence of NaBH4 was also carried out to evaluate the catalytic performance of Co4N@NC PNCs-500. The evolution of UV-vis spectra along with reaction time for the hydrogenation of MB was recorded. As presented in Figure 9 and Table 2, the blue color of MB solution vanished rapidly within 4 min to colorless when 5 × 10−5 g of Co4N@NC PNCs-500 was used, exhibiting an excellent catalytic performance for hydrogenation of MB. The kapp, TOF and Ea toward reduction of MB over Co4N@NC PNCs-500 at room temperature are 1.324 min−1, 6.207 × 1020 molecule g−1 min−1 and 26.42 kJ mol−1, respectively.

4. CONCLUSIONS In summary, we have successfully fabricated a unique Co4N@N-doped carbon hollow porous nanocages through a direct nitridation of Co-MOF derived Co3O4@C hollow porous nanocages. The obtained Co4N@NC PNCs-500 was employed as a heterogeneous hydrogenation catalyst. Benefiting from the intrinsic high activity and electron conductivity of metallic Co4N phase and the good permeability of porous hollow nanostructure as well as the efficient doping of N into the carbon layer, the Co4N@NC PNCs hydrogenation catalyst shows high catalytic activity and stability. In addition, the covalent bridge between active Co4N surface and hollow porous nanocages shells provides facile pathways for electron and mass transport as well as the strong interface interaction for protection. Furthermore, the magnetic property and the stable configuration make it the excellent recyclability. This work not only offers a facile method to synthesize cobalt nitride-based porous hollow nanocages, but also experimentally demonstrates the potential applications of this novel Co4N@NC PNCs catalyst in heterogeneous


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hydrogenation reaction, paving a new way to prepare other highly active transition metal nitrides nanocages for wide applications. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available for free of charge on the ACS Publications website at DOI: XXX XXX. High magnification TEM images of different samples, Figure S1. N2 adsorption-desorption isotherms and pore size distribution of Co4N@NC PNCs-500, Figure S2. XRD patterns and XPS O 1s spectra of samples obtained under different nitridation temperature, Figure S3 & S4. Timedependent UV-vis spectra of different solutions under various conditions, Figure S5 & S7. TEM, HRTEM and XRD patterns of Co4N@NC PNCs-500 after grinding, Figure S6. Schematic diagram of the catalytic mechanism of Co4N@NC PNCs, Figure S8. The reusability of Co-MOF, Figure S9. Comparison of the activity of various catalysts for catalyzing the reduction of 4-NP, Table S1. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (L. Deng); *E-mail: [email protected] (Y.-N. Liu). Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the National Natural Science Foundation of China (No. 21636010 and 21476266), and State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China. Notes The authors declare no competing financial interest. REFERENCES (1) Sun, H.; Liu, S.; Zhou, G.; Ang, H. M.; Tadé, M. O.; Wang, S. Reduced Graphene Oxide for Catalytic Oxidation of Aqueous Organic Pollutants. ACS Appl. Mater. Interfaces 2012, 4, 5466– 5471. (2) Dutta, S.; Sarkar, S.; Ray, C.; Roy, A.; Sahoo, R.; Pal, T. Mesoporous Gold and Palladium Nanoleaves from Liquid-liquid Interface: Enhanced Catalytic Activity of the Palladium Analogue toward Hydrazine-assisted Room-temperature 4-Nitrophenol Reduction. ACS Appl. Mater. Interfaces 2014, 6, 9134–9143. (3) Ye, W.; Yu, J.; Zhou, Y.; Gao, D.; Wang, D.; Wang, C.; Xue, D. Green Synthesis of Pt-Au Dendrimer-like Nanoparticles Supported on Polydopamine-functionalized Graphene and Their High Performance toward 4-Nitrophenol Reduction. Appl. Catal. B 2016, 181, 371–378. (4) Chen, W.-F.; Muckerman, J. T.; Fujita, E. Recent Developments in Transition Metal Carbides and Nitrides as Hydrogen Evolution Electrocatalysts. Chem. Commun. 2013, 49, 8896– 8909. (5) Sheng, J. P.; Baikenov, M. I.; Liang, X. Y.; Rao, X. H.; Ma, F. Y.; Su, X. T.; Zhang, Y. Rapid Separation and Large-scale Synthesis of β-FeOOH Nanospindles for Direct Coal Liquefaction. Fuel Process. Technol. 2017, 165, 80–86.


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Scheme

Scheme 1. Synthesis route of Co4N@NC PNCs.


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Figures

Figure 1. TEM images of Co-MOF (a); Co3O4@C PNCs (b) and Co4N@NC PNCs-500 (c), inset in (c) is HRTEM image of Co4N@NC PNCs-500; (d) XRD patterns of Co-MOF, Co3O4@C PNCs and Co4N@NC PNCs-500; (e) XPS spectra of Co3O4@C PNCs and Co4N@NC PNCs-500. The typical HAADF-STEM and element mapping images (f) and EDS images (g) of Co4N@NC PNCs-500. High resolution of Co 2p (h); N 1s (i) and C 1s (j) spectra of Co4N@NC PNCs-500.


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Figure 2. Hysteresis loops of Co4N@NC PNCs-500 measured at room temperature in the applied magnetic field sweeping from −20 to 20 kOe. The inset shows the partial enlarged details.


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Figure 3. TEM and HRTEM images of the products which obtained under different annealing temperatures: (a) 200 oC; (b) 300 oC; (c) 400 oC; (d) 500 oC and (e) 600 oC. (f) The corresponding XRD patterns to these synthesized products.


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Figure 4. High resolution Co 2p XPS spectra of the products which obtained under different annealing temperatures. The dominating peak positions were labeled by red and blue dash line respectively, a positive chemical shift can be seen obviously along with the increase of the nitridation temperature.


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Figure 5. (a) UV-vis spectra of 4-NP solution before and after adding NaBH4; (b) the reduction of 4-NP in the presence of Co4N@NC PNCs-500; the plots of Ct/C0 (c) and lnCt/C0 (d) versus reaction time; the kapp (e) and TOF values (f) of difference catalysts.


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Figure 6. (a) Plot of ln(Ct/C0) versus time; (b) Arrhenius plot of lnK versus 1/T for the catalytic reduction of 4-NP in the presence of Co4N@NC PNCs-500 at 293.15 K, 298.15 K, 303.15 K and 308.15 K.


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Figure 7. Catalytic performance of 4-NP reduction to various catalysts which obtained under different nitridation temperatures: The plots of Ct/C0 (a) and lnCt/C0 (b) versus reaction time; the kapp (c) and TOF values (d) of difference catalysts.


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Figure 8. The recyclability of Co4N@NC PNCs-500 catalyst for the reduction of 4-NP.


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Figure 9. (a) UV-vis absorption spectrum changes in MB solution catalyzed by Co4N@NC PNCs-500; (b) Plot of ln(Ct/C0) versus time corresponds to (a); (c) Plot of ln(Ct/C0) versus time and (d) Arrhenius plot of lnK versus 1/T for the catalytic reduction of MB in the presence of Co4N@NC PNCs-500 at 293.15 K, 298.15 K, 303.15 K and 308.15 K.


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Tables Table 1. Kapp and TOF values for the reduction of 4-NP with different catalysts (the concentration of the catalyst is 16.6 mg L−1).

No.

Samples

TOF×10−20 molecule g−1 min−1

kapp min−1

R2

1

Co-MOF

14.54

0.531

0.964

2

Co3O4@C PNCs

1.008

0.019

0.946

3

Co4N@NC NPs

8.078

0.191

0.984

4

Co3O4@NC PNCs-200

0.893

0.022

0.970

5

CoO@NC PNCs-300

15.98

0.514

0.987

6

Co4N@NC PNCs-400

21.21

1.133

0.981

7

Co4N@NC PNCs-500

52.01

2.106

0.973

8

Co4N@NC PNCs-600

15.83

0.293

0.932

Table 2. Thermodynamics parameters of the catalytic reduction of 4-NP and MB at different temperatures.

Sample

4-NP

T K

TOF×10−20 molecule g−1 min−1

kapp min−1

R2

293.15

42.02

1.724

0.996

298.15

46.84

2.106

0.993

303.15

51.62

2.473

0.996

308.15

54.17

2.726

0.987

293.15

5.592

1.031

0.983

298.15

6.207

1.324

0.963

303.15

6.595

1.534

0.974

308.15

8.198

1.763

0.991

MB

Ea kJ mol−1

A×10−4

23.53

2.712

26.42

5.418


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Page 35 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC

3.6 cm × 8.4 cm


35

A MOF-Templated Fabrication of Hollow Co4N@N-doped Carbon

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