Multiphase Porous Electrochemical Catalysts Derived from Iron-Based

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Multi-Phase Porous Electrochemical Catalysts Derived from Iron-Based Metal-Organic Framework Compounds Kai Liu, Menglin Yu, Haiying Wang, Juan Wang, Weiping Liu, and Michael R Hoffmann Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01143 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 12, 2019

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Environmental Science & Technology

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Multi-Phase Porous Electrochemical Catalysts Derived from Iron-Based

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Metal-Organic Framework Compounds

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Kai Liu,†, ‡ Menglin Yu,† Haiying Wang,† Juan Wang,† Weiping Liu,† Michael R. Hoffmann‡,*

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† College

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of Environmental and Resource Science

Zhejiang University, Hangzhou 310058, China ‡

Department of Environmental Science and Engineering

California Institute of Technology, Pasadena, California 91126, United States # Author Contributions K. Liu and M. Yu contributed equally to the work. *Corresponding

Author: Michael R. Hoffmann

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Tel: +1-626-395-4391. Fax: +1-626-395-4391. E-mail: [email protected]

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Table of Contents Art

E-Fenton

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O2 Reduction

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ACS Paragon Plus Environment


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Abstract

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Herbicide use has attracted attention recently due to potential damage to human health and

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lethality to the honey bees and other pollinators. Fenton reagent treatment processes can be applied

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for the degradation of herbicidal contaminants from water. However, the need to carry out the

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normal Fenton reactions under acidic conditions often hinders their practical application for

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pollution control. Herein, we report on the synthesis and application of multi-phasic porous

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electro-Fenton catalysts prepared from calcinated metal-organic framework compounds,

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CMOF@PCM, and their application for the mineralization of herbicides in aqueous solution at

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circum-neutral pH. CMOF nanoparticles (NPs) are anchored on porous carbon monolithic (PCM)

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substrates, which allow for binder-free application. H2O2 is electrochemically generated on the

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PCM substrate which serves as a cathode, while ·OH is generated by the CMOF NPs at low applied

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potentials (-0.14 V). Results show that the structure and reactivity of the CMOF@PCM electro-

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Fenton catalysts is dependent on the specific MOF precursor used during synthesis. For example,

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CMIL-88-NH2, which is prepared from MIL-88(Fe)-NH2, is a porous core-shell structured NP

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comprised of a cementite (Fe3C) intermediate layer that is sandwiched between a graphitic shell

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and a magnetite (Fe3O4) core. The electro-Fenton production of hydroxyl radical on the

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CMOF@PCM composite material is shown to effectively degrades an array of herbicides.

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Introduction

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Due to extensive application of herbicides, they are now considered to be pseudo-persistent

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pollutants.1 Among the widely used herbicides, glyphosate is considered to be carcinogenic2, 3.

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Furthermore, several herbicides are reported to impact pollinators resulting in a dramatic reduction

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of bee populations worldwide.4-7 Herbicides are also suspected of causing harm to crustaceans,

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which are an important component of freshwater and oceanic food webs.8 In spite of these growing

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concerns, there is little research on their removal from the aquatic environment compared to other

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contaminants of emerging concern (CECs).

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Advanced oxidation processes (AOP) including electrochemical oxidation are suitable for

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herbicide degradation and removal from contaminated water.9-11 However, electrochemical

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degradation of organic compounds is often energy intensive with high capital expenditure

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(CAPEX) costs. This is due to use of expensive material such as platinum group metals (PGM) or

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boron-doped diamonds as anodes or in the case of platinum as cathodes. In contrast, alternative

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oxidative approaches employing Fenton-reagent reactions (H2O2 + Fe(II) → ∙OH + Fe(III) + OH-

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) utilize iron, an earth abundant metal.12, 13 Fenton-like systems can achieve complete degradation

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of some of the most persistent contaminants such as perfluorinated surfactants and

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perfluorotelomer alcohols.14, 15 However, a major drawback of using the Fenton process is that

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above pH 5, Fe(II) is readily oxidized to Fe(III) in the form of Fe(OH)3 by dissolved oxygen, thus

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limiting the pH range to carry out the overall reactions. Electro-Fenton reaction, on the other hand,

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overcomes this problem by reducing Fe(III) to Fe(II) at cathode.16,

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concentrations as high as 0.01 M need to be applied for the effective removal of contaminants.15

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Unfortunately, Fe(II)

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We have shown previously that a composite Fe-NP and nitrogen co-doped graphite

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material can be used as an electrode for heterogeneous electro-Fenton oxidation.18 This approach

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only required a loading level of 0.36 wt% Fe in order to maximize ·OH production. However, the

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percentage of Fe that was accessible on the surface was limited.18 In order to explore the full

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potential of a heterogeneous electro-Fenton process, a porous electrode where all potential Fe sites

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are accessible is desired. Given this objective, inorganic porous nanomaterials derived from MOFs,

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or carbonized MOFs (CMOFs), appear to provide a solution that would achieve this. For example,

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metal NPs porous catalyst derived from MOFs have been shown to be useful for organic

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synthesis.19

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In order to be useful in practical applications, the electro-Fenton catalysts must also meet

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the following criteria: 1) the metal nano-catalyst supported on a substrate must be well dispersed

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in order to allow for sufficient contact of electrolyte and substrate with the active catalyst; 2) the

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metal catalyst must form a transient bond with the substrate; and 3) both the substrate and

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nanoparticulate metal catalysts must be attached to a highly porous material. The first criterion

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allows the utilization of the electrochemically-active nano-sized catalyst. The third criterion allows

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for the construction of 3D electrodes that have form-factor advantages, which can maximize the

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electrode surface to electrolyte volume ratio during application.

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Herein, we use binder-free multi-phase porous electro-Fenton catalysts made from

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pyrolyzed MOFs for the electrochemical degradation of an array of herbicides. The resulting

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catalysts are anchored onto porous carbon monoliths (PCM), which are denoted as CMOF@PCMs.

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In contrast to a bulk-phase catalyst, the narrow distribution of the pores within the CMOF@PCMs

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allows for facile access to the active Fe(II)/Fe(III) catalytic centers. In addition, the PCM substrate

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allows in-situ generation of hydrogen peroxide (H2O2) that is needed for the electro-Fenton

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reaction. The CMOF@PCMs are shown to be efficient at removing herbicides at neutral pH while

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Fe leaching is negligible. Furthermore, we discovered the resulting structure of CMOFs are

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independent of the topology of parent MOF. But the resulting structure appears to depend on the

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specific functional group and on the identity of the organic linker. A core-shell structured

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CMOF@PCM is obtained from a MIL-88(Fe)-NH2 precursor. The results provided herein provide

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insights into the development of stable and porous materials for use as sustainable electro-Fenton

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catalysts.

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Materials and Methods

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Materials

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All chemicals were obtained from Sinopharm Chemical Reagent Co. Ltd. and were used

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without further purification. Solvents were HPLC grade unless otherwise stated. Pesticides with

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chemical purity ≥ 97.0% (Sinopharm Chemical Reagent Co. Ltd.) used in this study are listed in

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Table S1.

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Preparation of Porous Carbon Substrate

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The PCM substrate was synthesized based on a previously reported procedure.20 Briefly,

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resorcinol (3.0 g, 27.3 mmol) and Pluronic F127 (1.25 g) were dissolved in a solvent mixture of

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ethanol (11.4 mL) and deionized water (9 mL). 1,6-diaminohexane (0.078 g, 0.67 mmol) was

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added to the above solution and stirred for 30 min. Subsequently, formalin (4.42 g, 54.5 mmol)

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was added. The reaction mixture was stirred for 40 min before sealed and heated at 90°C for 4 h.

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The polymer monolith, poly(benzoxazine-co-resol), was dried at 90°C for 48 h. Pyrolysis was

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performed at 800°C for 2 h under a nitrogen protection. Crack-free PCM was obtained after

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cooling down to room temperature.

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Preparation of MOFs

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A microwave-assisted synthesis of Fe-MOFs was based on a similar method using

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microwave synthesizer (CEM, Discover SP).21 In brief, FeCl3 (0.88 g, 0.54 mmol) and terephthalic

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acid (0.9 g, 0.54 mmol) were dissolved in N, N-dimethylformamide (DMF) (15 mL) inside a 30

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mL microwave tube. The reaction was carried at 150°C for predetermined time before cooled to

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room temperature. The nano-crystals of MIL-88(Fe) were separated by centrifugation at 11,000

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rpm, washed with DMF and methanol before drying in vacuo at 150°C. MIL-100(Fe), MIL-

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101(Fe), MIL-88(Fe)-NH2, and MIL-101(Fe)-NH2 were synthesized using the same method.

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Preparation of CMOF@PCM

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PCM was mixed with the Fe-MOF powder at a given mass ratio. The powdered MIL-

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88(Fe)@PCM was carbonized at 600 ºC for 0.5 h under a N2 atmosphere to produce CMIL-

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88@PCM. The same procedure was applied for the preparation of the other CMOF@PCMs using

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the other MOFs precursors. The CMOF@PCMs used in the current study were designated as

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CMIL-88@PCM, CMIL-100@PCM, CMIL-101@PCM, CMIL-88-NH2@PCM, and CMIL-101-

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NH2@PCM for the hybrid materials prepared from MIL-88(Fe), MIL-100(Fe), MIL-101(Fe),

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MIL-88(Fe)-NH2, and MIL-101(Fe)-NH2, respectively. Fe-MOF powder was replaced with Fe3O4

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nanoparticle to produce Fe3O4@PCM using similar method.

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Preparation of Electrodes

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Evaluation of electro-catalytic property of CMOF@PCMs required identical geometrical

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surface areas for the working electrode. As the result, rigid carbon paper was coated with these

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electro-catalysts and used as the working electrode in order to ensure identical electro-active areas.

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The ground CMOF@PCM substrates (50 mg) were suspended in 4.47 ml of mili-Q water, 1.5 ml

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of isopropanol, and 100 μl of a Nafion solution. The resulting ink was spray-coated onto carbon

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paper (HCP030P Hesen) at a catalyst loading of 0.6 mg/cm2. Both sides of carbon paper were

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uniformly coated (with total effective area of 8 cm2) to avoid exposing the supporting carbon paper

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substrate to the electrolyte.

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Characterization of Catalysts

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The surface morphology of the PCM substrate was examined using a Hitachi SU8010 field-

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emission scanning electron microscope (FE SEM), while the surface morphologies and elemental

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compositions of the CMOF@PCMs were examined using a ZEISS Merlin FE SEM equipped with

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an Oxford INCA X-MAX50 energy dispersive spectrometer (EDS). TEM micrographs and TEM

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EDS measurements were obtained using a FEI Tecnai G2 F20 (200 kV) transmission electron

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microscope (TEM). X-ray powder diffraction patterns were collected using a Bruker D8 A25

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Advance diffractometer with Cu-Kα radiation (λ=1.5418 Å). The element composition of catalysts

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was characterized by X-ray photoelectron spectroscopy (XPS) using Surface Science Instruments

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Thermo Scientific ESCALAB 250Xi surface spectrometer. Monochromatic Al-Kα radiation

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(1486.6 eV) was used.

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Electrochemical Experiments

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Electrochemical experiments were performed using a CH CHI660E C17171 potentiostat

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with a three-electrode beaker cell at room temperature. A platinum (Pt) wire and saturated calomel

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electrode (SCE) were used as the counter electrode and the reference electrode, respectively (Fig.

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S1). Recorded potentials were converted to the reversible hydrogen electrode (RHE) scale (RHE

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= SCE + 0.059 × pH + 0.241 V). Cyclic voltammetry (CV) was conducted in Ar- or O2-saturated

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electrolyte (0.1 M Na2SO4) at a scan rate of 10 mV/s. Constant potentials were applied to determine

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H2O2 yields obtained with the PCM substrate and electro-Fenton efficiencies of herbicide

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degradation by CMOF@PCMs. 0.1 M Na2SO4 was used as electrolyte, and solution pH was

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adjusted to 4-10 using H2SO4 and NaOH. H2O2 yield measurements were performed in triplicate.

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Electro-Fenton degradation experiments were performed in duplicate with the average value being

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reported.

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Analytical Methods

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To quantify the H2O2 produced, samples collected at certain time intervals were analyzed

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by titanium oxalate spectrophotometric method.22 Briefly, reaction sample was added into 3 M

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sulfuric acid solution before reacted with potassium titanium oxalate solution (0.5 wt% in water)

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to produce a yellow pertitanic acid (TiO(H2O2)2+) complex. The absorbance of the solution was

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measured using a JASCO V-750 spectrophotometer at 400 nm. H2O2 solutions with known

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concentration were used to construct a calibration curve.

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During the electro-Fenton process, the concentrations of herbicides were analyzed using a

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high-performance liquid chromatography (Waters Acquity UPLC) system coupled to e2998

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photodiode array detector and an e2695 separation module. Sunfire C18 column (4.6x50 mm; 5

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µm particles) was used. (A) 0.1% Phosphoric acid and (B) methanol were used as mobile phase

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with flow rate of 0.8 mL min-1.

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RESULTS AND DISCUSSION

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Synthesis of E-Fenton Catalysts from Fe-MOFs

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The iron-containing MOFs MIL-88, MIL-100, and MIL-101 were used as templates in the

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preparation of electro-Fenton catalysts. MIL-88(Fe) and MIL-101(Fe) were synthesized by using

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the same organic ligand, terephthalic acid (BDC), with FeCl3. MIL-88(Fe) consists of an infinite

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framework with three-dimensional channels formed by [Fe3(μ3-O)(COO)6(H2O)2Cl]6 secondary

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building units (SBUs) and BDC linker (Fig. 1). MIL-101(Fe), however, consists of quasi-

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spherical cages formed by Fe3O-carboxylate trimers SBU and BDC linkers (Fig. 1). The

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mesoporous cages with dimensions of 2.9 by 3.4 nm are accessible via microporous windows or

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openings (1.2 and 1.6 nm). Comparison between MIL-88(Fe) and MIL-101(Fe) allows us to study

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the impact of MOFs topology on the subsequent electrocatalytic properties of the CMOFs. In

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addition, MIL-100(Fe) was synthesized by replacing the BDC ligand with 1,3,5-

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benzenetricarboxylic acid (BTC) in order to investigate the effect of the ligand on the

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electrocatalytic properties of the CMOFs. MIL-101(Fe) and MIL-100(Fe) have mesoporous cages

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formed by the combination of iron octahedra trimers of [(Fe3(m3-O))(OH)(H2O)2]6 SBUs and BTC

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linkers (Fig. 1). The MIL-100(Fe) cages (2.5 and 2.9 nm) have microporous apertures of 0.55 and

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0.86 nm, which are significantly smaller than that of MIL-101(Fe)’s. In order to investigate the

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functional group effect, MIL-88(Fe)-NH2 and MIL-101(Fe)-NH2 were synthesized by using an

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aminated ligand, BDC-NH2. Since the MOF crystal size is directly proportional to the overall

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reaction time, the microwave reaction conditions needed to be optimized in order to produce nano-

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scaled MOF crystals. Thus, MIL-88(Fe) was synthesized as a function of microwave reaction time.

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The SEM images of the various MIL-88(Fe) products are summarized in Figure S2. Initially, the

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size of the MIL-88(Fe) crystals increased as the microwave irradiation time was lengthened. After

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15 mins, the average diameter of the MIL-88(Fe) crystals were stabilized at 500 nm which is most

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likely due to the depletion of starting materials. Since irradiation for 40 min produced the highest

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yield, we then used a 40 min microwave irradiation time for the synthesis of the other MOFs. The

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PXRD patterns of synthesized MOFs were consistent with the theoretically predicted PXRD

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patterns (Fig. S3); thus, the identities of MOFs used in this study were confirmed.

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The electro-Fenton catalysts were prepared by direct pyrolysis of the MOFs at 873 K for

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30 mins under a N2 atmosphere. The synthesized catalysts are denoted as CMIL-88, CMIL-100,

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CMIL-101, CMIL-88-NH2, and CMIL-101-NH2, for MIL-88(Fe), MIL-100(Fe), MIL-101(Fe),

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MIL-88(Fe)-NH2, and MIL-101(Fe)-NH2 respectively. SEM micrographs show that CMOFs

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retained crystal morphologies similar to that of parent MOFs (Fig. S4). PXRD pattern showed that

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the main iron containing species in the CMOFs is Fe3O4 (Fig. 2), except for CMIL-88-NH2 for

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which cementite (Fe3C) was also identified as an iron component. The compositions of the CMOFs

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were confirmed by energy dispersive X-ray spectroscopy (EDS). The results confirmed the

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relatively even distribution of Fe within the CMOFs (Fig. S5).

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The MOFs precursor and CMOFs were also analyzed by the FTIR; the results are shown

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in Figures S6 and S7. For MIL-88(Fe)-NH2, peaks at 3414 and 3370 are assigned to the symmetric

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and asymmetric stretching of the primary amine, while peaks at 1620 and 1392 cm−1 are assigned

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to the asymmetric and symmetric vibrations of the carboxyl groups, respectively. The absence of

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peak at 1700 cm-1 indicates that no uncoordinated BDC is present in MIL-88(Fe). In addition,

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doublet at 572, 487 cm-1 is most likely due to the Fe-O vibration in MIL-88(Fe). For CMIL-88, a broad

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peak at 3397 cm-1 is assigned to the O-H stretching vibrations of C-OH group. A newly formed

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peak at 1644 cm-1 is assigned to the vibration of C=C bonds in the graphitic carbon skeleton. A

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strong peak at 522 cm-1 is associated with the stretching vibration of Fe-O from the Fe3O4-NPs. These

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peaks are assigned based on previously reported assignments.23, 24 MIL-101(Fe)-NH2 and CMIL-

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101-NH2 gave similar FTIR spectra (Fig. S7). This result implies that the formation of graphitic carbon

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and Fe3O4 during MOFs pyrolysis is independent of the specific MOF topology.

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To prepare the PCM supported bi-functional electrochemical catalysts (CMOF@PCM),

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resorcinol-formaldehyde (RF) polymers were synthesized as a carbon precursor. The RF polymer

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was pyrolyzed at 1,073 K for 2 h under N2 atmosphere to produce PCM. After formation, the

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product PCM was impregnated with MOFs before carbonization at 873 K for 30 mins under N2.

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The MOFs loading amount was adjusted by changing the amount of the specific MOF mixed with

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PCM during impregnation. The MOFs loading of MOF@PCM catalyst precursor was fixed at 25

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wt%. Pyrolysis of MOF@PCM at 873 K for 30 mins under a N2 atmosphere produced mesoporous

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material designated as CMOF@PCM. SEM analysis indicates that the PCM substrate has an open

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porous network that is fully accessible to both electrolytes, substrates, and dissolved gas molecules

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(Fig. 3). In addition, SEM micrograph shows that the CMOF s were deposited inside the

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macropores of PCM material (Fig. S8). Based on these analyses, the loaded CMOFs have mean

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particle sizes ranging between 150 to 200 nm, 100 to 200 nm, 100 to 150 nm for CMIL-88@PCM,

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CMIL-100@PCM, and CMIL-101@PCM, respectively.

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Electro-generation of H2O2 by Porous Carbon Substrates

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The electrochemical activities of the PCM support for the oxygen reduction reaction (ORR)

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were investigated. Cyclic voltammetry (CV) was used to examine the oxygen reduction potential.

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As shown in Figure S9, the working electrode prepared from the PCM substrate was featureless

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for an Ar-saturated electrolyte solution, but showed a pronounced reduction peak in an O2-

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saturated electrolyte solution. The current density measured at the ORR peak was 2.3 mA/cm2. A

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similar value was found at pH 4, 7, and 10. The onset potentials at pH 4, 7, and 10 were 0.49,

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0.75, and 0.94 V vs. RHE, respectively. These results suggest the PCM substrate can

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electrochemically catalyze the ORR on site over a wide range of solution pH. Such a result bodes

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well for promoting the coupled Fenton reactions occurring under these conditions.

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Oxygen reduction taking place on graphitic materials are selective towards a two-electron

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reduction to produce H2O2 with a selectivity over 90%.25, 26 As a baseline, the amount of H2O2

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produced via O2 reduction on the PCM support was determined (Fig. 4). These results show that

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PCM substrate can produce H2O2 at 3 pH values which are consistent with the cyclic voltammetry

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results at each pH (Fig. S9). In addition, the rate of formation of H2O2 increases with increasing

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applied potential. However, once the applied potential exceeds to that of ORR peak potential, there

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is a declining increase in H2O2 productivity for all solution pH tested. This is due to a diffusion

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controlled limit for H2O2 release from the porous structure of PCM substrate. Thus, with the porous

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structure there is an elevated concentration of H2O2 that works against a further increase in H2O2

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production rate as measured in solution. The rates of H2O2 production on 6 mg of the PCM

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substrate at pH 4, 7, and 10 were 9.06, 9.41, and 9.48 mmol/L/h, respectively, which suggests

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solution pH has limited effect on its H2O2 productivity. In contrast, nonporous carbon material

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produced H2O2 at a rate of 4.41 mmol/L/h at pH 7 under similar reaction conditions.27 Since the

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oxygen reduction performance of carbon based materials is known to be unaffected by buffer,15

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this suggests solution pH can be easily maintained during the oxygen reduction reaction using

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PCM material.

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Electro-Fenton Degradation of Napropamide

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In order to demonstrate the feasibility of catalytically enhancing heterogeneous electro-

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Fenton reactions on CMOF@PCM, napropamide was selected as a model compound for testing

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the electro-Fenton process using CMIL-88@PCM. The initial concentration of napropamide was

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10 ppm in 50 mL 0.1 M Na2SO4; the effective surface area of the CMIL-88@PCM electrode was

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6 cm2. For comparison, we investigated the removal efficiency of napropamide via the E-Fenton

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process under multiple sets of conditions; these results are summarized in Figure 5. H2O2 alone

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was unable to degrade napropamide. The kinetics of napropamide removal using the CMIL-

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88(Fe)@PCM electro-Fenton process, electrosorption on CMIL-88(Fe)@PCM, electrosorption

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on PCM in an Ar-saturated 0.1 M Na2SO4 electrolyte, and electrosorption on PCM in an O2-

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saturated 0.1 M Na2SO4 electrolyte resulted in loss percentages of 75.5%, 35.4%, 14.6%, and

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16.0%, respectively. The electrosorption of napropamide by the PCM support material rapidly

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reaches equilibrium within 30 min due to the limited surface area of the PCM material. On the

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other hand, CMIL-88(Fe)@PCM removed substantially more napropamide by electrosorption due

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to the additional CMOFs embedded in the PCM support. The removal efficiency of napropamide

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on CMIL-88(Fe)@PCM in Ar saturated electrolyte is similar to that of the O2-saturated electrolyte

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at the presence of isopropanol (IPA) as a ·OH trap. This result suggests that ·OH is the primary

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oxidant generated during the electro-Fenton process.

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CMOF@PCM is a unique, multifunctional electrochemical catalyst in which the electro-

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Fenton reaction is catalyzed by CMOF-NPs while H2O2 is generated on site by the PCM support

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substrate. Because the electrochemical performance of CMOFs may be controlled by the MOFs

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precursors, various of MOFs with different topology including MIL-88(Fe), MIL-100(Fe) and

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MIL-101(Fe) were synthesized to investigate the MOFs topology effects on CMOF’s electro-

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Fenton performance. Among the MOFs, MIL-88(Fe) and MIL-101(Fe) were synthesized using an

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identical metal salt and a bidentate linker, BDC. In contrast, a tridentate linker, BTC, was used to

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synthesize MIL-100(Fe). The relative napropamide removal efficiency decreased in the following

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order: CMIL-100@PCM > CMIL-88@PCM > CMIL-101@PCM resulting in removal efficiencies

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of 82.3%, 64.1% and 60.56%, respectively, after 60 min of electrolysis (Fig. 6). The pseudo first-

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order kinetic plots versus time for napropamide degradation are shown in Figure S10. The

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corresponding pseudo first-order constants for napropamide removal were determined to be 1.70,

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0.99, and 0.87 h-1 for CMIL-100@PCM, CMIL-88@PCM, and CMIL-101@PCM, respectively

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(Table S2).

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In order to investigate the effects of MOF functional groups on CMOF@PCM electro-

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Fenton kinetics, amine functionalized MOFs using an aminated linker were synthesized. CMOFs

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were prepared from BDC and BDC-NH2. However, due to the steric hinderance of the tridentate

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trismic acid, CMIL-100-NH2@PCM was not prepared in the same fashion. A comparison of

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Napropamide electro-Fenton removal rates using CMOF@PCM and CMOF-NH2@PCM is given

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in Figure 7. The removal percentages after 60 min were 73.42%, 64.1%, 60.56%, and 53.63% for

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CMIL-88-NH2@PCM, CMIL-88@PCM, CMIL-101@PCM, and CMIL-101-NH2@PCM,

289

respectively. Again a pseudo first-order kinetic decay of napropamide was observed (Fig. S11)

290

with first-order constants of 1.26, 0.99, 0.87, 0.74 h-1 for CMIL-88-NH2@PCM, CMIL-88@PCM,

291

CMIL-101@PCM, and CMIL-101-NH2@PCM, respectively (Table S2).

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The effect loading levels of CMIL-100 on the observed electro-Fenton kinetics of

293

napropamide degradation was studied for samples that were prepared from mixing 10, 25, 50, 75

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wt% of CMIL-100 into the PCM support. These sample were designated as CMIL-100@PCM10,

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CMIL-100@PCM25, CMIL-100@PCM50, CMIL-100@PCM75, respectively. Napropamide

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degradation was carried out with CMIL-100 alone without PCM substrate for comparison. As

297

illustrated in Figure S12, the napropamide degradation percentage after 60 min is 11.8, 89.2, 97.0,

298

94.7, 92.6, and 91.3% for PCM, CMIL-100@PCM10, CMIL-100@PCM25, CMIL-100@PCM50,

299

CMIL-100@PCM75, and CMIL-100, respectively. Napropamide removal also follows apparent

300

pseudo first-order kinetics (Fig. S13). The first-order kinetic constants are summarized in Table

301

S2. The rate of napropamide degradation increased with the loading amount of CMIL-100 from

302

10 to 25 wt% but declined at higher doping concentrations. The maximum degradation efficiency

303

was obtained using CMIL-100@PCM25 (i.e., 97.0% after 60 min of electrolysis). In traditional

304

applications of the electro-Fenton process, contaminant removal efficiencies have been found to

305

increase with increasing amount of heterogeneous electro-Fenton catalyst until a maximum

306

efficiency is reached at higher loading levels.28 In these aforementioned systems, nonporous bulk-

307

phase Fe3O4 was used but there are limits in that non-surface accessible Fe(II) does not participate

308

in the Fenton reaction. Thus, a relatively low utilization efficiency of electro-Fenton catalyst

309

normally requires large amounts of Fe3O4 to be used in order to fully consume the added hydrogen

14


Page 14 of 30

Page 15 of 30


310

peroxide. In contrast, for CMOF@PCM, the main electro-Fenton active species is porous CMOFs.

311

The active form in our case is Fe3O4-NP that is embedded in graphitic carbon derived from the

312

specific organic linker. The highly uniform crystalline structure of the MOF precursors allows

313

even distribution of the Fe3O4-NPs throughout the CMOF-NPs after carbonization (Fig. S5).

314

Therefore, the embedded Fe3O4-NPs in the graphitic molecular support have similar local

315

coordination environments, which appear to be a prerequisite for maximizing the number of

316

electrocatalytically active sites.29 Since both the PCM support substrate and CMIL-100 NPs are

317

porous materials, they are able to maximize the interactions of H2O2 with the Fe3O4-NPs. However,

318

at a low loading level of CMOFs (e.g., CMIL-100@PCM5), the electro-Fenton reaction rate is

319

limited by the amount of Fe3O4 that was embedded in the multifunctional electrocatalysts,

320

CMOF@PCM. Low loading of Fe3O4 leads to an insufficient amount of Fe(II) available for the

321

reaction with H2O2 to produce a sufficient amount of ·OH radical. However, the electrochemical

322

reduction of O2 as catalyzed by the PCM substrate is the rate limiting step at higher loading levels

323

of CMOF in the form of CMIL-100@PCM75.

324

The impact of pH on napropamide degradation using CMIL-100@PCM was minimal as

325

shown in Figures S14. and S15. The corresponding pseudo first-order rate constants as given

326

Table S2 suggest the overall rate of degradation is somewhat higher at circum-neutral pH than at

327

low or high pH. The applied potential was set at -0.14 V, which is a potential that produces the

328

highest concentration of H2O2 (Fig. 4). Any pH dependence may be due to impact on the sorption

329

of napropamide onto the CMIL-100@PCM electrode.

330

Because actual wastewater has a wide range of conductivity, we have evaluated

331

napropamide removal efficiency at different electrolyte concentration. Figures S16 shows the

332

napropamide removal efficiency was slightly reduced at reduced electrolyte concentration. In

15



333

addition, Figure S16 suggests napropamide can be successfully removed by CMIL100@PCM in

334

simulated river water prepared according to Table S5.

335 336

Figure 8 shows that about 90% mineralization of napropamide can be achieved within 2 h of electrolysis via the electro-Fenton reaction as catalyzed by CMIL-100@PCM at pH 7.

337

The CMIL-100@PCM electro-Fenton was applied for the degradation of other herbicides

338

beyond (Fig. 9). The degradation of metolachlor, glyphosate, atrazine, acetochlor, and dichloprop

339

also followed apparent first-order kinetics (Fig. S17) with degradation percentages of 83.0, 78.5,

340

70.9, 91.1, and 76.2% after 60 min of electrolysis and kobs values of 1.50, 1.34, 0.99, 1.90, and

341

1.27 h-1, respectively. On the basis of reported electro-Fenton and Fenton degradation

342

mechanism,16 we propose a generalized mechanistic pathway for ·OH mediated degradation of

343

pesticides using CMOF@PCM electrodes (Fig. S18).

344

Reusability and Stability

345

The recyclability of the CMIL-100@PCM electro-Fenton catalyst has been assessed for

346

three consecutive cycles. No significant change to the napropamide degradation efficiency has

347

been observed (Fig. S19), demonstrating the excellent stability of CMOF@PCM. Over time

348

leaching of reactive Fe from CMIL-100@PCM may impact the durability of this system. However,

349

Figure S20 shows that only approximately 6.5 ppb of the Fe was detected in the solution, and the

350

Fe concentration remain unchanged throughout the reaction using CMIL-100@PCM. The initial

351

Fe concentration maybe due to the dissociation of CMIL-100@PCM that was loosely bonded to

352

the carbon paper substrate by nafion binding agent. In comparison, significant Fe leaching has

353

been observed for Fe3O4@PCM, which consists Fe3O4-NP embedded on PCM substrate. The

354

concentration of leached Fe in the electrolyte had increased linearly and reached about 200 ppb

355

after 2 h reaction.

16


Page 16 of 30

Page 17 of 30

356


The Primary Basis for High E-Fenton Reactivity

357

In order to increase the electrochemical catalytic efficiency, it is important to be able to

358

access the largest number of surface active sites that are capable of participating in an

359

electrocatalytic reaction. However, the reactivity of the electrochemical active sites is affected by

360

both the surface structure of the catalyst and its interactions with molecules or molecular moieties

361

close to the site of catalysis. For example, heterogeneous catalytic reactions are dependent on the

362

catalytically active surface area. In our system, it is difficult to accurately determine the actual

363

electro-chemically active surface area of the Fe3O4-NPs embedded within the porous carbon

364

matrix due to the dielectric behavior of magnetite. Thus, Brunauer-Emmett-Teller (BET) surface

365

area measurements are used as a surrogate. BET measurements of the CMOF surfaces were

366

determined at 77 K by N2 adsorption with the results summarized in Table S3. For non-

367

functionalized CMOFs, the BET surface areas ranged from 287.9, 340.9, and 361.6 m2/g for

368

CMIL-88, CMIL-100, and CMIL-101, respectively. Hysteresis (type H3) between adsorption and

369

desorption branches at medium pressure (P/P0 = 0.4) were observed for both CMIL-88 and CMIL-

370

101 (Fig. S21). This behavior is consistent with the presence of wide distribution of mesopores.

371

In contrast, CMIL-100 gaves a type H4 isotherm, which suggests the presence of narrow slit-like

372

pores that are dominated by micropores with a limited number of mesopores. In fact, the pore size

373

distribution (Fig. S22) showed that the average pore diameters were 13.12, 4.85, 13.65 nm for

374

CMIL-88, CMIL-100, and CMIL-101, respectively. This result indicates that both the BET surface

375

areas and the pore structures of CMOFs are dependent on the linker and core metal ion rather than

376

the topology of parent MOFs. The BET surface areas of the CMOFs synthesized from the aminated

377

MOFs are given in Table S3. These results show that amine functionalization resulted in slightly

378

lower BET surface areas for CMIL-88-NH2 and CMIL-101-NH2 of 217.88 and 211.85 m2/g,

17



379

respectively. Both CMIL-88-NH2 and CMIL-101-NH2 gave type H4 BET isotherms (Fig. S23)

380

that indicated that micropores control the pore structure. The average pore diameter for CMIL-88-

381

NH2 and CMIL-101-NH2 were determined to be 8.50 and 11.10 nm, respectively (Fig. S24). These

382

results clearly suggest that amine functional groups in the linker lead to a reduction in pore

383

diameters of the CMOFs. These results are also consistent with the location of amine functional

384

group in the MOFs precursor, which is at the edge of the aperture (Fig. S25). These observations

385

highlight the importance of BET surface area in evaluating the reactivity of electro-Fenton

386

catalysts. Furthermore, the results presented in Table S3 also show that the CMOF@PCMs have

387

greater BET surface area than CMOFs without PCM substrate. This is consistent with the higher

388

BET surface area of PCM. In light of these results, we have normalized the pseudo first- order rate

389

constants to the BET surface areas (kSA) to give surface area normalized heterogeneous rate

390

constants. However, since the BET surface area of the PCM substrate is the same for all of the

391

CMOF@PCMs, the BET surface areas of the CMOFs were used instead of CMOF@PCMs for

392

normalization. These results are summarized in Table S4, which show that the normalized rate

393

constants, kSA, follow the order of CMIL-88-NH2@PCM > CMIL-100@PCM > CMIL-101-

394

NH2@PCM = CMIL-88@PCM > CMIL-101@PCM.

395

The normalized rate constants for the electro-Fenton reactions show that the catalytic

396

activity of the CMIL-88-NH2@PCM catalyst is almost twice that of the CMIL-88@PCM in spite

397

of a lower BET surface area (Fig. S26). The PXRD spectra indicate that the CMIL-88 NPs are

398

present primarily as Fe3O4 without an additional phase (Fig. S27). This result is consistent with

399

the high-resolution TEM images that clearly identify nano-particulate Fe3O4 in Figure S28. The

400

observed lattice lines have a lattice spacing of 4.90 Å, which is consistent with the (110) plane of

401

Fe3O4. In contrast, the CMIL-88-NH2 NPs TEM images show the presence of two iron phases,

18


Page 18 of 30

Page 19 of 30


402

Fe3O4 and Fe3C. The high-resolution TEM images shown in Figure 10 show that the CMIL-88-

403

NH2 NPs are uniformly dispersed within the graphitic PCM substrate. CMIL-88-NH2 has a unique

404

core-shell structure. The graphic carbon shell thickness is ∼2 nm, while a lattice spacing of the

405

intermediate layer of 2.38 Å, which corresponds to the (210) plane of Fe3C. The core is Fe3O4 with

406

a lattice spacing of 2.53 Å. These results are also consistent with the PXRD spectrum (Fig. S27).

407

The difference in electro-Fenton reaction performance between CMIL-88-NH2@PCM and

408

CMIL-88@PCM is most likely due to the difference in their structures. In the case of CMIL-

409

88@PCM, Fe3O4 NPs are imbedded in the carbon substrate matrix. The unit cell for Fe3O4 has

410

cubic inverse spinel structure, each containing 8 Fe atoms occupying Wyckoff position 8a and 16

411

Fe atoms occupying Wyckoff position 18d in space group Fd3m (Fig. S29a). The Fe atoms are

412

directly accessible by the H2O2 for electro-Fenton reaction. In terms of CMIL-88-NH2@PCM,

413

core-shell structured NPs are imbedded in the graphitic carbon substrate. Within the core-shell

414

nanoparticulate structures, the Fe3C interfacial layer is sandwiched between the graphitic carbon

415

shell and Fe3O4 core. Fe3C has orthorhombic structure in a Pnma space group. Each unit cell

416

contains 12 Fe atoms and 4 C atoms. 8 Fe atoms occupy Wyckoff position 4c and the remaining

417

Fe atoms occupy Wyckoff position 8d (Fig. S29b). Compared to the base elemental metal, the

418

formation of a metal carbide (Fe3C) leads to a lengthening of the metal-metal bond distance due

419

to presence of carbon. As the result, a metal d-band contraction occurs, generating more density

420

of states (DOS) near the Fermi level.30-32 Therefore, metal carbides are quasi electron conductors.

421

Incorporation Fe3C as an intermediate layer within CMIL-88-NH2@PCM composite material

422

improves the electro-Fenton reactivity by: (1) facilitating the electron transfer between the

19



423

graphitic carbon shell and Fe3O4 core, (2) increasing the catalytic reactivity by reducing the lattice

424

mismatch between the graphitic carbon shell and Fe3O4 core,33 and (3) by fixing in three-

425

dimensions Fe(II) species within the Fe3O4 core as well as locking Fe atoms within Fe3C. The

426

latter effect essentially prevents Fe(II) leaching into solution and provides for long-term stability

427

of the electrode. This is confirmed by Fe leaching measurement from the composite catalyst (Fig.

428

S20).

429

In conclusion, the degradation of a set of herbicides has been achieved using an electro-

430

Fenton process based on multifunctional CMOF@PCM catalysts. In this version of an electro-

431

Fenton reaction system, porous carbonized Fe-based metal organic framework compounds

432

(CMOFs) optimize the availability of Fe(II) sites to initiate the initial oxidation of Fe(II) by H2O2

433

to Fe(III) and ∙OH. The reverse reaction, reduction of Fe(III) is achieved by the negative applied

434

potential on the composite carbon substrate electrode (Fig. S18). A low applied potential (-0.14

435

V) combined with a broad operational pH range may make this particle electro-Fenton reaction

436

system cost-effective for the degradation and mineralization of a wide range of herbicides or other

437

oxidizable organic chemical water contaminants.

20


Page 20 of 30

Page 21 of 30


438

Associated Contents

439

Supporting Information

440

Includes SEM, TEM images, XPS spectrum, and kinetic study.

441 442

Author Information

443

Corresponding Author

444

*(Michael

445

[email protected]

R.

Hoffmann)

Tel:

+1-626-395-4391.

Fax:

+1-626-395-4391.

E-mail:

446 447

Notes

448

The authors declare no competing financial interest.

449 450

Acknowledgments

451

This work was supported by the Bill and Melinda Gates Foundation (Grant OPP1149755),

452

National Natural Science Foundations of China (Grant No. 21777137), International Cooperation

453

Grant of China (Grant No. 21320102007), and the Caltech Rosenburg Innovation Initiative.

21



455

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Page 26 of 30

Figure 1. Structure of (a) MIL-88(Fe), (b) MIL-100(Fe), and (c) MIL-101(Fe).

555 (a)

(b)

(c)

556 557 558

Figure 2. PXRD spectrum of CMOFs used in the current study.

CMIL-101-NH2 CMIL-101 CMIL-100 CMIL-88-NH2 CMIL-88 Simulated Fe3O4 Simulated FeC3 Simulated Fe0 20

30

40

50

60

70

80

90



2  

559 560

Figure 3. SEM micrographs of (a) CMIL-88@PCM, (b) CMIL-100@PCM, and (c) CMIL-

561

101@PCM. (a)

(b)

(c)

26


Page 27 of 30

562


400 nm

563

Figure 4. The concentration of H2O2 produced by 6 mg of PCM substrate at (a) pH 4, (b) pH 7,

564

and (c) pH 10.

565

(b) 0.08V -0.12V -0.32V

9 6 3 0

0

30 60 Time (min)

90

(c) 0.26V 0.06V -0.14V

12 9 6 3 0

0

30 60 Time (min)

90

H2O2 Concentration (mmol/L)

12



(a)

0.42V 0.24V 0.04V

12 9 6 3 0

0

30 60 Time (min)

90

566 567 568 569 570

Figure 5. Napropamide (10 ppm) removal by H2O2 oxidation (400 ppm), PCM substrate (-0.14

571

V), and CMIL-88@PCM (-0.14 V). All experiments were performed in 0.1 M Na2SO4

572

electrolyte at pH 7.

573

27



1.00

C/C0

0.75 H2O2 PCM (O2) PCM (Ar2) CMIL-88@PCM (O2) CMIL-88@PCM (Ar2) CMIL-88@PCM (O2)/IPA

0.50 0.25 0

30 60 Time (min)

574

90

575 576

Figure 6. Napropamide (10 ppm) removal by electro-Fenton using CMOF@PCMs prepared

577

from MIL-88(Fe), MIL-100(Fe), and MIL-101(Fe). All experiments were performed in 0.1 M

578

Na2SO4 electrolyte at pH7 (-0.14 V).

579 1.0

CMIL-101@PCM CMIL-100@PCM CMIL-88@PCM

C/C0

0.8 0.6 0.4 0.2 0

580

15

30 45 Time (min)

60

581 582 583 584

Figure 7. Napropamide (10 ppm) removal by electro-Fenton using CMOF@PCMs and CMOFs-

585

NH2@PCMs (0.1 M Na2SO4, pH7, -0.14 V). 28


Page 28 of 30

Page 29 of 30


586 CMIL-101@PCM CMIL-101-NH2@PCM CMIL-88@PCM CMIL-88-NH2@PCM

1.0

C/C0

0.8 0.6 0.4 0.2

0

15

587

30 45 Time (min)

60

588

Figure 8. TOC removal efficiency of electro-Fenton removal of 10 ppm napropamide by CMIL-

589

100@PCM (0.1 M Na2SO4, pH 7, -0.14 V).

590

1.0

C/C0

0.8 0.6 0.4 0.2 0.0

591

0

30

60 90 120 150 180 Time (min)

592 593 594 595 596

Figure 9. Electro-Fenton degradation of metolachlor, glyphosate, atrazine, acetochlor, and

597

dichlorprop (10 ppm) by CMIL-100@PCM (0.1 M Na2SO4, -0.14 V, pH 7). 29



598

Metolachlor Glyphosate Atrazine Acetochlor Dichlorprop

1.00

C/C0

0.75 0.50 0.25 0.00

599

30 45 60 Time (min)

600

Figure 10. (a) TEM images of CMIL-88-NH2@PCM, (b) high-resolution TEM image of CMIL-

601

88-NH2@PCM core-shell structure. (a)

0

15

75

(b)

602 603

30


90

Page 30 of 30

Multiphase Porous Electrochemical Catalysts Derived from Iron-Based

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