Metal (Hydr)oxides@Polymer Core–Shell Strategy ... - ACS Publications

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Metal (Hydr)oxides@Polymer Core−Shell Strategy to Metal SingleAtom Materials Maolin Zhang,†,Δ Yang-Gang Wang,†,Δ Wenxing Chen,†,Δ Juncai Dong,‡ Lirong Zheng,‡ Jun Luo,§ Jiawei Wan,† Shubo Tian,† Weng-Chon Cheong,† Dingsheng Wang,*,† and Yadong Li*,† †

Department of Chemistry, Tsinghua University, Beijing 100084, China Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China § Center for Electron Microscopy, Tianjin University of Technology, Tianjin 300384, China ‡

S Supporting Information *

isolated state. Inspired by this accidental discovery, we report a novel core−shell strategy to fabricate metal single atoms dispersed on CN materials (SA-M/CN). By varying metal precursors or polymers, we can obtain various single- or multiple-component SA-M/CN (M = Fe, Co, Ni, Mn, FeCo, FeNi, etc.). Intriguingly, the SA-Fe/CN exhibits as an excellent catalyst toward the hydroxylation of benzene to phenol, the conversion of benzene is 45% and the selectivity is 94%. Our synthesis strategy includes three steps: (1) coating metal precursors with organic polymers; (2) carbonizing organic polymers coated metal precursors at high temperature; (3) etching unstable species by concentrated HCl. As illustrated in Scheme 1, first, α-FeOOH nanorods were synthesized by

ABSTRACT: Preparing metal single-atom materials is currently attracting tremendous attention and remains a significant challenge. Herein, we report a novel core−shell strategy to synthesize single-atom materials. In this strategy, metal hydroxides or oxides are coated with polymers, followed by high-temperature pyrolysis and acid leaching, metal single atoms are anchored on the inner wall of hollow nitrogen-doped carbon (CN) materials. By changing metal precursors or polymers, we demonstrate the successful synthesis of different metal single atoms dispersed on CN materials (SA-M/CN, M = Fe, Co, Ni, Mn, FeCo, FeNi, etc.). Interestingly, the obtained SA-Fe/ CN exhibits much higher catalytic activity for hydroxylation of benzene to phenol than Fe nanoparticles/CN (45% vs 5% benzene conversion). First-principle calculations further reveal that the high reactivity originates from the easier formation of activated oxygen species at the single Fe site. Our methodology provides a convenient route to prepare a variety of metal single-atom materials representing a new class of catalysts.

Scheme 1. Schematic Illustration of the Synthesis of SA-Fe/ CN

M

etal single-atom materials are emerging as a new research frontier because of their unique catalytic properties in catalysis.1−5 In this rapidly developing field, several strategies have been suggested for preparing metal single-atom materials, including atomic layer deposition (ALD),6 wet impregnation,7,8 coprecipitation9 and photodeposition.10 Accordingly, various supported metal singleatom catalysts, including noble metal Au,5,11 Pt,7−9,12,13 Pd,6,10 Ir14 and Ru,15 non-noble metal Fe,16,17 Co18−20 and Ni,21 have been demonstrated to show good catalytic performance. However, wide applications of metal singleatom materials still require the development of advanced synthetic methodology. Recently, we intend to synthesize a hollow nitrogen-doped carbon (CN) materials by carbonizing polydopamine (PDA) coated α-FeOOH nanorods and acid leaching. In our primary characterizations, we found that the CN materials still had residual stable existed Fe species. After further investigations, surprisingly, the residual Fe species existed in the form of single atoms. That means, through a pyrolysis and acid leaching process, the formed CN can firmly anchor metal atoms in an © 2017 American Chemical Society

hydrothermal method. Then, α-FeOOH@PDA was obtained by self-polymerizing dopamine monomers in the presence of αFeOOH nanorods. The α-FeOOH@PDA was further annealed at 700 °C under an inert atmosphere. During this time, PDA layers converted into CN shell and α-FeOOH was reduced into iron slowly by PDA layers, and strong interaction also formed between Fe atoms and the CN shell. Consequently, acid leaching was introduced to remove the unstable species and the stable anchored Fe single atoms were exposed on the inner wall of CN materials. After filtered and washed thoroughly by deionized water, the uniform SA-Fe/CN was last obtained. Transmission electron microscopy (TEM, Supporting Information Figure S1) shows that the rod-shaped α-FeOOH has the length of 500 nm−1 μm and the diameter of 50−100 nm. Figure 1a illustrates that PDA forms a uniform coating layer on the surface of α-FeOOH nanorods with a thickness of about 10 nm. SA-Fe/CN (Figure 1b and Figure S2) displays a hollow pipe-like morphology with a shell thickness of about 5 Received: May 24, 2017 Published: July 31, 2017 10976

DOI: 10.1021/jacs.7b05372 J. Am. Chem. Soc. 2017, 139, 10976−10979

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Figure 2. (a) XANES spectra at the Fe K-edge of SA-Fe/CN, FeO, Fe2O3 sample and Fe foil; the inset is the magnified image of pre-edge XANES spectra. (b) Fourier transform (FT) at the Fe K-edge of SAFe/CN, Fe2O3 sample and Fe foil. Wavelet transform (WT) of (c) SAFe/CN; (d) Fe foil; (e) Fe2O3 samples. (f, g) Corresponding EXAFS fitting curves of SA-Fe/CN at k space and R space, respectively. The inset of panel g is the schematic model of SA-Fe/CN, Fe (pink), N (blue) and C (gray).

Figure 1. TEM images of (a) α-FeOOH@PDA; (b, c) SA-Fe/CN. (d) HAADF-STEM image and corresponding EDX mapping of SA-Fe/ CN, C (pink), N (green) and Fe (yellow). (e, f) AC HAADF-STEM image and enlarged view of SA-Fe/CN.

FeO and Fe2O3 references. The absorption edge of SA-Fe/CN is located between the FeO and Fe2O3, suggesting the Fe atom valence is situated between that of Fe2+ and Fe3+. In addition, in the inset of Figure 2a, the pre-edge peak at 7115.1 eV (derived from 1s→3d transition) is the fingerprint of Fe−X 4 coordination, which is coincident with the Fe−N4 stucture.23 Figure 2b shows the Fourier-transformed (FT) k3-weighted extended X-ray absorption fine structure (EXAFS) spectra of SA-Fe/CN. Interestingly, it just displays one main peak at 1.5 Å, which is thought to correspond to the Fe−N/C first coordination shell, and no appreciable Fe−Fe coordination peak or other high shell peaks are detected. In addition, wavelet transform (WT) was used to analyze Fe K-edge EXAFS oscillations. As illustrated in Figure 2c, the WT maximum at 5 Å−1 for SA-Fe/CN could be assigned to the Fe−N(C) bonding and no intensity maximum corresponded to Fe−Fe is observed, compared with the WT plots of Fe foil and Fe2O3 in Figure 2d,e. Consequently, combining AC HAADF-STEM and XAFS results, and further evidenced by the WT analysis, we can conclude that Fe atoms are atomically dispersed on the SA-Fe/ CN. To obtain the quantitative chemical configuration of Fe atom, EXAFS fitting was also performed to extract the structure parameters. The obtained coordination number of center atom Fe is about 4 and the mean bond length of Fe−N/C is 2.04 Å (Table S1). The fitting curves are exhibited in Figure 2f,g and Figure S7. The data indicate that in the SA-Fe/CN the Fe atom is coordinated by 4 N atoms. To validate the generality of this approach, we extended this core−shell strategy to other metal precursors, such as Co(OH)2 nanoplates, Ni(OH)2 nanoplates and MnO2 nanowires, or even commercial available multicomponent CoFe2O4 and NiFe2O4 nanoparticles. After characterized by AC HAADF-STEM, as shown in Figure 3a−e, many individual bright dots are also

nm and a length range from 500 nm to 1 μm. Figure 1c shows the high magnification TEM image of SA-Fe/CN, in which no Fe particles are observed. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image and corresponding Energy-dispersive X-ray (EDX) mapping analysis of SA-Fe/CN (Figure 1d) reveal that C, N and Fe are homogeneously distributed along the CN. The content of Fe, measured via inductively coupled plasma optical emission spectrometry (ICP-OES) analysis is about 0.9 wt %. To characterize the dispersion of Fe in the SA-Fe/CN, aberration-corrected HAADF-STEM (AC HAADF-STEM) was used to check SA-Fe/CN. As shown in Figure 1e,f, high density bright dots (highlighted by yellow circles) corresponding to Fe single atoms are observed. When we examined more hollow pipes, bright dots are also observed (Figure S3a,b). Nitrogen adsorption−desorption isotherms (Figure S4) show that SA-Fe/CN exhibits a high Brunauer−Emmett−Teller (BET) surface area of 891 m2 g−1. The average pore diameter is about 2.4 nm. X-ray diffraction (XRD) pattern of SA-Fe/CN only displays a broad peak in the range 15−35° (2θ) related to the (002) plane of graphitized carbon (Figure S5). The X-ray photoelectron spectroscopy (XPS) spectrum of SA-Fe/CN (Figure S6a) indicates the presence of C, N and O in the CN. The highresolution N 1s XPS spectrum (Figure S6b) can be deconvoluted into two main peaks at 398.4 and 400.8 eV, corresponding to pyridinic-N and quaternary-N, respectively.22 X-ray absorption fine structure (XAFS) measurements were carried out to further investigate the structure of Fe species in atomic level. Figure 2a shows Fe K-edge X-ray absorption nearedge structure (XANES) spectra of SA-Fe/CN with Fe Foil, 10977

DOI: 10.1021/jacs.7b05372 J. Am. Chem. Soc. 2017, 139, 10976−10979

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Journal of the American Chemical Society

Figure 4. (a) Benzene conversion catalyed by the SA-Fe/CN, Fe nanoparticles/CN, CN, heme iron, phthalocyanine iron and ferric chloride, respectively. (b) Reaction mechanisms for H2O2 activation and benzene oxidation on both SA-Fe/CN and Fe nanoparticles/CN. The values in bracket denote energy barriers and the other values (not in bracket) denote the relative energy referenced to the initial state. Ben, bezenze; Phe, Phenol; TS, transition states.

Figure 3. AC HAADF-STEM images of (a) SA-Co/CN; (b) SA-Ni/ CN; (c) SA-Mn/CN; (d) SA-FeCo/CN; (e) SA-FeNi/CN and (f) SA-Fe/CN. Single atoms are highlighted by the yellow circles.

observed in the SA-M/CN (M = Co, Ni, Mn, FeCo, FeNi). HAADF-STEM, XRD and XAFS (Figures S8−S16) were further used to comfirm as-obtained atomically dispersed metal atoms. In addition to PDA, polyaniline (PANI) and polypyrrole (PPy) are also used widely to synthesize CN matrix. Herein, we further demonstrated that PANI (Figure 3f and Figures S17− S21) or PPy (Figures S22−S27) could also be used to synthesize SA-M/CN materials. As we know, metal single-atom material is a kind of attractive catalyst that can provide a unique opportunity to tune the reaction activity and selectivity. For example, single Fe atoms anchored on N-doped carbon exhibit as efficient ORR catalysts.17 Here, we used SA-Fe/CN to examine the catalytic activity of the hydroxylation of benzene to phenol, an important reaction in chemical industry. As shown in Figure 4a, after reaction for 24 h, we achieved 45% benzene conversion and 94% selectivity for phenol by GC−MS and GC, which is one of the best results for the hydroxylation of benzene to phenol based on Fe-contained catalyst.16,24−28 When the reaction was processed without addition of SA-Fe/CN, we obtained 1% conversion of benzene. If the SA-Fe/CN was replaced by CN (Figure S28a, CN was obtained by carbonizing PDA spheres) or Fe nanoparticles/CN (Figure S28c, CN loaded with Fe nanoparticles), only 1.4% and 5% benzene conversion were obtained, respectively. Furthermore, FeCl3, phthalocyanine iron and heme iron were also chosen as reference, low conversion of benzene reached, that is, 0.9% for FeCl3, 3% for phthalocyanine iron and 6% for heme iron. It is therefore evident that it is indeed SA-Fe/CN that can catalyze direct oxidation of benzene to phenol well. TEM images (Figure S29), EDX mapping (Figure S30), AC HAADF-STEM image (Figure S31) and EXAFS spectrum (Figure S32) all

show that Fe atoms are still atomically dispersed after reaction and no aggregation is observed, demonstrating the stability of single Fe atoms on the SA-Fe/CN. To further understand the high catalytic activity of the SAFe/CN, we performed comparative density functional theory calculations on the reaction mechanisms of benzene oxidation on both SA-Fe/CN and Fe nanoparticles/CN, as shown in Figure 4b and Figures S33−S35. The reaction pathway is considered to experience two steps: (i) the formation of activated oxygen species by the decomposition of H2O2 (i.e., H2O2 → H2O + O); (ii) the oxidation of benzene (Ben) to form phenol (Phe) by the activated oxygen species (i.e., C6H6 + O → C6H5OH). On SA-Fe/CN catalyst, it is found that the oxidant H2O2 initially dissociates into two hydroxyls (2HO*) coadsorbing at the single Fe site in accordance with a recent theoretical report.29 After that, the two hydroxyls easily form the active oxygen species (O*) by releasing one water molecule. This process only needs to overcome a small barrier of 0.79 eV and is highly exothermic by −2.23 eV. Once the activated oxygen species is formed, it can oxidize benzene to phenol by overcoming a barrier of 1.08 eV. On Fe nanoparticles/CN, H2O2 is also found to dissociate into two hydroxyls; however, the two hydroxyls are found extremely hard to form the oxygen species because the process needs to experience high activation barriers (1.67 and 1.54 eV) and is highly endothermic by 1.99 eV. Moreover, the formed oxygen species has to oxidize benzene with a high barrier of 1.73 eV. By comparing the two reaction pathways, it is easily concluded that SA-Fe/CN possesses much higher reactivity for benzene oxidation than Fe nanoparticles/CN. This is traced back to the fact that the Fe particle bind both the hydroxyl and the 10978

DOI: 10.1021/jacs.7b05372 J. Am. Chem. Soc. 2017, 139, 10976−10979

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oxygen species much stronger than single Fe atom and made them too stable to be catalytically active. Mulliken charge analysis shows that the two hydroxyl species can attain 0.90 e− from Fe6 particle but only attain 0.68 e− from single Fe atom, suggesting that the charge interaction on Fe6 particle is stronger than that on single Fe atom. The electronic chemical potential calculations further confirm that the single Fe atom has a higher electronic chemical potential (3.89 eV) than that of Fe particle (2.96 eV) and thus is harder to transfer charges to the adsorbed species. We note that recently Deng et al. reported an additional H2O2 can also create a second adsorbed oxygen species on Fe single-atom catalyst with one oxygen preadsorbed and oxidize benzene further, which provides a supplementary for benzene oxidation mechanism but does not affect our understanding on the reactivity difference between SA-Fe/CN and Fe nanoparticles/CN.16 To support the density functional theory (DFT) calculations, we used electron paramagnetic resonance spectroscopy to characterize the reaction process. As shown in Figure S36, the spectrum of a frozen reaction solution contains an obvious signal at g = 4.3, which belongs to FeIVO species.30 The observed formation of FeIVO species is usually condisdered to be the active intermediates in this kind of reaction, which verifies the DFT calculations that H2O2 could be activated on the single Fe atom. In conclusion, we have developed a novel core−shell strategy to prepare stable SA-M/CN materials. The obatined SA-Fe/ CN catalyst performed high conversion of 45% and selectivity of 94% in the direct hydroxylation of benzene to phenol, which is much higher than Fe nanoparticles/CN (only 5% conversion). This work indicates that novel SA-M/CN materials can be prepared by this efficient route, which may have broad applications, such as organic reaction or energy conversion.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05372. Detailed experimental procedures, characterization methods, and additional tables and figures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Yang-Gang Wang: 0000-0002-0582-0855 Dingsheng Wang: 0000-0003-0074-7633 Yadong Li: 0000-0003-1544-1127 Author Contributions Δ

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S Supporting Information *

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Communication

M.Z., Y.-G.W. and W.C. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by China Ministry of Science and Technology under Contract of 2016YFA (0202801), and the National Natural Science Foundation of China (21521091, 21390393, U1463202, 21471089, 21671117). We thank the 1W1B station for XAFS measurement in Beijing Synchrotron Radiation Facility (BSRF). 10979

DOI: 10.1021/jacs.7b05372 J. Am. Chem. Soc. 2017, 139, 10976−10979