Interface Stabilization of Undercoordinated Iron Centers on

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Interface Stabilization of Undercoordinated Iron Centers on Manganese Oxides for Nature-Inspired Peroxide Activation Li Yu, Gong Zhang, Chunlei Liu, Huachun Lan, Huijuan Liu, and Jiuhui Qu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03338 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Interface Stabilization of Undercoordinated Iron Centers on Manganese Oxides for Nature-Inspired Peroxide Activation Li Yu,†,§,ǁ Gong Zhang,‡,ǁ Chunlei Liu,†,§ Huachun Lan,‡ Huijuan Liu,*,†,‡,§ and Jiuhui Qu†,‡,§ †

State Key Laboratory of Environmental Aquatic Chemistry, Key Laboratory of Drinking Water Science and

Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, P. R. China. ‡

School of Environment, Tsinghua University, Beijing 100084, P. R. China.

§

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

1

ACS Paragon Plus Environment

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ABSTRACT: Coordinatively unsaturated metal centers constitute a key element in natural catalytic cycles. Construction of analogues of these ensembles on heterogeneous supports may aid in the innovative development of artificial catalysts showing efficient and stable reaction patterns. We herein stabilized naturally prevalent undercoordinated iron (UCI) centers on manganese oxides via the interface confinement effect between transition metal oxides. The created

heterostructure

showed

efficient

activation

of

peroxy-bonds

containing

peroxymonosulfate (PMS) molecules, with aqueous organic contaminant oxidation efficacy several times that of reference metal oxides. The combined spectroscopic, electrochemical, and in-situ measurement results revealed that these interfacial oxygen-deficient UCI sites not only benefited thermodynamically favored PMS accumulation, but also facilitated surface-to-surface electronic communication across atomic interface-bonding channels, thus providing a feasible platform to give rise to highly oxidizing (Fe, Mn)-oxo intermediates. Such PMS-activating metal centers in transitional states were sequentially reduced via either direct oxidation of organic substrates or electrophilic attack of other PMS molecules, with reactive singlet oxygen (1O2) generation. This reaction pattern guaranteed preservation of the catalyst structure after the reversible redox cycle, enabling a stable, kinetics-enhanced catalytic process. KEYWORDS: undercoordinated iron center, manganese oxide, peroxymonosulfate, interface engineering, electron transfer

INTRODUCTION Efficient catalysis requires a catalyst capable of preferential accumulation and successive dissociative activation of reactants, which are difficult to simultaneously optimize at a single site. Heterointerface architecture can fulfil this goal for high-performance catalyst design, where rationally selected components are intentionally arranged for the spatial separation of stepwise 2


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reactions and durable synergy.1-3 Active site stabilization and surface-to-surface electronic exchange are of great concern for these heterostructures, as they often suffer structure deactivation and limited diffusion-controlled electron transfer.4 In situ established chemical bonds at the interface will benefit the structural and electronic coupling between different sites for highly active and robust catalysts.5 Based on these principles, well-designed Bi−S bonds in the [Cl2]–[Bi12O17-x]–[MoS2] Janus bilayer,6,7 Pt−Fe bonding in a FeO/Pt heterostructure,8,9 and the Fe-LiH interface10,11 boost the multi-step processes in activated species transfer and reactant conversion at different centers with stable performance. Typically, the following criteria thus can be applied to fulfill the requirements for the desired catalysts: (i) establishment of interface-bonding channels to enhance charge transfer and avoid interfacial structure detachment; (ii) creation of preferential sites for reactant binding and dissociative activation via defect engineering12-14 or electronic effect tuning;15,16 and (iii) maintenance of appreciable stability in aqueous systems.17 The principles obtained herein will aid targeted catalyst design and can be extended to other catalytic systems. Transition-metal-initiated Oxone (active ingredient peroxymonosulfate, PMS) activation,18-20 is an efficient, green, and environmentally benign Fenton-like system. The radical generation processes involved, however, can cause undesirable catalytic durability due to an energetically unfavorable redox cycle and difficulty in the regeneration of active metal centers. In addition, the adsorption properties of catalytic sites, closely related to activation potential, have not received sufficient attention in many PMS-mediated systems. An ongoing question is whether a directional structure arrangement can be devised to simultaneously optimize active species binding, transfer, and activation, as well as tune reactive species generation. This remains challenging as PMS-bound complexes are generally unstable, short-lived, and instantaneously converted. 3


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Real-time reaction dynamics remain problematic for further independent assessment and optimization of these stepwise processes. Such restrictive effects also limit the application of many Fenton-like systems. Natural catalytic cycles may provide insights into novel artificial catalyst design to meet these challenges, given the efficient and stable redox cycling of the central metal ions in natural and biomimetic environments.21-23 Undercoordinated iron (UCI) centers have superior reactive potential to drive most homogeneous and metalloenzymatic catalytic processes when stabilized by ligands or proteins.21-26 Construction of analogues of these ensembles on heterogeneous supports may be applicable to other systems to enable stable reaction patterns.8,27 For Fenton-like systems, we rationalize the possibility of stabilizing UCI oxides to deliberately construct interfacial unsaturated-unsaturated sites, that is, oxygen-vacancy accommodating Lewis-base centers for dissociative adsorption of peroxide, and adjacent undercoordinated iron sites for interface bonding with transition metal oxides as electron shuttle channels. In this structural arrangement, the electron localization state at the oxygen-deficient surface and the facilitated charge transfer could synergistically regulate the exchange, rearrangement, and final activation of peroxide. Although coordination and stabilization of UCI sites on heterogeneous catalysts remain challenging8,27,28 especially in aqueous reactions, elaborate interface-tailoring can provide a platform for both heterostructure activity and durability, as well as facilitate fundamental understanding of catalysis processes. We attempted to stabilize UCI oxides via interface confinement effects on manganese oxide supports for PMS activation, considering their rich surface-redox chemistry. Based on a series of physicochemical techniques and in situ measurements, multifunctional reaction centers were conclusively identified as preferential sites for PMS accessibility and successive activation, which 4


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were further facilitated across the created interface-bonding channels in facile electronic exchange. This nature-inspired and robust catalysis route breaks the limitations imposed by the undesirable redox cycling of metal centers in traditional Fenton-like systems due to the well-recovered catalyst structure, thereby creating stable reaction patterns and ensuring durable synergy. The dual-functionality of the UCI sites established herein can be extended to different heterostructure architectures, with the goal of achieving diverse capabilities.

RESULTS AND DISCUSSION To construct a well-defined interfacial structure, one-dimensional MnO2 nanorods first served as the platform for the accommodation of FeOOH deposition in a surfactant-free aqueous system. According to interfacial energy theory, development of well-assembled architectures generally requires interfacial compatibility, where chemical interaction or bonding is essential between dissimilar crystal lattices. High degrees of acidity and ionic strength were employed to increase the surface charge density by proton adsorption or electrostatic repulsion screening to control the interfacial tension of the system.29 X-ray diffraction (XRD) patterns (Figure 1) confirmed the formation of tetragonal-phase β-MnO2 (JCPDS No. 24-0735) and orthorhombic-phase α-FeOOH (JCPDS No. 81-0464) in the composite. During controlled reduction under hydrogen flow at 350 °C, the heterostructure was finally transformed into FeMn-350 with mixed phases of cubic MnO (JCPDS No. 75-0257) and Fe3O4 (JCPDS No. 74-0748). Scanning electron microscope (SEM) and transmission electron microscope (TEM) images and corresponding selected area electron diffraction (SAED) (Figure 1a and b) of the as-synthesized MnO2 manifested a nanorod-like morphology with single crystallinity. Most tiny FeOOH nanorods were homogeneously decorated and arranged in defined orientations perpendicular to the long axis of the backbone, as illustrated in the zoomed-in SEM image (Figure 1c). Furthermore, the original 5


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smooth surface of the MnO2 adopted a uniformly rough texture after the deposition of FeOOH. A close-up view of the interfacial region using high-resolution TEM (HRTEM) (Figure 1d) further visualized the structural distortion, which was likely related to strain release lowering the surface energy and enhancing structural stability.30 The superposition of two sets of diffraction spots were visibly identified and assigned to MnO2 and FeOOH, respectively (Figure 1e). Their epitaxial —

—

relationship was established and defined by [1 1 0]MnO2//[001]FeOOH and (110)MnO2//( 1 20)FeOOH, with a calculated lattice mismatch of about 7.8%. This quasi-coherent growth benefited the generation of a well-defined interfacial structure during the sequentially controlled reduction process, with maintained morphology (Figure 1f) and strong interfacial interactions in the homogeneous atomic reconstruction. Typical TEM images and elemental line scan profiles (Figure 1g and h) further confirmed a porous structure with no obvious interfacial boundary, pointing to highly uniform and abundant interfacial sites originating from the facilitated lattice oxygen mobility and exchange in thermal treatment. The highly uniform interfacial centers could act as multifunctional locations for catalytic processes, with the potential for PMS activation therefore evaluated. The optimal FeMn-350 oxide exhibited enhanced activity in PMS catalysis compared to that prepared at different temperatures (FeMn-300, FeMn-400), which was not directly dependent on the specific surface area or pore distribution (Figure S1, Table S1). It also outperformed other catalysts, including MnO2, FeOOH/MnO2, Co3O4 (a benchmark PMS activator20), as well as Fe3O4-x, MnO, and their mixture (58/42 by weight, FeMn-Mix) (Figure 2a). Given the similarity in the chemical states of iron and manganese species, Fe3O4-x and MnO were used herein to represent the individual constituents of FeMn-350 for investigation of the isolated catalytic effect (Figure S2). Specifically, bisphenol A (BPA) was almost completely and rapidly removed (25 min) in the FeMn-350-based 6


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process, with exceptional activity and accelerated reaction kinetics (estimated initial- and second-stage reaction rate constants k1 = 0.295 min–1 and k2 = 0.110 min–1). The sustained and intensified reaction even proceeded at a 6‒55 times higher rate (k2) than that in other comparable heterocatalytic systems (Table S2). The combined liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS) analyses showed the conversion of BPA initiated from cleavage of the C─C bond between the isopropyl and benzene rings, followed by dehydration, demethylation, and hydroxylation (Scheme S1). The continuous oxidation effect in the system transformed the intermediates into small-molecule products, thereby benefiting the mineralization reaction, with total organic carbon (TOC) removal efficacy above 80% (Figure S3). Similar synergetic catalysis was also obtained for 2,4-dichlorophenol (DCP) and rhodamine B (RhB), with different structures and properties from those of BPA (Figure 2b, c and S3). Stepwise dechlorination, hydroxylation, and oxidative ring-opening reactions were involved in the catalytic conversion of DCP, whereas decolorization of RhB was achieved via successive N-de-ethylation followed by chromophore destruction. Additional tests showed that aqueous-leached metal ions had a negligible influence on the reaction, confirming the surface sites as the catalytic locations for the reaction process (Figure S4a). As anticipated, FeMn-350

showed

a

durable

oxidation

potential

and

the

capability

for

facile

magnetization-driven separation in recycling tests, retaining superior performance throughout five successive runs (Figure S4b and S5). Specific treatments were then performed to probe the responsible reactive species in this synergetic catalysis process. In the presence of ethanol or tert-butanol, two common quenchers of sulfate and hydroxyl radicals, the degradation efficacy of BPA remained high (Figure S6). However, after the addition of furfuryl alcohol for quenching 1O2 in the PMS activation process, 7


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the oxidation ability for BPA was greatly suppressed, with the k1 and k2 values lowered by more than four and seven times, respectively (Figure S6 and Table S2). Reactive 1O2 was then directly trapped as TMP-1O2 adducts (2,2,6,6-tetramethyl-4-piperidinol-N-oxyl, TMPN, aN = 16.9 G and g = 2.0054). The recorded 5,5-dimethyl-2-oxo-pyrroline-1-oxyl intermediates (DMPO-X, aN = 7.2 G, aH = 3.8 G, and g = 2.0065) suggested a highly reactive system, partly from 1O2 inducing DMPO oxidation.31 Notably, MnO alone exhibited some activation capability, whereas Fe3O4-x was catalytically inert due to the almost undetectable electron spin resonance (ESR) signals (Figure 2d, e). The dominant diffusion process accounted for its weak activation potential in charge transfer, as reflected by the corresponding linear Nyquist plot in the electrochemical impedance spectroscopy data (EIS, Figure 2f). By contrast, the distinctly enhanced ESR signal and decreased resistance in the FeMn-350-based process implied appreciably accelerated kinetics in the interfacial reaction and a shortened diffusion distance, resulting in lower hindrance to electron shuttling. The well-defined interfacial sites and synergetic activation motivated our interest to unravel the underlying structure-activity relationship. First, the 57Fe Mössbauer spectra clearly identified non-stoichiometric and distorted-coordination characteristics of magnetite Fe3O4-x, with a relative abundance of tetrahedral:octahedral coordination (A-site:B-site) of 0.9:1 (Figure 3a, Table S3). This probably stemmed from the oxygen defects induced by formation of undercoordinated Fe1-xO, in which the tetrahedral Fe(III) clusters exhibited close structural similarity to that in magnetite. It was difficult to differentiate their separate signals, thus giving rise to the observed A-site-like contributions.32-35 This increase in the A:B ratio has also been observed in other unsaturated iron oxide heterostructures.32,36 The created low-coordination iron sites, on the other hand, behaved as reliable Lewis acid centers, prone to binding with manganese oxides via 8


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surface-terminated oxygen atom bridging. The changed A:B ratio (0.9:1 to 0.5:1, Table S3) showed that the establishment of the Fe–O–Mn bonds influenced the iron coordination in FeMn-350. In addition, Mn atom incorporation induced a charge imbalance to the structure due to its different chemical states compared to Fe. The oxygen defects and coordinatively unsaturated iron structure were thus maintained for charge compensation,37 as revealed by the dominant component of paramagnetic Fe(II) relative to Fe(III) (3.6:1, two additional doublets, Figure 3b). The presence of both lattice oxygen (Olatt) and adsorbed oxygen (Oads) associated with vacancies, derived by X-ray photoelectron spectroscopy (XPS) (Figure S7), further indicated structural oxygen loss during hydrogen treatment. Such defect-mediated interfacial interactions were also verified from their inherent magnetization properties. Different from the smooth patterns in a characteristic ferrimagnetic (FiM) system, such as in FeMn-Mix, two magnetic transition points emerged in the zero field cooled-field cooled (ZFC–FC) curves of FeMn-350, as marked by arrows in Figure 3c. In addition to the maximum feature at 120 K associated with the Néel temperature TN of antiferromagnetic (AFM) MnO,38 the slight kink appearing at about 42 K was believed to stem from the interaction between AFM (MnO) and FiM (Fe3O4) interfaces,39 which caused spin fluctuations and thus a discontinuous change of magnetization.40,41 Furthermore, noticeable asymmetric magnetization reversals (hysteresis shifts along H- and M-axes) and an exchange bias field (HE, 70 Oe), typically expressed as HE = |HC-Left + HC-Right| / 2 (HC is coercivity),42 revealed exchange coupling and strong interaction at the AFM/FiM interface (Figure 3d). This strong interfacial interplay further affected the phase transformation of FeMn-350 compared to its individual components due to the acceleration of redox cycles across the interfacial Fe–O–Mn bonds during thermal treatment (Figure S8).43 In addition, the enhanced ligand-to-metal charge 9


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transfer transitions from oxygen to iron and manganese accounted for the much more intense absorption in the ultraviolet (UV) region (250–400 nm, Figure S9).44-46 The absorption edge of FeMn-350 was estimated to be in the near-infrared (NIR) region, between that of Fe3O4-x and MnO, further indicating the chemically bonded hybrid structure. The possibility for interfacial charge transfer to Mn sites was demonstrated again from the negatively shifted (30 mV) anodic Mn oxidation peak in the cyclic voltammetry (CV) analysis (Figure S10).47 The interfacial interactions and specially arranged dual-active-centers empowered the heterostructure with exceptional performance. Generally, the dissociative adsorption of reactants is a decisive step for catalysis, while also being inherently dependent on the compositional and geometrical arrangement of the surface structure as well as the adsorption energy and configuration of reactants. An oxygen-deficient surface in an electron-rich state would affect the electronic structure of catalysts with strong affinity for many specific molecules such as O2, H2O, N2, CO2, and H2O2.12,14,48-50 Owing to the interactions with surface anion defects (oxygen vacancies), the peroxy-bonds containing HSO5− (PMS) herein tended to preferentially accumulate at UCI sites. However, the single sites with undesirable surface redox properties toward adsorbed-PMS showed very limited activation potential, as clarified above. After chemical stabilization by adjacent manganese oxides, the modulated interface-PMS interaction mode further induced charge rearrangement in the changed geometric and electronic structure of the activated metal centers, thus facilitating PMS dissociation. The interfacial Fe–O–Mn bonds could also act as dual-active-center linkages and electron shuttle channels to form the interface and enable both structural and electronic coupling. Similar interface engineering strategies are also reported to benefit either charge flow steering or electron interflow between two components using elaborately designed Bi−S6 or Mn–O–Co species47 for stable and high-performance 10


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catalysis. Based on this structural arrangement, energetically and spatially favorable reactions were facilitated to finally boost the generation of reactive species. In situ Raman spectra showed direct evidence of the preferential affinity for HSO5− on the undercoordinated iron oxides. As depicted in Figure 4a, three Raman bands centered at 881, 978, and 1060 cm−1 (Figure 4a) were generally associated with the adsorbed peroxide species (HSO5−) and symmetric stretching vibration mode of S=O bonds vs(S=O) for SO42− and HSO4−, respectively.51-53 The progressive increases in Raman intensity manifested the enhanced binding of reactant Oxone with reaction time. A relatively high consumption of aqueous HSO5− further confirmed its preferential affinity (Figure S11a). An additional signal at 838 cm−1, arising on a somewhat longer time scale, stemmed from the interactions of the peroxy-bonds with surface vacancies on Fe3O4-x.54 This surface-bound peroxide-bridge structure was also determined from the slight chemical shift in the Olatt peak toward a higher binding energy in the catalytic process (Figure S7). When confined in FeMn-350 heterostructures, these UCI sites with low-valent Fe species maintained excellent structural stability, as shown by the lack of measurable differences in the Fe K-edge X-ray absorption near edge structure (XANES) profiles regarding edge energies and features (Figure 5a and S12). Fourier transform (FT) k3χ(k) analysis also gave rise to almost constant coordination numbers in the Fe−O and Fe−Fe shells during the activation process (Figure S13, S14 and Table S4). Examination of high-resolution XPS spectra revealed no observable variation in Fe 2p states (Figure S15a) or binding energy of Oads associated with vacancies (Figure S7a). Further Raman analysis gave a characteristic signature for magnetite without any adjacent shoulder bands ascribed to Fe(III)-oxides (Figure S15), indicating the stable chemical structure of undercoordinated Fe and oxygen-defect sites after complete catalytic cycles. On the other hand, manganese oxide-involved systems showed facile activation and conversion of 11


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HSO5− accompanied by the formation of (HSO4−)2 anionic dimers, based on the assignment of the Raman band at 1027 cm−1 (Figure 4b).53,55-57 Even much higher concentrations of (HSO4−)2 dimers were detected in the FeMn-350-activated system (Figure 4c) due to synchronous HSO5− binding and rapid activation contributing to enhanced catalysis. The continuous conversion of HSO5− to (HSO4−)2 also accounted for the gradual decrease in pH (Figure S11b). In addition, the PMS activation process caused partial structural conversion from the edge-sharing MnO6 network to a corner-sharing one, considering the XANES spectral change associated with a less intense peak B.58,59 However, a stable oxidation state of Mn was still maintained in the first catalytic cycle, as seen by the lack of an obvious energy shift on the edge-rise (Figure 5b and S12). As expected, the interfacial oxygen-deficient sites proved to be thermodynamically superior for PMS capture, whereby excess localized electrons were accessible for the Fe–O interaction, as revealed by the in situ Raman band at 838 cm−1. Different from Fe3O4-x alone, which suffered defect conversion and structural oxidization (Figure S7 and S16), the Fe-Mn-based heterostructure was hypothesized to first undergo interfacial charge rearrangement, with PMS as the oxygen donor. The sequential electron and proton transfer resulted in the production of highly oxidizing (Fe, Mn)-peroxo or -oxo species after O–O heterolysis. As powerful electrophilic oxidants, these hereby activated metal centers of higher valence states were not stable, and thus highly reactive for either direct oxidation of organic substrates or electron withdrawal from other PMS molecules with generation of reactive 1O2 and (HSO4−)2 anionic dimers. Such a favored PMS dissociation mode with lowered activation barrier further back-donated electrons to the metastable Fe-Mn-based metal centers, leading to active site recovery and considerable resistance toward structural oxidation. This reaction pattern was very similar to that of iron- and manganese-complex-induced catalytic cycles in natural and biomimetic systems.21-23 In particular, 12


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the inner-sphere electron circulation across atomic interface-bonding channels showed decreased resistance (EIS, Figure 2f) for defect-centered substrate oxidation and catalytic PMS disproportionation. Overall, these multi-step catalytic processes promoted the final electron transfer from one PMS molecule to another or to an organic substrate, accompanied by reversible redox cycling on Fe-Mn centers. PMS activation on the stabilized interface was thus energetically favored and kinetically enhanced for durable catalytic generation of reactive species. Even though total structural stability of Mn was still difficult after recycling tests, manganese oxides in different states maintained impressive potential for PMS activation,60 and similar reaction pathways would be expected to enable sustained synergy. The remarkably low amplitude of FT peaks in the Mn K-edge extended X-ray absorption fine structure (EXAFS) profiles (Figure S14b) implied an increased degree of local structural disorder in the Mn arrangement, associated with surface dangling bonds and distortion to maintain structural stability.14 This distinct local atomic coordination state likely originated from the interaction with Fe, inducing geometric and electronic structure change,61 which could also be deduced from the slight peak shift. Such strong interactions arising from the interfacial coordination of Fe-Mn oxides further optimized the thermodynamically and kinetically accessible pathways in preferential reactant accumulation, and facilitated electron shuttling and activation, with improved stability against structural detachment and oxidation.

CONCLUSIONS Inspired from natural catalytic cycles, functional UCI sites were rationally coordinated and stabilized on manganese oxide supports via the interface confinement effect between transition metal oxides. The heterostructure proved to be highly effective and robust in PMS catalysis for water

purification,

with

promoted

aqueous

organic

13


contaminant

removal.

Multiple

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physicochemical techniques and in situ measurements demonstrated the thermodynamically favored PMS binding at UCI sites, and simultaneous surface-to-surface electron shuttling across Fe-Mn centers. The PMS-mediated high valence metal-oxo species in metastable states showed strong reactivity for electrophilic oxidation of either organic substrates or PMS molecules, with active 1O2 generation. This dual-active-center architecture further maintained durable catalytic synergy due to the well-preserved defect structure in the reversible redox cycle. The rational immobilization of active centers inspired by natural catalytic cycles will open many possibilities and can be further generalized to different systems for feasible catalytic design strategies or other applications.

ASSOCIATED CONTENT Supporting Information Additional content includes experimental procedures, catalysis tests, reaction pathways, as well as methodological details and discussions on general characterizations referred to in the main text. Figures S1−S16, Tables S1−S5 and Scheme S1 (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions ǁ

L.Y. and G.Z. contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant 14


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ACS Catalysis

Nos. 51438011, 51708543, and 51722811).

REFERENCES (1) Valenti, G.; Boni, A.; Melchionna, M.; Cargnello, M.; Nasi, L.; Bertoni, G.; Gorte, R. J.; Marcaccio, M.; Rapino, S.; Bonchio, M.; Fornasiero, P.; Prato, M.; Paolucci, F. Nat. Commun. 2016, 7, 13549. (2) Chen, G.; Zhao, Y.; Fu, G.; Duchesne, P. N.; Gu, L.; Zheng, Y.; Weng, X.; Chen, M.; Zhang, P.; Pao, C.-W.; Lee, J.-F.; Zheng, N. Science 2014, 344, 495-499. (3) Yao, S.; Zhang, X.; Zhou, W.; Gao, R.; Xu, W.; Ye, Y.; Lin, L.; Wen, X.; Liu, P.; Chen, B.; Crumlin, E.; Guo, J.; Zuo, Z.; Li, W.; Xie, J.; Lu, L.; Kiely, C. J.; Gu, L.; Shi, C.; Rodriguez, J. A.; Ma, D. Science 2017, 357, 389-393. (4) Li, X.-B.; Gao, Y.-J.; Wang, Y.; Zhan, F.; Zhang, X.-Y.; Kong, Q.-Y.; Zhao, N.-J.; Guo, Q.; Wu, H.-L.; Li, Z.-J.; Tao, Y.; Zhang, J.-P.; Chen, B.; Tung, C.-H.; Wu, L.-Z. J. Am. Chem. Soc. 2017, 139, 4789-4796. (5) Liu, J. ACS Catal. 2017, 7, 34-59. (6) Li, J.; Zhan, G.; Yu, Y.; Zhang, L. Nat. Commun. 2016, 7, 11480. (7) Li, J.; Li, H.; Zhan, G.; Zhang, L. Acc. Chem. Res. 2017, 50, 112-121. (8) Fu, Q.; Li, W.-X.; Yao, Y.; Liu, H.; Su, H.-Y.; Ma, D.; Gu, X.-K.; Chen, L.; Wang, Z.; Zhang, H.; Wang, B.; Bao, X. Science 2010, 328, 1141-1144. (9) Chen, Z.; Mao, Y.; Chen, J.; Wang, H.; Li, Y.; Hu, P. ACS Catal. 2017, 7, 4281-4290. (10) Wang, P.; Chang, F.; Gao, W.; Guo, J.; Wu, G.; He, T.; Chen, P. Nat. Chem. 2017, 9, 64-70. (11) Wang, P.; Xie, H.; Guo, J.; Zhao, Z.; Kong, X.; Gao, W.; Chang, F.; He, T.; Wu, G.; Chen, M.; Jiang, L.; Chen, P. Angew. Chem. Int. Ed. 2017, 56, 8716-8720. (12) Li, H.; Shang, J.; Ai, Z.; Zhang, L. J. Am. Chem. Soc. 2015, 137, 6393-6399. (13) Jiao, X.; Chen, Z.; Li, X.; Sun, Y.; Gao, S.; Yan, W.; Wang, C.; Zhang, Q.; Lin, Y.; Luo, Y.; Xie, Y. J. Am. Chem. Soc. 2017, 139, 7586-7594. (14) Gao, S.; Sun, Z.; Liu, W.; Jiao, X.; Zu, X.; Hu, Q.; Sun, Y.; Yao, T.; Zhang, W.; Wei, S.; Xie, Y. Nat. Commun. 2017, 8, 14503. (15) Kitano, M.; Inoue, Y.; Yamazaki, Y.; Hayashi, F.; Kanbara, S.; Matsuishi, S.; Yokoyama, T.; Kim, S.-W.; Hara, M.; Hosono, H. Nat. Chem. 2012, 4, 934-940. (16) Chen, G.; Xu, C.; Huang, X.; Ye, J.; Gu, L.; Li, G.; Tang, Z.; Wu, B.; Yang, H.; Zhao, Z.; Zhou, Z.; Fu, G.; Zheng, N. Nat. Mater. 2016, 15, 564-569. (17) Ran, J.; Gao, G.; Li, F.-T.; Ma, T.-Y.; Du, A.; Qiao, S.-Z. Nat. Commun. 2017, 8, 13907. (18) Anipsitakis, G. P.; Stathatos, E.; Dionysiou, D. D. J. Phys. Chem. B 2005, 109, 13052-13055. 15


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(39) Song, H.-M.; Zink, J. I.; Khashab, N. M. J. Phys. Chem. C 2015, 119, 10740-10748. (40) Muscas, G.; Concas, G.; Cannas, C.; Musinu, A.; Ardu, A.; Orrù, F.; Fiorani, D.; Laureti, S.; Rinaldi, D.; Piccaluga, G.; Peddis, D. J. Phys. Chem. C 2013, 117, 23378-23384. (41) Lei, S.; Liu, L.; Wang, C.; Shen, X.; Guo, D.; Wang, C.; Zeng, S.; Cheng, B.; Xiao, Y.; Zhou, L. CrystEngComm 2014, 16, 1322-1333. (42) Fontaíña Troitiño, N.; Rivas-Murias, B.; Rodríguez-González, B.; Salgueiriño, V. Chem. Mater. 2014, 26, 5566-5575. (43) Wang, H.; Qu, Z.; Xie, H.; Maeda, N.; Miao, L.; Wang, Z. J. Catal. 2016, 338, 56-67. (44) Morris, R. V.; Lauer, H. V.; Lawson, C. A.; Gibson, E. K.; Nace, G. A.; Stewart, C. J. Geophys. Res. 1985, 90, 3126-3144. (45) Klokishner, S. I.; Reu, O.; Chan-Thaw, C. E.; Jentoft, F. C.; Schlögl, R. J. Phys. Chem. A 2011, 115, 8100-8112. (46) Meng, Y.; Genuino, H. C.; Kuo, C.-H.; Huang, H.; Chen, S.-Y.; Zhang, L.; Rossi, A.; Suib, S. L. J. Am. Chem. Soc. 2013, 135, 8594-8605. (47) Guo, C.; Zheng, Y.; Ran, J.; Xie, F.; Jaroniec, M.; Qiao, S.-Z. Angew. Chem. Int. Ed. 2017, 56, 8539-8543. (48) Nowotny, J.; Alim, M. A.; Bak, T.; Idris, M. A.; Ionescu, M.; Prince, K.; Sahdan, M. Z.; Sopian, K.; Mat Teridi, M. A.; Sigmund, W. Chem. Soc. Rev. 2015, 44, 8424-8442. (49) Ling, T.; Yan, D.-Y.; Jiao, Y.; Wang, H.; Zheng, Y.; Zheng, X.; Mao, J.; Du, X.-W.; Hu, Z.; Jaroniec, M.; Qiao, S.-Z. Nat. Commun. 2016, 7, 12876. (50) Li, H.; Li, J.; Ai, Z.; Jia, F.; Zhang, L. Angew. Chem. Int. Ed. 2018, 57, 122-138. (51) Li, W.; Gibbs, G. V.; Oyama, S. T. J. Am. Chem. Soc. 1998, 120, 9041-9046. (52) Jariwala, M.; Crawford, J.; LeCaptain, D. J. Ind. Eng. Chem. Res. 2007, 46, 4900-4905. (53) Ribeiro, M. C. C. J. Phys. Chem. B 2012, 116, 7281-7290. (54) Zhang, T.; Zhu, H.; Croué, J.-P. Environ. Sci. Technol. 2013, 47, 2784-2791. (55) Goypiron, A.; De Villepin, J.; Novak, A. J. Raman Spectrosc. 1980, 9, 297-303. (56) Pham-Thi, M.; Colomban, P.; Novak, A.; Blinc, R. J. Raman Spectrosc. 1987, 18, 185-194. (57) Varma, V.; Rangavittal, N.; Rao, C. N. R. J. Solid State Chem. 1993, 106, 164-173. (58) Hwang, S.-J.; Kwon, C.-W.; Portier, J.; Campet, G.; Park, H.-S.; Choy, J.-H.; Huong, P. V.; Yoshimura, M.; Kakihana, M. J. Phys. Chem. B 2002, 106, 4053-4060. (59) Lee, Y. R.; Kim, I. Y.; Kim, T. W.; Lee, J. M.; Hwang, S.-J. Chem. Eur. J. 2012, 18, 2263-2271. 17


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ACS Catalysis

Figure 1. Morphological characterization, epitaxial interface structures, and phase corroboration. Representative SEM and low-magnification TEM overviews of (a, b) MnO2, (c, d) FeOOH/MnO2, and (f, g) FeMn-350 heterostructures. Red marked squares in (b), (d), and (g) represent reference regions for the refinement of the phase profiles, characterized by corresponding HRTEM images or SAED patterns of MnO2 (inset of b), FeOOH/MnO2 (e), and FeMn-350 heterostructure (h), respectively. Circle in (e) highlights the presence of interfacial structure distortion with the superposition of two sets of diffraction spots. Inset in (g) shows typical elemental line scan profiles. (i) Corresponding XRD patterns.

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ACS Catalysis

0

0.0

(a)

0.0

(b)

-1 -2 -3

Oxone MnO2

-0.4

FeOOH/MnO2

-0.8

(c)

-0.4 -0.8

Fe3O4-x MnO FeMn-Mix Co3O4

-1.2

Oxone Fe3O4-x

FeMn-350

-1.6

MnO FeMn-350

0

5

10

15

20

25

Time (min)

0

5

25

MnO

Fe3O4-x

Fe3O4-x

Oxone

Oxone

3520

3540

Magnetic field (G)

30

0

5

10

15

20

25

30

Time (min)

(f)

MnO

3500

20

2.0 1.5

120 1.0

FeMn-350

FeMn-350

3480

15

MnO FeMn-350

Time (min)

(e)

(d)

10

Oxone Fe3O4-x

-1.2

-Z'' (kohm)

ln(Ct/Co)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

0.5 0.0 0.0 0.5 1.0 1.5

40 FeMn-Mix FeMn-350

0

3460 3480 3500 3520 3540 3560

Magnetic field (G)

Fe3O4-x MnO

0

30

60

90

120

Z' (kohm)

Figure 2. Comparative catalysis kinetics and characterization results highlight the superior activity of FeMn-350. (a)−(c) Experimental (solid symbols) and calculated (solid lines) time-dependent conversion of (a) BPA, (b) DCP, and (c) RhB in different systems based on a two-step consecutive first-order reaction model. Initial reaction conditions: each pollutant 80 mg L−1, catalyst loading 0.5 g L−1, Oxone 0.6 mM, and pH 7.5. (d), (e) Identification of the induced reactive species probed with (d) DMPO- and (e) TMP-trapping in situ ESR. (f) EIS response derived from Oxone activation in different systems, with inset showing enlarged Nyquist plots at high frequency.

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M (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Normalized Transmission (%)

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(a)

(b)

100

100

96

96

Fe3O4-x

92

FeMn-350

92

88

88 -10

(c)

-5

0

5

Velocity (mm/s)

10

H = 100 Oe 10

(d)

-10

40

60 30

20

0 -30

0

-5

0

5

Velocity (mm/s)

10

T=5K

-60 -10 -5 0

5 10

-20

9 0

FeMn-Mix -40 FeMn-350 50 100 150 200 250 300 -1.0

-0.5

Temperature (K)

FeMn-Mix FeMn-350 0.0 0.5 1.0

H (kOe)

Figure 3. Mössbauer spectra and magnetometry analysis reflecting defect-mediated interfacial interactions. (a, b) Room temperature Mössbauer spectra of Fe3O4-x (a) and FeMn-350 (b) fitted with ferrimagnetic sextets or paramagnetic doublets. (c) Temperature-dependent ZFC and FC magnetization curves for FeMn-Mix and FeMn-350. (d) Enlarged view of low-temperature (5 K) magnetization hysteresis loops after cooling in 1 T. Inset reveals the corresponding M-H analysis in the full field.

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ACS Catalysis

(a) Scattering Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(b)

0 min

(c)

0 min

0 min

1027

2 min

5 min 978

10 min 838 881

800

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900

1000

1060

978

2 min

2 min

5 min

5 min

10 min

10 min

1100 800

900

1000

-1

1100 800

900

1027 1060

1000

1100

Raman Shift (cm ) Figure 4. In situ Raman progression recording the dynamics of surface binding and activation of the reactive HSO5− species in (a) Fe3O4-x-, (b) MnO-, and (c) FeMn-350-based processes.

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(a) Fe K-edge

FeO Fe3O4

Normalized Absorption

(b) Normalized Absorption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Fe2O3

FeMn-350 FeMn-350 -used

7110 7120 7130 7140 7150 7160 MnO Mn K-edge B Mn2O3 MnO2 A

FeMn-350 A FeMn-350 -used 6540

6545

6550

6540 6550 6560 6570 6580 6590

Photon Energy (eV) Figure 5. Comparison of experimental normalized Fe, Mn K-edge XANES spectra recorded for FeMn-350 samples and standards. Insets in (a) and (b) highlight corresponding energy inflection profiles. FeMn-350 shows a close structural similarity to undercoordinated magnetite in low-valent Fe species, as well as spectral features of the partially oxidized MnO structure. Two discernible peaks A and B in the main-edge profiles arise from the dipole-allowed 1s to 4p transitions. A nearly negligible peak A in FeMn-350 suggests inhibited formation of Jahn-Teller active Mn or comparatively reduced long-range crystal order after Fe incorporation, whereas the intense and sharp peak B reveals a dominant edge-sharing MnO6 octahedral coordination relative to a corner-sharing one.

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TOC Graphic

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Interface Stabilization of Undercoordinated Iron Centers on

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