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Single Cobalt Atoms Anchored on Porous N-Doped Graphene with Dual Reaction Sites for Efficient Fenton-like Catalysis Xuning Li, Xiang Huang, Shibo Xi, Shu Miao, Jie Ding, Weizheng Cai, Song Liu, Xiaoli Yang, Hongbin Yang, Jiajian Gao, Junhu Wang, Yanqiang Huang, Tao Zhang, and Bin Liu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05992 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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Journal of the American Chemical Society

Single Cobalt Atoms Anchored on Porous N-Doped Graphene with Dual Reaction Sites for Efficient Fenton-like Catalysis Xuning Li,†,‡ Xiang Huang,§ Shibo Xi,! Shu Miao,† Jie Ding,† Weizheng Cai,‡ Song Liu,†,‡,# Xiaoli Yang,†,# Hongbin Yang,‡ Jiajian Gao,‡ Junhu Wang,†,!! Yanqiang Huang,†,* Tao Zhang,†,# and Bin Liu‡,* †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China ‡

School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore

§

Department of Physics, Southern University of Science and Technology, Shenzhen 518055, PR China

!

Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Singapore 627833, Singapore

!!

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

Supporting Information Placeholder ABSTRACT: Fenton-like process presents one of the most promising strategies to generate reactive oxygen containing radicals to deal with the ever-growing environmental pollution. However, developing improved catalysts with adequate activity and stability is still a long-term goal for the practical application. Herein, we demonstrate single cobalt atoms anchored on porous N-doped graphene with dual reaction sites as highly reactive and stable Fenton-like catalysts for efficient catalytic oxidation of recalcitrant organics via activation of peroxymonosulfate (PMS). Our experiments and density functional theory (DFT) calculations show that the CoN4 site with single Co atom serves as the active site with optimal binding energy for PMS activation, while the adjacent pyrrolic N site adsorbs organic molecules. The dual reaction sites greatly reduce the migration distance of the active singlet oxygen produced from PMS activation and thus improve the Fenton-like catalytic performance.

INTRODUCTION Fenton-like process, as a clean and efficient approach to generate reactive oxygen-containing radicals for the elimination of recalcitrant organic pollutants, has been regarded as a promising strategy to deal with the evergrowing environmental pollution and the scarcity of fresh water resources resulted from rapid civilization and industrialization.1-2 The hydroxyl radical (•OH) based Fenton system, using hydrogen peroxide (H2O2) as the oxidizing agent, is highly efficient to degrade nearly all organic compounds.3 However, the low utilization rate of H2O2 and narrow work pH range remarkably limit its practical applications.4-5 Recently, sulfate radical (SO4•-) based Fenton-like system via activation of peroxymonosulfate (PMS) has received increasing attention for the degradation of recalcitrant pollutants in water because of the strong oxidizing capability of SO4•- at wider pH ranges.6-8 Over the past few years, various transition metal-based materials have been studied as Fenton-like catalysts for PMS activation.9-13 Unfortunately, most of them suffer from the problems of metal ions leaching and low catalyt-

ic performance.14 Alternatively, although a number of exciting advances have been achieved using N-doped graphene for PMS activation through a non-radical reaction process,15-18 the performances of such non-metal based catalysts are always depressed and development of improved catalysts with adequate activity and stability is still a long-term goal to realize their practical application. As a radical predominant reaction, the half-life periods of most reactive oxygen-containing radicals generated from PMS activation are quite short, which are less than 1 μs for •OH and 30 - 40 μs for SO4•-.19-20 In this regard, approaches to minimize the migration distance of the reactive radicals to the target organic molecules are therefore highly desirable for maximizing the catalytic performance. To this end, construction of catalysts with dual reaction sites, which are able to independently recognize/activate the respective target functional groups of substrates, is highly attractive to realize the simultaneous reactive radical generation on the catalytic site and efficient target pollutant degradation on the adsorption site. Single-atom catalyst (SAC) with atomically distributed active metal center has recently emerged as a new research frontier in various catalytic reactions for maximum

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Figure 1. The preparation route and model of single-Coatom catalyst. Scale bars: (left) 50 nm, (middle) 4 nm, and (right) 2 nm.

atom efficiency due to the unique electronic property of a single metal active site.21-23 Our previous study has also confirmed that the N coordination could dramatically enhance the Fenton performance due to the reduced adsorption energy and facilitated electron transfer for PMS activation catalyzed on Mn4N.24 Taking into consideration of these two factors as well as the excellent adsorption property of carbon materials, anchoring single transition metal atoms on N-doped graphene are thus expected to be effective to improve the Fenton-like catalysis. Furthermore, a clear cognition on the reactive sites of SAC for PMS activation is highly desired for further insight into the underlying mechanism of Fenton chemistry, and it may shed light on the development of other “singleatom catalysts”. Herein, we report single cobalt atoms anchored on porous N-doped graphene as highly reactive and stable Fenton-like catalysts for efficient catalytic oxidation of bisphenol A (BPA) via activation of PMS. We used Fe doping and changed the calcination temperature to modulate the contents of pyrrolic N in N-doped graphene, which was verified as the adsorption site for binding BPA based on both experiments and density functional theory (DFT) calculations. The optimized single-cobalt-atom catalyst exhibits impressive catalytic performance with TOF as high as 12.52 min-1 for BPA degradation. The DFT calculations further helped to attribute the high catalytic activity to the CoN4 site with single Co atom anchored on Ndoped graphene with optimal adsorption energy to facilitate electron transfer for PMS activation. The 5’5Dimethyl-1-Pyrroline N-oxide (DMPO) trapped electron paramagnetic resonance (EPR) and radical scavenger experiments confirmed the singlet oxygen produced from the activation of PMS on the CoN4 sites as the critical active species for the rapid degradation of BPA that was adsorbed on the pyrrolic N sites. Our study details the first insights into the single-cobalt-atom based catalyst with dual reaction sites for efficient and sustainable remediation of organic pollutants.

RESULTS AND DISCUSSION

Figure 2. Structural characterization of the FeCo-NC-2. (a) FESEM image, (b, c) TEM images, (d) HRTEM image, (e) EDX mappings, and (f-i) HAADF-STEM images of FeCo-NC2. Scale bars: (a) 500 nm, (b) 200 nm, (c) 50 nm, (d) 2 nm, (e) 50 nm, (f) 5 nm, (g, h) 2 nm, and (i) 1 nm.

As schematically illustrated in Figure 1, pyrolyzing Fe-Co Prussian blue analogue (Fe0.5Co0.5[Co(CN)6]0.67·nH2O, FeCo PBA) nanospheres (Figure S1) in N2 resulted in the formation of nitrogen-doped graphene encapsulated FeCo bimetal nanocages (FeCo@NC).25-26 The single-Co-atom catalysts (FeCo-NC-1, FeCo-NC-2, and FeCo-NC-3) could be obtained by further soaking FeCo@NC (calcined at 500, 650 and 800 °C, respectively) in 1 M H2SO4 at 80 °C for 4 h. To understand the structure and reactive sites transformation of catalysts before and after acid treatment, we focus on the comparison of FeCo-NC-2 with FeCo@NC. As displayed in Figure 2a and Figure S2, the FeCo-NC-2 has very uniform morphology, well inheriting the morphology of Fe-Co PBA. Compared to FeCo@NC obtained at 650 °C, the X-ray diffraction (XRD) pattern of FeCo-NC-2 shows diminished FeCo peaks (JCPDS No.500795), while a new (002) diffraction peak of graphene (JCPDS No.01-1578) appears (Figure S3). The specific surface area of FeCo-NC-2 (375 m2 g-1) is much larger than that of the corresponding FeCo@NC (103 m2 g-1), moreover, the inflection “knee” at 0.45 - 0.5 P/P0 in the N2 desorption isotherm (Figure S4) indicates dramatically increased porosity.27 All of these results suggest that most of the metallic FeCo cores have been removed after acid treatment (Figure S2). Additionally, the similar XRD patterns (Figure S5), N2 adsorption-desorption isotherms (Figure S4) and Brunauer-Emmett-Teller (BET) surface areas (Table S1) obtained for Fe-NC and Co-NC, prepared by replacing the Fe-Co PBA precursor with Fe-Fe and CoCo PBA, indicate the universality of the preparation method. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images of FeCo-NC-2 (Figure 2b-d) show the existence of a large number of


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Figure 3. Structural characterizations of single-Co-atom catalyst by X-ray absorption spectroscopy. (a) Normalized Co K-edge XANES spectra of Co foil, Fe-Co PBA, FeCo-NC3 2, and FeCo@NC obtained at 650 ºC. (b) The k -weighted Fourier transform spectra from Co K edge EXAFS. (c) The corresponding EXAFS fitting curves for FeCo-NC-2. (d) The structure of the CoN4 site in FeCo-NC-2.

hollow graphene spheres, with only a small proportion of residual FeCo nanocrystals completely coated by seamless graphene shells after acid treatment. These graphene shells prevent the metallic FeCo nanocrystals in the core from direct contact with the external solution, as evidenced by the cyclic voltammograms shown in Figure S6. Energy-dispersive X-ray spectroscopy (EDX) mappings of FeCo-NC-2 as shown in Figure 2e illustrate the distribution of the N, Fe, and Co species in both nanocrystals and porous graphene shells. The dispersion of single Co/Fe atoms anchored on the porous N-doped graphene could be clearly observable by aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). As shown in Figure 2f-i, the bright spots corresponding to heavy atoms (single Co/Fe atoms in this case) are well dispersed across the entire graphene spheres. X-ray absorption fine structure (XAFS) measurements were performed to probe the possible bonding between the Fe/Co atoms and the light elements. As shown in the normalized Co K-edge X-ray absorption near edge structure (XANES) spectra (Figure 3a), the position of the rising edge for FeCo-NC-2 is located between those for Co foil and the Fe-Co PBA precursor, indicating that the average oxidation state of cobalt in FeCo-NC-2 is between Co0 and Co2+.23, 28 Compared to FeCo@NC, the increase of cobalt oxidation state agrees well with the high resolution X-ray photoemission spectroscopy (XPS) spectra of Co 2p3/2 (Figure S7), which is ascribed to the leaching of metallic Co with a corresponding increase of the relative content of single Co atom after acid treatment. Figure 3b displays the k3-weighted Fourier transform spectra of the Co K edge extended X-ray absorption fine structure (EXAFS). The peak at approximately 1.41 Å, corresponding to the Co–N first-shell,29-30 is clearly observable in both

Figure 4. Fenton-like catalytic performance. (a) The evolution of BPA in different reaction systems catalyzed by the leaching solution (black, ), Fe-NC (olive, ), FeCo@NC obtained at 650 ºC (pink, ), Co-NC (purple, ), FeCo-NC-3 (blue, ), FeCo-NC-1 (cyan, ), and FeCo-NC-2 (red, ) within 6 min. (b) The relationship between the reaction rate constants and the contents of Co and Fe in different catalysts. (c) Comparison on reaction rate under different quenching conditions. (d) EPR spectra with and without FeCo-NC-2. Reaction condition: −1 −1 −1 [BPA] = 20 mg L , [PMS] = 0.2 g L , catalyst = 0.1 g L , T = 298 K, initial solution pH = 6.0, [Methanol] = 0.5 M (if needed), [TBA] = 0.5 M (if needed), [NaN3] = 0.02 M (if needed), and [KI] = 0.04 M (if needed).

FeCo-NC-2 and Fe-Co PBA. Furthermore, as shown in Figure 3c, the Fourier transforms of the Co K edge EXAFS spectra of FeCo-NC-2 can be well fitted using the scattering path of Co-Co from metallic Co structure and Co-N from CoN4 structure as presented in Figure 3d (optimized by DFT). Table S2 summarizes the fitting results of the EXAFS spectra of FeCo-NC-2. Only Fe–Fe backscattering signal at approximately 2.09 Å observed in the Fourier transforms of the Fe K edge EXAFS spectra (Figure S8) reflects the minimal content of FeN4.31 All of these results indicate that single Co atoms with CoN4 configuration have been incorporated in FeCo-NC-2, which match well with the HAADF-STEM results. Fenton-like catalytic performance of the single-Coatom catalysts was evaluated for BPA removal via activation of PMS. As shown in Figure 4a, the control experiment using FeCo@NC as the catalyst reveals less than 40 % degradation of BPA in 6 min. Surprisingly, as high as 100 % BPA removal could be achieved in 4 min using FeCo-NC-2 as the catalyst. In addition, the concentration of leached Co (0.23 mg L-1) during reaction was significantly lower using FeCo-NC-2 as the catalyst when compared to the situation of FeCo@NC (3.30 mg L-1), which is far below the permissible limit (1 mg L-1) according to the Chinese National Standard (GB 25467-2010). The leaching solution only contributes to ~3 % of BPA removal, indicating that the degradation of BPA was mainly induced by heterogeneous catalytic reaction on the FeCo-NC-2. To give a clear comparison of the catalytic performance, the BPA removal kinetics was fitted by the first-order reaction. As shown in Figure S9, the apparent rate constant



Figure 5. Identification of the adsorption site in Fentonlike reaction. (a) High-resolution XPS N 1s spectra of FeNC, FeCo-NC-1, FeCo-NC-2, FeCo-NC-3, and Co-NC (from top to bottom). (b) Structure-property relationships between BPA adsorption efficiency and normalized N species content. (c) High-resolution XPS N 1s spectra of FeCo-NC2 after reaction (top) and regeneration by annealing in N2 rd after the 3 reaction cycle (bottom).

(k) of FeCo-NC-2, as high as 1.252 min-1 (R2 = 0.992), is approximately 10 times higher than that of FeCo@NC, and 3 times higher than that of the homogeneous reaction catalyzed by the same concentration of Fe2+ and Co2+ ions (Figure S10). Even by considering the 3.7 times larger surface area for FeCo-NC-2 (375 m2 g-1) as compared to FeCo@NC (103 m2 g-1), the significant enhancement on the Fenton performance of FeCo-NC-2 suggests the trivial role of metallic FeCo for catalytic PMS activation. In addition, it is worthwhile mentioning that the optimal activity of FeCo-NC-2, with a TOF as high as 12.52 min-1 for BPA degradation, is by far the most impressive catalytic performance among all reported catalysts (Table S3). To help understanding the Fenton-like catalysis, we performed a series of control experiments to explore the main active site(s) for the excellent catalytic performance of single-Co-atom catalysts. As shown in Figure 4a, without introducing the element of cobalt in the single-atom catalyst, Fe-NC only gives a BPA removal efficiency of ~22 %, indicating the crucial role of the single cobalt atom site for the Fenton-like reaction. The homogeneous reaction catalyzed by Fe2+ and Co2+ ions (Figure S10) and calculations of their thermodynamic feasibility for PMS activation (Note S1) further evidence the single cobalt atom as the active site for catalytic PMS activation. Furthermore, the order of the reaction rate constant for FeCo-NC-1 (1.18 min-1), FeCo-NC-2 (1.25 min-1), and FeCo-NC-3 (0.47 min-1) is found to match well with the relative content of cobalt in the catalyst (Figure 4b and Figure S11), while the reaction rate constant is randomly dependent on the content of iron. However, when Co-NC (Figure 4b), which has the highest Co content (19.5 wt.%), was used as the catalyst for PMS activation, only less than 73 % efficiency of BPA removal was resulted, suggesting that besides the single cobalt atom site, other factors might also dominate the Fenton-like catalytic reaction.

Other than the aforementioned catalytic trends, we observed another interesting phenomenon of greatly enhanced BPA adsorption efficiency after acid treatment of FeCo@NC. As shown in Figure S12, a much faster adsorption equilibrium within 5 min with BPA adsorption efficiency as high as 24 % could be achieved on FeCo-NC-2, and the BPA adsorption efficiency was found to be correlated well to the Fenton-like performance (Figure S13, a higher adsorption efficiency would always lead to a higher value of k in BPA removal). These results suggest that the excellent Fenton-like performance of FeCo-NC-2 may result from the balanced adsorption of BPA and activation of PMS. Radical quenching experiments were conducted to identify the active radical species generated during PMS activation. As shown in Figure 4c and Figure S14, if tertbutanol (TBA) or methanol (radical scavenger for •OH and SO4•-, respectively) was added into the reaction mixture,32 the BPA degradation efficiency decreased slightly, which indicates the presence of active species other than •OH and SO4•- during the reaction. However, when NaN3, a unique scavenger for singlet oxygen (1O2),33 was added into the reaction system, the efficiency of BPA degradation was significantly reduced, suggesting the crucial role of singlet oxygen. Furthermore, addition of KI, a strong quencher for surface-bound free radicals,34 almost completely quenched the reaction, demonstrating that BPA was likely decomposed through a surface catalytic process by reacting with 1O2 that was locally generated via PMS activation. To further verify the active species in the Fenton-like reaction, DMPO trapped electron paramagnetic resonance (EPR) experiments were conducted. As shown in Figure 4d, without catalyst, the characteristic signals for DMPO-•OH adducts (with aN = aH = 14.9 G) were detectable,4 indicating that •OH could be produced by hydrolysis of HSO5-.8 However, the characteristic signals for DMPOX adducts (5, 5-dimethyl-2-pyrrolidone-N-oxyl, with aN = 7.2 G and aHr = 4.1 G),35 always resulted from the reaction between 1O2 and DMPO,36-37 could be detected if FeCo-NC-2 was added into the reaction system. These results clearly indicate the generation of active 1O2 in the Fenton-like reaction catalyzed on single-Co-atom catalysts. Previous studies have suggested that the defects (vacancies and/or zigzag/armchair edges) at the boundaries of graphene could serve as the active sites to break the OO bond for PMS activation.38 To identify the adsorption and catalytic sites of single-cobalt-atom catalysts in Fenton-like reaction, Raman spectra were collected and the results are shown in Figure S15. All prepared catalysts show similar intensity ratio of the D-band/G-band (ID/IG) (Table S1), suggesting similar disorientated degree of graphene. Doping nitrogen is considered as an efficient way to improve the electronic/catalytic properties of graphene.15-16 Therefore, it is imperative to understand the types and contents of nitrogen dopants that influence the catalytic performance, albeit the conclusion is still under debate. To this end, we used Fe doping (Figure S16 and Table S4) and changed the calcination temperature during catalyst preparation to tune the content of nitrogen


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Figure 6. Unraveling the mechanism of the Fenton-like reaction on single-Co-atom catalyst. Local adsorption configurations of BPA on (a) pyrrolic, (b) pyridinic, and (c) graphitic N doped graphene. Distance between the H and N atoms is indicated below the configuration. Values in bracket are the distances with vdW correction. Optimized configurations of PMS adsorbed on (d) Co (0001), (e) CoO (100), (f) CoN4-graphene, (g) graphitic N doped graphene, and (h) graphene, respectively. (i) Charge 3 density difference in CoN4-graphene (ρtotal - ρsubstrate - ρPMS). Isosurface contour is 0.001 e/bohr . The light purple and light green denote the electron accumulation and electron depletion, respectively. (j) Trends in reaction rate plotted as a function of the adsorption energy of PMS on Co (0001), CoO (100), CoN4-graphene, graphitic N doped graphene, and graphene. (k) The proposed overall Fenton-like reaction mechanism on single-Co-atom catalyst.

dopant to investigate the structure-property relationship for BPA adsorption (Figure S17). High-resolution XPS N 1s spectra of the as-prepared catalysts were collected to quantify the types and contents of nitrogen dopants as shown in Figure 5a. Three peaks with binding energy at 398.4, 399.8 and 401.1 eV are deconvoluted, which can be assigned to pyridinic, pyrrolic, and graphitic N, respectively.16 The content of pyrrolic N is observed to associate with the BPA adsorption efficiency (Figure 5b and Figure S17), which indicates that pyrrolic N may serve as the adsorption site for BPA. To further confirm this conjecture, high-resolution XPS N 1s spectra of used catalysts were examined. As shown in Figure 5c, the BPA removal efficiency was observed gradually decreased to 69 % after three reaction cycles (Figure S18), accompanying with a reduction in the content of surface pyrrolic N. The pyrrolic N was protonated to pyrrolic N-H during reaction, which shifted the original XPS peak position of pyrrolic N to overlap with that of the graphitic N (Figure S19).39 Additionally, the oxygen level of the catalyst increases from 5.93 to 11.97 at. % after the reaction, and no degradation intermediates of BPA in the reaction solution could be detected as evidenced by timedependent evolution of the HPLC spectra (Figure S20). These results suggest that the adsorptions of BPA and the intermediates on the pyrrolic N sites are the main cause

for the gradual deactivation of single-Co-atom catalysts. Furthermore, the catalytic activity of the catalyst is observed to follow the BPA adsorption efficiency (Figure S21) and could be well recovered via a simple thermal treatment in N2 to remove the surface adsorbates (Figure 5c and Figure S18). The similar morphologies and Co3+ contents of FeCo-NC-2 before and after reaction and thermal reactivation suggest high stability of the structure of the catalytic sites (Figure S22), making it easy to be reactivated. The adsorption of BPA on single-Co-atom catalyst most probably follows the “donor-acceptor complex” mechanism,40 involving pyrrolic N as the electron donor and -OH in BPA as the electron acceptor, which was verified by DFT calculations. Figure 6a-c and Table S5 show the optimized configurations for BPA adsorption on pyrrolic, pyridinic and graphitic N, among which, the adsorption of BPA on pyrrolic N is the strongest with the shortest H to N distance (1.78 Å) and the highest adsorption energy (-0.31 eV). DFT calculations were further performed to shed light on the theoretical insights of the CoN4 site with single Co atom for PMS activation. Figure 6d-h and Figure S23 show the fully relaxed atomic configurations of PMS on Co (0001), CoO (100), CoN4, graphitic N, and graphene, respectively with the corresponding adsorption energies ( and Bader charges listed in Table S6. Using the



adsorption energy of PMS as the descriptor, the reaction rate is found to follow a volcano relation as shown in Figure 6i. Thus, the low reactivity of PMS on metallic cobalt and cobalt oxide are due to the strong binding of PMS, which results in the active site poisoning. On the other hand, binding of PMS on graphitic N and graphene are too weak to effectively activate the PMS molecules. Additionally, as evidenced from the charge density analysis (Figure 6j), there exists significant electron transfer between PMS and CoN4, reflecting the chemisorption of PMS on the CoN4 site. Based on the above investigations, we propose a dualreaction-site mechanism for the Fenton-like reaction that takes place on single-Co-atom catalysts as displayed in Figure 6k. Firstly, PMS prefers to bond to the CoN4 site accompanying with fast electron accumulation and depletion, which results in effective PMS activation to produce 1 O2. Meanwhile, BPA adsorbs onto the pyrrolic N site via a “donor-acceptor complex” mechanism. The migration distance of 1O2 for one half-life period was calculated to be around 90 nm in the reaction solution (Note S2), therefore, as evidenced by the HAADF-STEM results (Figure 2g), the dual reaction sites within the single-Coatom catalyst shall greatly reduce the migration distance of the active 1O2. As a result, the rapid reaction of in-situ generated 1O2 with adjacent adsorbed BPA on single-Coatom catalyst gives rise to excellent Fenton-like catalytic performance.

CONCLUSIONS In summary, we have demonstrated the high Fentonlike activity and stability of single-Co-atom catalysts for remediation of organic pollutants. The single-Co-atom catalysts prepared in this work not only provide abundant CoN4 sites with optimal binding energy for PMS activation to produce singlet oxygen as active species, but also afford rich pyrrolic N sites for organics adsorption. The dual reaction sites greatly reduce the migration distance for the active species and thus improve the Fenton-like catalytic performance. Our study details the first insights into single-atom based catalysts for the Fenton-like reaction. We anticipate that our proposed dual-reaction-site mechanism shall shed light on the development of other “single-atom catalysts” in future.

EXPERIMENTAL SECTION Chemicals. Potassium hexacyanocobaltate (III) (98%) was purchased from Beijing J&K Co., Ltd, China. PMS (KHSO5·0.5KHSO4·0.5K2SO4) was obtained from Alfa Aesar. Cobalt (99.9%, 30 nm) and sodium azide were purchased from Sigma-Aldrich. BPA, and 5,5-dimethyl-1-pyrroline-Noxide (DMPO) were purchased from Aladdin Co., China. Ferrous chloride, sodium sulfite, cobaltous chloride, tbutanol (TBA), poly(vinylpyrrolidone) (PVP) and methyl alcohol were obtained from Tianjin Kermel Chemical Reagent Co., Ltd, China. All chemical reagents were used without further purification. Catalyst preparation. Fe-Co Prussian blue analogue (Fe0.5Co0.5[Co(CN)6]0.67·nH2O, Fe-Co PBA) nanosphere was 25 prepared according to our previous published method . Typ-

ically, calculated amounts of FeCl2·4H2O and CoCl2·6H2O (total 9 mM) were dissolved in 40 mL of deionized water with PVP (1.2 g) under vigorous stirring, followed by slowly adding 40 mL of K3[Co(CN)6] aqueous solution (5 mM). Subsequently, the obtained colloidal solution was further stirred for another 30 min and aged for 20 h. The resulting precipitates were then centrifuged and washed at least three times with ethanol and deionized water, and then dried in an oven at 60 °C for 20 h. To obtain nitrogen-doped graphene encapsulated FeCo bimetal (FeCo@NC) nanocages, Fe-Co PBA nanospheres were heated at different temperatures (500, 650 26 and 800 °C) for 1 h in N2. To prepare FeCo-NC-1, FeCo-NC-2, and FeCo-NC-3, the asobtained FeCo@NC nanocages at 500, 650 and 800 °C were treated in 1 M H2SO4 at 80 °C for 4 h, respectively. The final products were then collected, washed with copious amounts of deionized water, and dried. For comparison, the Fe-NC and Co-NC were obtained at 650 °C by replacing the precursor with Fe-Fe and Co-Co PBA, respectively. Characterization. Crystal structure and morphology of the catalysts were examined by powder X-ray diffraction (XRD, PANalytical X’Pert-Pro X-ray diffractometer) equipped with Cu Kα radiation (λ = 0.15406 nm), field-emission scanning electron microscopy (FESEM, JSM 7800F), and highresolution transmission electron microscopy (HR-TEM, JEM2100). The elemental compositions were analyzed by inductively coupled plasma atomic emission spectroscopy (ICPOES). Specific surface area was measured based on the BET method on a Micromeritics ASAP 2010 instrument at liquidnitrogen temperature. The Co and Fe K-edge X-ray absorption fine structure (XAFS) spectra were collected at the Beijing Synchrotron Irradiation Facility (BSRF, 1W1B) with stored electron energy of 2.2 GeV using transmission mode. The Co and Fe K-edge spectra were processed following the conventional procedure using the IFEFFIT package. Atomic high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization was conducted on a probe corrected JEOL JEM-ARM200F STEM/TEM. The X-ray photoelectron spectroscopy (XPS) spectra fitted by the XPSPEAK41 software using Shirley-type background were recorded on the ESCALAB 250 X-ray photoelectron spectroscope equipped with a monochromated Al Kα source. The 57 room temperature Fe Mössbauer spectra were obtained using a proportional counter and a Topologic 500A spec57 trometer with Co (Rh) as a γ-ray radioactive source. The electron paramagnetic resonance (EPR) spectra were collected on a Bruker EPR I200 spectrometer with a center field at 3320 G and a sweep width of 140 G at room temperature. Catalytic activity. The Fenton performance was assessed -1 in a 100 mL reactor containing 20 mg L BPA with an initial solution pH of 6.0. The reaction temperature was maintained at 25 °C using a water bath. In a typical experiment, 2.5 mg of catalyst was added into 25 mL of BPA solution and stirred for 30 min to establish the adsorption-desorption equilibrium. The reaction was initiated by adding a certain amount of PMS aqueous solution. At predetermined time intervals, 0.5 mL of the reaction solution was withdrawn and immediately quenched by 0.3 mL sodium sulfite solution (0.1 M). For cycling test, the catalyst was recycled after each run of the experiment by filtration and washed thoroughly with copious amounts of deionized water. Thermal treatment on the recycled catalyst was conducted at 650 °C for 1 h in N2 to regenerate the catalytic activity.


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Journal of the American Chemical Society The concentration of BPA was analyzed by high performance liquid chromatography (HPLC, Agilent, 1260-Infinity) at a detection wavelength of 230 nm with methanol/water mixture (70:30, v/v) as the mobile phase. The concentrations of leached iron and cobalt were quantified by inductively coupled plasma atomic emission spectroscopy (ICP-OES). The reaction rate was evaluated by a pseudo first order kinetics model (Eq. 1) and the BPA adsorption efficiency (R%) were calculated using Eq. 2: ln(C0/Ct) = kt (1) R% = 100 × (C0 - Ce)/C0 (2) where C0 is the initial pollutant concentration, Ct is the concentration at a certain time t during the degradation process, Ce is the concentration of adsorbate at equilibrium, and k is the reaction rate constant. Computational framework. Spin-polarized DFT calculations were performed with Vienna ab initio simulation pack41-42 age (VASP), using projector augmented wave (PAW) 43 pseudopotential for the core electrons, a cutoff energy (480 eV) for the valence electrons, and the generalized gradient approximation (GGA) in the form of Perdew-Burke44-45 Ernzerhof (PBE) for the exchange-correlation potentials. The (3 × 3) and (2 × 2) supercells with four atomic layers were cleaved to mimic Co (0001) and CoO (100) surfaces, respectively. The vacuum layer was about 13 Å. The upper two atomic layers of slabs and adsorbates were free to move in all directions, while the bottom two layers were fixed at the ground-state bulk positions. The surface Brillouin-zone of Co (0001) and CoO (100) surfaces were sampled using the k-meshes of 6 × 6 × 1 and 4 × 4 × 1, respectively. Note that CoO is an antiferromagnetic oxide, the antiferromagnetic calculations of PMS adsorption on CoO (100) surface with spin alignment planes on the (111) and (100) planes were considered. The adsorption of PMS on graphitic N doped graphene and CoN4-graphene was simulated using a (8 × 8) supercell with a vacuum layer of 20 Å. K-mesh of 2 × 2 × 1 was used for the sampling of the Brillouin-zone. The adsorption of BPA on pyrrolic, pyridinic and graphitic N doped graphene were carried out in slab models in dimensions of 30.76 Å × 17.11 Å × 20 Å, 17.29 Å × 28.11 Å × 20 Å, and 19.76 Å × 19.76 Å × 20 Å with k-meshes of 1 × 2 × 1, 2 × 1 × 1, and 2 × 2 × 1 to sample the first Brillouin-zone, respectively. The atoms were relaxed fully until the force acting on each atom is less than 0.02 eV/Å. van der Waals (vdW) interaction was taken into 46 account at DFT-D2 level as proposed by Grimme. The adsorption energy is defined as Eads = Etotle – Esubstrate - Emolecule where Etotle, Esubstrate, and Emolecule denote the total energy of substrate with adsorbate, substrate, and free molecule, respectively.

ASSOCIATED CONTENT Supporting Information Supplementary data associated with this article is available free of charge via the Internet at http://pubs.acs.org. Characterization and catalytic performance of materials, simulation details, Figures S1−S23, Tables S1−S6, and Notes S1-S2.

AUTHOR INFORMATION Corresponding Authors

*[email protected] *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Key Projects for Fundamental Research and Development of China (2016YFA0202804), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17000000), Dalian Institute of Chemical Physics (DICP DMTO201408), Nanyang Technological University (M4080977.120), Ministry of Education of Singapore (AcRF Tier 1 M4011021.120 and 2015-T1-002108), Agency for Science, Technology and Research (A*Star, M4070178.120 and M4070232.120) and the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

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