Surface Facet of CuFeO2 Nanocatalyst: A Key Parameter for H2O2

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Surface Facet of CuFeO2 Nanocatalyst: A Key Parameter for H2O2 Activation in Fenton-like Reaction and Organic Pollutant Degradation Chu Dai, Xike Tian, Yulun Nie, Hong-Ming Lin, Chao Yang, Bo Han, and Yanxin Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01448 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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Surface Facet of CuFeO2 Nanocatalyst: A Key Parameter for H2O2 Activation in

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Fenton-like Reaction and Organic Pollutant Degradation

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Chu Dai†, Xike Tian*, †, Yulun Nie†, Hong-Ming Lin‡, Chao Yang†, Bo Han†, Yanxin

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Wang§ †

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Faculty of Material Science and Chemistry, China University of Geosciences,

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Wuhan, 430074, P.R. China. ‡

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§

Department Materials Engineering, Tatung University, 104 Taipei, Taiwan.

School of Environmental Studies, China University of Geosciences, Wuhan, 430074, P. R. China.

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ABSTRACT: The development of efficient heterogeneous Fenton catalysts is mainly

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by “Trial-and-Error” concept and the factor determining H2O2 activation remains

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elusive. In this work, we demonstrate that suitable facet exposure to elongate O-O

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bond in H2O2 is the key parameter determining the Fenton catalyst’s activity. CuFeO2

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nanocubes and nanoplates with different surface facets of {110} and {012} are used to

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compare the effect of exposed facets on Fenton activity. The results indicate that

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ofloxacin (OFX) degradation rate by CuFeO2 {012} is four times faster than that of

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CuFeO2 {110} (0.0408 vs. 0.0101 min-1). In CuFeO2 {012}-H2O2 system, OFX is

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completely removed at a pH range 3.2-10.1. The experimental results and theoretical

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simulations show that •OH is preferentially formed from the reduction of absorbed

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H2O2 by electron from CuFeO2 {012} due to suitable elongation of O-O (1.472 Å)

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bond length in H2O2. By contrast, the O-O bond length is elongated from 1.468 Å to

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3.290 Å by CuFeO2 {110} facet, H2O2 tends to be dissociated into -OH group and

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passivates {110} facet. Besides, the new formed ≡Fe2+* on CuFeO2 {012} facet can

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accelerate the redox cycle of Cu and Fe species, leading to excellent long-term

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stability of CuFeO2 nanoplates.

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 INTRODUCTION

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The manipulation over the electron states of various oxidants (O2, H2O2 and O3 etc.)

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holds the key to modulate their conversion into radicals in a wide variety of oxidation

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reactions.1-5 For example, activation of H2O2 on the surface of catalysts, so called the

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heterogeneous Fenton-like reaction, plays a central role in the efficient abatement of

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organic pollutants in water.6-8 Generally, the interaction of organic compounds and

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H2O2 is essentially limited due to the low redox potential of H2O2 (1.77 eV).9,10 As

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well-known, the efficient degradation and even mineralization of these pollutants can

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be obtained by •OH radical with high redox potential (2.8 eV) generated from the

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reduction of H2O2 with different Fenton-like catalysts, such as Fe3O4, Fe2O3, FeOOH

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and FeOOH@GO etc.11-16 However, H2O2 can react with catalyst surface via redox

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reactions for •OH radical or catalytic decomposition into O2.17,18 For the reason above,

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it is urgent to provide a heterogeneous Fenton catalyst to inhibit O2 formation but

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favor the H2O2 conversion into •OH.

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Based on the previous reports, the conversion of H2O2 to •OH over Fenton catalyst is

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a spontaneous process depending on the reactive sites rather than total surface area,

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the oxidation state of active metals and solid-surface-area to solution-volume-ratio

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(SA/V).19 However, most of catalysts are characterized by extensive randomness in

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terms of surface structure and defects.20-22 Hence, in depth understanding of the

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surface Fenton reaction process needs to be explored. The issues include hydrogen

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bonding between H2O2 and the hydroxylated surfaces of catalysts; the degree of

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interaction between the O atoms of H2O2 and the surface exposed metal cations within

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catalysts; and the exact reactive sites for efficient H2O2 decomposition. All these

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concerns will determine the reaction mechanism that H2O2 decomposition into •OH or

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O2.23,24

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In fact, the surface H2O2 activation and efficient electron transfer from active metal to

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absorbed H2O2 are the key steps that determine the performance of heterogeneous

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Fenton catalyst. Hence, the crystal surface of nanoparticle is of paramount importance

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for the catalyst’s reactivity. The surface facets on the crystals have a huge impact on

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the molecular adsorption process due to the different crystal energies in the different

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atomic arrangements. Hence, it is possible to tailor the surface properties of Fenton

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catalyst by manipulating the surface density of atomic steps, ledges, and kinks.25 Via

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the Arrhenius and the transition state theory (TS), the activation energies and

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enthalpies of Fenton reactions help to understand the metal-molecule interactions,23

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however, it is still difficult to describe the chemistry at the atomic scale. Therefore,

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the design and synthesis of singlet-facet nanocrystals is a big challenge to explore the

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complex surface chemistry.

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Herein, the nanocube CuFeO2 {110} and nanoplate CuFeO2 {012} are synthesized to

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study the effect of facet exposure on Fenton reaction performance. Their Fenton-like

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activity was evaluated by the OFX degradation rate and degradation efficiency. It

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show that OFX can be more efficiently degraded on {012} facets with a reaction rate

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constant of 0.0408 min-1 vs. 0.0101 min-1 on {110} facets. The theoretical simulations

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and characterizations were conducted to clarify the difference of {012} and {110}

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facets in H2O2 activation into reactive oxygen species (ROS). Moreover, the effect of

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initial solution pH and long-term stability of CuFeO2 {012} were further explored.

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The intrinsic electron transfer between Cu and Fe element, especially the new formed

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Fe2+* species for the excellent Fenton activity and stability of CuFeO2 {012} was also

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discussed. Therefore, this study demonstrates the facet control is a critical parameter

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to design a heterogeneous Fenton catalyst in water treatment.

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 EXPERIMENTAL SECTION

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Nanocrystal Synthesis. CuFeO2 nanoplates with exposed {012} facets (denoted as

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CuFe-012) is prepared according to the following procedure. 0.1 M Cu(NO3)2·3H2O

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and Fe(NO3)3·9H2O solution is added dropwise to the 0.8 M NaOH solution under

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constant stirring for full precipitation. After washing to a neutral pH, the precipitation

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is re-dispersed into 50 mL distilled water and 5 mL ethylene glycol is then added as

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reducing agent. The well mixed dispersion is transferred into Teflon-lined autoclave

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and heated in an oven at 200 °C for 12 h. The obtained black precipitates are collected

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and repeatedly washed with distilled water and absolute ethyl alcohol. Finally, the

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samples are vacuum-dried at 60°C overnight. CuFeO2 nanocubes with exposed {110}

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facets (CuFe-110) are synthesized under the similar conditions. The precipitation is

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transferred into Teflon-lined autoclave directly without washing step. For comparison,

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Fe2O3 and Cu2O are prepared individually using Fe(NO3)3 and Cu(NO3)2 as precursor,

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respectively, following the same procedure of CuFe-110.

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Characterization. Powder X-ray diffraction (XRD) with monochromatic Cu Kα

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(λ=1.5406 Å) is recorded by a Bruker AXS D8-Focus diffractometer. The surface

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morphology is studied using field emission scanning electron microscopy (FESEM,

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Hitachi SU-8010) equipped with an attached Oxford Link ISIS energy-dispersive

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X-ray spectroscopy (EDS) and transmission electron microscopy (TEM, Philips CM

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12). X-ray photoelectron spectroscopy (XPS) is examined by the MULTILAB2000

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electron spectrometer with 300W Al Kα radiation. The Brunauer-Emmett-Teller (BET)

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surface area of the prepared sample is tested using Micromeritics ASAP 2020 HD88

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adsorption analyzer. Electron spin resonance (ESR) spectra are obtained using a

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FA200 ESR spectrometer equipped with a quanta-Ray Nd: YAG laser system as the

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irradiation light source (λ=532 nm). The settings of center field, microwave frequency,

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and power are 3480.00 G, 9.79 GHz, and 5.05 mW, respectively. The leaching of iron

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is determined by ICP-MS (iCAP Qc, Thermo Scientific).

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Computational Methods. The chemisorption and reaction of H2O2 on CuFeO2

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surface are investigated by spin-polarized density functional theory (DFT) using the

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Vienna ab-initio simulation package (VASP). The projector augmented wave (PAW)

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method is used to describe electron-ion interaction with Perdew-Burke-Ernzerh (PBE)

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of functional for exchange-correlation energy. An energy cutoff of 400 eV is used for

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the plane-wave expansion of the electronic wave function. The optimized crystal

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structure of CuFeO2 (CCSD No. 00-039-0246) is cleaved as a slab model under

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periodic boundary condition to represent the surface structure. The vacuum region is

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set 15 Å. The surface is fully equilibrated prior to H2O2 adsorption except the bottom

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2 layers, which are always fixed during our simulation, until the total energy of the

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system converged to within 10–3 eV. Electronic energies are calculated using a

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self-consistent-field (SCF) with the tolerance of 10–4 eV. The Brillouin zone

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integration is treated with a 2 × 2 × 1 Monkhorst-Pack k-point mesh.

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Experimental Procedure and Analysis. The heterogeneous Fenton activity of

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CuFe-012 and CuFe-110 is evaluated by the degradation efficiency of ofloxacin

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(OFX). Unless otherwise specified, 30 mg catalyst is added into 50 mL 10 mg/L of

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OFX solution and the suspensions are magnetically stirred for 60 min to obtain

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adsorption/desorption equilibrium between catalyst and OFX solution. The Fenton

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reaction then starts upon the addition of desired amount of 30% H2O2. The reaction

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solution is not buffered and the pH changes during the reaction process are monitored

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by a pH meter. The pH is adjusted by a diluted aqueous solution of NaOH or HCl to

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keep the pH within 0.3 units at the end of reaction. At given time intervals, 2 mL

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sample is taken and filtered immediately to remove the catalyst for analysis. Then 0.1

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mL 0.04 mol/L Na2S2O4 is added to quench the residual •OH. The concentration of

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OFX in filtrate is determined by high performance liquid chromatography (HPLC,

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Hitachi L-2130) with an UV-DAD detector. The chromatographic separation is

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performed by a reverse-phase C18 column (250 mm×4.6 mm, 5 μm). The mobile

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phase composed of 15% acetonitrile and 85% ultrapure water is acidified by 1%

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phosphoric acid with a flow rate of 0.5 mL/min. The injection volume is 20 μL.

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Temperature of the column chamber is maintained at 25 °C and the detection

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wavelength is 288 nm. All the experiments are repeated three times and the data

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represent the average of the triplicates with a standard deviation less than 5%.

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Tert-butyl alcohol (TBA) and Benzoquinone are used as scavengers for •OH and •O2-,

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respectively. The changes of H2O2 concentration during Fenton reaction is determined

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by KMnO4 titration. The Arrhenius activation energies are obtained from the slope of

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the logarithm of the first-order rate constants versus the inverse absolute temperature.

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 RESULTS AND DISCUSSION

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Reactivity of CuFeO2 with Different Exposed Facets. The catalytic performance of

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CuFeO2 nanocube (CuFe-110) and nanoplate (CuFe-012) is compared in Figure 1A.

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Almost no OFX degradation is observed under H2O2 oxidation alone without catalyst

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(curve a). Both CuFe-110 and CuFe-012 are negligible in OFX adsorption (below 5%,

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data not shown). In comparison, OFX was rapidly degraded in the CuFe-110/H2O2,

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and the degradation rate is even faster in the CuFe-012/H2O2 (curves d and e).

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However, both Fe2O3 and Cu2O have much lower efficiency for OFX degradation

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(curves b and c). Besides, the leached Cu and Fe ions from CuFeO2 shows almost no

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OFX degradation (Figure S1 of the SI) and the contribution of homogeneous Fenton

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reaction to OFX removal is negligible when CuFeO2 is used as Fenton catalyst. Hence,

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CuFeO2 is proven to be a promising heterogeneous Fenton catalyst. As depicted in

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Figure S2 of the SI, the reaction condition is optimized as followed: 0.6 g/L CuFe-012,

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0.06 mol/L H2O2 dosage. However, as shown in Figure 1B, the OFX degradation can

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be fit well with pseudo-first-order kinetics (R2 > 0.99). The rate constant (k) of OFX

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degradation in the CuFe-012/H2O2 is found to be 4.04 times of that in the

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CuFe-110/H2O2, with values of 0.0408 and 0.0101 min-1, respectively. As shown in

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Table S1 of the SI, Turn Over Frequency (TOF) of OFX degradation over CuFeO-012

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is also two times higher than that of CuFeO-110, which is consistent with the results

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in Figure 1. Although the same composition of CuFeO2, nanocube (CuFe-110) and

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nanoplate (CuFe-012) exhibited different Fenton activity towards OFX degradation.

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It was reported that the catalytic feasibility of heterogeneous Fenton catalysts was

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governed by the thermodynamic favorability of reducing M(n+1)+ to Mn+ by H2O2.26,27

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XRD patterns of CuFe-012 and CuFe-110 in Figure 2A match well with the standard

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data for high pure bulk CuFeO2 with a rhombohedral structure (JCPDS card No.

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75-2146, space group, R-3M). 28,29 XPS spectrum in Figure S3 of the SI indicates that

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the valence states of Cu and Fe are +1 and +3 in both CuFe-012 and CuFe-110. The

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peaks at 951.8 eV and 931.8 eV indicated in Figure S3A correspond to the Cu 2p1/2

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and Cu 2p3/2 of Cu+, while the binding energies at 725.3 eV and 711.2 eV indicated in

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Figure S3B are the characteristic peaks of Fe3+ with Fe 2p1/2 and Fe 2p3/2, respectively.

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30,31

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catalytic activity between CuFe-012 and CuFe-110 is then negligible. The surface area

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of CuFe-012 and CuFe-110 analyzed by BET is 15.23 m2/g and 12.76 m2/g in Figure

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S4 of the SI, respectively. Therefore, the surface area of CuFe-012 and CuFe-110 is

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also not related to the difference in degradation rate of both Fenton system.

The contribution of element composition and their chemical states to the different

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Exposed {012} Facets of CuFeO2 for High H2O2 Utilization Efficiency. The

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above results show that the heterogeneous Fenton activity of CuFeO2 should be not

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only dependent on the chemical valance of Cu, Fe and surface area. It is necessary to

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investigate the reaction between H2O2 and CuFeO2 with different crystal orientation.

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Firstly, the morphology and structure of CuFe-012 and CuFe-110 are examined by

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SEM, TEM and HRTEM. Figure 2B and 2C depicted that two CuFeO2 architectures

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were well-defined shape. The average width and thickness of CuFe-012 nanoplate is

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about 1 µm and 50 nm. The lattice fringe of 0.251 nm in HRTEM is consistent with

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{012} plane (Figure 2E), which meant the plate facet of CuFe-012 is mainly in {012}

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crystal phase. CuFe-110 was mainly composed of cube with well-defined edges and

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corners. The lattice fringe of 0.152 nm in HRTEM image (Figure 2F), which indicates

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the cube facet of CuFe-110 is mainly in {110} orientation.

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The surface structure of Cu, Fe, and O on the facets of CuFeO2 is expected to be

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vital for its Fenton activity and the H2O2 decomposition. To confirm this anticipation,

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the theoretical simulations of the surface structure of CuFeO2 and H2O2 adsorption on

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the both facets is carried out in this study. The chemisorption state of H2O2 on

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different facets is simulated to verify the effect of adsorption process on the Fenton

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activity of CuFeO2. The simulation of CuFeO2 structure indicates there are more iron

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and copper atoms to be exposed on the surface of {110} than that of {012} facet as

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showed in Figure 3A and 3B. Table S2 of the SI reveals that the O-O bond length in

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H2O2 can be greatly elongated from 1.468 Å to 3.290 Å. Long O-H-O bond (2.472 Å)

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may inhibit electron transfer from Cu and Fe to absorbed H2O2, hence, H2O2 tends to

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be dissociated into -OH group and passivates the {110} facet. In comparison with

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{110} facet, Cu, Fe, and O atoms are exposed evenly on {012} facet, where the most

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favorable configuration between H2O2 and CuFeO2 {012} facet was formed with a

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slightly larger O-O bond length of 1.472 Å than that of free H2O2 (1.468 Å). Besides,

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the length of O-H-O bond (1.749 Å) on CuFe-012 is smaller than that on {110} facet,

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which also favors the H2O2 activation and •OH radical formation due to easy electron

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transfer.32,33 The concentration of •OH during the Fenton-like reaction process was

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determined by the reaction with dimethyl sulfoxide (DMSO).34,35 Figure S5 of the SI

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clearly indicates that •OH was generated in both CuFe-012 and CuFe-110 Fenton

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systems. However, the •OH amount over CuFe-012 was much higher than that over

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CuFe-110 under the same reaction condition. It provides a direct proof of the higher

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Fenton activity of CuFe-012 catalyst. The effect of exposed facet on Fenton

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performance of CuFeO2 is studied by comparing the activation energy (Ea) of OFX

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degradation according to Arrhenius equation.36,37 Figure S6 of the SI shows that OFX

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removal efficiency increases with the reaction temperature for both FeCu-110 and

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FeCu-012 catalysts. It means the Fenton reaction process for OFX degradation is

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mainly governed by thermodynamics. However, as shown in Figure 4A, {012} facet

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has a much higher H2O2 decomposition rate (0.0166 min-1) and efficiency (100% at

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120 min) than that of {110} facet (0.0039 min-1, 50% at 120 min) at 25 oC. The

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activation energy (Ea) of {012} and {110} facet is calculated as 22.92 and 34.52

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kJ/mol by Arrhenius equation (Figure 4B). It agreed well with their vertical distance

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between H2O2 and CuFeO2 surface, 1.749 Å for {012} vs. 2.472 Å for {110}

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indicated in Figure 3C and 3D. The H2O2 utilization efficiency of {012} facet and

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{110} facet was 65.85% and 45.75%, respectively in Figure S7 of the SI. Hence, a

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suitable facet exposure for appropriate elongation of O-O bond in H2O2 play a key

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role in determining the heterogeneous Fenton activity of CuFeO2.

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Radicals Identification and Good Stability of CuFe-012. The electron transfer

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process between CuFeO2 and H2O2 can generate •OH and O2•-. The ESR spectrum in

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Figure 5A indicates a strong 4-fold characteristic peak of DMPO-•OH adducts with an

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intensity ratio of 1:2:2:1.38,39 At the same time, a relatively weak sextet peaks of

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DMPO-HO2•/O2•- adducts was also observed. Moreover, tert-butyl alcohol has a much

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higher inhibition effect on OFX degradation compared with benzoquinone as shown

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in Figure 5B. In comparison with the control experiment (no scavenger is present), the

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OFX degradation efficiency was decreased by about 70% in the presence of 50 mM

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tert-butyl alcohol, whereas only about 15% inhibition is observed in the presence of

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the same amount of benzoquinone. Therefore, •OH is the dominant reactive oxygen

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species and responsible for OFX degradation in CuFeO2 {012} facet/H2O2 system.

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Based on the traditional Fenton mechanism, •OH is generated from the reaction Mn+ +

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H2O2 → M(n+1)+ + •OH + OH- (Reaction 1), while HO2• is produced from the reaction

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M(n+1)+ + H2O2 → Mn+ + HO2• + H+ (Reaction 2). The redox process should occur near

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stoichiometrically 1:1 for an ideal Fenton catalyst. Actually, much more amount of

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•OH

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Cu+ in CuFeO2 are finally 100% consumed but the recycle of Fe3+/Fe2+, Cu2+/Cu+ was

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indeed inhibited. Then, CuFeO2 nanoplates should have a poor reuse stability.

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However, no obvious deactivation of CuFe-012 is observed in Figure 6. OFX is

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always completely degraded in the five successive degradation experiments. The XRD

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patterns and SEM images of CuFe-012 after 5th cycle shown in Figure S8 further

was detected than HO2• in this study. It means that the reductive species such as

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illustrates the stability of CuFe-012 catalyst. There was almost no change on the Cu,

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Fe chemical valance in CuFe-012 based on XPS spectra (Figure S9 of the SI).30 Based

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on the above results, CuFeO2 nanoplates had an excellent stability and its crystalline

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structure did not change. There must exist an intrinsic reason for the interaction of Cu

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and Fe within CuFeO2 during heterogeneous Fenton reaction process.

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Discussion on the Mechanism of CuFe-012 for Efficient H2O2 Activation. The

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concentration of leaching Cu and Fe ions from CuFe-012 are determined by ICP-MS

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analysis. As shown in Figure S10 of the SI, there is no significant Cu ion detected

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(below 0.30 mg/L). While the concentration of Fe ion in solution increases rapidly

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and reaches a maximum value of 5.45 mg/L at 30 minutes and then decreases with the

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reaction time. It indicated that Cu is stable within CuFe-012 but Fe has a release and

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re-confinement process during the Fenton reaction. Based on the experimental results

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and previous literature, the possible heterogeneous Fenton reaction mechanism over

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CuFeO2 {012} facet is proposed in Figure 7. ≡ Cu+ + H2 O2 →≡ Cu2+ + OH ∙ + OH −

Reaction 3

Fe2+ + ≡→≡ Fe2+∗

Reaction 5

≡ Cu+ + ≡ Fe3+ →≡ Cu2+ + ≡ Fe2+

Reaction 7

≡ Fe3+ + H2 O2 →≡ Fe2+ + HO∙2 + H +

Reaction 4

≡ Fe2+∗ + ≡ Cu2+ →≡ Cu+ + ≡ Fe3+

Reaction 6

≡ Fe2+∗ + H2 O2 →≡ Fe3+ + OH ∙ + OH −

Reaction 8

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During the H2O2 activation process on {012} facet, ≡Cu+ is oxidized to ≡Cu2+,

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accompanied by the generation of •OH (reaction 3). At the same time, some ≡Fe3+ is

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reduced to ≡Fe2+ and HO2• is formed by reaction 4. It is worth noting that most of the

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released Fe2+ are re-confined on CuFeO2 {012} facet via oxygen-iron bond (≡Fe2+*,

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reaction 5). ≡Fe2+* is reported to have a much higher reductive ability than ordinary

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Fe2+, which can induce the reduction of high valent UVI, CrVI, AsV and substituted

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nitrobenzene.40-45 Similarly, ≡Fe2+* can reduce ≡Cu2+, yielding a Cu redox cycle

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(reaction 6). Of course, part of ≡Fe3+ and ≡Cu+ was converted to ≡Fe2+ and ≡Cu2+ via

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reaction 7.46 The new formed ≡Fe2+* plays an important role for the excellent

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long-term stability of CuFe-012. As depicted in Figure 3C and 3D, H2O2 is preferably

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absorbed on Fe sites in CuFe-012 by theoretical simulations. Due to the strong

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electron attraction of ≡Fe3+, the -OH bond on CuFeO2 {012} facet can decrease the

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electron density of neighboring H-O bond, resulting in the O-O bond in H2O2 to

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become weaker. It will favor the •OH formation via the electron transfer from exposed

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metal to H2O2 according to reaction 3 and 8.

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It has been widely accepted that the activity of most heterogeneous Fenton catalysts

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will decrease greatly at neutral or even alkaline condition because a small fraction of

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H2O2 is converted into •OH radicals. In this study, CuFe-012 shows satisfied oxidation

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efficiency for OFX degradation at a pH range of 3.2-10.1 with the best performance at

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pH 6.5 (Figure S11A of the SI). Although the OFX removal efficiency decreases by

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about 40% in the local pond water in comparison with the control experiment (in

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distilled water, Figure S11B), it is because of the competitive effect of total dissolved

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organic carbon and co-existing anions in consumption of •OH radicals. Figure S12 of

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the SI further depicted that the zero charge point of CuFe-012 is close to pH 6. Since

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the pKa1 and pKa2 of OFX are 6.05 and 8.11 respectively,

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adsorption to OFX removal should increase with the initial solution pH. The intrinsic

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electron transfer between Cu and Fe, especially the new formed ≡Fe2+* species could

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overcome the inhibited cycle of M(n+1)+/Mn+. The suitable {012} facet exposure and

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O-O bond elongation in H2O2 can further enhance the H2O2 utilization efficiency in

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Fenton-like reaction and OFX degradation. The findings in this study provide a deep

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understanding on the enhanced reactivity by facets exposure, and also shed light on

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the design of high efficient heterogeneous Fenton catalysts.

289

ASSOCIATED CONTENT

290

Supporting Information

291

Additional experimental data, such as the characterization of CuFeO2 by XRD, XPS,

292

BET; contribution of homogeneous Fenton reaction to OFX degradation caused by

293

leached Cu and Fe from CuFe-012, optimization of CuFe-012 and H2O2 dosage;

294

pseudo-first-order kinetic rate plot; TOF values; OFX adsorption experiments; H2O2

295

decomposition and utilization efficiency; and the effect of solution pH and water

296

characteristics on OFX degradation are provided in the supplementary information.

297

This material is available free of charge via the Internet at http://pubs.acs.org.

298

AUTHOR INFORMATION

299

Corresponding Authors: *Prof. Xike Tian. Phone: 86-27-67884574. Fax:

300

86-27-67884574. E-mail: [email protected].

301

Notes

302

The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS

304

This work is supported by the National Natural Science Foundation of China (Grant

305

No. 41773126), the Foundation for Innovative Research Groups of the National

306

Natural Science Foundation of China (No. 41521001) and the “Fundamental Research

307

Funds for the Central Universities”.

308

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456 457

Figure Captions

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Figure 1. OFX degradation under different conditions (A): (a) H2O2 oxidation alone,

459

(b) Fe2O3/H2O2, (c) Cu2O/H2O2, (d) CuFe-110/H2O2, (e) CuFe-012/H2O2; and (B) the

460

pseudo-first-order kinetic rate plot of OFX degradation in CuFe-110/H2O2 and

461

CuFe-012/H2O2. (Reaction condition: [OFX]=10 mg/L, pH=6.53, [catalyst]=0.6 g/L,

462

[H2O2]=0.06 mol/L, T=25℃)

463

Figure 2. XRD patterns of CuFe-110 and CuFe-012 (A); SEM image of CuFe-110 (B),

464

CuFe-012 (C) and the corresponding TEM images (inset); (D) SAED pattern of

465

CuFe-012; HRTEM image of CuFe-012 (E) and CuFe-110 (F).

466

Figure 3. The optimized surface structures of CuFeO2 catalyst: (A) CuFe-012 and (B)

467

CuFe-110; (C) H2O2 adsorption on CuFe-012 and (D) H2O2 adsorption on CuFe-110.

468

Figure 4. H2O2 decomposition as a function of reaction time over CuFe-110 and

469

CuFe-012 (A) Calculation of activation energy for CuFe-110 and CuFe-012 by the

470

plot of lnk against 1/T for a range of temperatures (B). (Reaction condition:

471

[OFX]=10 mg/L, [catalyst]=0.6g/L, [H2O2]=0.06 mol/L, pH=6.53, T=25℃)

472

Figure 5. ESR spectrum of DMPO-•OH and DMPO-O2•- in CuFe-012/H2O2 system

473

(A) using DMPO as radical trapping agents; (B) Effects of radical scavengers on OFX

474

degradation.

475

Figure 6. Stability of CuFe-012 in the multicycle degradation of OFX in the presence

476

of H2O2.

477

Figure 7. Proposed heterogeneous Fenton reaction mechanism over CuFe-012.

478 21 ACS Paragon Plus Environment

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479 480

481 482

Figure 1. OFX degradation under different conditions (A): (a) H2O2 oxidation alone,

483

(b) Fe2O3/H2O2, (c) Cu2O/H2O2, (d) CuFe-110/H2O2, (e) CuFe-012/H2O2; and (B) the

484

pseudo-first-order kinetic rate plot of OFX degradation in CuFe-110/H2O2 and

485

CuFe-012/H2O2. (Reaction condition: [OFX]=10 mg/L, pH=6.53, [catalyst]=0.6 g/L,

486

[H2O2]=0.06 mol/L, T=25℃)

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488 489

Figure 2. XRD patterns of CuFe-110 and CuFe-012 (A); SEM image of CuFe-110 (B),

490

CuFe-012 (C) and the corresponding TEM images (inset); (D) SAED pattern of

491

CuFe-012; HRTEM image of CuFe-012 (E) and CuFe-110 (F).

492 493

Figure 3. The optimized surface structures of CuFeO2 catalyst: (A) CuFe-012 and (B)

494

CuFe-110; (C) H2O2 adsorption on CuFe-012 and (D) H2O2 adsorption on CuFe-110.

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495 496

Figure 4. H2O2 decomposition as a function of reaction time over CuFe-110 and

497

CuFe-012 (A) Calculation of activation energy for CuFe-110 and CuFe-012 by the

498

plot of lnk against 1/T for a range of temperatures (B). (Reaction condition:

499

[OFX]=10 mg/L, [catalyst]=0.6g/L, [H2O2]=0.06 mol/L, pH=6.53, T=25℃)

500

501 502

Figure 5. ESR spectrum of DMPO-•OH and DMPO-O2•- in CuFe-012/H2O2 system

503

(A) using DMPO as radical trapping agents; (B) Effects of radical scavengers on OFX

504

degradation.

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507 508

Figure 6. Stability of CuFe-012 in the multicycle degradation of OFX in the presence

509

of H2O2.

510

511 512

Figure 7. Proposed heterogeneous Fenton reaction mechanism over CuFe-012.

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Table of Contents graphic

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