Nitrogen-Coordinated Cobalt Embedded in a Hollow Carbon Polyhedron for Superior Catalytic Oxidation of Organic Contaminants with Peroxymonosulfate

  • Yaowen Gao
    Yaowen Gao
    Institute of Environmental Research at Greater Bay, Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou University, Guangzhou 510006, China
    More by Yaowen Gao
  • Yue Zhu
    Yue Zhu
    Institute of Environmental Research at Greater Bay, Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou University, Guangzhou 510006, China
    More by Yue Zhu
  • Zhenhuan Chen
    Zhenhuan Chen
    Institute of Environmental Research at Greater Bay, Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou University, Guangzhou 510006, China
  • , and 
  • Chun Hu*
    Chun Hu
    Institute of Environmental Research at Greater Bay, Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou University, Guangzhou 510006, China
    Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
    *Phone: +86-20-39346609. E-mail: [email protected], [email protected]
    More by Chun Hu
Cite this: ACS EST Engg. 2021, 1, 1, 76–85
Publication Date (Web):October 1, 2020
https://doi.org/10.1021/acsestengg.0c00039
Copyright © 2020 American Chemical Society
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Abstract

The search for high-active and long-lasting materials is of paramount significance for elimination of aqueous organic contaminants. Herein, a nitrogen-coordinated cobalt embedded in hollow carbon polyhedron (marked as NCoHCP) has been synthesized by thermal transformation of core–shell-architectured [email protected] X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and theoretical calculations confirmed the coordination of a Co atom with one N atom in NCoHCP. The resultant NCoHCP catalyst delivers extremely superior catalytic performance in peroxymonosulfate (PMS) decomposition into active species toward complete oxidation of organic pollutants in less than 45 s. Experimental data and theoretical computations disclosed the formation of Co–pyridinic N moiety not only creates the Co species with higher electron density as active sites but also increases the C atom neighboring pyridinic N with lower electron density as binding sites for PMS conversion toward reactive oxygen species generation to oxidize organic pollutant. On the other hand, the donation of electron from organic contaminant toward the C atom adjacent to pyridinic N also induces organic contaminant oxidation. The dual-pathway degradation contributes to superior catalytic oxidation of organics over NCoHCP. This study furnishes an effective strategy for developing high-performance and robust metal/carbon hybrid materials toward wide environmental applications.

 Note

This paper was published ASAP October 1, 2020, with incorrect Figures 4 and 5 introduced during production. The corrected version was posted on December 22, 2020.

  Note

Made available for limited times for personal research and study only license.

Introduction

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To meet the requirement of current clean water, multiple advanced technologies have been exploited to eliminate recalcitrant organic contaminants in aqueous media. (1) In recent years, the persulfate-based advanced oxidation processes have exhibited enormous effectiveness in organic pollutant degradation by virtue of sulfate radical (SO4•–) with powerful oxidation capacity. (2−4) For the generation of SO4•–, the couple of transition metal ions with peroxymonosulfate (PMS) demonstrates high effectiveness, (5) and the cobalt ion (Co2+) has been proven to be the most efficient activator for the conversion of PMS into SO4•–. (6) However, the shortcomings of the homogeneous process associated with precipitation of metallic ion and the potential carcinogenic effect of excessive cobalt ion substantially restrict the practical application of the Co2+/PMS system. (7) In this context, heterogeneous cobalt-based materials have attracted increasingly considerable concern as more efficient PMS activators. (8−11) Unfortunately, these Co-based catalysts, particularly the Co-containing nanoparticles and the supported ones, are plagued with easy aggregation and sluggish kinetics, which largely retard their implementation for large-scale applications in PMS activation. (7)
In order to overcome the demerits, encapsulation of the Co species into the porous nitrogen-doped carbon materials appears to be an effective and promising approach, since carbon frameworks can alleviate the aggregation of metal nanoparticles and protect the embedded active metal species from the external harsh environment, (12) impeding the leakage of metal ions. Moreover, the modulation of electron density and charge distribution by the cobalt ion and nitrogen species can create more active sites in the metal/carbon hybrid materials for catalytic reaction. (13,14) The zeolite-like zeolitic imidazolate framworks (ZIFs) with 3D topological structures and abundant contents of carbon, nitrogen, and transition metal ions (15,16) have been demonstrated to be desired platforms for fabrication of the metal-embedded carbon composites via pyrolysis. For instance, Wang and co-workers (17) reported the utilization of ZIF-67-derivative cobalt/carbon hybrid as the heterogeneous catalyst for the PMS-mediated organic pollutant destruction. However, this cobalt/carbon composite suffers from a low degree of graphitization, which hampers mutual electron shuttle between the catalyst and PMS molecule, resulting in dissatisfactory catalytic activity. Moreover, the underlying PMS activation mechanism by such cobalt/carbon nanocomposite, especially the identification of active sites, has been not well elucidated, which needs further exploration.
Aiming at improving the catalytic performance of metal/carbon hybrid materials, bimetallic [email protected] MOFs with core–shell structure have recently been constructed and utilized as the precursor to fabricate the cobalt-embedded nitrogen-doped carbon catalyst due to the following reasons. First, the lattice-matched feature of ZIF-8 and ZIF-67 facilitates the growth of [email protected] with core–shell architecture. (18) Further, the zinc (Zn) evaporation from Zn-based ZIF-8 core at elevated temperature can offer carbon with rich micro/mesopores and high nitrogen content. (13) Moreover, the unique core–shell nanostructure of [email protected] benefits the encapsulation of cobalt species in the in situ formed nitrogen-doped carbon nanotubes catalyzed by cobalt and the formation of Co–Nx moiety owing to the high affinity of nitrogen to cobalt species. (14,19) Benefiting from rich porosity, high conductivity, and plentiful active sites, the hybrid cobalt-encapsulated nitrogen-doped carbon catalysts deliver excellent activity in various electrocatalytic reactions. With these merits, it is reasonable to conceive that the nitrogen-coordinated cobalt embedded in the graphitic carbon framework derived from core–shell (Zn, Co)-bimetallic ZIFs can behave as an ideal activator for PMS conversion, which has not yet been reported so far. On the other hand, an unambiguous cognition on the active sites of cobalt/carbon hybrid catalyst toward the activation of PMS can also benefit the in-depth understanding of activation mechanism.
Herein, we have successfully synthesized the nitrogen-coordinated cobalt embedded in hollow carbon polyhedron (marked as NCoHCP) through the transformation of core–shell [email protected] at 920 °C under Ar atmosphere. Various analytical techniques along with theoretical computations revealed a unique coordination path of a Co atom with one N atom for the construction of Co–pyridinic N species, resulting in high-degree graphitization and charge redistribution of NCoHCP. Therefore, the NCoHCP catalyst delivers superior catalytic performance in PMS activation with complete oxidation of organic pollutants within 45 s, which was also evidenced by the significantly higher specific activity of NCoHCP than that of existing state-of-the-art counterparts. By combining experimental results and theoretical calculations, the origin of high reactivity of NCoHCP in PMS-mediated destruction of organic contaminants has been clarified.

Experimental Section

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The details including chemicals, catalyst preparation and characterization, catalytic tests, analyses, and theoretical computations are presented in the Supporting Information. The mobile phases and detection wavelengths, and molecular structures for various organic contaminants are displayed in Table S1 and Figure S1, respectively.

Results and Discussion

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Characterizations

The X-ray diffraction (XRD) patterns of samples (Figure S2) show that ZIF-67 shares a diffraction mode similar to that of ZIF-8 because of their analogical unit cells and crystal lattices, (15,19) which can afford epitaxial growth of ZIF-67 on ZIF-8 resulting in the construction of core–shell-architectured [email protected] with the parallel ZIF-8-type crystal phase. The scanning electron microscopy (SEM) (Figure S3a,c,e) and transmission electron microscopy (TEM) (Figure S3b,d,f) images illustrate that monometallic ZIF-8 and ZIF-67 and bimetallic [email protected] display the well-shaped rhombic polyhedral morphologies with slippy surfaces. Moreover, the TEM together with corresponding energy-dispersive spectroscopy (EDS) elemental mapping images (Figure S4) suggest the presence of carbon (C), nitrogen (N), Co, and Zn elements in [email protected], of which Co and Zn atoms are distributed at the outside and center, respectively, confirming that [email protected] with core–shell architecture has been triumphantly fabricated. After thermal annealing under an Ar atmosphere, a hollow polyhedron structure appears in NCoHCP, with multiple carbon nanotubes (CNTs) being attached on the rough surface, which can be visualized by the SEM image (Figure 1a). The EDS elemental mapping image (Figure 1b) manifests the main components of C, N, and Co elements in NCoHCP with N and Co atoms being homogeneously distributed across the carbon frameworks. The TEM image (Figure 1c) verifies that NCoHCP possesses the hollow polyhedron architecture with the attachment of CNTs on its surface, and that the Co species is primarily distributed at the edge of hollow polyhedron without apparent aggregation. Previous works (20,21) reported that the Co nanoparticles-driven catalytic behavior accounted for the development of CNTs. Nonetheless, the direct thermal treatment of individual Co-based ZIF-67 gives rise to no production of CNTs but results in severe agglomeration of Co nanoparticles in Co-NC (annealed ZIF-67) (Figure S5). This highlights the particular merit of core–shell architecture of bimetallic ZIFs such as [email protected] in the preparation of fine cobalt-embedded hollow carbon polyhedron. To be specific, the Zn evaporation from ZIF-8 core under elevated pyrolysis temperature can facilitate the movement of Co species from ZIF-67 shell toward the edge for avoidance of Co aggregation, which leaves rich micro/mesopores and a nitrogen source and thus promotes the Co species-triggered catalytic formation of CNTs on the external surface. (13) High-resolution TEM (HRTEM) image demonstrates the well-defined graphitic carbon structure of NCoHCP, with the Co species being encapsulated by graphitic carbon layers (Figure 1d), which should be attributable to the Co nanoparticles-driven catalytic carbon graphitization. (22) The sharp diffraction peak located at 25.9° in NCoHCP (Figure 1e), which is associated with the (002) plane of graphitic carbon, (23,24) suggests the graphitic structure of NCoHCP. Besides, the other three peaks centered at 44.0°, 51.2°, and 75.7° are respectively indexed to the (111), (200), and (220) facets of metallic Co (JCPDS 01–1225). The lower ID/IG value of NCoHCP compared with the Co-NC (annealed ZIF-67) and Zn-NC (annealed ZIF-8) (Figure 1f) further corroborates the high-degree graphitization of NCoHCP. This confirms that the Zn evaporation during high-temperature pyrolysis facilitates the carbon graphitization that is catalyzed by the Co components, which can enhance the electron mobility and corrosion resistance of the catalyst in catalytic reaction. The typical type IV curve in the N2 adsorption–desorption isotherm (Figure S6) suggests the micro/mesoporosity of NCoHCP, with the specific surface area of 302.9 m2 g–1. Meantime the pore size distribution curve, which is inserted in Figure S6 further attests to the mesoporous structure of NCoHCP.

Figure 1

Figure 1. (a) SEM, (b) EDS elemental mapping, (c) TEM, and (d) HRTEM images of NCoHCP. (e) XRD pattern of NCoHCP. (f) Raman spectra of samples.

The X-ray photoelectron spectroscopy (XPS) survey spectrum (Figure S7a) substantiates the presence of C, N, O, and Co atoms in NCoHCP. However, no peak of the Zn signal is noted, as also supported by the high-resolution XPS (HR-XPS) Zn 2p spectrum of NCoHCP (Figure S7b), indicating the complete evaporation of Zn species during the synthesis process. The low Co content (0.44 at %) in NCoHCP suggests that the acid etching in the final step of synthesis procedure removes the surface accessible Co species. Moreover, the practical Co mass content in this catalyst is measured to be 3.32 wt % via inductively coupled plasma-optical emission spectrometry (ICP-OES). Figure S8 presents the HR-XPS C 1s spectrum containing C–C/C=C and C–N peaks, which illuminates nitrogen incorporation into the graphitic carbon host of NCoHCP. The HR-XPS N 1s spectrum of NCoHCP (Figure 2a) exhibits five subpeaks at 398.2, 399.2, 400.3, 401.3, and 403.6 eV, which respectively refer to pyridinic N, Co–N, pyrrolic N, graphitic N, and oxidized N. The band at 399.2 eV is attributable to the nitrogen interaction with the metallic cobalt. (25) In the HR-XPS Co 2p spectrum (Figure 2b), two main peaks appear at 779.8 and 795.3 eV corresponding to binding energy of Co 2p3/2 and Co 2p1/2, which can be further split into subpeaks of Co0 (779.4, 795.1 eV), Co2+ (781.9, 798.5 eV), and satellite Co (785.2 eV). It is worthwhile noting that the proportion ratio of Co0/Co2+ is calculated to be 1.81, indicating that the Co0 is the main form of Co component in NCoHCP.

Figure 2

Figure 2. HR-XPS spectra of (a) N 1s and (b) Co 2p for NCoHCP. (c) Normalized Co K-edge XANES and (d) EXAFS spectra of NCoHCP and reference samples. (e) FT EXAFS first-shell fitting curve for NCoHCP at R space. (f) Optimized structures and corresponding calculated formation energy (Ef) of the models of Co–pyridinic N and Co–pyrrolic N. Blue, white, and brown represent Co, N, and C atoms, respectively.

To further explore the structure of NCoHCP at the atomic level, X-ray absorption spectroscopy (XAS) was undertaken. The X-ray absorption near-edge structure (XANES) spectra (Figure 2c) illustrate the energy absorption margin of NCoHCP situating between Co foil and CoO reference, but approaching more closely to Co foil, which suggests that the Co atom holds a positive charge with Co0 of being the dominant contributor, consistent with the above XPS results. The Fourier-transformed (FT) k3-weighted extended X-ray absorption fine structure (EXAFS) spectrum of NCoHCP exhibits two main peaks at 1.5 and 2.1 Å (Figure 2d), which respectively correspond to Co–N and Co–Co coordination peaks according to the EXAFS spectra of the reference ZIF-67 and Co foil. The appearance of the Co–N bond in NCoHCP verifies the coordination of Co and N atoms. On the other hand, the Co–Co coordination peak in the EXAFS spectrum of NCoHCP is slightly shifted to low-R as compared to that in Co foil, reflecting that the Co component has been embedded in the carbon matrix, which is in line with TEM observation and has been reported elsewhere. (22,26) The EXAFS fitting analysis was then carried out for the first coordination shell to achieve the quantitative structural parameters of Co atom in NCoHCP, as displayed in Figure 2e and Table S2. The coexistence of Co–Co and Co–N shells corroborates the formation of metallic Co and the Co–N bond in the NCoHCP catalyst. Moreover, the Co–N coordination number of approximately 1 means that a Co atom is coordinated with one N atom. Density functional theory (DFT) calculations were undertaken to further unravel the particular configuration of Co–N species. Considering the high graphitization degree (low ID/IG value for NCoHCP in Figure 1f), namely, the low defect of NCoHCP, two possible coordination sites in terms of pyridinic N or pyrrolic N at the edge of a monolayer graphene-like model for the Co atom were explored. As depicted in Figure 2f, the Co–pyridinic N configuration shows a much lower formation energy (Ef) compared to the configuration of Co–pyrrolic N, suggesting that the Co atom prefers to be bonded to pyridinic N with one-coordinate configuration such as Co–pyridinic N. In this regard, we simply consider the Co–pyridinic N configuration in the following discussion on PMS activation mechanism.

Catalytic Performance of NCoHCP

The performance of NCoHCP is investigated via bisphenol A (BPA) degradation with PMS, and the corresponding results are illustrated in Figure 3a. The individual addition of PMS causes no degradation of BPA, ruling out the ability of PMS in oxidizing BPA directly. The NCoHCP alone gives a BPA removal efficiency of ∼30% in 60 s. However, when NCoHCP and PMS are presented simultaneously, complete BPA oxidation is fulfilled in less than 45 s, reflecting the prominent reactivity of NCoHCP in PMS decomposition and conversion. Additionally, the PMS activation behaviors over Zn-NC and Co-NC were assessed. As depicted in Figure S9, Zn-NC exhibits limited catalytic activity in PMS activation, whereas Co-NC is extremely active for the activation of PMS. Nonetheless, the Co leaching from the Co-NC material during reaction is severe, with the concentration of leached Co ion higher than 10 mg L–1, signifying that the rapid BPA oxidation in the Co-NC/PMS system is due to the leached Co-involved homogeneous catalytic reaction. With respect to NCoHCP, the concentration of leached Co ion after catalytic reaction is found to be as low as 0.14 mg L–1, which is considered negligible from the catalytic viewpoint, indicating that the PMS activation process over NCoHCP is intrinsically heterogeneous. In addition, the rate constant based on the pseudo-first-order kinetics is calculated as to 0.13 s–1 (equivalent to 7.80 min–1) for NCoHCP (Figure S10), which appears to be the highest value among the existing catalysts toward the activation of PMS (Table S3). In view of different properties of catalysts and disparate reaction conditions, the specific activity that is obtained by rate constant normalized to specific surface area and dosage of the catalyst was utilized as a descriptor to better understand the catalytic activity of NCoHCP. (27) As can be observed from Figure 3b, the specific activity calculated for the NCoHCP is significantly higher than those for the currently reported catalysts, clearly confirming the superior catalytic performance of NCoHCP in activating PMS. The excellent reactivity of NCoHCP for the activation of PMS is also supported by the correspondingly high level of PMS decomposition, as displayed in Figure 3c and Table S3. Furthermore, NCoHCP exhibits robust stability in the activation of PMS, as presented by the achievement of 92% BPA removal efficiency after at least six consecutive cycles (Figure 3d). Moreover, the rather low concentration of Co leaching from the catalyst during catalytic reaction (as indicated above) also substantiates the admirable stability of NCoHCP. To gain further insights into the effectiveness of NCoHCP/PMS, the oxidation of other organics including 2-chlorophenol (2-CP), 2,4-dichlorophenoxyacetic acid (2,4-D), ciprofloxacin (CIP), and diphenhydramine (DP) by NCoHCP with PMS was explored. As shown in Figure 3e, these pollutants can be all efficiently oxidized, though the degradation efficiency of DP is relatively lower, suggesting the high effectiveness of the NCoHCP/PMS system in catalytic oxidation of organic contaminants.

Figure 3

Figure 3. (a) Catalytic activity of NCoHCP. (b) Catalytic specific activity comparison of NCoHCP with existing catalysts. (c) PMS decomposition over NCoHCP. (d) Stability of NCoHCP. (e) Catalytic oxidation of multiple organic contaminants over NCoHCP with PMS. Reaction conditions: [pollutant] = 0.1 mM; [NCoHCP] = 0.2 g L–1; [PMS] = 4 mM; [Temp] = 30 °C; initial pH = 6.7 (if needed).

Mechanism behind PMS Activation by NCoHCP

To explore the PMS activation mechanism by NCoHCP, the reactive oxygen species formed during the reaction was first detected by means of the electron paramagnetic resonance (EPR) technique. From Figure 4a, the representative signals of both DMPO–OH and DMPO–SO4•– adducts clearly appear in the EPR spectrum of NCoHCP/PMS with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a capture reagent, manifesting that OH and SO4•– are produced from PMS decomposition over NCoHCP. However, no superoxide radical (O2•–) is generated in the NCoHCP/PMS system, which can be evidenced by the inappreciable change in the intensity of O2•– signal in the suspension of NCoHCP and PMS compared with that in PMS alone (Figure S11a). Besides radicals, nonradical species such as singlet oxygen (1O2) is detected in the system of NCoHCP/PMS, as supported by the appearance of representative 1O2 signal with strong intensity in the spectrum of NCoHCP/PMS with 2,2,6,6-tetramethyl-4-piperidinol (TMP) (Figure 4b). Moreover, the quenching experiment results that (i) the injection of tert-butyl alcohol (TBA), ethanol, and β-carotene all inhibits BPA degradation and (ii) the presence of benzoquinone (BQ) hardly suppresses BPA oxidation (Figure S11b) further validate the yield of OH, SO4•–, and 1O2, where TBA, ethanol, β-carotene, and BQ are respectively typical scavengers for OH, SO4•–, 1O2, and O2•–.

Figure 4

Figure 4. EPR spectra for (a) OH/SO4•– and (b) 1O2 of different systems. (c) Charge density distribution image of the Co–pyridinic N–C model. (d) Bader charge of Co, pyridinic N, and C atoms neighboring pyridinic N. (e) Adsorption energy of PMS or BPA at the sites of Co, C2, and C5. (f) Bader charge of Co, C2, and C5 sites before and after PMS or BPA adsorption.

The production of radicals and nonradical species means that the electron exchange takes place between NCoHCP and PMS, which is in close connection with the electronic structure of NCoHCP. DFT calculations were undertaken to analyze the electronic property of the catalyst and the interfacial electron transfer process of NCoHCP with the PMS molecule. The two-dimensional charge density distribution image of the Co–pyridinic N–C model shown in Figure 4c reveals the redistribution of electron density in NCoHCP after the coordination of Co atom with pyridinic N. It is found that the Co atom has the maximum electron probability distribution than pyridinic N and C atoms, while pyridinic N has better electron probability distribution relative to the C atoms, which is attributable to the highest electronegativity of N atom among these three atoms. Bader charge analysis was further conducted for precise illumination of electron density of different sites. As depicted in Figure 4d, pyridinic N gets 1.071 e with the Bader charge increasing from 6.000 e for the mean Bader charge of an N atom to 6.071 e. On the other hand, the Co atom loses 0.359 e from the intrinsic Bader charge of 9.000 to 8.641 e. Similarly, the attenuation in Bader charge is also noticeable for C2 and C5 atoms that are most adjacent to pyridinic N, with their Bader charges reducing from the inherent value of 4.000 e to 3.513 and 3.517 e, respectively. Despite the electron depletion of the Co atom, it still delivers the highest electron density because of the intrinsically high number of valence electron. In view of higher and lower electron density, the Co atom, pyridinic N, and the C2 and C5 atoms in NCoHCP may engage as catalytic sites to bond with PMS by means of electron transfer.
Aiming at further confirmation of binding sites for PMS accessibility, the PMS adsorption behavior onto the different sites was explored by DFT computations. As illustrated in Figure 4e, the calculated PMS adsorption energy (Eads) at the Co site is the highest among all the catalytic sites, where the adsorption energy was defined as Eads = ENCoHCP+moleculeENCoHCPEmolecule (ENCoHCP+molecule, ENCoHCP, and Emolecule represented the total energy of NCoHCP surface adsorbed molecule, the NCoHCP surface and free molecule, respectively). In addition, the Eads value for the C2 or C5 atom is very close but still outperforms that for pyridinic N. This result rules out the possibility of pyridinic N as the active site to anchor the PMS molecule. In addition to PMS adsorption energy, the change in the O–O bond length (lO–O) in the active component of peroxymonosulfate (SO4–OH) before and after PMS adsorption is another descriptor for identification of active sites toward PMS immobilization. Compared with the lO–O in free PMS (1.412 Å), the lO–O values are stretched to 1.471, 1.460, and 1.467 Å after PMS adsorption on the Co, C2, and C5 atoms, respectively. The stretch trend of lO–O in PMS is in accordance with the adsorption energy of PMS onto the Co and C atoms, indicating the Co atom and C5 atom neighboring pyridinic N as preferential sites for PMS attachment. The mutual electron shuttle of PMS with the Co or C5 site in NCoHCP was further revealed by the alteration in Bader charge of the Co or C5 site upon interaction with PMS. As depicted in Figure 4f, the Co atom delivers a pronounced decrease in Bader charge after the adsorption of PMS. On the contrary, the C2 or C5 atom shows an enhancement in Bader charge when PMS was anchored onto this site, in which the Bader charge of the C5 atom mounts more significantly relative to that of the C2 atom. The decreased electron density of the Co site upon PMS adhesion signifies the electron movement from the Co atom toward PMS, namely, PMS reduction occurs around the Co atom with higher electron density to produce OH and SO4•–. On the other hand, the enhanced electron density of the C5 site with PMS accessibility reflects that the electron flows from PMS in the direction of the C5 site, which enables PMS oxidation over the C5 atom with less charge density to generate PMS anion radical (SO5•–), which afterward transforms into 1O2 via reacting with water molecules. (28) Apart from PMS, the theoretical insights into the affinity between the organic pollutant such as BPA and the active sites were also analyzed. The Eads value of BPA at the C5 site approaches that at the C2 site but is higher than that at the sites of the Co atom and pyridinic N (Figure 4e), suggesting the C5 atom is more prone to BPA approachability. The increased Bader charge of the C5 site with BPA accessibility (Figure 4f) implies the electron transport from BPA to the C5 atom, which can induce the BPA oxidation. On the other hand, the lower Eads values of BPA on the active sites as compared to those of PMS (also see Figure 4e) suggest the stronger affinity of catalytic sites to PMS adhesion. In other words, the active sites of the catalyst are more inclined to anchor PMS for ROS generation toward the oxidation of BPA.
It is well-known that the oxidation potential of 1O2 is apparently lower in comparison with that of OH/SO4•–, and 1O2 with the electrophilic character prefers to attack the organic compound with high charge density. (29,30) As indicated above, the NCoHCP/PMS system exhibits great effectiveness in the degradation of BPA, 2-CP, 2,4-D, and CIP but is relatively less active for DP oxidation, which is likely to be associated with the molecular structures of different organic contaminants. In particular, besides OH/SO4•–-induced oxidation, BPA, 2-CP, 2,4-D, and CIP containing phenolic hydroxyl group and halogen groups with high charge densities (see Figure S1) can be also oxidized by 1O2, whereas DP without such groups in its molecule seems to be inert to oxidation via 1O2 attack. In this context, the ROS evolution during PMS decomposition over NCoHCP with BPA or DP was analyzed to substantiate PMS reduction and oxidation around different sites. From Figure 5a,b, the injection of BPA or DP reduces the OH/SO4•– signal intensities but enhances the intensity of 1O2 peak. This can be due to the preferential degradation of BPA and DP by OH/SO4•– with higher oxidation potential around the Co atom, which consumes OH and SO4•– and thus stimulates more electron migration from the Co atom toward PMS to decrease the electron density of the Co site. The reduction of electron density of the Co atom would induce more electron transport from C atoms alongside pyridinic N through the Co–pyridinic N–C bond, which weakens electron density of these C atoms and hence motivates electron shuttle from the PMS molecule toward the C5 site for promotion of 1O2 generation. In the case of DP degradation by OH and SO4•– instead of 1O2, a more significant reduction in the strengths of OH and SO4•– signals means a more pronounced increase in the 1O2 signal intensity. To further attest to the involvement of the Co–pyridinic N–C moiety in PMS conversion, the XPS analysis of NCoHCP after catalytic reaction was carried out. As can be seen from the HR-XPS N 1s spectra (Figure 5c), the pyrrolic N and graphitic N peaks in NCoHCP and the used one undergo inappreciable change. However, the positions of pyridinic N and Co–N in NCoHCP both shift toward higher binding energies after PMS activation process. In the HR-XPS C 1s spectra (Figure 5d), a positive shift of the C–N species is noted in NCoHCP after catalytic reaction. This substantiates the participation of the Co–pyridinic N configuration and the C atom bonded to pyridinic N in PMS activation. Moreover, the Co0/Co2+ ratio decreases from 1.81 in NCoHCP to 0.20 in the used NCoHCP (Figure 5e), verifying the occurrence of PMS reduction over the Co atom, (31) which is consistent with the above DFT calculations and EPR measurements. Therefore, the mechanism of PMS activation by NCoHCP for organic pollutant oxidation can be proposed in Figure 5f. In particular, the Co atom with higher charge density and the C atom next to pyridinic N with lower electron density serve as catalytic sites toward synchronous PMS reduction and oxidation to produce OH/SO4•– and 1O2 to oxidize BPA. Besides ROS-mediated oxidative degradation, the donation of electron from BPA toward the C atom neighboring pyridinic N also induces partial BPA oxidation.

Figure 5

Figure 5. EPR spectra for (a) OH/SO4•– and (b) 1O2 of NCoHCP/PMS with organic contaminant. HR-XPS spectra of (c) N 1s, (d) C 1s, and (e) Co 2p for NCoHCP before and after catalytic reaction. (f) Proposed PMS activation mechanism by NCoHCP for organic pollutant oxidation. (g) 3D-EEM fluorescence spectroscopy of realistic industrial water sample before and after catalytic oxidation. Reaction conditions: [NCoHCP] = 0.2 g L–1; [PMS] = 4 mM; [Temp] = 30 °C.

Moreover, the applicability of NCoHCP was further tested through the treatment of realistic industrial wastewater via PMS activation with three-dimensional excitation and emission matrix (3D-EEM) fluorescence spectroscopy to assess the treatment efficiency. (27,32) The actual industrial wastewater was collected from the effluent after biological treatment in an Industrial Garden in Shanxi Province, China. The constituents of the wastewater sample are shown in Table S4, where the BOD5/COD ratio is calculated to be 0.13, suggesting the low biodegradability of wastewater sample. That is to say, this wastewater sample is difficult to be further biodegraded. The fluorescence spectrum of the original water sample (Figure 5g) shows two primary peaks situating at Ex/Em of 305–325/420–440 nm (peak A) and 220–235/405–445 nm (peak B), which are relevant to the fulvic acid analogues. (32) As the catalytic oxidation reaction proceeds, the fluorescence strengths of these two peaks reduce significantly, while peak B even disappears completely after 60 s, which demonstrates admirable catalytic performance of NCoHCP for actual industrial wastewater treatment through PMS activation, rendering it a highly active catalyst for environmental catalysis.

Environmental Implications

This work reports an effective approach to synthesize nitrogen-coordinated cobalt embedded in hollow carbon polyhedron using the core–shell-featured [email protected] as a precursor. The thermal annealing of bimetallic ZIFs at elevated temperature induces the evaporation of zinc from the ZIF-8 core to form the hollow carbon polyhedron and blow the cobalt species to the edge of the hollow rhombohedron, which avoids cobalt aggregation and affords the hollow carbon polyhedron with high porosity and conductivity, and sufficient nitrogen incorporation as well as massive Co–N moiety. As a persulfate activator, the resultant NCoHCP exhibits superior performance in the activation of PMS toward catalytic organic pollutant oxidation with ultrahigh specific activity, surpassing all catalysts reported so far. In addition, the NCoHCP catalyst delivers excellent catalytic durability. The exceptional catalytic activity and robust stability of NCoHCP are experimentally and theoretically revealed to be associated with the engineered interface and active site regulations. Specifically, the high affinity of pyridinic N to metallic Co species can facilitate the interfacial electron shuttle of the catalyst with reactants. Moreover, the graphitic carbon shell not only protects the active Co species from the harsh reaction condition but also functions as a catalytic site for PMS conversion into reactive oxygen species owing to the charge redistribution of the C atoms modulated by the Co and N atoms. The understanding of PMS activation over NCoHCP will be of scientific significance in the metal-catalyzed persulfate-based advanced oxidation process. Different from the conventional metal-based catalysts most emphasizing the catalytic behavior of metal species for PMS activation, the formation of Co–pyridinic N moiety in NCoHCP creates the Co atom with higher electron density and the C atom next to the pyridinic N with lower electron density as active sites, which increases the number of catalytic sites and hence greatly enhances the catalytic specific activity of NCoHCP for PMS activation toward organic pollutant degradation. On the other hand, the donation of electron from the organic contaminant toward the C atom neighboring pyridinic N also induces partial organic pollutant oxidation. Our findings shed innovative mechanistic insights into persulfate activation and organic contaminant degradation and, more importantly, pave a useful avenue for developing MOFs-derived metal/carbon hybrid material with controllable morphology and high catalytic performance toward wide environmental applications.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsestengg.0c00039.

  • Experimental section; parameters for measurement of pollutants by HPLC; EXAFS fitting results; detailed specific activity comparison of NCoHCP with existing catalysts; constituents of actual wastewater sample; molecular structures of various organic contaminants; XRD patterns and SEM, TEM, and EDS images of samples; N2 adsorption–desorption isotherm and XPS spectra of NCoHCP; catalytic activity of Zn-NC and Co-NC; pseudo-first-order kinetic for BPA degradation; EPR spectra for O2•–; quenching experiment data (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
    • Chun Hu - Institute of Environmental Research at Greater Bay, Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou University, Guangzhou 510006, ChinaKey Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, Chinahttp://orcid.org/0000-0003-3217-7671 Email: [email protected] [email protected]
  • Authors
    • Yaowen Gao - Institute of Environmental Research at Greater Bay, Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou University, Guangzhou 510006, Chinahttp://orcid.org/0000-0003-4032-9436
    • Yue Zhu - Institute of Environmental Research at Greater Bay, Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou University, Guangzhou 510006, China
    • Zhenhuan Chen - Institute of Environmental Research at Greater Bay, Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou University, Guangzhou 510006, China
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors acknowledge financial support from the National Natural Science Foundation of China (51808142, 51838005), the introduced innovative R&D team project under the “The Pearl River Talent Recruitment Program” of Guangdong Province (2019ZT08L387), and the National Key Research and Development Plan (2016YFA0203200).

References

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This article references 32 other publications.

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  • Abstract

    Figure 1

    Figure 1. (a) SEM, (b) EDS elemental mapping, (c) TEM, and (d) HRTEM images of NCoHCP. (e) XRD pattern of NCoHCP. (f) Raman spectra of samples.

    Figure 2

    Figure 2. HR-XPS spectra of (a) N 1s and (b) Co 2p for NCoHCP. (c) Normalized Co K-edge XANES and (d) EXAFS spectra of NCoHCP and reference samples. (e) FT EXAFS first-shell fitting curve for NCoHCP at R space. (f) Optimized structures and corresponding calculated formation energy (Ef) of the models of Co–pyridinic N and Co–pyrrolic N. Blue, white, and brown represent Co, N, and C atoms, respectively.

    Figure 3

    Figure 3. (a) Catalytic activity of NCoHCP. (b) Catalytic specific activity comparison of NCoHCP with existing catalysts. (c) PMS decomposition over NCoHCP. (d) Stability of NCoHCP. (e) Catalytic oxidation of multiple organic contaminants over NCoHCP with PMS. Reaction conditions: [pollutant] = 0.1 mM; [NCoHCP] = 0.2 g L–1; [PMS] = 4 mM; [Temp] = 30 °C; initial pH = 6.7 (if needed).

    Figure 4

    Figure 4. EPR spectra for (a) OH/SO4•– and (b) 1O2 of different systems. (c) Charge density distribution image of the Co–pyridinic N–C model. (d) Bader charge of Co, pyridinic N, and C atoms neighboring pyridinic N. (e) Adsorption energy of PMS or BPA at the sites of Co, C2, and C5. (f) Bader charge of Co, C2, and C5 sites before and after PMS or BPA adsorption.

    Figure 5

    Figure 5. EPR spectra for (a) OH/SO4•– and (b) 1O2 of NCoHCP/PMS with organic contaminant. HR-XPS spectra of (c) N 1s, (d) C 1s, and (e) Co 2p for NCoHCP before and after catalytic reaction. (f) Proposed PMS activation mechanism by NCoHCP for organic pollutant oxidation. (g) 3D-EEM fluorescence spectroscopy of realistic industrial water sample before and after catalytic oxidation. Reaction conditions: [NCoHCP] = 0.2 g L–1; [PMS] = 4 mM; [Temp] = 30 °C.

  • References

    ARTICLE SECTIONS
    Jump To

    This article references 32 other publications.

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    2. 2
      Oh, W. D.; Dong, Z.; Lim, T. T. Generation of sulfate radical through heterogeneous catalysis for organic contaminants removal: Current development, challenges and prospects. Appl. Catal., B 2016, 194, 169201,
    3. 3
      Wacławek, S.; Lutze, H. V.; Grübel, K.; Padil, V. V. T.; Černík, M.; Dionysiou, D. D. Chemistry of persulfates in water and wastewater treatment: A review. Chem. Eng. J. 2017, 330, 4462,
    4. 4
      Wang, J.; Wang, S. Activation of persulfate (PS) and peroxymonosulfate (PMS) and application for the degradation of emerging contaminants. Chem. Eng. J. 2018, 334, 15021517,
    5. 5
      Anipsitakis, G. P.; Dionysiou, D. D. Radical generation by the interaction of transition metals with common oxidants. Environ. Sci. Technol. 2004, 38, 37053712,
    6. 6
      Anipsitakis, G. P.; Dionysiou, D. D. Degradation of organic contaminants in water with sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ. Sci. Technol. 2003, 37, 47904797,
    7. 7
      Hu, P.; Long, M. Cobalt-catalyzed sulfate radical-based advanced oxidation: A review on heterogeneous catalysts and applications. Appl. Catal., B 2016, 181, 103117,
    8. 8
      Anipsitakis, G. P.; Stathatos, E.; Dionysiou, D. D. Heterogeneous activation of oxone using Co3O4. J. Phys. Chem. B 2005, 109, 1305213055,
    9. 9
      Ren, Y.; Lin, L.; Ma, J.; Jing, Y.; Fan, Z. Sulfate radicals induced from peroxymonosulfate by magnetic ferrospinel MFe2O4 (M = Co, Cu, Mn, and Zn) as heterogeneous catalysts in the water. Appl. Catal., B 2015, 165, 572578,
    10. 10
      Hu, L.; Yang, F.; Lu, W.; Hao, Y.; Yuan, H. Heterogeneous activation of oxone with CoMg/SBA-15 for the degradation of dye Rhodamine B in aqueous solution. Appl. Catal., B 2013, 134–135, 718,
    11. 11
      Zeng, T.; Zhang, X.; Wang, S.; Niu, H.; Cai, Y. Spatial confinement of a Co3O4 catalyst in hollow metal-organic frameworks as a nanoreactor for improved degradation of organic pollutants. Environ. Sci. Technol. 2015, 49, 23502357,
    12. 12
      Wang, J.; Xu, F.; Jin, H.; Chen, Y.; Wang, Y. Non-noble metal-based carbon composites in hydrogen evolution reaction: Fundamentals to applications. Adv. Mater. 2017, 29, 16058381605872,
    13. 13
      Chen, Z.; Wu, R.; Liu, Y.; Ha, Y.; Guo, Y.; Sun, D.; Liu, M.; Fang, F. Ultrafine Co nanoparticles encapsulated in carbon-nanotubes-grafted graphene sheets as advanced electrocatalysts for the hydrogen evolution reaction. Adv. Mater. 2018, 30, 18020111802020,
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      Wang, X. R.; Liu, J. Y.; Liu, Z. W.; Wang, W. C.; Luo, J.; Han, X. P.; Du, X. W.; Qiao, S. Z.; Yang, J. Identifying the key role of pyridinic-N-Co bonding in synergistic electrocatalysis for reversible ORR/OER. Adv. Mater. 2018, 30, 18000051800014,
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      Park, K. S.; Ni, Z.; Côté, A. P.; Choi, J. Y.; Yaghi, O. M. Exceptional chemical and thermal stability of zeolitic imidazolate frameworks. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 1018610191,
    16. 16
      Chen, Y. Z.; Wang, C.; Wu, Z. Y.; Xiong, Y.; Xu, Q.; Yu, S. H.; Jiang, H. L. From bimetallic metal-organic framework to porous carbon: High surface area and multicomponent active dopants for excellent electrocatalysis. Adv. Mater. 2015, 27, 50105016,
    17. 17
      Li, H.; Tian, J.; Zhu, Z.; Cui, F.; Zhu, Y. A.; Duan, X.; Wang, S. Magnetic nitrogen-doped nanocarbons for enhanced metal-free catalytic oxidation: Integrated experimental and theoretical investigations for mechanism and application. Chem. Eng. J. 2018, 354, 507516,
    18. 18
      Pan, Y.; Sun, K.; Liu, S.; Cao, X.; Wu, K.; Cheong, W. C.; Chen, Z.; Wang, Y.; Li, Y.; Liu, Y.; Wang, D.; Peng, Q.; Chen, C.; Li, Y. Core-shell [email protected] CoP nanoparticle-embedded N-doped carbon nanotube hollow polyhedron for efficient overall water splitting. J. Am. Chem. Soc. 2018, 140, 26102618,
    19. 19
      Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. High-throughput synthesis of zeolitic imidazolate frameworks and application to CO2 capture. Science 2008, 319, 939943,
    20. 20
      Chen, Y.; Li, X.; Park, K.; Song, J.; Hong, J.; Zhou, L.; Mai, Y. W.; Huang, H.; Goodenough, J. B. Hollow carbon-nanotube/carbon-nanofiber hybrid anodes for Li-ion batteries. J. Am. Chem. Soc. 2013, 135, 1628016283,
    21. 21
      Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X. A metal-organic framework-derived bifunctional oxygen electrocatalyst. Nat. Energy 2016, 1, 1500615013,
    22. 22
      Zhang, Y.; Lin, Y.; Jiang, H.; Wu, C.; Liu, H.; Wang, C.; Chen, S.; Duan, T.; Song, L. Well-defined cobalt catalyst with N-doped carbon layers enwrapping: The correlation between surface atomic structure and electrocatalytic property. Small 2018, 14, 17020741702082,
    23. 23
      Zheng, F.; Yang, Y.; Chen, Q. High lithium anodic performance of highly nitrogen-doped porous carbon prepared from a metal-organic framework. Nat. Commun. 2014, 5, 52615270,
    24. 24
      Gao, Y.; Zhu, Y.; Chen, Z.; Zeng, Q.; Hu, C. Insights into the difference in metal-free activation of peroxymonosulfate and peroxydisulfate. Chem. Eng. J. 2020, 394, 123936123946,
    25. 25
      Fei, H.; Dong, J.; Arellano-Jiménez, M. J.; Ye, G.; Dong Kim, N.; Samuel, E. L. G.; Peng, Z.; Zhu, Z.; Qin, F.; Bao, J.; Yacaman, M. J.; M. Ajayan, P.; Chen, D.; M. Tour, J. Atomic cobalt on nitrogen-doped graphene for hydrogen generation. Nat. Commun. 2015, 6, 86688675,
    26. 26
      Li, X.; Bi, W.; Zhang, L.; Tao, S.; Xie, Y. Single-atom Pt as co-catalyst for enhanced photocatalytic H2 evolution. Adv. Mater. 2016, 28, 24272431,
    27. 27
      Gao, Y.; Chen, Z.; Zhu, Y.; Li, T.; Hu, C. New insights into the generation of singlet oxygen in the metal-free peroxymonosulfate activation process: Important role of electron-deficient carbon atoms. Environ. Sci. Technol. 2020, 54, 12321241,
    28. 28
      Zhang, B. T.; Zhao, L.; Lin, J. M. Determination of folic acid by chemiluminescence based on peroxomonosulfate-cobalt(II) system. Talanta 2008, 74, 11541159,
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      Cheng, X.; Guo, H.; Zhang, Y.; Wu, X.; Liu, Y. Non-photochemical production of singlet oxygen via activation of persulfate by carbon nanotubes. Water Res. 2017, 113, 8088,
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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsestengg.0c00039.

    • Experimental section; parameters for measurement of pollutants by HPLC; EXAFS fitting results; detailed specific activity comparison of NCoHCP with existing catalysts; constituents of actual wastewater sample; molecular structures of various organic contaminants; XRD patterns and SEM, TEM, and EDS images of samples; N2 adsorption–desorption isotherm and XPS spectra of NCoHCP; catalytic activity of Zn-NC and Co-NC; pseudo-first-order kinetic for BPA degradation; EPR spectra for O2•–; quenching experiment data (PDF)


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