Topotactic Transformation of Metal–Organic Frameworks to Graphene

Topotactic Transformation of Metal−Organic Frameworks to Graphene-Encapsulated Transition-Metal Nitrides as Efficient Fentonlike Catalysts Xuning Li,†,‡ Zhimin Ao,§ Jiayi Liu,†,‡ Hongqi Sun,⊥ Alexandre I. Rykov,† and Junhu Wang*,† †

Mössbauer Effect Data Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China University of Chinese Academy of Sciences, Beijing 100049, China § Institute of Environmental Health and Pollution Control, School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou, 510006, China ⊥ School of Engineering, Edith Cowan University, Joondalup, Western Australia 6027, Australia ‡

S Supporting Information *

ABSTRACT: Innovation in transition-metal nitride (TMN) preparation is highly desired for realization of various functionalities. Herein, series of graphene-encapsulated TMNs (FexMn6−xCo4−N@C) with well-controlled morphology have been synthesized through topotactic transformation of metal−organic frameworks in an N2 atmosphere. The as-synthesized FexMn6−xCo4−N@C nanodices were systematically characterized and functionalized as Fenton-like catalysts for catalytic bisphenol A (BPA) oxidation by activation of peroxymonosulfate (PMS). The catalytic performance of FexMn6−xCo4−N@C was found to be largely enhanced with increasing Mn content. Theoretical calculations illustrated that the dramatically reduced adsorption energy and facilitated electron transfer for PMS activation catalyzed by Mn4N are the main factors for the excellent activity. Both sulfate and hydroxyl radicals were identified during the PMS activation, and the BPA degradation pathway mainly through hydroxylation, oxidation, and decarboxylation was investigated. Based on the systematic characterization of the catalyst before and after the reaction, the overall PMS activation mechanism over FexMn6−xCo4− N@C was proposed. This study details the insights into versatile TMNs for sustainable remediation by activation of PMS. KEYWORDS: graphene encapsulated, Mn4N, sulfate radicals, activation mechanism, DFT calculation

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and morphology of the spinels inherited the topology of metal−organic frameworks (MOFs) of PBAs precursors.10−12 Furthermore, topotactic transformation was applied as a feasible approach for fabrication of various functional nanocomposites such as monocrystalline bimetal hydroxides, porous metal phosphates, and oxides, etc.13−16 However, whether TMNs-based materials with well-controlled morphology could be fabricated from such strategy is still unknown. Sulfate radical (SO4•−) based advanced oxidation processes (SR-AOPs) through peroxymonosulfate (PMS) activation have received increasing attention for the degradation of recalcitrant

ransition-metal nitrides (TMNs) represent an important category of materials with interesting optoelectronic, catalytic, electrochemical, and structural functions.1−5 However, the synthesis of TMNs is often challenging due to the generally unreactive nature of the most common nitrogen sources, including N2 and NH3, at low temperature and pressure.5 Although various methods, including atomic layer deposition and physical vapor deposition techniques, have been developed,6,7 facile and environmentally friendly approaches for the preparation of nanoscale TMNs with high surface areas are still highly desirable. Previously, we reported a facile “copolymer-co-morphology” concept for shape-control synthesis of various Prussian blue analogues (PBAs).8 Porous spinel MnxFe1.8−xCo1.2O4 nanodices and FexCo3‑xO4 nanocages could be successfully synthesized via a one-step thermal decomposition of the PBAs in air.8,9 The resulting composition © 2016 American Chemical Society

Received: November 8, 2016 Accepted: December 9, 2016 Published: December 9, 2016 11532

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ACS Nano pollutants in water due to the strong oxidizing capability of SO4•− at a wide pH range.17−20 Among various transition-metal catalysts, cobalt has been proven as the most efficient PMS activator to produce SO4•−. However, the potential carcinogenic effects of Co remarkably limit its practical application, and innovation of the catalysis for PMS activation is highly desirable.21−24 Manganese-based materials, with the unique Mn2+/Mn3+ redox loop involving a single electron transfer and less toxic than cobalt to the environment, have been widely developed for applications in SR-AOPs to produce sulfate radical.25−31 It has been reported that the activity of different oxidation states of manganese for PMS activation follows the order: Mn2O3 > MnO > Mn3O4 > MnO2.28 An α-Mn2O3 cube was found to present better catalytic activity than octahedral and truncated octahedral α-Mn2O3, suggesting that shape-controlled synthesis is an efficient route to improve the catalytic performance.29 However, the activity of manganese oxide for PMS activation is usually much lower and requires a higher PMS dosage when compared to the cobalt-based catalysts. Because of Mn4N’s face-centered (FCC) cubic cell structure with one N atom in the center, the Mn atoms in it could lose electrons to N ions and thus increase its reducing capacity,32 which may have great significance for efficient activation of PMS to produce SO4•−. However, as far as we know, there are few reports of functionalized TMNs for PMS activation in environmental applications. In addition, the PMS activation processes by nitrogen-coordinated metals have not been well illustrated. Herein, we have successfully synthesized a series of graphene-encapsulated TMNs (FexMn6−xCo4−N@C; 0 < x < 6) with well-controlled morphology by topotactic transformation of MnyFe1−y[Co(CN)6]0.67·nH2O (MnyFe1−y−Co PBAs; 0 < y < 1) in N2 atmosphere. The morphology, textural property, and crystalline structure of the prepared FexMn6−xCo4−N@C were thoroughly characterized by various techniques. The series of FexMn6−xCo4−N@C nanodices were functionalized as Fenton-like catalysts for bisphenol A (BPA) oxidation by activation of PMS. The PMS activation processes over FexMn6−xCo4−N@C were systematically investigated, and density functional theory (DFT) calculations were applied for further confirming the superiority of Mn4N compared to MnO. Both sulfate and hydroxyl radicals were identified, and the degradation pathway of BPA was further investigated. Moreover, the overall PMS activation mechanism over FexMn6−xCo4−N@C was systematically studied and proposed.

Scheme 1. Preparation Route and Model of the Graphene Encapsulated TMNs (FexMn6−xCo4−N@C) with WellControlled Morphology

crystal system (cubic) and space group (Pm−3m) of FeCo and Mn4N. The final obtained FexMn6−xCo4−N@C particles, constructed from the small FeCo and Mn4N nanocrystals encapsulated in graphene layers, will inherit the morphology of MnyFe1−y−Co PBAs. Figure 1A−D display typical SEM images of the obtained FexMn6−xCo4−N@C. Extremely uniform morphologies of the FexMn6−xCo4−N@C particles, like nanodices, which well inherited the morphologies of MnyFe1−y−Co PBAs, were obtained. In addition, with the increasing content of Mn, a regular change of the morphology of FexMn6−xCo4−N@C nanodices could be realized, suggesting the flexibility of the strategy for shape-control synthesis. The EDX mappings of Fe4Mn2Co4−N@C are shown in Figure 1E, illustrating that the Mn, Fe, and Co species were homogeneously distributed in the particles. The TEM image of Fe4Mn2Co4−N@C further confirmed the porous structure of the nanodices (Figure 1F). The high-resolution transmission electron microscopy (HRTEM) images of single nanocrystals (Figure 1G−I) clearly show that the FeCo and Mn4N/Fe4N nanocrystals with interlayer distances of 0.20 and 0.22 nm, respectively, were well coated by graphene shells with an interlayer distance of 0.34 nm.40 The element composition of the products was recorded by energy dispersive X-ray spectrometer (EDS) and is shown in Table S1. Powder X-ray diffraction (XRD) was applied to confirm the crystallographic structure and phase of the products (Figure 2B). As can be seen, the FeCo bimetal alloy phase (JCPDS No.44-1433) was observed in the pattern of Fe5Mn1Co4−N@C. However, with increasing content of Mn, two peaks at 41.0° and 47.8° could be observed and the peak intensity became stronger, suggesting the existence of Mn4N (JCPDS No.89-7380), which gradually appeared as a main phase in FexMn6−xCo4−N@C. Based on the Debye− Scherrer equation (D = 0.89 λ/B cos θ), the average crystalline size of FexMn6−xCo4−N@C was estimated (Table S1), indicating that the particle of the samples was constructed by small nanocrystals with an average crystal size of about 28 nm. 57 Fe Mössbauer spectroscopy was applied to investigate the coordination environment and oxidation state of Fe ions in the samples (Figure 2A). The four spectra were fitted mainly with two sextets, which could be assigned to two kinds of metallic irons in the FeCo alloy with different chemical environment and/or different crystallite size.41 The corresponding Mössbauer parameters are shown in Table S2. According to the isomer shift (IS) and hyperfine field (Bhf), the magenta doublet and the green sextets in the upper three spectra could be assigned as Fe4N.42 The Bhf value of the green sextet in Fe5Mn1Co4−N@C (9.0 T) was found to be much smaller than the literature

RESULTS AND DISCUSSION The MOF precursors, MnyFe1−y-Co PBAs, were prepared similarly to our previous report.8 The SEM images of the PBAs (Figure S1) reveal the well-controlled morphology of the four samples like nanodices with good dispersity. A TG-DSC profile of Mn0.4Fe0.6−Co PBA under N2 atmosphere indicates two decomposition stages (Figure S2). The weight loss at the first stage (below 215 °C) could be ascribed to the loss of water molecules from the MOFs structure. The following weight loss at the second stage (over 455 °C) could be ascribed to the conversion of MOFs structure to the final products. As illustrated in Scheme 1, through thermal decomposition of MnyFe1−y−Co PBAs at 650 °C in N2, the CN− group of PBAs will serve as nitrogen and carbon sources to form nitrogendoped graphene layers.37−39 Meanwhile, Co and Fe atoms will form an FeCo alloy, while Mn atoms will form Mn4N nanocrystals inside, which are most probably due to the same 11533

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Figure 1. FESEM images of (A) Fe5Mn1Co4−N@C, (B) Fe4Mn2Co4−N@C, (C) Fe3Mn3Co4−N@C, and (D) Fe1Mn5Co4−N@C; (E) EDX mappings, (F) TEM, (G, H) HRTEM images of Fe4Mn2Co4−N@C; and (I) HRTEM image of Fe1Mn5Co4−N@C.

Figure 2. (A) Room-temperature 57Fe Mössbauer spectra of FexMn6−xCo4−N@C nanodices. The latter three spectra were fitted with two sextets, corresponding to two kinds of metallic irons in the FeCo alloy with different chemical environments and/or different crystallite sizes. The magenta doublet and the green sextet in the upper three spectra could be assigned to Fe4N. (B) XRD patterns (C) High-resolution XPS spectrum of N 1s of FexMn6−xCo4−N@C nanodices.

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Figure 3. (A) Removal efficiency of BPA in different reaction systems within 12 min. (B) Rate constant of FexMn6−xCo4−N@C compared with that of FexMn6−xCo4−H@C. (C) Time-dependent EPR spectra in activation of PMS by Fe1Mn5Co4−N@C. (D) Time-dependent evolution of the HPLC diagrams for BPA oxidation catalyzed by Fe1Mn5Co4−N@C. Reaction conditions: [BPA] = 20 mg L−1, [PMS] = 0.2 g L−1, catalyst = 0.1 g L−1, T = 298 K, initial solution pH = 6.0.

promising potential for sustainable remediation by activation of PMS. The catalytic performance of the FexMn6−xCo4−N@C nanodices was evaluated for BPA removal by activation of PMS. As shown in Figure 3A, less than 2% of BPA could be degraded by PMS, indicating the negligible radical production through thermal activation of PMS at current reaction conditions. A control test using only Fe1Mn5Co4−N@C nanodices reveals that about 6% of BPA could be adsorbed in 12 min. However, the simultaneous presence of FexMn6−xCo4−N@C and PMS led to a significant enhancement on the BPA removal efficiency, which could achieved 51%, 74%, 97%, and 100% in 12 min catalyzed by Fe5Mn1Co4− N@C, Fe 4 Mn 2 Co 4 −N@C, Fe 3 Mn 3 Co 4 −N@C, and Fe1Mn5Co4−N@C, respectively. These results suggest that the FexMn6−xCo4−N@C nanodices are highly efficient catalysts for PMS activation to produce reactive radicals, which could be attributed mainly to coexistence of Fe, Mn, and Co species in the core metals. The well-developed graphene shells, on the other hand, most probably act as a protective layer, thus increasing the stability of the catalysts.45 As shown in Figure S6, the catalytic performance of regenerated Fe1Mn5Co4−N@C remained well after four-cycle runs, suggesting the high stability of FexMn6−xCo4−N@C nanodices as Fenton-like catalysts for organic pollutant degradation. In addition, as BPA removal efficiency could be largely enhanced with the increase of Mn

reported value (21.6T), which is most probably due to the substitution of the Fe by Mn.43 In addition, the high-resolution X-ray photoelectron spectra (XPS) of Co 2p3/2 and Mn 2p3/2 of FexMn6−xCo4−N@C are shown in Figure S3, and the binding energies at 639.5 and 778.5 eV corresponded to metallic manganese and cobalt, respectively. The high-resolution XPS spectra of N 1s of FexMn6−xCo4−N@C nanodices were fitted with three individual peaks, corresponding to nitrogen bound to the metal (Mn−N and Fe−N; 396.6 eV), pyridinic N (398.4 eV), and graphitic N (400.7 eV), respectively (Figure 2C).44 All these results suggest that the graphene-encapsulated TMNs (FexMn6−xCo4−N@C) with Mn4N and FeCo alloy as the main phase and well-controlled morphology could be successfully synthesized through the one-step thermal decomposition of MnyFe1−y−Co PBAs. The TG-DSC profile of the Fe3Mn3Co4−N@C under air atmosphere is shown in Figure S4. The increase of the weight at the first stage (197−332 °C) was due to the oxidation of Fe/ Co/Mn to oxides. The following weight loss (over 332 °C) could be ascribed to the combustion of graphene layers. It was interesting to see that the weight remained unchanged until the temperature reached 197 °C, suggesting the good stability of the FexMn6−xCo4−N@C nanodices. In addition, considering the large specific surface areas (Figure S5) and the magnetic separation performance of all four samples, they may have 11535

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ACS Nano content, the Mn4N in FexMn6−xCo4−N@C is therefore most probably the main factor for the higher activity. To further confirm the role of Mn4N, Fe1Mn5Co4−H@C, with MnO and FeCo alloy as the main phase (Figure S7), was prepared using the same precursor by thermal decomposition at 650 °C and kept for 1 h under H2 atmosphere. The fieldemission scanning electron microscopy (FESEM) images of the as-prepared Fe1Mn5Co4−H@C are shown in Figure S8, indicating that the morphology of the PBAs could also be inherited after heating under H2 atmosphere. A first-order kinetic model46 was applied to calculate the apparent rate constant (k) of BPA removal kinetics catalyzed by FexMn6−xCo4−N@C, and Fe1Mn5Co4−H@C nanodices. As shown in Figure 3B, the values of k increased from 0.055 to 0.480 min−1 with an increase of the content of Mn in FexMn6−xCo4−N@C. The optimal activity of Fe1Mn5Co4−N@ C, with a k value equal to 0.480 min−1, was much higher than that of Fe1Mn5Co4−H@C (0.065 min−1) as well as most of the recent reported catalysts (Table S3),9,27,31,46−49 further confirming the crucial role of Mn4N for the largely enhanced PMS activation efficiency. To investigate the activation mechanism, the involved radicals produced during the PMS activation were investigated using radical scavenger experiments. As shown in Figure S9, the reaction rate was largely decreased when TBA, a common used radical scavenger for •OH,50 was added into the reaction system, indicating •OH as one of the main reactive radicals involved. The reaction rate was further decreased when methanol, an effective radical scavenger for both •OH and SO4•−, was added into the reaction system, suggesting that both SO4•− and •OH were generated in the reaction system. The electron paramagnetic resonance (EPR)/DMPO experiments were carried out to further confirm the involved radicals. As shown in Figure 3C, characteristic signals for both DMPO− SO4•− and DMPO−•OH adducts were observed at the different reaction times.51,52 The decrease of the relative intensity of the EPR signals at 4 min was most probably due to the fast consumption of PMS during the reaction. All of these results further reveal that both •OH and SO4•− were involved as the main reactive radicals during the PMS activation using FexMn6−xCo4−N@C as the catalyst. In addition, the amount of sulfate radical produced during the PMS activation at various time was quantified based on the quantitation of BQ (Figure S10).53 Results demonstrated that at least 35.9 μmol L−1 SO4•− was generated from PMS activation catalyzed by Fe1Mn5Co4− N@C nanodices. The main degradation intermediates of BPA during its degradation by SO4•− and •OH radicals were detected by the time-dependent evolution of the HPLC diagrams (Figure 3D) and identified by analyzing the m/z data obtained by LC−MS (Figures S11 and 12). The main intermediate (product I), with a retention time (tR) of 6.95 min and m/z of 242, was produced through hydroxylation and oxidative of BPA.54 As can be seen, product I was accumulated at first and then decomposed and gradually formed product I (tR 5.46), further confirming that the BPA degradation pathway was mainly through hydroxylation, oxidation, and decarboxylation of the aromatic ring.9,55 Furthermore, as shown in Figure S13, more than 43% of BPA was mineralized within 12 min, suggesting that BPA was not only decomposed to small organic compounds but even mineralized to CO2. To better understand the effect for PMS activation on MnO and Mn4N, density functional theory (DFT) calculations are

performed to investigate the adsorption of PMS on MnO and Mn4N surfaces. On the basis of experimental results, MnO (200) and Mn4N (111) were chosen for the calculations. All of the possible adsorption sites and orientation of PMS were considered, and Figure 4 shows the relaxed atomic structures of

Figure 4. Atomic configuration of PMS on MnO (200) [panel (a)] and Mn4N (111) surfaces [panel (b)], respectively. The red, yellow, white, purple, and blue spheres are O, S, H, Mn, and N atoms, respectively.

a PMS molecule adsorption on MnO (200) (panel a) and Mn4N (111) surfaces (panel b), respectively. For adsorption on the MnO (200) surface, it shows that the PMS is standing on the MnO (200) surface with the two O atoms on the −SO4 side bonding with two Mn atoms on surface. The two bond lengths are 2.094 and 2.277 Å, respectively. For the adsorption of PMS on the Mn4N (111) surface, the PMS lays down with the OH group on the left, and two O atoms from −SO4 group bind with two Mn atoms on the surface. To better understand the interaction between the surfaces and PMS, also for the activation of PMS, Table S4 provides the adsorption energy of PMS (Eads) on the two different surfaces, charge transfer between PMS and surface Q, and the bond length lO−O between the −OH group and −SO4 group. The adsorption energy (or binding energy for this case) Eads can be determined by Eads = Etot − EMnO − EPMS, where Etot, EMnO, and EPMS are the energies of the MnO surface with PMS adsorption, the MnO surface, and the PMS molecule, respectively. As shown in Table S4, the adsorption on both surface is quite strong with Eads being −4.0 and −5.23 eV for MnO (200) and Mn4N (111) surfaces, respectively. This also agrees with the formation of covalent bonds between PMS and the two surfaces as shown in Figure 4. Comparing the two surfaces, the adsorption of PMS on Mn4N (111) is stronger with stronger Eads, longer lO−O, and more electrons received on PMS. Therefore, the PMS on the Mn4N (111) is more active. 11536

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Figure 5. High-resolution XPS spectra of (A) Co 2p3/2, (B) N 1s, (C) Mn 2p3/2, and (D) O 1s of Fe1Mn5Co4−N@C before and after reaction. Reaction conditions: [BPA] = 20 mg L−1, [PMS] = 0.2 g L−1, catalyst = 0.1 g L−1, T = 298 K, initial solution pH = 6.0.

of the catalyst before and after the reaction are shown in Figure 5B. Negligible changes could be found on the nitrogen bound to the metal (Mn−N and Fe−N) as well as the pyridinic N and graphitic N, indicating the better stability of Mn4N for the activation of PMS. The XPS spectra of Mn 2p3/2 (Figure 5C) could be deconvoluted into three contributions with binding energy located at 639.5, 641.3, and 643.1 eV, attributed to the metallic manganese (Mn0), Mn2+, and Mn3+, respectively.8,58 The relative proportion of Mn0 was decreased after the reaction, suggesting the partial oxidation of Mn0 to Mn2+ and Mn3+ on the Fe1Mn5Co4−N@C surface. In addition, the increase of the relative proportion of lattice O after reaction (Figure 5d) could further confirm the oxidation of Co0, Fe0, and Mn0 during the activation of PMS. However, 12% of the relative contributions of Mn0 to the overall Mn intensity after reaction further suggest the better stability of Mn4N for the activation of PMS. Therefore, the overall mechanism of PMS activation over FexMn6−xCo4−N@C nanodices could be proposed as follows (Figure 6). First, Co0/Fe0/Mn0 in FexMn6−xCo4−N@C could activate PMS to produce SO4•− and •OH radicals through eqs 1,2 with itself oxidized to Co2+/Fe2+/Mn2+ and Co3+/Fe3+/Mn3+, respectively.17,44 Second, the generated  Co2+/Fe2+/Mn2+ could be quickly oxidized by PMS and

For a better insight into the mechanism of PMS activation over FexMn6−xCo4−N@C nanodices, a number of characterizations on the used catalyst were carried out to clarify the structural changes and the redox cycling of the Fe, Co, and Mn species during the reaction process. The XRD patterns of Fe1Mn5Co4−N@C before and after reaction are shown in Figure S14, while negligible changes could be found on the diffraction peaks of FeCo and Mn4N. More importantly, the morphology of Fe1Mn5Co4−N@C remained (Figure S15). All of these results suggest the good stability of the FexMn6−xCo4− N@C nanodices. High-resolution XPS spectra of the used catalyst were applied to further explore the PMS activation mechanism. Figure 5A shows the high-resolution XPS spectra of Co 2p3/2 of Fe1Mn5Co4−N@C before and after the reaction. Three peaks with binding energy located at 778.5, 780.5, and 782.1 eV could be assigned to the alloyed cobalt (Co0), Co2+, and Co3+, respectively. 56,57 After the reaction, the peak of Co 0 disappeared, and the relative contents of Co2+ and Co3+ increased. A similar phenomenon was found on the highresolution XPS spectra of Fe 2p3/2 of Fe1Mn5Co4−N@C (Figure S16). All of these results suggest that the Co0 and Fe0 on Fe1Mn5Co4−N@C surface were oxidized during the activation of PMS. The high-resolution XPS spectra of N 1s 11537

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activation mechanism over FexMn6−xCo4−N@C was proposed on the basis of the systematic characterization of the catalyst before and after the reaction. This study details the insights into versatile TMNs for sustainable remediation by activation of PMS and may shed some light for the development of highperformance catalyst for PMS activation.

EXPERIMENTAL METHODS Chemicals. Potassium hexacyanocobaltate(III) (98%) was purchased from Beijing J&K Co., Ltd., China. PMS (KHSO5·0.5KHSO4· 0.5K2SO4) was purchased from Alfa Aesar. BPA, p-hydroxybenzoic acid (HBA), benzoquinone (BQ), and 5,5-dimethyl-1-pyrroline Noxide (DMPO) were purchased from Aladdin Co., China. Ferrous chloride, manganous nitrate, tert-butyl alcohol (TBA), methyl alcohol, and poly(vinylpyrrolidone) (PVP) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd., China. Sample Preparation. MnyFe1−y[Co(CN)6]0.67·nH2O (MnyFe1−y− Co PBAs; 0 < y < 1) were synthesized similarly to our previous report.8 Typically, desired amounts of FeCl2·4H2O and Mn(NO3)2 (9 mM) were first dissolved in 40 mL of PVP (1.2 g) solution under vigorous stirring. K3[Co(CN)6] aqueous solution (40 mL, 5 mM) was then added into the above-mentioned solution slowly. The colloid solution was stirred for 30 min and then aged for 20 h. The precipitates obtained were centrifuged and washed at least three times using a mixture of deionized water and ethanol, followed by drying at 333 K in an oven for 20 h. FexMn6−xCo4−N@C (0 < x < 6) and Fe1Mn5Co4−H@C nanodices were obtained by heating MnyFe1−y−Co PBAs at 650 °C and kept for 1 h under N2 and H2 atmosphere, respectively. Characterization. The structures of the as-prepared samples were characterized by XRD equipped with a PANalytical X’Pert-Pro X-ray diffractometer. The morphological and textural properties were studied by HR-TEM (JEM-2100) and FESEM (JSM 7800F). The element compositions were measured on an EDS. The X-ray photoelectron spectra (XPS) recorded on the ESCALAB 250 X-ray photoelectron spectroscope were fitted using XPSPEAK41 software. The specific surface areas were measured on a Micromeritics ASAP 2010 instrument using the Brunauer−Emmett−Teller (BET) N2 adsorption−desorption method. The thermogravimetric (TG) studies were conducted on a Setaram Setsys 16/18 thermo-analyzer with a heating rate of 10 K min−1 in N2 or air flow. The room-temperature 57 Fe Mössbauer spectra were obtained from a proportional counter and a Topologic 500A spectrometer equipped with 57Co (Rh) as γ-ray radioactive source. The EPR spectra were collected with a center field at 3350 G using a Bruker EPR I200 spectrometer. Catalytic Activity Measurements. The performance of the catalysts was tested in a 200 mL reactor containing 20 mg L−1 BPA with a initial solution pH of 6.0. The reaction temperature was maintained at 25 °C using a water bath. In a typical test, 50 mL of BPA solution containing 5 mg of catalyst was first stirred for 10 min to establish the adsorption−desorption equilibrium. A certain amount of PMS aqueous solution was then added into the above-mentioned solution to initiate the reaction. One milliliter of solution was immediately withdrawn and quenched with 0.5 mL of methanol at predetermined time intervals. The BPA and BQ concentrations were analyzed using HPLC (Agilent, 1260-Infinity) at detection wavelengths of 230 and 244 nm, respectively. A methanol/water mixture (70:30, v/v) was used as the mobile phase. The BPA degradation intermediates were detected using LC−MS with an Agilent 1290 Infinity ultrahigh-performance liquid chromatography system equipped with an Agilent C18 column in combination with an Agilent 6540 quadrupole time-of-flight mass spectrometer. The total organic carbon (TOC) was detected by a total organic carbon analyzer (TOCVCPH/CPN, Shimadzu, Japan). Computational Models and Methodology. The DFT calculations are carried out using the CASTEP module.33 Exchange− correlation functions are used as local density approximation with CAPZ.34,35 The convergence tolerance of energy is 2.0e−5 eV/Atom, and the maximum-allowed displacement and force are 0.002 Å and 0.05

Figure 6. Proposed mechanism for PMS activation over FexMn6−xCo4−N@C nanodices.

produced SO4•− radicals (eq 3). Both PMS and Co0/Fe0/ Mn0 could further reduce Co3+/Fe3+/Mn3+ to Co2+/Fe2+/ Mn2+ and thus makes the reaction proceed cyclically until PMS was completely consumed.59−61 In addition, the stronger Eads, longer lO−O, and more electrons received on PMS catalyzed by Mn4N largely enhanced the catalytic activity of FexMn6−xCo4− N@C. Co0 /Fe0 /Mn 0 + 2HSO5− → Co2 +/Fe2 +/Mn 2 + + 2SO4•− + 2OH−

(1)

Co0 /Fe0 /Mn 0 + 3HSO5− → Co3 +/Fe3 +/Mn 3 + + 3SO4 2 − + 3•OH

(2)

Co2 +/Fe2 +/Mn 2 + + HSO5− → Co3 +/Fe3 +/Mn 3 + + SO4•− + OH−

(3)

Co3 +/Fe3 +/Mn 3 + + HSO5− → Co2 +/Fe2 +/Mn 2 + + SO5•− + H+

(4)

Co0 /Fe 0 /Mn 0 + 2Co3 +/Fe3 +/Mn 3 + → 3Co2 +/Fe 2 +/Mn 2 + •

OH/SO4•− + BPA → intermediates → CO2

(5) (6)

CONCLUSIONS In summary, topotactic transformation was developed as a facile strategy for shape-controlled synthesis of graphene-encapsulated TMNs (FexMn6−xCo4−N@C) via a one-step thermal decomposition of MOFs in N2 atmosphere. The catalytic oxidation performance of the as-synthesized FexMn6−xCo4− N@C nanodices in BPA degradation by PMS activation was found to be largely enhanced with the increasing of the content of Mn4N. The optimal activity of Fe1Mn5Co4−N@C was much higher than that of Fe1Mn5Co4−H@C, which contains MnO and FeCo alloy as the main phase. DFT calculations illustrated that Mn4N is able to dramatically reduce the adsorption energy and facilitate electron transfer for PMS activation. Through radical scavenger and EPR/DMPO experiments, both •OH and SO4•− were confirmed as the main reactive radicals involved during the PMS activation. Moreover, the overall PMS 11538

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ACS Nano eV/Å, respectively. Considering the van der Waals forces, the DFT-D method is used for all calculations.36 To avoid the interaction of surfaces in different supercells along C direction, a 20 Å vacuum above the surface is taken. All atoms are allowed to relax expected the bottom one layer atoms to represent the bulk material surface.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07522. Other characterizations along with additional supporting data (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Hongqi Sun: 0000-0003-0907-5626 Junhu Wang: 0000-0003-1987-2522 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21476232) and the Chinese Academy of Sciences Visiting Professorships for Senior International Scientists (2011T1G15). Z.A. acknowledges financial support from the “1000 plan” for Young Professionals Program of China, the “100 Talents” Program of Guangdong University of Technology, and the National Natural Science Foundation of China (21607029). Many thanks to the anonymous reviewers who have helped improve this paper. REFERENCES (1) Chhowalla, M.; Unalan, H. E. Thin Films of Hard Cubic Zr3n4 Stabilized by Stress. Nat. Mater. 2005, 4, 317−322. (2) Lee, B. S.; Yi, M.; Chu, S. Y.; Lee, J. Y.; Kwon, H. R.; Lee, K. R.; Kang, D.; Kim, W. S.; Lim, H. B.; Lee, J.; Youn, H.-J.; Chi, D. Y.; Hur, N. H. Copper Nitride Nanoparticles Supported on a Superparamagnetic Mesoporous Microsphere for Toxic-Free Click Chemistry. Chem. Commun. 2010, 46, 3935−3937. (3) Yang, M.; Zakutayev, A.; Vidal, J.; Zhang, X.; Ginley, D. S.; DiSalvo, F. J. Strong Optical Absorption in Cutan2 Nitride Delafossite. Energy Environ. Sci. 2013, 6, 2994−2999. (4) Cui, Z.; Yang, M.; DiSalvo, F. J. Mesoporous Ti0.5cr0.5n Supported Pdag Nanoalloy as Highly Active and Stable Catalysts for the Electro-Oxidation of Formic Acid and Methanol. ACS Nano 2014, 8, 6106−6113. (5) Vaughn, D. D., II; Araujo, J.; Meduri, P.; Callejas, J. F.; Hickner, M. A.; Schaak, R. E. Solution Synthesis of Cu3pdn Nanocrystals as Ternary Metal Nitride Electrocatalysts for the Oxygen Reduction Reaction. Chem. Mater. 2014, 26, 6226−6232. (6) Yang, M.; DiSalvo, F. J. Template-Free Synthesis of Mesoporous Transition Metal Nitride Materials from Ternary Cadmium Transition Metal Oxides. Chem. Mater. 2012, 24, 4406−4409. (7) Veith, G. M.; Baggetto, L.; Adamczyk, L. A.; Guo, B.; Brown, S. S.; Sun, X.-G.; Albert, A. A.; Humble, J. R.; Barnes, C. E.; Bojdys, M. J.; Dai, S.; Dudney, N. J. Electrochemical and Solid-State Lithiation of Graphitic C3n4. Chem. Mater. 2013, 25, 503−508. (8) Li, X.; Yuan, L.; Wang, J.; Jiang, L.; Rykov, A. I.; Nagy, D. L.; Bogdan, C.; Ahmed, M. A.; Zhu, K.; Sun, G.; Yang, W. A ″CopolymerCo-Morphology″ Conception for Shape-Controlled Synthesis of Prussian Blue Analogues and as-Derived Spinel Oxides. Nanoscale 2016, 8, 2333−2342. 11539

DOI: 10.1021/acsnano.6b07522 ACS Nano 2016, 10, 11532−11540

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DOI: 10.1021/acsnano.6b07522 ACS Nano 2016, 10, 11532−11540