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Selective Nitrate Reduction to Dinitrogen by Electrocatalysis on Nanoscale Iron Encapsulated in Mesoporous Carbon Wei Teng, Nan Bai, Yang Liu, Yupu Liu, Jianwei Fan, and Wei-Xian Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04775 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 8, 2017

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Environmental Science & Technology

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Selective Nitrate Reduction to Dinitrogen by Electrocatalysis on Nanoscale

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Iron Encapsulated in Mesoporous Carbon

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Wei Teng,† Nan Bai, † Yang Liu,‡ Yupu Liu, ‡ Jianwei Fan† and Wei-xian Zhang*,†

5 6 7 8 9

†

State Key Laboratory for Pollution Control, School of Environmental Science and

Engineering, Tongji University, Shanghai, China 200092 ‡

Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and

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Innovative Materials, and Advanced Materials Laboratory, Fudan University, Shanghai,

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China 200433

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ACS Paragon Plus Environment


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ABSTRACT

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Excessive nutrients (N and P) are among the most concerned pollutants in surface and

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ground waters. Herein, we report nanoscale zero-valent iron supported on ordered

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mesoporous carbon (nZVI@OMC) for electrocatalytic reduction of nitrate (NO3-) to nitrogen

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gas (N2). This material has a maximum removal capacity of 315 mg N/g Fe and nitrogen

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selectivity up to 74%. The Fe-C nanocomposite is prepared via a postsynthetic modification

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including carbon surface oxidation, in-situ ammonia prehydrolysis of iron precursor and

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hydrogen reduction. The synthesized materials have large surface areas (660 – 830 m2/g) and

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small iron nanoparticles (3 – 9 nm) uniformly dispersed in the carbon mesochannels. The iron

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loading can be adjusted in the range of 0 to 45%. Results demonstrate that the reaction

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reactivity of electrocatalysis can be fine-tuned by manipulating iron nanoparticle size, degree

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of crystallization, as well as porous structure. Meanwhile, the small, uniform, and stable iron

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nanoparticle promotes fast hydrogen generation for rapid cleavage of the N-O bond.

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Furthermore, this material can maintain its high performance over repetitive experimental

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cycles. Results suggest a new approach for fast and eco-friendly nitrate reduction and a novel

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nZVI application.

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TOC

31 32 33


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INTRODUCTION

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Nitrate is ubiquitous in nature waters. Excessive nitrate leads to eutrophication and algae

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bloom.1 Together with phosphorus, nutrients are the top pollutants in surface waters in China

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and to a large degree among the most significant water pollutants globally.2-5 A common

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approach for remediating water contaminated with nitrate is via denitrification, i.e.,

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conversion of nitrate to nitrogen gas (N2) by biological and chemical/catalytic reduction.5

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Biological denitrification is effective for removing nitrate through a stepwise formation of N2.

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However, this process is slow and sensitive to varying treatment conditions such as dissolved

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oxygen, temperature and dissolved organic matters.6 Chemical reduction of nitrate can also

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be achieved by using metallic powder (e.g., Fe,7,

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excessive generation of ammonium rather than nitrogen, and additional pH adjustment.

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Meanwhile, catalytic reduction garners increasing attentions, but this process requires

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hydrogen gas as a reductant or electron donor, and transport and use of pressurized hydrogen

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have safety issues, which has limited the application in large scale.10 Combined with the

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advantages of no chemical input, high-efficiency catalysis, ambient operating conditions and

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minimal sludge generation, electrocatalysis is a promising method for denitrification.11, 12

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Al9). This approach often results in

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Various anode electrodes including Cu, Ni, Sn, Ti and Pt have been investigated for the

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nitrate electroreduction.11, 13, 14 Typical catalytic mechanism is known that a promoter metal,

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such as Cu, Sn or In,15-17 transforms nitrate to nitrite, which is then reduced to nitrous oxides,

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nitrogen and ammonium by use of the precious metal (Pd or Pt).6, 18-20 Understandably, the

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use of precious metal as catalyst comes with high costs. Recently, nanoscale Zero-Valent Iron

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(nZVI) has been extensively documented for effective transformation of a wide variety of

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pollutants in water21-23 including various reports on nitrate reduction,7, 24 due to its excellent

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electron-donating capability. Many studies suggest that iron plate can be considered as a

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non-precious metal catalyst for electrochemical reduction of nitrate.25 Mechanism is

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nonetheless unknown concerning electrocatalysis over the iron nanoparticles for nitrate

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reduction. Virtually all work published so far has focused on Fe(0) as a reducing agent.

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Catalytic reduction capacity and selectivity for nitrogen gas generation are vital factors for

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nitrate reduction, which depend on the catalyst properties, e.g., metal loading, metal ACS Paragon Plus Environment


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nanoparticles size, crystallization and electronic properties of the catalyst support.6, 10 Many

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studies reported that the nanoparticles can be dispersed onto various solid supports to

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enhance the catalytic performance, including conventional activated carbon,26 alumina,27

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silica28 and novel supports, such as TiO2,29 ZrO2,30 CeO2,30 graphene oxide32 and so on.

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Support plays an important role in stabilizing nanoparticles and minimizing particle size to

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create more active sites. For common support, it is difficult to control the size and uniform

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dispersity of metal nanoparticles by reasons of irregular pore structures.33-35 The ordered

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mesoporous carbon is a promising support material due to its unique advantages, involving

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uniform pore size (2 – 50 nm), regular mesostructure, good conductivity and large surface

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area and pore volume for metal loading36-39.

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Herein, we designed and demonstrated an electrocatalytic denitrification using mesoporous

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carbon supported nanoscale zero-valent iron as catalyst for efficient nitrate reduction

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(Scheme 1). The novel nanocomposites (nZVI@OMC) are prepared via post-synthetic route.

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Uniform and ultra-small iron nanoparticles can be well-dispersed into the carbon matrix, and

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the amount of metal loading, iron particle size and constituents are regulated by varying the

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amount of iron precursor and hydrogen reduction temperature. The nanocomposites were

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used as electrode materials for electrocatalytic reduction of nitrate in sodium chloride

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electrolyte. By additional voltage, the whole reaction carried out in a mild and controllable

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condition, which is better than direct hydrogen sparging in terms of operational safety. We

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evaluated the reduction ability and selectivity of nitrogen on a number of nanocomposites

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with varied nanoparticle sizes, loadings and constituents. The long-term performance was

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investigated through multiple electrocatalytic cycles. Furthermore, the reduction mechanisms

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were proposed and verified.

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EXPERIMENTAL SECTION

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Preparation of nZVI@OMC Materials

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Ordered bimodal mesoporous carbon as a matrix was prepared first according to the

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method reported by Liu et al.40 The obtained mesoporous carbon was processed by a surface


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oxidation in a 1.0 M of acidic ammonia persulfate solution (in 2 M H2SO4) at 60°C for 3 h to

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obtain a surface oxygen-containing mesoporous carbon.

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A series of composite materials of nZVI@OMC were then prepared by impregnation of

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iron precursor into the surface oxygen-containing mesoporous carbon and followed with

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ammonia-atmosphere prehydrolysis and thermal reduction in a H2 atmosphere. Typically, 0.6

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g of the oxygen-containing mesoporous carbon was dispersed in 8 mL of ethanol. Iron

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precursor Fe(NO3)39H2O (0.9 g) was dissolved in ethanol (8 mL) and then added into the

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carbon ethanolic solution for continuous stirring at room temperature. Until solvent

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evaporation, the sample was dried at 60°C under vacuum, and then put into a glass tube,

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which was in a teflon bottle filled with 10 mL of NH3H2O (14 wt. %), to avoid direct contact

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between sample and NH3H2O. The bottle was sealed and placed at 60°C for 3h. After being

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washed with deionized water and ethanol to remove the generated NH4NO3, the collected

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sample was dried at 100°C overnight. Finally, the sample was heated and reduced at 300 -

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500°C for 2 h under H2 atmosphere in a tube furnace and the composites of nZVI@OMC

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were obtained (referred as nZVI@OMC-T). Moreover, the nanocomposites with different

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iron contents were prepared by adjusting the amount of iron precursor (all heated in H2 at

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400°C), labeled as nZVI@OMC-x%, where x is the iron content.

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Reduction of Nitrate

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A 200 mg/L of sodium nitrate stock solution was prepared. Different volumes of the stock

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solution were added into a beaker to get 50 mL of reaction solution with various

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concentrations (20-200 mg/L). NaCl was added (0.02 M) as the electrolyte. All the

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electrochemical tests were performed on a CHI 660D electrochemical analyzer system

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(Shanghai) at -1.3 V for 24 h. At a predetermined time, about 2 mL of solution was taken out

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for analysis of the concentration of nitrate, nitrite, and ammonium ions.

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Platinum and saturated calomel electrodes were used as the counter and reference

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electrodes in a three-electrode cell, respectively. The material of working electrode was

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prepared by a mixture of nZVI@OMC composites, acetylene black and polyvinylidene

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fluoride (dissolved in N-methyl-2-pyrrolidone, 10 g/L) with the ratio of 8:1:1. Then, the



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mixture was coated on a nickel foam (1 × 1 cm), followed by dried at ~ 50°C for 6 h and then

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~ 120°C under vacuum for 12 h. The final working electrode sheet was obtained by pressing

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the prepared nickel foam under 20 MPa.

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Characterization

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Transmission electron microscopy (TEM) images were taken by a JEOL JEM 2011 or a

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JEM 2100F microscope at 200 kV. The samples for TEM analysis were first dispersed in

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ethanol and then dropped onto carbon films supported on Cu or micro grids. X-ray diffraction

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(XRD) patterns were carried out with a D8 X-ray diffractometer using Ni-filtered Cu Kα

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radiation (40 kV, 40 mA), in a scanning range of 20 – 80°. Nitrogen sorption isotherms were

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measured with a Micromeritics Tristar 3020 analyzer at 77 K. Before the measurements, the

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samples were degassed under vacuum at 120°C for at least 8 h. The concentration of leaching

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metal iron in aqueous solution was analyzed by Inductive coupled plasma-atomic emission

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spectrometer (ICP-AES, Agilent 720ES). More details in characterization are listed in

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supporting information.

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Analytical Methods and Data Analysis

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During the nitrite reduction, different products are formed including nitrite, nitrous oxide,

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nitrogen and ammonium. The gas products such as nitrous oxide and nitrogen are sparged

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from solution during reaction. Previous literature reported that nitrous oxide eventually

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reduced to dinitrogen exclusively with excess hydrogen in a sealed batch reactor.6,

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Therefore, in our experiments, the generated gases were regarded as nitrogen. The

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concentration of nitrate, nitrite and ammonium was detected by UV-Vis spectrophotometer.

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The detailed procedures based on previous literature.42 Nitrate removal capacities (Q) of

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different materials was calculated by the following equation: Q=(C0-Ct)V/mFe. Where C0

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(mg/L) is the initial concentration, Ct (mg/L) is the concentration at time t, V (L) is the

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volume of nitrate solution, and mFe (g) stands for the mass of iron in nanocomposites coated

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on the nickel foam.

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The selectivity was evaluated by the equation:


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S (%) = 149

∆C ( NO3− ) − ∆C ( NO2− ) − ∆C ( NH 4+ ) ×100 ∆C ( NO3− )

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Where ∆C ( NO3− ) , ∆C ( NO2− ) and ∆C ( NH 4+ ) are the absolute difference of nitrate, nitrite

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and ammonium concentration before and after reaction, respectively. All reduction

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experiments were performed at least three times.

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RESULTS AND DISCUSSION

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Properties of the Nanocomposites

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Ultra-small zero-valent iron nanoparticles in the carbon mesochannels with high iron

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content are obtained by following steps (Figure 1A): (i) A bimodal mesoporous carbon matrix

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is selected and surface-oxidized for an easy loading of metal precursor (Fe(NO3)39H2O) into

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the mesopores. (ii) The precursor is first converted into hydroxides by the in-situ hydrolysis

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under ammonia atmosphere to localize the upcoming metal nanoparticles without aggregation.

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(iii) The sample is subsequently treated at 300 - 500°C in hydrogen leading to nucleation,

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growth and reduction of metal nanoparticles. Transmission electron microscope (TEM)

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images (Figure 1B-D) show ordered stripe-like carbon frameworks and very small

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nanoparticles are uniformly confined in the mesochannels. With the hydrogen reduction

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temperatures increasing from 300 to 500°C, the sizes of the nanoparticles gradually become

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larger (~ 3.8, 6.2 and 8.9 nm), but still uniformly distributed across the whole carbon matrix.

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The size of nanoparticles is too small to be observed in the composite nZVI@OMC-8% with

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low iron content (Figure S1a), whereas the nanoparticles grow up to larger ones (> 20 nm) in

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nZVI@OMC-45% with high content by twice impregnation of iron precursor (not shown in

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article). High-resolution transmission electron microscopy (HRTEM) image clearly shows

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separated nanoparticles with ultra-small particle sizes (5 - 6 nm) uniformly distributed across

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the whole mesostructured matrix (Figure 1E). The nanoparticle is crystallized to some extent

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with the lattice fringe of ~ 2.02 Å, matched to the d110 of Fe phase (Figure 1E inset). Energy

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dispersive spectroscopy (EDS) spectrum displays the obvious signal of Fe, further

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conforming the existing of iron (Figure S1b). Field-emission scanning electron microscopy

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(FESEM) images (Figure S2) exhibit ordered mesostructures in large domains with fully



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open pore channels on the surface of carbon matrix without any large particles outside of the

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mesopores. The dispersed nZVI nanoparticles are elucidated by elemental mapping, further

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displaying uniformly distributed iron across the matrix (Figure S3). Additionally, surface

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sensitive X-ray photoelectron spectra (XPS) of the nZIV@OMC exhibits weak peak of iron,

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demonstrating that most of the metal nanoparticles are located in the carbon frameworks

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(Figure S4).

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Wide-angle XRD patterns of the nZVI@OMC nanocomposites at different temperatures in

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hydrogen reduction show an evolution of the nanoparticles lattice (Figure 1F). At 300°C, no

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obvious diffraction peaks suggest that the generation of iron nanoparticle is small and

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well-dispersed. When the nanocomposite is treated at 400°C, the characteristic diffraction

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peaks at 36 and 44° appear, indexed to the maghemite phase (JCPDS 19-0629) and

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body-centered cubic α-Fe (JCPDS 06-0696), respectively. After the treatment of hydrogen

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reduction at 500°C, the intensity of the diffraction peak enhances, indicating a better

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crystallization. On the basis of the Debye-Scherrer formula, the crystalline sizes of the nZVI

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nanoparticles are ~ 5.8 and 10.1 nm for nZVI@OMC-400 and 500, in accordance with the

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TEM results. Furthermore, the XRD pattern (Figure S5) shows that the peak at around 44° is

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still present in a sample exposed in air for 8 months, suggesting that the nZVI dispersed in the

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mesopores is more stable than that bare nZVI.

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Thermogravimetric (TG) curves (Figure S6) in air flows show the change in weight with

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the increased temperature. Notably, a rapid rise occurs at 200-400°C, implying that

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zero-valent iron reacts with oxygen to generate iron oxides. The rising percentage is

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correlated to the iron content. The following stage at 400-550°C shows a fast weight loss,

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mainly attributed to the carbon frameworks combustion. The platform after 550°C with no

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obvious change in the weight indicates the carbon is completely removed and only iron

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oxides left. Based on the results, the iron contents in the nanocomposites can be inferred

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about ~8, 25 and 45% for nZVI@OMC-8%, -400 and -45%, respectively.

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The SAXS patterns (Figure S7) of the nanocomposites nZVI@OMC present three

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scattering peaks assigned to the 10, 11 and 21 reflections, indicating a highly ordered

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hexagonal mesostructure (space group p6m). N2 sorption-desorption isotherms of all the

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mesoporous composites nZVI@OMC show typical type-IV curves with distinct hysteresis ACS Paragon Plus Environment

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loops, indicating similar pore structures and features to the pristine carbon matrix (Figure

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S8A). The decrease of the surface area from ~ 1970 to 660, 770, 830, 1410 and 210 m2/g and

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pore volume from ~ 1.5 to 0.41, 0.52, 0.57, 1.16 and 0.1 cm3/g is observed for the

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nanocomposites nZVI@OMC-300, 400, 500, -8% and -45% compared with those of the

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carbon matrix (Table S1), respectively. Nevertheless, the synthesized materials still possess

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high surface areas and large mesoporosities. The pore size distribution curves show the

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primary mesopores decrease from 6.0 to 5.2 - 4.6 nm due to the occupation of the iron

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nanoparticles and no changes in the secondary mesopores (1.8 nm) after the iron loading

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(Figure S8B). As a result, the proper iron content can maintain superb pore features for

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application.

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Nitrate Reduction

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To identify the electrocatalytic efficiency and selectivity of nitrogen, a series of

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nanocomposites are synthesized and evaluated as cathode materials for nitrate reduction

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(Figure 2). Compared with the composites nZVI@OMC pyrolyzed and reduced at different

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temperatures (300-500°C), the sample nZVI@OMC-400 shows the maximum nitrate removal

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about 315 mg N/g Fe and the highest nitrogen selectivity of 74%. This phenomenon can be

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explained that when the nZVI@OMC treated at 300°C, the iron precursors is only partially

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converted to nZVI, and thus the removal capacity is only 155 mg N/g Fe. Whereas the

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temperature increased to 500°C, although the degree of crystallization increases a lot, the iron

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particles grow up to larger ones (~8.9 nm) resulting in a decline in surface active sites, and as

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a result the removal capacity reduces to 230 mg N/g Fe. Meanwhile, the performances of the

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nanocomposites

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nZVI@OMC-400-8% with low iron content shows a relative lower removal of 65 mg N/g Fe.

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Although this material has a large surface area and pore volume, the iron content is too low to

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offer enough active sites, resulting in low overall removal efficiency. When the iron content is

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too much (~ 45%), the removal ability of nZVI@OMC-400-45% reaches ~ 200 mg N/g Fe,

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but still lower than that of nZVI@OMC-400 with 25% of iron, mainly because too much

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filling of iron to decrease the surface area (210 m2/g) and pore volume (0.1 cm3/g), leading to

236

obstruction and afterwards that a large number of active iron sites are unavailable in the

with

different

iron

contents

are

also


compared.

The

sample


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internal mesochannels. The results suggest that the suitable nanoparticle size and

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crystallization of iron, and proper pore structural property guarantee sufficient catalytic sites

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for excellent reduction of nitrate. In addition, the mesoporous carbon loaded with iron oxide

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nanoparticles is used for comparison. With the same iron content, the sample iron

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oxide@OMC-400 shows relatively lower removal capacity of ~ 120 mg N/g Fe. That iron

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species being reduced to zero valence rather than oxide is a key factor for the nitrate

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reduction by electrochemical catalysis. Finally, the mesoporous carbon matrix without any

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metal loading (OMC) is employed as electrode material. The removal capacity of nitrate is

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very low, only 20 mg N/g C. Meanwhile, the nickel foam without any coating shows

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negligible effect on the removal of nitrate (from initial 50 mg/L to final 48.7 mg/L in 24

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hours). Results further suggest that the nitrate reduction is mainly contributed to the

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zero-valent iron but not the carbon matrix or nickel foam. The bare nZVI (about 60 nm) is

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prepared by sodium borohydride of Fe(III) salts for comparison. Removal capacity of about

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376 mg N/g Fe and lower nitrogen selectivity (~ 43%), indicating that the mesoporous carbon

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as the carrier plays a very important role to disperse the nZVI nanoparticles from growth and

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aggregation, thereby increasing the surface area and active sites for enhanced performance.

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Additionally, the same system without electrolysis is tested. It is found that the nitrate is not

254

removed, indicating that the adoption of the electrolysis is needed.

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Among these materials, small differences of nitrogen selectivity (65-75%) are displayed

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when the nZVI@OMC nanocomposites treated at or above 400°C in hydrogen, whereas the

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sample nZVI@OMC-300 shows only 40% (Figure 2b). This demonstrates that the treated

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temperature is important for nitrogen selectivity, mainly due to the better reduction and

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crystallization of iron nanoparticles at higher temperature. Moreover, the selectivity of iron

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oxide@OMC-400 is only ~ 35%, further certifying the importance of zero valent iron

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nanoparticle for the nitrate reduction.

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The evolution of nitrate, nitrite, ammonia, nitrogen and its selectivity with reaction time is

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demonstrated during the electrochemical catalysis on the nZVI@OMC-400 (Figure 3). The

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concentration of nitrate gradually decreases with final removal efficiency of 65%. Reduction

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products including nitrite, ammonium and nitrogen generate accordingly and their amounts

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increase gradually. The contents of NH4+ increase at the beginning, reach the maximum about ACS Paragon Plus Environment

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12 h and then decrease to ~10%. More than half products are nitrogen gas and its selectivity

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from initial 45% to the final ~ 74%. The tendency suggests that nitrate is mainly reduced to

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NH4+ at first, and gradually transformed to nitrogen. During the process, the concentration of

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the iron ion in the solution is detected to determine the dissolution of nZVI nanoparticles in

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the nanocomposite. The dissolvable iron can be negligible (< 1%, initial pH 6.8), implying

272

that the obtained electrons in the process of nitrate reduction is not directly derived from the

273

nZVI, which is very different from the traditional chemical redox through iron consuming,

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but as a catalyst to speed up the conversion of nitrate into nitrogen.

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The effect of the nitrate concentration on the removal efficiency is evaluated by using the

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nanocomposite nZVI@OMC-400. Results (Figure 4a) show that the removal capacity

277

enhances from ~ 130 to 920 mg N/g Fe as the initial concentration of nitrate increases from

278

20 to 200 mg/L. In order to assess the long-term performance, the sample nZVI@OMC-400

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is employed for reduction of nitrate several times. Results exhibit that the nitrate removal and

280

the nitrogen selectivity decrease somewhat after 5 times of recycling (Figure 4b). But the

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overall removal capacity remains at over 270 mg N/g Fe (< 20% drop), and nitrogen

282

selectivity is still around 60%. The nanocomposites used repeatedly can be considered as a

283

catalyst and the iron nanoparticles are not consumed in the reduction process, consistent with

284

the result that the soluble iron is low.

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After electrocatalysis, the original structure of the nZVI@OMC-400 remains from TEM

286

imaging observation (Figure S9). The XPS spectra of Fe(2p) exhibit three peaks at 707, 710.6

287

and 724 eV, corresponding to Fe(0), Fe(II) and Fe(III), respectively (Figure S10). The Fe(II)

288

increases from 49 to 61%, indicating that Fe(0) is partially oxidized during the

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electrocatalysis. Because XPS is surface sensitive, the majority of iron nanoparticles are

290

protected in the mesochannels and few exposed to bulk solution, which may have

291

over-estimated the percentages of Fe(II) and Fe(III). The XRD patterns were carried out by

292

the electrode slice after use because of very little amount of materials (2-3 mg) are coated

293

(Figure S11). Despite the presence of Ni, the main peaks of the material are almost

294

maintained, indicating little compositional change of iron. Results suggest that generated H2

295

can protect the nZVI from oxidation and reduce the Fe(II) to regenerate Fe(0).

296

As aforementioned the electrochemical catalysis, by additional voltage on the ACS Paragon Plus Environment


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nZVI@OMC nanocomposites, nitrate can be easily and effectively removed and converted to

298

harmless nitrogen without iron consumption. The likely mechanisms are as follows (Scheme

299

1): When the electrolysis begins, a large amount of hydrogen (H2 or H·) and chlorine (Cl2)

300

generate on the surface of working and counter electrode, respectively, in the presence of

301

NaCl as electrolyte

302

44

303

protects the zero-valent iron from oxidation. First, NO3- is reduced at cathode due to the

304

presence of high valence N (+5). If there is only electrolysis in the absence of nZVI (i.e., on

305

OMC or nickel foam), nitrate reduction is negligible with the generated H2 quickly escaped

306

from the solution. When zero-valent irons are on cathode, the nitrate reduction is accelerated

307

with 45% of nitrogen selectivity at the beginning of the reaction. Fe is a relatively reactive

308

metal. Its 3d orbit is unstable and easy to pair with extranuclear electron of oxygen in nitrayte

309

(N-O) first to form Fe(II)O [or Fe(III)O]. The Fe(II)O is likely the intermediate rather than

310

iron oxide, because the iron oxide@OMC sample is observed to exhibit low efficiency for

311

nitrate removal (120 mg N/g Fe). Using iron as catalyst also promotes other reductants, for

312

example the generated hydrogen to break the N-O bond and yields NO2- (NO3- + Fe(0) →

313

Fe(II)O + NO2-). Following a similar mechanism, the O in NO2- pairs sequentially with iron

314

and the N-O bond is broken again by the H2 attack, thus achieving the nitrate reduction to

315

generate NO2-, N2 and NH4+ step by step (NO2- + H+ + 0.5H2 → NO + H2O, 2NO + H2 →

316

N2O + H2O, N2O +H2 → N2 + H2O and NO2- + 5H2O + 6e- → NH3 + 7OH-).45 The optimal

317

particles size (5-6 nm), crystallization and stable dispersion of the nZVI are crucial for the

318

nitrate reduction by offering abundant, effective and stable catalytic sites. Furthermore, the

319

Fe(II)O can be rejuvenated to zero-valent iron by the active hydrogen as the dissolution of

320

iron is not detected. During the whole reaction, nZVI mainly acts as a catalyst rather than a

321

reducing agent. Due to the sustained consumption of nitrate, the remainder constantly

322

migrates to the cathode for reduction due to concentration gradient difference, resulting in the

323

eventually high removal capacity. On the other hand, the generated product NH4+ diffuses to

324

the solution during the reaction. Based on break point chlorination, a part of NH4+ is further

325

oxidized to N2 by generated HClO (Cl2 + H2O → HClO + Cl- + H+ and NH4+ + HClO → N2

326

+ H2O + Cl- + H+).13 After reaction for 24 h, the percentage of ammonia reverses from

(cathode surface: 2H+ → H2 - 2e and anode surface: 2Cl- → Cl2 + 2e).43,

The generated H2 not only provides the reducing agent for nitrate reduction, but also


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increase to a modest decline. This is accompanied with continuous nitrogen generation to a

328

maximal yield of 74%. Additionally, the ordered mesoporous carbon cooperated with nZVI

329

enhances the nitrate reduction, because it is not only served as a matrix to confine and

330

disperse nanoparticles, but also offers interconnected space for fast molecular diffusion and

331

transportation to the iron active sites.

332

Compared with conventional electrochemical reduction with precious metals or chemical

333

reduction of nitrate by nZVI, electrocatalysis with nZVI@OMC exhibits significant

334

advantages (Table S2).11, 44 A sample of polluted lake water is taken for electrocatalytic

335

denitrification experiments. The nitrate concentration decreases within 24 hours from 22.5 to

336

9.6 mg/L (268 mg N/g Fe) with the nitrogen selectivity of 91%. The results demonstrate this

337

material has potential for nitrate removal.

338 339

ASSOCIATED CONTENT

340

Supporting Information

341

Supporting information includes chemicals, characterization of the nanocomposites, and

342

consists of 18 pages, 11 figures and 2 tables. This material is available free of charge via the

343

Internet at http://pubs.acs.org.

344 345

AUTHOR INFORMATION

346

Corresponding Author

347

*E-mail: [email protected]. Tel: +86-21-65985885.

348

Notes

349

The authors declare no competing financial interest.

350 351

ACKNOWLEDGEMENTS

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This work was supported by National Natural Science Foundation of China (NSFC Grants

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21707104 and 51578398), Fundamental Research Funds for the Central Universities, and

354

State Key Laboratory of Pollution Control and Resource Reuse Foundation (NO. PCRRK



355

16009).

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Scheme 1. Mechanism of nitrate reduction by electrocatalysis on nanoscale zero-valent iron

468

supported on ordered mesoporous carbon (nZVI@OMC).

469



470 471

Figure 1. Illustration of synthetic route for the preparation of nZVI@OMC nanocomposites

472

(A), TEM images of nZVI@OMC after pyrolysis and reduction at (B) 300°C, (C) 400°C, and

473

(D) 500°C, HRTEM image (E) of nZVI@OMC-400 and wide-angel X-ray diffraction (XRD)

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patterns of nZVI@OMC (F) at (a) 300, (b) 400, and (c) 500°C, respectively.

475


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476 477

Figure 2. Removal performance (a) and selectivity for N2 (b) of different materials as

478

cathodes for electrocatalytic reduction of nitrate (50 mg/L of nitrate, 0.02 M NaCl and 24 h).

479



480 481 482

Figure 3. Product distributions of nitrate reduction (a) and selectivity for N2 (b) with the

483

reaction time (50 mg/L of nitrate, 0.02 M NaCl).


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484 485

Figure 4. The nitrate concentrations before and after reactions and removal capacity at

486

different initial concentration (a, 0.02 M NaCl and 24 h), and capacity and selectivity for N2

487

of consecutive cycles on nanocomposite nZVI@OMC-400 (b, 50 mg/L of nitrate, 0.02 M

488

NaCl and 24 h).

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Selective Nitrate Reduction to Dinitrogen by Electrocatalysis on

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