Discriminating Catalytically Active FeNx Species of Atomically

Article pubs.acs.org/JACS

Discriminating Catalytically Active FeNx Species of Atomically Dispersed Fe−N−C Catalyst for Selective Oxidation of the C−H Bond Wengang Liu,§,‡,† Leilei Zhang,§,† Xin Liu,§ Xiaoyan Liu,§ Xiaofeng Yang,§ Shu Miao,§ Wentao Wang,§ Aiqin Wang,*,§ and Tao Zhang*,§ §

State Key Laboratory of Catalysis, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China ‡ University of Chinese Academy of Sciences, Beijing, 100049, China S Supporting Information *

ABSTRACT: Nanostructured Fe−N−C materials represent a new type of “platinum-like” non-noble-metal catalyst for various electrochemical reactions and organic transformations. However, no consensus has been reached on the active sites of the Fe−N−C catalysts because of their heterogeneity in particle size and composition. In this contribution, we have successfully prepared atomically dispersed Fe−N−C catalyst, which exhibited high activity and excellent reusability for the selective oxidation of the C−H bond. A wide scope of substrates, including aromatic, heterocyclic, and aliphatic alkanes, were smoothly oxidized at room temperature, and the selectivity of corresponding products reached as high as 99%. By using sub-ångström-resolution HAADF-STEM in combination with XPS, XAS, ESR, and Mössbauer spectroscopy, we have provided solid evidence that Fe is exclusively dispersed as single atoms via forming FeNx (x = 4−6) and that the relative concentration of each FeNx species is critically dependent on the pyrolysis temperature. Among them, the medium-spin FeIIIN5 affords the highest turnover frequency (6455 h−1), which is at least 1 order of magnitude more active than the high-spin and low-spin FeIIIN6 structures and 3 times more active than the FeIIN4 structure, although its relative concentration in the catalysts is much lower than that of the FeIIIN6 structures.

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adsorption configurations after a thorough XANES analysis.23 Su et al. argued that the real active site for the ORR in the Fe− N−C catalysts was FeN6, a [FeIII(porphyrin)(pyridine)2] complex, on the basis of Mössbauer spectroscopy.24 One key reason for the discrepancy about the active sites of M−N−C materials is that the catalysts are heterogeneous in both composition and particle size as a result of an uncontrollable pyrolysis process. In fact, in most cases multispecies of M, such as metallic M nanoparticles, MOx nanoparticles, and monodispersed and aggregated M−Nx, are simultaneously present in one catalyst,24−27 which poses a significant challenge for the identification of the active site. On the other hand, single-atom catalyst (SAC), in which catalytically active metal is exclusively dispersed as single atoms,2,28−37 provides a good entry to identify the structure of the active site without interference from other metal species. M−N−C catalysts with M dispersed as exclusively single atoms can be viewed as a special SAC if the surrounding N and C atoms with which the metal atoms bond are considered as a ligand.16,38 In spite of the distinctive advantages of uniform dispersion of active sites, the successful synthesis of M−N−C SAC remains challenging and critically depends on the rational

INTRODUCTION Noble metals are widely used as catalysts in energy conversion, chemicals production, and emission control.1−3 The scarcity and high cost of noble metals, however, constitute major barriers to large-scale practical applications. Searching for earthabundant yet efficient catalysts to replace noble metals has therefore become one of the central tasks in the catalysis community.4−7 Among others, M−N−C (M typically refers to Fe and Co) materials, which were originally obtained by pyrolysis of transition-metal-containing macrocycles and later could be prepared by simply heating mixtures of more available and cheaper metal and nitrogen precursors on carbon supports, have attracted a great deal of attention thanks to their promising catalytic performances in a variety of typical platinum-catalyzed reactions, such as the oxygen reduction reaction (ORR),8−11 CO2 electroreduction reaction,12 and hydrogen evolution reaction (HER),13−16 as well as fine chemical synthesis.17−21 However, despite extensive investigations of M−N−C catalysts, no consensus has been reached on the active sites. For example, Mukerjee and co-workers stated that the active site of Fe−N−C materials was FeN4C10 in crystallographic atomic defects or FeN4C8 in armchair edges of graphitic surfaces.22 Zitolo et al. proposed that the Fe−N4 moieties in Fe−N−C catalysts should be assigned to in-plane Fe−N 4 porphyrinic architectures with two different O2 © 2017 American Chemical Society

Received: May 24, 2017 Published: July 26, 2017 10790

DOI: 10.1021/jacs.7b05130 J. Am. Chem. Soc. 2017, 139, 10790−10798

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Journal of the American Chemical Society design of the metal−support interaction as well as controllable pyrolysis. Previously, we employed Mg(OH)2 instead of carbon as the sacrificial support for the preparation of self-supporting Co−N−C SAC, which exhibited excellent performances in the hydrogenative coupling of nitroarenes.39 By using aberrationcorrected HAADF-STEM, X-ray absorption, and DFT calculations, we were able to identify, without much ambiguity, the structure of the active site as CoN4C8-1−2O2. Chen et al. reported atomically dispersed Fe−Nx on N and S codecorated hierarchical carbon layers that showed good performances in both OER and ORR reactions.40 Also aiming for the ORR, Zhu et al. prepared Fe−Nx SAC by using a one-step hydrothermal method.41 Nevertheless, in the above two examples and other previous literature,20,38 the exact structure of the active sites is yet to be determined. Moreover, in comparison with the extensively studied electrocatalysis, the atomically dispersed M−N−C catalyst has rarely been explored for organic transformations, especially for the challenging reaction of C− H bond activation. In this contribution, we report the preparation, characterization, and catalytic performances of atomically dispersed Fe− N−C catalyst. With nano-MgO as a sacrificial template, atomically dispersed self-supporting Fe−N−C catalyst was successfully prepared, and its structure was identified by a combinatorial use of aberration-corrected HAADF-STEM, electron spin resonance (ESR), X-ray absorption spectroscopy (XAS), and Mössbauer spectroscopy. To our surprise, even if all the Fe atoms are atomically dispersed, there are four types of different FeNx (x = 4−6) structures, and the relative concentration is critically dependent on the pyrolysis temperature. The most active site for the selective oxidation of the C− H bond is the medium-spin FeIIIN5 structure, which is at least 1 order of magnitude more active than the high-spin and low-spin FeIIIN6 structures and 3 times more active than the FeIIN4 structure. To the best of our knowledge, this is the first report that different FeNx structures are well-discriminated and correlated with the catalytic activity in the atomically dispersed Fe−N−C material.

diffraction peaks corresponding to Fe or FeOx were detected, excluding the presence of any large crystalline particles of Fecontaining species. In accordance with the XRD result, the Raman spectra showed only two bands characteristic of carbon materials (G band at 1600 cm−1 and D band at 1350 cm−1, Figure S2, SI) and no formation of any FeOx species.43 The N2 adsorption−desorption isotherms and pore size distributions revealed that the three samples all had large surface areas (620−835 m2/g depending on the pyrolysis temperature) and contained both micro- and mesopores (Figure S3, SI). By scanning electron microscopy (SEM) under different detector modes we could confirm that there were no visible particles larger than 10 nm in the three samples (Figure S4, SI).44 Nevertheless, from HAADF-STEM images one can clearly see that small particles with sizes of 3−5 nm are sparsely dispersed in the Fe−N−C-800 sample (Figure 1c inset and Figure S5a,b,

RESULTS AND DISCUSSION Preparation of Atomically Dispersed Fe−N−C Catalyst. The Fe−N−C catalyst with atomically dispersed Fe was prepared by a template-sacrificial approach [Scheme S1, Supporting Information (SI)].39 Briefly, the Fe(phen)x (phen = 1,10-phenanthroline) complex supported on nano-MgO template was pyrolyzed at 600−800 °C under N2 atmosphere, followed by acid leaching to remove the MgO template (for more details, see the Supporting Information). The samples pyrolyzed at different temperatures are denoted as Fe−N−C-X (X = pyrolysis temperature). ICP-AES (inductively coupled plasma atomic emission spectroscopy) analysis revealed Fe loadings of 1.8, 1.6, and 1.4 wt % for the Fe−N−C-600, Fe− N−C-700, and Fe−N−C-800 samples, respectively. A slight decrease of the Fe loading with pyrolysis temperature suggested that more metallic Fe particles are formed at higher pyrolysis temperatures and they are subsequently removed by acid leaching, leading to a decrease of the Fe loading. Identification of Atomic Dispersion of Fe. The XRD patterns of the samples showed only two broad peaks at 25° and 43° (Figure S1, SI), which were assigned to reflections of the (002) and (004) planes of carbon.42 The carbon was formed as a result of pyrolysis of 1,10-phenanthroline that was not fully ligated to iron cations. It is noted that not any

Figure 1. HAADF-STEM images of Fe−N−C-600 (a), Fe−N−C-700 (b), and Fe−N−C-800 (c) and elemental mapping of Fe−N−C-700 (d). The inset in part c shows the Fe-containing particles highlighted with red circles.

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SI) while they are completely absent in the other two samples (Figure S6, SI). From the image contrast we could assign the very small particles in Fe−N−C-800 to the Fe-containing particles. HRTEM images of Fe−N−C-800 (Figure S5c,d, SI) further reveal that the Fe-containing particles are encapsulated with carbon layers of 3−5 nm thickness, which protect them from being leached by acid. The results demonstrate an aggregation tendency of Fe atoms upon elevating the pyrolysis temperature. Since no visible clusters/particles are observed in either Fe−N−C-600 or Fe−N−C-700 with both SEM and normal HAADF-STEM, we then resort to sub-ångströmresolution aberration-corrected HAADF-STEM to identify the dispersion of Fe species. As shown in Figure 1, a high density of Fe single atoms are uniformly dispersed in both Fe−N−C-600 and Fe−N−C-700, while both Fe single atoms and nanoparticles are observed in Fe−N−C-800. On the basis of the Fe loading and surface area of each sample, we could roughly estimate the density of Fe single atoms0.31, 0.21, and 0.20 Fe atoms per nm2 for Fe−N−C-600, Fe−N−C-700, and Fe− 10791


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Journal of the American Chemical Society N−C-800, respectivelywhich are in good agreement with those estimated based on HAADF-STEM images. Moreover, the elemental mapping of Fe−N−C-700 (Figure 1d) indicates that the signals of C, N, O and Fe are superimposed on each other, suggesting the homogeneous distribution of these elements at the nanoscale. The atomic dispersion of Fe−N−C catalysts is further approved by EXAFS analysis. Figure 2a shows the Fourier-

Figure 3. XPS Fe 2p (a) and N 1s (b) spectra of Fe−N−C samples.

results of the three samples are similar, showing two peaks at binding energies of 711.2 eV (Fe 2p3/2) and 724.5 eV (Fe 2p1/2), respectively. These binding energy values and the associated shakeup structure point to Fe3+ species.45,46 The absence of Fe0 in the XPS of Fe−N−C-800 appears to be contradictive with the result of HAADF-STEM. Nevertheless, if one considers the fact that the small particles of Fe are encapsulated with thick carbon layers (Figure S5c,d, SI), it is reasonable that these embedded Fe/FexC particles are not detectable by the XPS technique. Therefore, only the surface or subsurface single-atom Fe species are detected by XPS, and they are assigned to Fe3+. The XPS spectra of N 1s in the three samples can be deconvoluted into four peaks with binding energies of 398.4, 399.8, 401.1, and 405.2 eV, which are assigned to pyridinic N, pyrrolic N, graphitic N, and N-oxide (N+−O−), respectively.16,39 The relative concentrations of different N species are listed in Table S1 (SI). According to previous studies, in Fe−N−C materials the Fe atoms coordinate with pyridinic N to form catalytically active Fe− Nx sites.15,47,48 For our three samples, the concentration of pyridinic N species follows the order of Fe−N−C-600 > Fe− N−C-700 > Fe−N−C-800, well agreeing with surface density of Fe single atoms. The predominance of Fe3+ in the three Fe−N−C samples is corroborated by the XANES spectra at the Fe K-edge. As shown in Figure 2b, the pre-edge peak at 7117 eV in FePc (navy line), which arises from the 1s → 4pz transition with simultaneous ligand-to-metal charge transfer, is regarded as the fingerprint of square-planar Fe−N4 moieties (D4h symmetry).22,23,39,49 It is noted that a similar pre-edge peak also appears in the three Fe−N−C samples, whereas it is absent from the Fe(phen)x precursor, suggesting that the Fe−N−C samples contain a similar Fe−N4 planar structure, which is quite different from the Co−N−C structure we previously reported.39 On the other hand, a comparison of E0 values (the first inflection point) shows that Fe−N−C-600 has an E0 value of 7124.3 eV, which is close to that of the Fe3O4 reference (Table S2, SI), suggesting the copresence of Fe2+ and Fe3+ in this sample. In comparison, both Fe−N−C-700 and Fe−N−C800 have an E0 value of 7126.7 eV, very close to those of ferric samples, indicating the predominance of Fe3+ in the two samples. It is also interesting to note that the Fe(II)Pc reference sample presents an E0 value similar to that of Fe−N− C-600, which is probably caused by the partial oxidation of Fe2+ to Fe3+ upon exposure to ambient atmosphere. This will be validated by the Mössbauer spectroscopy below. Mö ssbauer spectroscopy is a powerful technique to discriminate different Fe species.24 Figure 4 shows the Mössbauer spectra of the three Fe−N−C samples, and Table 1 lists the Mössbauer parameters as well as the relative areas of different Fe species. For both Fe−N−C-600 and Fe−N−C-700 samples, the spectra can be well-fitted with three doublets. The

Figure 2. (a) The k2-weighted Fourier transform spectra of Fe−N−C catalyst as well as reference samples. (b) The normalized XANES spectra at the Fe K-edge of different samples. The dotted ellipse shows the pre-edge peak at 7117 eV.

transformed k2-weighted EXAFS spectra at the Fe K edge. In contrast to the reference samples of Fe foil and Fe2O3, neither Fe−N−C-700 nor Fe−N−C-600 presents any prominent peak at the position of Fe−Fe coordination, in agreement with the above HAADF-STEM result that all the Fe species are atomically dispersed without any aggregation. However, a small peak of Fe−Fe coordination can be clearly observed in Fe−N−C-800, suggesting the formation of Fe aggregates. Moreover, all three Fe−N−C samples show a main peak at 1.47 Å, which is due to Fe−N coordination in the first shell, resembling the FePc and Fe(phen)x reference samples. Chemical State and Coordination Environment of Fe Single Atoms. The chemical state of single-atom Fe as well as its local environment is the key to the catalytic performances. Therefore, we subsequently employed X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and Mössbauer spectroscopy to determine the exact chemical state of Fe. As shown in Figure 3, the XPS Fe 2p 10792


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Figure 4. samples.

Fe−N−C-700, while they may change into oxygen molecules (O2) in Fe−N−C-800 due to the breaking of axial Fe−N bonds at higher temperatures and the weak adsorption of ambient oxygen molecules on the central Fe.23,54 In contrast to D2, D3 has a similarly low IS value (0.2−0.3) but a much larger QS value (1.3−1.6) and can be assigned to the N−(FeIIIN4)−N low-spin structure,24 in which the central Fe atom is strongly coordinated with four nitrogen atoms in graphitic layers and the other two nitrogen atoms in the axial direction perpendicular to the Fe−N4 plane. Such a perfect saturated coordination structure is robust and therefore cannot be acting as the catalytically active site.24,55 On the other hand, the D4 species, which featured the smallest IS value (0.12−0.13) and the largest QS value (2.8−3.3) among the four doublets, can be assigned to N−(FeIIIN4) medium-spin species with a pentacoordinated rhombic monopyramidal structure.58,59 Its smaller IS value and larger QS value than the Fe−N4 structure in FePc (Figure S8, Table S3, SI) is caused by the additional N coordinating with the central Fe in the apical position. The unsaturated coordination structure of D4 will enable it to be catalytically active in chemical reactions. This will be validated in the subsequent catalytic tests. It is also noted that the relative amount of each Fe species is critically dependent on the pyrolysis temperature (Table 1). D1 (Fe IIN4) only exists in the Fe−N−C-600 sample, suggesting that this structure is not stable at higher temperatures. In comparison, D2 (high-spin X−FeIIIN4−Y) exists in all the three samples and even becomes the predominant Fe species in both Fe−N−C-600 and Fe−N−C-800 samples, while it appears to be the least populated species in Fe−N−C-700. The reason for such an irregular change with pyrolysis temperature is not clear yet, but the decreased QS value with increasing pyrolysis temperature indicates that the X−FeIIIN4−Y structure becomes more symmetric. On the other hand, the concentration of D3 [low-spin N−(FeIIIN4)−N] increases with the pyrolysis temperature, until it completely disappears in Fe−N−C-800; concurrently, nanoparticles of γ-Fe and FexC are formed in Fe− N−C-800. This result seems to suggest that the D3 structure is destroyed and then aggregation of Fe0 occurs as a consequence of pyrolysis at higher temperatures (800 °C or above). It is also interesting to note that the concentration of D4 [medium-spin N−(FeIIIN4)] gets maximized in Fe−N−C-700, which might bring about activity enhancement. Catalytic Performance of Fe−N−C for Selective Oxidation of the C−H Bond. The selective oxidation of the C−H bond is one of the most important organic transformations for construction of high-value-added carbonyl compounds, such as aldehydes, ketones, and carboxylic acid and esters, which are widely used as key intermediates in the fine chemical industry.60,61 However, due to the high dissociation energy of the C−H bond and the overoxidation side reactions, the direct oxidation of the C−H bond has been a long-standing challenge accompanied by low conversion and poor selectivity.62−65 Considering that the Fe−N−C catalysts possess the unique feature of single-atom dispersion and high oxidation state of Fe, we expect them to be efficient catalyst in the selective oxidation reactions. To study the catalytic performances of the atomically dispersed Fe−N−C catalysts for the selective oxidation of C−H bond, ethylbenzene was first chosen as a substrate to explore the efficiency. First, in a blank experiment, 1,10phenanthroline and nonpyrolyzed Fe(phen)x−MgO precursor were tested; however, only negligible conversion of ethyl-

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Fe Mössbauer spectra of the Fe−N−C-600/700/800

Table 1. Summary of the Mössbauer Parameters and Assignments to Different Iron Species in Fe−N−C-600/ 700/800 Catalysts Fe species

IS/mm s‑1

QS/mm s‑1 area %a

600-D1 600-D2

0.61 0.21

2.62 0.82

14.2 66.3

600-D3 700-D2

0.31 0.25

1.60 0.78

19.5 28.3

700-D3 700-D4 800-D2

0.24 0.13 0.19

1.37 2.81 0.71

53.8 17.9 59.0

0.12 −0.12 0.13

3.28

7.8 10.0 19.5

800-D4 800-Singlet 800-Sext

0

assignmentb II

Fe N4, like FePc, MS X−FeIIIN4−Y, (X, Y ligands like O/N), HS N−(FeIIIN4)−N, LS X−FeIIIN4−Y, (X, Y ligands like O/N), HS N−(FeIIIN4)−N, LS N−(FeIIIN4), MS X−FeIIIN4−Y, (X, Y ligands like: O/N), HS N−(FeIIIN4), MS γ-Fe FexC (H0 = 27.9 T)

a

The relative absorption area of each iron species in Fe−N−C samples. bLS, MS, and HS denote low-spin, medium-spin, and highspin, respectively.

absence of sextet and singlet indicates the absence of Fe0 species. By contrast, in the Fe−N−C-800 sample, besides two doublets, one singlet (S1) and one sextet (Sext1) corresponding to γ-Fe and FexC species appear in the spectrum.50 This result is well-consistent with the HAADF-STEM and EXAFS results that both Fe−N−C-600 and Fe−N−C-700 exclusively consist of Fe single atoms whereas aggregates of Fe0 particles are present in Fe−N−C-800. According to the isomer shift (IS) and quadrupole splitting (QS) values (Table 1), four different doublets (D1−D4) can be identified in the above three samples. D1, which has relatively larger values of IS and QS and appears only in the Fe−N−C600 sample, can be assigned to FePc-like FeIIN4 species.51−53 D2, with an IS value of ∼0.2 and a QS value of 0.7−0.8, appears in all the three samples and can be assigned to ferric X− FeIIIN4−Y high-spin species (X and Y refer to O or N ligand), in which the Fe atom is slightly pulled out of the N4 plane, resulting in a deformed octahedral structure.54,55 This assignment is further confirmed by EPR (electron paramagnetic resonance) characterization results. As shown in Figure S7 (SI), an isotropic signal at g = 4.24 is clearly observed, which can be assigned to high-spin ferric species in a rhombic symmetry.52,56,57 It is also noted that the apical atoms (X and Y) may differ with the calcination temperature. For example, both X and Y are most probably nitrogen atoms in Fe−N−C-600 and 10793


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Journal of the American Chemical Society Table 2. Catalytic Activity of Different M−N-C Samples for the Oxidation of Ethylbenzene in Watera

selectivity (%) entry

catalyst

time (h)

conv (%)

2a

2b

2c

2d

1 2 3 4 5 6 7 8c 9

none phenb Fe(phen)x−MgO Fe−N−C-600 Fe−N−C-700 Fe−N−C-800 Fe−N−C-700 Fe−N−C-700 Co−N−C-700

8 8 8 5 5 5 7 7 7

0.7 1.1 1.5 94 98 81 99 99 74

77 55 71 96 97 94 99 95 85

0 13 8.6 0.7 0.4 2.1 0.3 0.3 1.7

23 32 20 2.6 2.5 4.5 0.3 4.7 13

0 0 trace 0 0 0 0 0 0

a Reaction conditions: 10.0 mg of catalyst, 0.5 mmol of ethylbenzene, room temperature, 500 μL of TBHP as a 70 wt % aqueous solution (7 equiv) diluted with 6.5 mL of H2O. The conversion and selectivity were determined by GC−MS and GC analysis with dodecane as an internal standard. b phen = 1,10-phenanthroline. cTwo equivalents of TBHP (1 mmol) was used.

benzene was observed (Table 2, entries 1−3). In remarkable contrast, the three Fe−N−C catalysts afforded high activity and excellent selectivity; among them, Fe−N−C-700 gave the best performance with 98% ethylbenzene conversion and 97% acetophenone selectivity (Table 2, entries 4−6). When the reaction time was prolonged to 7 h, both the ethylbenzene conversion and the acetophenone selectivity reached 99% at the ambient temperature (Table 2, entry 7). More interesting, even when the tert-butyl hydroperoxide (TBHP) was reduced to a stoichiometric amount (2 equiv), the conversion of ethylbenzene could still reach 99% with an acetophenone selectivity of 95% (entry 8), indicating a nearly 100% TBHP efficiency. It is also noted that, under identical conditions, the atomically dispersed Co−N−C catalyst we previously developed gave only moderate conversion and selectivity (Table 2, entry 9), demonstrating the superiority of the Fe−N−C single-atom catalysts. Indeed, when compared with reports in the literature, it is found that our Fe−N−C-700 catalyst has the highest activity at room temperature (Table S4, SI). On the basis of the conversion−time profiles (Figure S9, SI) and the total Fe contents of the three Fe−N−C catalysts, the initial turnover frequencies (TOFs) were calculated to be 682.5, 1932, and 850.5 h−1 for the Fe−N−C-600, Fe−N−C-700, and Fe−N−C-800 catalysts, respectively. Evidently, the pyrolysis at different temperatures resulted in different Fe-centered structures, as identified with 57Fe Mössbauer spectra (Figure 4 and Table 1), which consequently led to remarkable difference in intrinsic activities. The Fe−N−C-700 sample is 2−3 times more active than the other two samples because it contains a greater number of unsaturated coordination sites (Fe−N5). This will be discussed in the following section. The excellent performance of Fe−N−C-700 catalyst was also tested for the selective oxidation of C−H bonds of a broad scope of substrates at room temperature. As shown in Table 3, various aromatic hydrocarbons with an electron-donating group (−OMe) or electron-withdrawing group (−NO2) were transformed smoothly into the corresponding ketone compounds with >98% selectivities at high conversions (Table 3, entries 2 and 3). Bulky and sterically hindered substrates, such as cumene, diphenylmethane, and fluorene, could also be smoothly activated by the atomically dispersed Fe−N−C-700 catalyst with high conversion and selectivity (Table 3, entries

Table 3. Selective Oxidation of Hydrocarbons in Water with Fe−N−C-700 as Catalysta

a

Reaction conditions: 10.0 mg of catalyst, 0.5 mmol of substrates, room temperature for 7 h, 500 μL of TBHP as a 70 wt % aqueous solution (7 equiv) diluted with 6.5 mL of H2O. Conversion and selectivity are determined by GC−MS and GC analysis with dodecane as an internal standard.

4−7). Moreover, the heterocyclic substrate could also be tolerated, and the corresponding product was obtained with excellent selectivity (Table 3, entry 8). Notably, the selective oxidation of aliphatic hydrocarbon was also tested. For example, 10794


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On the other hand, based on the 57Fe Mö ssbauer spectroscopy results (Figure 4 and Table 1), there are four types of Fe−Nx species (D1−D4) depending on the pyrolysis temperature. We assume that D3 species with a N−(FeIIIN4)− N low-spin structure is catalytically inactive due to the saturated and strong coordination of central Fe with six N atoms, and in this case, we can calculate the intrinsic activities of the other three Fe species (D1, D2, and D4) on the basis of the TOFs per Fe atom as well as their relative areas in Mössbauer spectra (for calculation details, see the Supporting Information). The result shows that the TOFs are 2063, 588, and 6455 h−1 for D1, D2, and D4 sites, respectively. Clearly, as we expect, the D4 species with a N−(FeIIIN4) medium-spin structure is the most catalytically active species, and it is 1 order of magnitude more active than the high-spin 6-fold D2 species and 3 times more active than the D1 FePc-like FeIIN4 species. This result is in good agreement with the KSCN titration result, where the first poisoned species should be D4 with an estimated TOF of 5350 h−1, followed by the second (D2, 533 h−1) and third (D3, 160 h−1). The slight difference in the exact TOF values should be caused by the fitting errors in both methods, but the trend is well-consistent. On the other hand, to approve the catalytic activity of D1 species, we tested FePc, which has a similar structure as D1, for the same reaction. As expected, the FePc is active for the reaction and the TOF based on Fe content is 373 h−1. The lower activity of FePc than the D1 species is probably due to the poor accessibility of FePc. In summary, both the KSCN titration method and the Mössbauer spectroscopy method demonstrate independently that there are different Fe−Nx sites on the Fe−N−C catalysts, and the D4 site with a N−(FeIIIN4) medium-spin structure is the most catalytically active species. For the best-performing catalyst Fe−N−C-700, the intrinsic activity follows the order of D4 ≫ D2 ≫ D3; in particular, D4 is 33 times more active than D3 and 1 order of magnitude more active than D2 on the basis of the KSCN titration method, confirming the reasonability of our assumption in the Mössbauer spectroscopy method that D3 is inactive due to the saturated and strong coordination of the central Fe with six N atoms. Meanwhile, it is noted that the most active D4 site is unfortunately the least abundant in the catalysts (Table 1), suggesting that there is much room to enhance the activity by increasing the number of D4 species. To further shed light on the reaction mechanism, DMPO (5,5-dimethyl-1-pyrroline N-oxide) spin-trapping EPR experiments were conducted. As shown in Figure 6, the •OH radical (aN = aH = 15.1 G), alkyl free radical (aN = 16.3, aH = 23 G), oxidized DMPO radical, and t-Bu-OO• radical are all detected;67 meanwhile, t-BuOH, 1-phenylethyl alcohol, and benzaldehyde are detected as intermediates of the reaction by GC−MS analysis. On the basis of these experimental results and the reports in the literature,68,69 a plausible mechanism is proposed, as shown in Figure 7. First, TBHP is adsorbed and activated at Fe−N5 sites via the interaction between the lone pair of electrons of the distant oxygen in TBHP and the electron-deficient Fe(III) center as well as the hydrogen bonding between N and H atoms, leading to homolytic cleavage of TBHP to produce tert-butyl oxygen radical (tBuO•) and hydroxyl radical (•OH). At the same time, ethylbenzene can be adsorbed on N/O-containing functional groups adjacent to the Fe−Nx active site via hydrogen bonding (e.g., in the form of O/N···H).69 The presence of such functional groups on the catalyst surface is revealed by the N 1s (Figure 3b) and O 1s (Figure S10, SI) XPS spectra. Then, the t-

62% conversion of cyclohexane was obtained after 7 h reaction at room temperature, with 99% selectivity to cyclohexone and cyclohexanol (Table 3, entry 9). Understanding the Origin of Catalytic Activity. It has been reported that heteroatom-doped carbon materials could catalyze the oxidation of the C−H bond.64,66 In our Fe−N−C catalysts, there are also such N-doped carbon sites in addition to Fe−Nx sites, as shown by the high N/Fe atom ratios in XPS result (N/Fe = 30−60). To discriminate the role played by the N-doped carbon and the Fe−Nx sites, we conducted titration experiments by using KSCN to poison the Fe−Nx sites, because SCN− could form a stable chelate complex with Fe cations.15 The titration experiment was performed with Fe− N−C-700 catalyst within a kinetically controlled region, that is, the catalyst amount was reduced to 2 mg while the reaction time was shortened to 5 min. As shown in Figure 5, with an

Figure 5. Titration experiment of active sites in Fe−N−C-700 with KSCN. Before the reaction, a specified equivalent of KSCN aqueous solution (1 mg/mL) was injected into the reaction system. Reaction conditions: 2 mg of Fe−N−C-700 catalyst (Fe 1.6 wt %, containing 0.0006 mmol of Fe), 0.5 mmol of substrates, room temperature, 500 μL of TBHP as a 70 wt % aqueous solution diluted with 6.5 mL of H2O, 5 min.

increase of the SCN− amount that was introduced into the reaction system, one can clearly observe a three-stage decrease in the ethylbenzene conversion. At the first stage, ethylbenzene conversion drastically decreased from 14.5% to 3.8% upon introducing 0.2 equiv of SCN−. At the second stage, ethylbenzene conversion decreased less fast, from 3.8% to 2.2% after introducing an additional 0.3 equiv of SCN−. At the third stage, the conversion decreased at a much slower rate, until it remained constant (1.4%) at 1.0 equiv of SCN−. Such a nonlinear decrease behavior strongly suggests that there are three types of active sites on the catalyst surface, which is in good agreement with the Mössbauer spectroscopy results (D2, D3, and D4, Table 1). On the basis of the amount of SCN−, we can calculate the intrinsic activity (TOFs) of different sites: the most active site has a turnover frequency (TOF) of 5350 h−1, while the less active site has a TOF of 533 h−1, and the least active one has a TOF of only 160 h−1 (for calculation details, see the Supporting Information). Moreover, the titration result indicates that the contribution of Fe-free sites accounts for only 9.7%. Therefore, Fe−Nx sites should play a predominant role in catalysis. 10795


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is composed of a central Fe ion coordinating with four N atoms in the square plane and with the fifth N atom in the imidazole ring of histidine residue in the axial direction (Figure S11, SI). Such a Fe−N5 under-coordinated moiety can reversely bind oxygen and thus can transport O2 for our life activities. In this regard, our present atomically dispersed Fe−Nx structure acts as a mimic of the enzyme in organic transformations, demonstrating the great potential of a single-atom catalyst in bridging heterogeneous and homogeneous catalysis. Stability of the Fe−Nx Structure. The main advantage of heterogeneous catalyst lies in its good stability, recyclability and ease of separation. For this purpose, the stability and recyclability of the Fe−N−C-700 catalyst was investigated. As shown in Figure 8a, the catalyst could be reused at least five Figure 6. DMPO (5,5-dimethyl-1-pyrroline N-oxide) spin-trapping EPR experiment. The spectrum was recorded after 1 min upon introducing DMPO to the reaction system. Ethylbenzene was used as the substrate.

Figure 8. (a) Reusability of the Fe−N−C-700 catalyst. (b) HAADFSTEM image of the Fe−N−C-700 catalyst after five cycles of reaction. (c) The 57Fe Mössbauer spectra of Fe−N−C-700 before and after different cycles of reaction. (d) The relative absorption area of each iron species in the Fe−N−C-700 catalyst before and after different cycles of reaction.

times without decay in either activity or selectivity. The ICP analysis of the reaction medium after five runs shows no loss of Fe. The BET surface areas remain almost the same before and after the reaction (835 vs 818 cm2/g). Moreover, XRD (Figure S12, SI), XAS (Figure S13, SI), and HAADF-STEM (Figure 8b) characterizations show that the atomic dispersion of Fe species is well-preserved after the five repetitive runs. To probe any changes of the Fe−Nx sites after different cycles of reaction, we further performed Mössbauer spectroscopy characterizations for the three used catalysts (after the first, third, and fifth reaction cycles). As shown in Figure 8c, the Mössbauer spectra of all three used catalysts show similar peak profiles to that of the fresh catalyst, which can be well-fitted with three doublets assignable to the D2 site (ferric X−FeIIIN4−Y highspin species), D3 site (ferric N−FeIIIN4−N low-spin species), and D4 site (ferric N−FeIIIN4 medium spin species), respectively. Further quantification reveals that the relative content of each Fe−Nx species remains the same before and after the reaction (Figure 8d), indicating that all the Fe−Nx sites (including D2, D3, and D4) are rather stable during cycles of reaction. These results unambiguously demonstrate the excellent stability and recycling ability of the single-atom dispersed Fe−N−C catalyst.

Figure 7. Proposed reaction mechanism of ethylbenzene oxidation on the Fe−N−C-700 catalyst.

BuO• abstracts one α-H of ethylbenzene to yield the corresponding α-ethylbenzene radical, which reacts quickly with •OH to produce 1-phenylethyl alcohol. Subsequently, tBuO• captures H from the hydroxyl group of 1-phenylethyl alcohol to form 1-ethylbenzene oxygen radical, which then reacts with hydroxyl radical (•OH) to form the 1-hydroperoxyethylbenzene intermediate. Finally, acetophenone as the desired product and benzaldehyde as a byproduct were produced via carbonyl-forming elimination of 1-hydroperoxyethylbenzene. Among these elementary reaction steps, the dissociation of the C−H bond of methylene in ethylbenzene (step 3 in Figure 7) is believed to be the rate-determining step due to the high bond energy of α-H in ethylbenzene (412 kJ mol−1). The above reaction model is reminiscent of the activation of oxygen by the hemoglobin molecule,70 in which the active site 10796



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REFERENCES

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By using nano-MgO as a sacrificial template we have successfully prepared atomically dispersed Fe−N−C catalysts that exhibited promising activity, selectivity, and stability for the selective oxidation of the C−H bond of a broad scope of substrates at room temperature. Mössbauer spectroscopy and KSCN titration experiments reveal that there are four different FeNx species (x = 4−6) in the atomically dispersed Fe−N−C catalysts, the relative concentrations of which are critically dependent on the pyrolysis temperature. Among them, the most active Fe−N−C-700 catalyst is comprised of high-spin FeN6 (28.3%), low-spin FeN6 (53.8%), and medium-spin FeN5 (17.9%) species, where the FeN5 is at least 1 order of magnitude more active than the other two species, although it accounts for only 28.3% of the total Fe species. Pyrolysis at a higher temperature (Fe−N−C-800) resulted in a further decrease in the concentration of medium-spin FeN5 species to less than 10%, thus leading to significant loss in the activity based on total Fe atoms. This work demonstrates clearly the heterogeneity of the Fe−N−C catalysts, although all the Fe species are atomically dispersed, and more importantly, provides a guide to fabricate single-atom Fe−N−C catalyst with more medium-spin Fe−N5 sites.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05130.

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Details of the catalyst preparation and characterization results (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Xiaoyan Liu: 0000-0003-2694-2306 Aiqin Wang: 0000-0003-4552-0360 Tao Zhang: 0000-0001-9470-7215 Author Contributions †

W.L. and L.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the support from the National Natural Science Foundation of China (21690080, 21690084, 21373206, 21522608, 21503219, 21672210, and 21673228), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020100), the National Key Projects for Fundamental Research and Development of China (2016YFA0202801), and the BL 14W beamline at the Shanghai Synchrotron Radiation Facility (SSRF). We also thank Prof. Hongxian Han from Dalian Institute of Chemical Physics and Prof. Jihu Su from The University of Science and Technology of China, for their help in EPR analysis, and Prof. Junhu Wang from Mössbauer Effect Data Center of Dalian Institute of Chemical Physics. 10797


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Discriminating Catalytically Active FeNx Species of Atomically

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