Boosting Fenton-Like Reactions via Single Atom Fe Catalysis

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Remediation and Control Technologies

Boosting Fenton-Like Reactions via Single Atom Fe Catalysis Yu Yin, Lei Shi, Wenlang Li, Xuning Li, Hong Wu, Zhimin Ao, Wenjie Tian, Shaomin Liu, Shaobin Wang, and Hongqi Sun Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b03342 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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Environmental Science & Technology

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Boosting Fenton-Like Reactions via Single Atom Fe Catalysis

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Yu Yin†,‡,#, Lei Shi‡, Wenlang Li§, Xuning Li∥, Hong Wu‡, Zhimin Ao§, Wenjie Tian⊥, Shaomin

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Liu#, Shaobin Wang⊥, *, and Hongqi Sun‡, *

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†School

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Zhenjiang 212003, China.

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‡School

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Australia.

of Environmental and Chemical Engineering, Jiangsu University of Science and Technology,

of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027,

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§Guangdong

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Environmental Health and Pollution Control, School of Environmental Science and Engineering,

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Guangdong University of Technology, Guangzhou, 510006, China.

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∥School

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Drive, Singapore, 639789, Singapore.

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⊥School

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#Department

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Australia.

Key Laboratory of Environmental Catalysis and Health Risk Control, Institute of

of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang

of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia. of Chemical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845,

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1

ACS Paragon Plus Environment


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ABSTRACT

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Maximizing the numbers of exposed active sites in supported metal catalysts is important for

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achieving high reaction activity. In this work, a simple strategy for anchoring single atom Fe on SBA-

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15 to expose utmost Fe active sites was proposed. Iron salts were introduced into the as-made SBA-

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15 containing the template and calcined for simultaneous decomposition of the iron precursor and the

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template, resulting in single atom Fe sites in the nanopores of SBA-15 catalysts (SAFe-SBA). X-ray

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diffraction (XRD), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), high-angle annular

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dark-field scanning transmission electron microscopy (HAADF-STEM) and extended X-ray

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absorption fine structure (EXAFS) imply the presence of single atom Fe sites. Furthermore, EXAFS

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analysis suggests the structure of one Fe center with four O atoms, and density functional theory

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calculations (DFT) simulate this structure. The catalytic performances of SAFe-SBA were evaluated

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in Fenton-like catalytic oxidation of p-hydroxybenzoic acid (HBA) and phenol. It was found that the

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single atom SAFe-SBA catalysts displayed superior catalytic activity to aggregated iron sites (AGFe-

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SBA) in both HBA and phenol degradation, demonstrating the advantage of SAFe-SBA in catalysis.

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KEYWORDS: Single atom Fe catalysts, confined space; Fenton-like reactions; advanced oxidation

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processes; hydroxyl radicals.

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1. INTRODUCTION

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Single atom catalysts are attracting increasing attention in electrochemical catalysis1-3 and gas phase

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catalysis4-6 owing to the atomic dispersion of active sites and high catalytic activities. Single atom

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catalysis is now regarded as a new frontier in heterogeneous catalysis. Compared to gas phase and

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electrochemical catalysis, much less attention has been paid to single atom catalysis in aqueous phases,

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for instance, catalytic oxidation of organic pollutants in water. Over the past decades, remediation

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technologies for refractory organic contaminants have been receiving considerable interests in both

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scientific and industrial fields, in view of the increasingly restrictive environmental regulations

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worldwide and the strong desire of the society for a healthy environment.7-9 It is generally accepted

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that Fenton reaction, producing highly reactive hydroxyl radicals (•OH) from H2O2, is one of the most

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efficient advanced oxidation processes (AOPs) for degradation of organic contaminants.10-12

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Homogeneous catalysts of Fe2+ or Fe3+ ions are usually employed for the Fenton or Fenton-like

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reactions, yet suffering from iron leaching, sludge formation, recovery and narrow pH working range

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issues.13-15 Heterogeneous Fenton-like oxidation was subsequently suggested to be an alternative to

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alleviate these problems, in which supported iron catalysts are widely used, yet sacrificing the high

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activities in homogeneous catalysis.16-18 In the iron based Fenton-like catalysis, the reactions undergo

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three steps: (i) iron sites contact with H2O2, (ii) Fe3+/Fe2+/H2O2 system generates • OH radicals, and

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(iii) •OH radicals degrade organics. In these steps, the interaction between iron sites and H2O2 is the

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imperative step to determine the activity of •OH generation.19 However, due to the natural tendency of

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metal sites to aggregate, supported metal catalysts still suffer from the low numbers of exposed metal

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sites. Maximizing the dispersion degree of the active iron sites will improve the activity of Fenton-like

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catalysis. Single atom catalysts are highly expected to be employed in the catalytic reactions because 3



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the metal sites can reach the atomic dispersion degree. Recently, some supporting materials, carbon,20-

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22

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develop other supports like mesoporous silica for fabrication of single atom catalysts.

carbon nitride23-24 and graphene25-26 for loading single atoms are reported. It remains a challenge to

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Because of the ordered nanoporous structure, large surface area and moderate pore diameter,

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mesoporous silica SBA-15 has been employed as a support material in a variety of applications, for

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example, gas adsorption,27-29 oxygen evolution reaction,30-31 catalytic hydrogenation32-34 and catalytic

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hydrodeoxygenation35-37. In Fenton-like reactions, SBA-15 shows a great potential to be a support as

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well.38-39 Great efforts have been made to load iron sites on SBA-15. For example, Fe2O3 immobilized

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alumina coated mesoporous silica was prepared and it exhibited a wide working pH range for

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heterogeneous Fenton-like reactions.40 Beside, Fe2O3 has been reported to be introduced onto SBA-15

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for Fenton-like oxidation via a traditional method.41 This traditional route for modifying SBA-15

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generally contains two steps: (i) the as-made SBA-15 is firstly annealed to remove the template of

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Pluronic P123 in the pores and then (ii) the iron precursors are introduced into the open pores. Besides

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Fe2O3, a similar synthesis has been used for other SBA-15 supported metal oxides, for instance, CuO42

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and Co3O443 for AOPs. In the traditional method, metal sites on SBA-15 usually tend to aggregate into

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large particles, leading to the poor dispersion degree. The AOPs performances of these catalysts are

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considerably limited. Thus, for improving the heterogeneous Fenton-like reactions, fabrication of

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atomically dispersed metal sites to avoid the particle aggregation on SBA-15 is needed. It is worthwhile

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to note that there is an extra confined space between the silica walls and template P123 in the as-made

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SBA-15 before calcination, which has been employed to accommodate amines to facilitate CO2

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capture.29 The confined space full of silicon hydroxyl (Si-OH) groups is expected to facilitate the

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formation of metal species at an atomic level.

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In the present study, for the first time, single atom Fe sites are decorated in the nanopores of SBA-

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15 (SAFe-SBA) through a facile strategy via directly loading Fe species on the as-made SBA-15 before

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calcination. In this strategy (Scheme 1), an iron precursor of Fe(NO3)3 was thoroughly mixed with the

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as-made SBA-15 through a solid-state grinding. During the subsequent calcination, Fe(NO3)3

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conversion and template removal can be finished in one step. The decomposition of Fe(NO3)3 within

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the confined space with substantial Si-OH groups endows the iron sites dispersed in the nanopores at

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an atomic level. The single atom SAFe-SBA catalysts show superior activity compared with

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aggregated iron sites (AGFe-SBA) on HBA and phenol degradation. Our strategy of anchoring single

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atom Fe sites on SBA-15 opens an avenue for designing and fabricating heterogeneous AOP catalysts.

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Scheme 1. Decoration of single atom Fe sites on SBA-15 to obtain SAFe-SBA (green route) and with

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aggregated Fe2O3 to obtain AGFe-SBA (black route).

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2. EXPERIMENTAL SECTION 5



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2.1. Chemical reagents. Pluronic P123 (EO20PO70EO20, Mn = ~5,800), hydrochloric acid (HCl, 37%),

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tetraethylorthosilicate (TEOS, 98%), iron (III) nitrate nonahydrate (Fe(NO3)3∙9H2O, 98%), p-

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hydroxybenzoic acid (HBA, 97.0%), phenol (99%), methanol (99.9%), and 5,5-dimethyl-1-pyrroline

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(DMPO) were purchased from Sigma-Aldrich. Hydrogen peroxide (H2O2, 30%) and tert-butyl alcohol

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(TBA, 99.5%) were obtained from Rowe Scientific. All the chemicals were used directly without

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further purification.

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2.2. Synthesis of SBA-15, SAFe-SBA and AGFe-SBA catalysts. Synthesis of SBA-15. An ordered

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nanoporous material of SBA-15 was synthesized according to the literature.44-45 In the synthesis, 2 g

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of triblock copolymer P123 was dissolved in 75 g of HCl (1.6 M). The solution was vigorously stirred

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to form a homogeneous system and then 4.25 g of TEOS was slowly added in the solution under

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continuous stirring at 40 oC. The solution was further stirred at 40 oC for 24 h, followed by the

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hydrothermal treatment at 100 oC for 24 h in a Teflon-line autoclave. The as-made SBA-15 containing

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the template P123 was recovered by filtration and dried at room temperature for 3 d. Removal of the

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template P123 was conducted by the calcination of the as-made SBA-15 at 550 oC in an air flow to

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obtain SBA-15.

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Synthesis of single atom Fe sites loaded SBA-15 (xSAFe-SBA). The prefix x is the iron content (mmol)

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per gram of SBA-15. Take 1.0SAFe-SBA for instance, 0.162 g of Fe(NO3)3∙9H2O was mixed with

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0.672 g of the as-made SBA-15. In an agate mortar, Fe(NO3)3∙9H2O was mixed with the as-made SBA-

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15 before template removal, followed by manually grinding in the ambient condition for 30 min. The

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thoroughly mixed powder was transferred to a tube furnace for calcination at 500 oC for 5 h in an air

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flow.

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Synthesis of aggregated Fe sites loaded SBA-15 (yAGFe-SBA). The prefix y is the iron content

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(mmol) per gram of SBA-15. As for 1.0AGFe-SBA, 0.162 g of Fe(NO3)3∙9H2O was mixed with 0.4 g

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of SBA-15. AGFe-SBA was prepared similarly to the SAFe-SBA except for the use of SBA-15 not

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as-made SBA-15 for grinding.

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Synthesis of bulk Fe2O3. Bulk Fe2O3 was derived from calcination of Fe(NO3)3∙9H2O in a tube furnace at 500 oC for 5 h in an air flow.

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2.3. Characterizations. Powder X-ray diffraction (XRD) patterns were acquired on a Bruker D8

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Advance diffractometer with a Cu K X-ray source over a range from 0.7 to 4o (for low-angle patterns)

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or 5 to 80o (for wide-angle patterns). The tube voltage and current were set at 40 kV and 40 mA,

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respectively. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an Escalab 250Xi

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(ThermoFisher Scientific) equipped with an Al K X-ray source. UV-visible diffuse reflectance

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spectroscopy (UV-vis DRS) was tested in the wavelength range of 200 – 800 nm on an Agilent Cary

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300 UV-Vis spectrophotometer. The Brunauer-Emmett-Teller (BET) specific surface area and pore

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size distribution of the catalysts were measured by N2 adsorption-desorption using a Micrometrics

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Tristar II Plus instrument at -196 oC. Prior to analysis, the catalysts were degassed at 150 oC for 4 h in

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the vacuum. Transmission electron microscopy (TEM) images were obtained on a JEM-200CX

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electron microscope operated at 200 kV. High-angle annular dark-field scanning transmission electron

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microscopy (HAADF-STEM) was performed on FEI Titan G2 80-200 TEM/STEM at 200 kV. The

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aberration-corrected (AC) HAADF-STEM analysis was performed on FEI Titan G2-600 at 300 kV.

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X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)

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measurements at the Fe K-edge were carried out on Beijing Synchrotron Irradiation Facility (BSRF,

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1W1B) with stored electron energy of 2.2 GeV using a transmission mode. More information on data

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processing and fitting can be found in Supporting Information.

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2.4. Catalytic oxidation. The activities of SBA-15, SAFe-SBA and AGFe-SBA catalysts were

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evaluated by H2O2 activation for HBA and phenol degradation. The Fenton-like reactions were

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conducted in a batch reactor using a glass flask (500 mL). The pH was adjusted with H2SO4 (0.2 M).

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In a typical run, 0.025 g of catalyst was added into 250 mL of 20 mg∙L−1 HBA (or phenol) solution at

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a specific pH of 3. Then adsorption was carried out in 30 min to achieve adsorption-desorption

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equilibrium. After that, H2O2 was added into the solution to initiate the Fenton-like reactions. At fixed

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time intervals, 1.0 mL of the reaction solution was withdrawn by a syringe and transferred into a HPLC

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vial after the filtration through a 0.22 m Millipore film, and then 0.5 mL of methanol was injected

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into the vial immediately to quench the reaction. The concentration of HBA (or phenol) was analyzed

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using the UHPLC (Shimadzu Nexera X2) with the C-18 column (100 mm × 2.7 mm) with the

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column temperature at 30 oC and a UV detector set at a wavelength of 270 nm. The mobile phase for

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HBA was acetic solution (pH = 3.5, 0.27 mL∙min−1) and methanol (0.03 mL∙min−1). The total organic

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carbon (TOC) content was measured on a TOC-5000 CE analyzer (Shimadzu, Japan). The rate

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constant of the Fenton-like reactions was derived from a first order kinetic model in Eq. (1).46-47 In the

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equation, c0 and c are the concentrations of pollutants at initial and time t, respectively.

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c0

( ) = kt

ln

(1)

c

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The concentration of iron ions in the reaction solution was measured by an inductively coupled

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plasma optical emission spectrometer (ICP-OES) from PerkinElmer. The reactive radicals generated

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in H2O2 activation were detected by electron paramagnetic resonance (EPR) on a Bruker EMS-plus

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instrument with DMPO (0.08 M) as a spin-trapping agent. The classical quenching tests were

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performed to determine the radical species formed from H2O2.

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3. RESULTS AND DISCUSSION

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Figure 1. (A) Low-angle and (B) wide-angle XRD patterns for the samples of SBA-15, SAFe-SBA

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and AGFe-SBA; (C) XPS spectra of Fe region for the 1.0SAFe-SBA and 1.0AGFe-SBA samples; (D)

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The UV-visible spectra of the SAFe-SBA and AGFe-SBA samples; (E) N2 adsorption-desorption

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isotherms and pore size distributions calculated by (F) adsorption and (G) desorption branches of the

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isotherms of SBA-15, SAFe-SBA, and AGFe-SBA samples, Curves are plotted offset for clarity.

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3.1. Physicochemical properties of iron based catalysts. Figures 1A-B and S1 show the XRD

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patterns of SBA-15 and the supported iron catalysts. It can been seen from the low-angle XRD patterns

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(Figures 1A and S1A) that all the samples show three diffraction peaks, indexed as the (100), (110)

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and (200) reflections. The XRD pattern is the characteristic of a highly ordered hexagonal mesoporous

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structure of p6mm symmetry, suggesting there is no structural damage to SBA-15 after the introduction

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of iron species. Besides, the peaks of the (100) reflections of the supported iron samples show a slight

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shift as compared to SBA-15. The d-spacing of SAFe-SBA samples shifts to a higher value, while

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AGFe-SBA samples present lower d-spacing values. The values of unit cell constant can be calculated

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based on the d-spacing values and are shown in Tables 1 and S1. Pristine SBA-15 support shows a

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lattice size of 10.8 nm. Owing to the shrinkage of silica walls during the repeated calcination for

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template removal and conversion of iron species, the AGFe-SBA samples show a lower value (10.2

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nm) than that of SBA-15. It is worth noting that the SAFe-SBA samples show higher values (11.3 or

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11.2 nm) than other samples, suggesting that the introduction of iron species into the as-made SBA-

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15 can weaken the structural shrinkage during the calcination of template removal.

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The wide angle XRD patterns are given in Figures 1B and S1B. The broad diffraction peak at 2 of

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23o on the patterns of SBA-15 and iron based catalysts is assigned to amorphous silica. Notably, for

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SAFe-SBA samples, no new diffraction peaks are observed. Xie et al. reported that the dispersion

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degree of the active sites on a support could be deduced from the wide-angle XRD results.48 No new

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XRD diffractions on 1.0SAFe-SBA and 3.0SAFe-SBA catalysts suggest that the iron sites are well

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dispersed with no sign of aggregation. In contrast, new diffraction peaks at 2 of 24.1 and 33.2o

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corresponding to -Fe2O3 (JCPDS no. 33-0664) can be observed on the patterns of 3.0AGFe-SBA.

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This indicates that the large aggregation of the iron particles occurs on 3.0AGFe-SBA.

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The XPS results of 1.0SAFe-SBA and 1.0AGFe-SBA catalysts in Figure 1C reveal that the binding

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energies (BE) of Fe 2p1/2 and Fe 2p2/3 are at 725.4 and 711.8 eV, respectively, indicating the presence

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of trivalent state of iron species in the samples. It is well accepted that UV-vis DRS spectra are useful

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to determine the dispersion degree of supported iron species. The absorption bands can be subdivided

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into three major regions: (i) below 300 nm for isolated Fe(III) sites, (ii) 300 – 400 nm for octahedral

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Fe(III) ions within [FeO4]– tetrahedron, and (iii) around 500 nm for aggregated iron sites.49 Figure 1D

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shows the UV-vis DRS spectra of SAFe-SBA and AGFe-SBA catalysts. On the spectrum of 1.0SAFe-

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SBA sample, the absorbance is predominantly below 300 nm, suggesting the highly dispersed isolated

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Fe(III) sites. 3.0SAFe-SBA sample shows intense absorbance below 300 nm, 300 – 400 nm and weak

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absorbance around 500 nm, suggesting an occurrence of weak aggregation of iron sites. AGFe-SBA

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samples display more intense absorption bands in the region of around 500 nm than SAFe-SBA at an

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identical iron content. This confirms the iron aggregation on AGFe-SBA samples, and the aggregation

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becomes serious at an increasing iron content.

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Figures 1E-G and S2 present N2 adsorption-desorption isotherms and pore size distributions of SBA-

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15, SAFe-SBA and AGFe-SBA catalysts. SBA-15 shows a type IV isotherm with an H1 hysteresis

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loop, being the ordered mesoporous structure with a cylinder shape. The pore sizes of SBA-15 are

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centered at 8.0 nm from adsorption and 6.3 nm from desorption, respectively (Tables 1 and S1). SAFe-

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SBA samples display the similar isotherms and pore size distributions to those of SBA-15, suggesting

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little changes in the pore structures and thus good dispersion of the iron guest species. AGFe-SBA

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samples show “tails” on the isotherms due to the desorption delay. As a result, the close of the

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hysteresis loop moves to a lower relative pressure (0.45) than that of SAFe-SBA (0.60), implying pore

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blocking by the aggregated guests. It is noted that AGFe-SBA samples show smaller surface areas and

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pore volumes than SAFe-SBA samples at an identical iron content. In addition, AGFe-SBA samples

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show quite different pore size distributions as compared to SAFe-SBA. The pore sizes of AGFe-SBA

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(7.7 – 7.8 nm) derived from adsorption are smaller than SAFe-SBA (8.0 – 8.3 nm). In the case of

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desorption, there are two peaks of pore sizes (6.2 – 6.3 nm and 3.8 nm) observed on AGFe-SBA.

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Therefore, we can verify the aggregation of iron species in the nanopores of SBA-15 for the AGFe-

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SBA samples and good iron dispersion on SAFe-SBA.

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Table 1. Textural properties of SBA-15, FeAS and FeCS samples

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Dp (nm)

Vp

a0 b

Fe quantity

SBET

(mmol∙g1 (SBA-15)) a

(m2·g−1)

SBA-15

0

805

1.03

8.0

6.3

10.8

1.0SAFe-SBA

1.010

738

0.92

8.2

6.4

11.3

3.0SAFe-SBA

3.011

512

0.67

8.0

6.3

11.2

1.0AGFe-SBA

0.996

587

0.68

7.8

6.3, 3.8

10.2

3.0AGFe-SBA

3.010

425

0.49

7.7

6.2, 3.8

10.2

Sample (cm3·g−1) adsorption desorption

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a

Derived from ICP-OES results.

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b

Unit cell constant was calculated according to a0 = 2  31/2  d100.50

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12


(nm)

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Figure 2. TEM images of the samples of (A) 1.0AGFe-SBA, (B) 3.0AGFe-SBA, (C) 1.0SAFe-SBA

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and (D) 3.0SAFe-SBA; (E, F) High angle annular dark field scanning TEM images (HAADF-STEM)

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and energy-dispersive X-ray spectroscopy (EDX) mapping images of the 1.0SAFe-SBA sample; (G,

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H) Aberration-corrected (AC) HAADF-STEM image of 1.0SAFe-SBA.

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TEM images of the obtained samples are depicted in Figures 2A-D and S3. SBA-15 support exhibits

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a highly ordered hexagonal mesoporous structure. SAFe-SBA and AGFe-SBA catalysts display the

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similar ordered mesopores to SBA-15, confirming that the structure of SBA-15 remains unaltered after

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the modification of iron sites. On 3.0AGFe-SBA, some irregular particles can be identified in blocking

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the channels. These irregular particles are the aggregated iron species. On the contrary, no particles

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are observable on the TEM images of SAFe-SBA samples. HAADF-STEM images and corresponding

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element mapping, and EDX spectra of 1.0SAFe-SBA are presented in Figures 2E-F and S4. 1.0SAFe-

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SBA catalyst clearly shows the hexagonal arrays of mesopores and no particles on the HAADF-STEM

246

images, coinciding with the TEM results. Besides, the guest iron sites can be identified by the element

247

mapping and EDX spectra even though their particles are not observable in the images. Furthermore, 13



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the element mapping presents a homogeneous dispersion degree of the iron sites in the nanopores. The

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AC-HAADF-STEM images (Figures 2G-H) also do not show any aggregation of particles in the

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nanopores.

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Figure 3. (A) K-edge XANES spectra, (B) R-space EXAFS magnitudes, (C) WT-EXAFS of 1.0SAFe-

254

SBA and references, and (D) the corresponding EXAFS R, q, k space fitting curves of 1.0SAFe-SBA.

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Figure 3A provides Fe K-edge X-ray absorption near-edge structure spectra (XANES). The

257

1.0SAFe-SBA catalyst exhibits the pre-edge peaks at 7114.3 eV, confirming the presence of Fe(III)

258

sites,23 in accordance with the XPS analysis. Extended X-ray absorption fine structure (EXAFS) of

259

1.0SAFe-SBA catalyst is illustrated in Figure 3B. The first shell peak at ~1.5 Å is referred to the Fe-

260

O bond as in Fe2O3.51 It is worthwhile mentioning that Fe2O3 shows a large peak at ~2.6 Å, however,

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this peak is absence in 1.0SAFe-SBA catalyst. The decrease of this shell peak at ~2.6 Å indicates that

262

the coordination state of the iron sites in 1.0SAFe-SBA is different from that in Fe2O3. The wavelet

263

transform (WT) plot of 1.0SAFe-SBA (Figure 3C) displays different images from Fe foil, Fe2O3 and

264

FeO, implying an unusual coordination state of iron sites in 1.0SAFe-SBA. The EXAFS fitting curves

265

indicate that the length of Fe-O bonds is 1.89 Å, and the quantitative coordination configuration of

266

iron sites in 1.0SAFe-SBA is deduced to be 4.5, which is distinct from Fe foil, Fe2O3 and FeO due to

267

the EXAFS fitting results (Figures 3D, S5 and Table S2). These observations, associated with the XRD,

268

UV-vis DRS, TEM and HAADF-STEM results, indicate the iron species are probably single atom Fe

269

sites. Figure S6 simulates the relaxed structure of SBA-15 surface, where SBA-15 has an amorphous

270

structure and the surface Si atoms bonding with O and -OH. With the loading of Fe (SAFe-SBA), four

271

H atoms of Si-OH on the surface are predicting to be substituted by Fe atoms and Fe-O bonds form.

272

The simulation of four Fe-O bonds with an average of 1.81 Å is matching to the experimental data of

273

Fe with 4.5 coordination numbers and Fe-O bond length of 1.89 Å.

274 275

From the above characterizations, 1.0SAFe-SBA sample shows no peaks of iron phase in the wide-

276

angle XRD patterns, no observable particles in the TEM and HAADF-STEM images, and the similar

277

N2 adsorption-desorption isotherms and hysteresis loops to SBA-15. But, EDX, XPS and Fe K-edge

278

XANES spectra confirm the presence of Fe(III) species on SBA-15. Moreover, EDX mapping images

279

show the homogeneous dispersion of iron sites, UV-vis DRS spectra indicate isolated iron sites, and

280

EXAFS results imply single Fe atom sites in 1.0SAFe-SBA. All the characterizations suggest that the

281

iron species in 1.0SAFe-SBA reach an excellent uniform dispersion, which can be denoted as single

282

atom Fe sites on SBA-15. DFT calculations predict the structure to prove the possibility of the presence

15



Page 16 of 31

283

this kind of single atom Fe catalyst by calculations. The successful fabrication of single atom Fe sites

284

can be attributed to the substantial Si-OH groups and the confined space of the as-made SBA-15, which

285

control the interactions between iron sites and the silica walls and prevent the aggregation of the guest

286

metals. The formation mechanism of single atom Fe sites is discussed in Supporting Information.

287

288 289

Figure 4. (A, B) HBA, (C) phenol adsorption and oxidation by H2O2 activation on catalysts; Effects

290

of reaction parameters on the HBA removal for 1.0SAFe-SBA: (D) H2O2 dosage, (E) pH value and

291

(F) temperature. Except for the investigated parameters, other parameters were fixed. ([Catalyst]0 =

292

0.1 g∙L−1, [H2O2]0 = 1000 ppm, pH = 3, [T] = 25 oC, and [pollutant]0 = 20 mg∙L−1).

293 294

3.2. Performances of catalysts in Fenton-like reactions. The activities of SBA-15, SAFe-SBA and

295

AGFe-SBA catalysts were investigated in Fenton-like reactions. Bulk Fe2O3 was also synthesized for

296

comparison, which was unsupported, undispersed and possessed a low BET surface area of 10 m2∙g1

297

(Figure S9). As it can be seen from Figures 4A-B, all the samples showed negligible HBA removal 16


Page 17 of 31


298

during the initial 30 min of adsorption. After H2O2 was added, the catalysts exhibited varying activities.

299

Pristine SBA-15 and bulk Fe2O3 gave less than 3% of HBA removal in 180 min, indicating that they

300

can hardly activate H2O2 to degrade HBA. H2O2 alone hardly degraded the pollutant. The SBA-15

301

supported iron catalysts showed a great improvement on HBA degradation. 0.25AGFe-SBA,

302

0.5AGFe-SBA, 1.0AGFe-SBA and 3.0AGFe-SBA catalysts achieved HBA removals at 58.9%, 60.8%,

303

72.4% and 80.3% in 180 min, respectively. Notably, every SAFe-SBA catalyst demonstrated a

304

superior catalytic activity to its counterpart AGFe-SBA catalyst. 0.5SAFe-SBA, 1.0SAFe-SBA and

305

3.0SAFe-SBA samples provided 100% HBA degradation in 150, 90 and 120 min, respectively. While

306

the catalytic activity of 0.25SAFe-SBA was slightly inferior to its homologues, degrading 88.1% of

307

HBA in 180 min. It is known that oxidant utilization efficiencies are as important as the reaction rates

308

in the heterogeneous Fenton-like systems. To this concern, we calculated the H2O2 consumption index

309

(XH2O2), which was defined as moles of H2O2 consumed per mole of HBA degraded until 50%

310

conversion of HBA was achieved.52 The results are depicted in Figure S10 and Table S3. It showed

311

that the use of higher dosage of H2O2 gave rise to a lower H2O2 efficiency. Besides, with the H2O2

312

dosage of 50 mg∙L−1, 1.0SAFe-SBA achieved superior H2O2 efficiency to 1.0AGFe-SBA. However,

313

with the higher H2O2 dosages of 200 and 1000 mg∙L−1, 1.0SAFe-SBA exhibited inferior H2O2

314

efficiencies to 1.0AGFe-SBA. The removal efficiencies of TOC after 180 min of reactions were also

315

investigated, and the results displayed that 19.6% of TOC was removed on 1.0SAFe-SBA, while only

316

10.7% on 1.0AGFe-SBA. In Figure S11, recycling 1.0SAFe-SBA catalyst exhibited some decreasing

317

activities on HBA oxidation. In more detail, the fresh catalyst removed 100% of HBA in 90 min,

318

however, it only degraded 90.6% and 49.0% of HBA in 180 min, respectively, in the second and third

319

runs. Afterwards, a heat treatment was adopted, and the catalyst degraded 99.5% of HBA in 180 min.

17



Page 18 of 31

320

The used 1.0SAFe-SBA after the first run was recovered and further characterized to investigate its

321

stability in water since it was an important concern.53 As shown in Figure S12 and Table S4, the XRD

322

patterns, N2 sorption isotherms and corresponding BET surface area, pore volume, and pore

323

distributions were found to be similar to the fresh 1.0SAFe-SBA, suggesting that the mesoporous

324

structure of SBA-15 was stable during the Fenton-like catalysis. Moreover, the XPS peaks of Fe region

325

at 711.8 and 725.4 eV indicated that the trivalent state of iron species was retained after catalysis.

326

A further investigation on phenol degradation also suggested that 1.0SAFe-SBA exhibited a

327

significantly enhanced degradation rate (99.96% in 180 min) than that of 1.0AGFe-SBA (8.9% in 180

328

min) (Figure 4C). The concentration of leaching iron ions in the solution after 180 min reaction of

329

phenol degradation was measured to be 1.7 mg∙L−1 for 1.0SAFe-SBA, below the environmental

330

regulation of 2.0 mg∙L−1 imposed by the European Union. To distinguish the catalytic role of single

331

Fe atoms against the leaching iron ions, homogeneous Fe2+ and Fe3+ ions were employed for HBA and

332

phenol degradation, respectively. On one hand, the same iron level as that of 1.0SAFe-SBA in the

333

reaction solution (5.19 mg∙L−1) was investigated. The results in Figure S13 revealed that Fe2+ or Fe3+

334

ions degraded 100% of pollutants in less than 30 min, much better than heterogeneous 1.0SAFe-SBA.

335

This indicated that there was still a gap to bridge between heterogeneous and homogeneous catalysis.

336

On the other hand, it was found that over 90% of HBA or phenol was removed on 1.0SAFe-SBA in

337

90 min. Due to the iron leaching increased with the reaction time, the concentration of iron ions after

338

90 min of pollutant degradation was also measured, exhibiting a low level leaching of 0.54 mg∙L−1.

339

The catalytic reactions were thus conducted on 0.54 mg∙L−1 of Fe2+ and Fe3+ ions respectively, and the

340

results showed no more than 15% of HBA or phenol can be degraded in 90 min. Thus, the catalytic

341

performances of 1.0SAFe-SBA were dominated by single Fe atoms rather than the leaching ions. It

18


Page 19 of 31


342

must also be mentioned that compared with other supported iron based Fenton-like catalysts, the

343

optimal 1.0SAFe-SBA catalyst still showed a superior catalytic activity (99.96% in 180 min) on the

344

phenol degradation as described in follows. On the iron phthalocyanine modified graphene, 77.1% of

345

phenol was degraded in 180 min in the presence of H2O2 under the additional visible light

346

irradiations.54 An iron-containing zeolitic material degraded 68.8% of phenol in 240 min under photo-

347

Fenton-like oxidation.55 The counterpart catalyst of 1.0AGFe-SBA-15 showed inferior activity of 8.9%

348

in 180 min to 1.0SAFe-SBA as well. We calculated the rate constant by Eq. (1) and further normalized

349

the value by the mass of Fe. It revealed that the mass-normalized rate constant increased from 3.09

350

(1.0AGFe-SBA) to 35.58 min−1∙g−1 (1.0SAFe-SBA). The performance of Fe-impregnated catalyst

351

under similar conditions was also introduced for comparison of the mass-normalized rate constants.

352

The Fe2O3/SBA-15 catalyst derived from the traditional impregnation method removed 18.5% of

353

phenol in 240 min upon adding H2O2 with the mass-normalized rate constant of only about 0.12

354

min−1∙g−1.56

355

It is commonly known that the concentration of H2O2, pH value and temperature are the decisive

356

factors to determine the activity of Fenton-like reactions. Thus the influences of these parameters were

357

studied. Figure 4D showed the degradation of HBA on 1.0SAFe-SBA at varying initial concentrations

358

of H2O2. When the H2O2 concentration increased from 200 to 1000 ppm, the time for achieving 100%

359

HBA removal decreased from 120 to 90 min. Further increasing H2O2 concentration to 2000 ppm gave

360

a significantly reduced degradation rate (98.0% in 180 min). A possible explanation was that an

361

excessive H2O2 caused scavenging of •OH radicals to much less reactive HO2• species. The effect of

362

pH value was also depicted in Figure 4E. As seen, a lower pH value would benefit the Fenton-like

363

reactions. While at pH 6, the catalyst can hardly show activity. Reducing the pH value to 4, 100%

19



Page 20 of 31

364

HBA can be removed after 150 min. The optimum pH value for HBA degradation was observed to be

365

3 (100% in 90 min). Further acidification of the solution to pH = 2 led to dramatic decline of the HBA

366

removal rate. Expanding the working pH range is also a great motivation in Fenton-like catalysis.

367

Actually, there has been some reported Fe-based catalysts that are active at neutral pH, like Fe-ZSM552

368

and Fe based carbon nanotube57. Our future direction is to fabricate single atom catalysts that are active

369

at neutral pH. The possible approach might be adjusting properties of the supports, exploring

370

multifarious supports, and designing other single atom metal sites. Figure 4F displayed the effect of

371

reaction temperature. HBA removal completed at a faster rate with a higher temperature. It took 90

372

min to degrade 100% HBA at 25 °C, 60 min at 35 °C, and 30 min at 45 °C. With an elevated

373

temperature, the rate constants gradually increased at 0.050, 0.101 and 0.217 min−1 (Eq. (1)). Based

374

on the well-known Arrhenius equation, the activation energy (Ea) of 1.0SAFe-SBA for the HBA

375

degradation was obtained to be 57.7 KJ∙mol−1.

376

20


Page 21 of 31


377 378

Figure 5. (A) EPR spectra for 1.0SAFe-SBA catalyzed Fenton-like reactions with different pH values

379

at 1 min; (B) Effect of the quenching agent (TBA) on HBA degradation for the 1.0SAFe-SBA catalyst;

380

(C) HPLC spectra of HBA oxidation by H2O2 activation on 1.0SAFe-SBA; and (D) Proposed

381

mechanism of Fenton-like reactions on single atom Fe sites. ([Catalyst]0 = 0.1 g∙L1, [H2O2]0 = 1000

382

ppm, pH = 3, [T] = 25 oC, and [HBA]0 = 20 mg∙L1).

383 384

3.3. Mechanistic studies on Fenton-like reactions. The radicals generated during Fenton-like

385

reactions on 1.0SAFe-SBA catalyst were investigated by the EPR technique, in which DMPO was

386

used as the spin trapping agent. As shown in Figure 5A, no peaks were observed in the 1.0SAFe-

387

SBA/DMPO system. However, H2O2/1.0SAFe-SBA/DMPO system showed a series of 4-fold 21



Page 22 of 31

388

characteristic peaks with an intensity ratio of 1:2:2:1, which can be assigned to the • OH radicals.

389

Taking into account that the H2O2/DMPO system did not produce any radicals,57 it is concluded that

390

1.0SAFe-SBA catalyst can induce H2O2 to generate •OH radicals. For the influence of •OH radicals

391

on the Fenton-like HBA degradation, tert-butyl alcohol (TBA) was employed as the quenching agent

392

to scavenge • OH radicals. As it can be seen from Figure 5B, the reaction rate severely reduced after

393

adding 5 mM of TBA into the solution, and only 25.8% HBA can degrade in 180 min. When the

394

concentration of TBA was further increased to 10 – 50 mM, HBA can hardly be removed. Therefore,

395

• OH

396

In addition, as presented in Figure 5A, the pH values had great influences on the intensities of the

397

OH-DMPO signals, showing the order of pH3 > pH4 > pH2/pH6. Not surprisingly, the pH value of 3

398

was the optimal condition to generate active radicals of •OH, leading to the optimal degradation rates.

399

As shown in the HPLC spectra of HBA oxidation (Figure 5C), the HBA peaks gradually disappeared

400

with the oxidation progress. Meanwhile, the peaks of intermediates increased and then decreased,

401

suggesting their generation and decomposition during the catalytic reactions. Hence, the possible

402

reaction mechanisms for HBA and phenol degradation in H2O2/1.0SAFe-SBA can be proposed (Figure

403

5D) as follows:

404

Si - O - Fe(III) + H2O2 → Si - O - Fe(III) - H2O2∙ → Si - O - Fe(II) +HO2∙

405

(2)

406

Si - O - Fe(II) + H2O2 → Si - O - Fe(III) + 2 ∙ OH

407

(3)

408

HBA +

409

Phenol +

radicals were the key to the HBA degradation in 1.0SAFe-SBA catalyzed Fenton-like reactions.

∙

(4)

OH → intermediates → CO2 + H2O ∙

(5)

OH → intermediates → CO2 + H2O

22


•

Page 23 of 31


410 411

It can be concluded that in the Fenton-like reactions, the SAFe-SBA catalysts showed superior

412

performances to AGFe-SBA in HBA and phenol degradation. The enhanced activities were mainly

413

attributed to the single atomic dispersion degree of iron sites, which maximized the activity of iron

414

atom use. The simple strategy to load single atom Fe sites on SBA-15 might offer a new avenue for

415

the development of new catalysts for a variety of catalytic reactions.

416 417

ASSOCIATE CONTENT

418

Supporting Information

419

The Supporting Information is available free of charge on the ACS Publications website at DOI:

420

Detailed information including formation mechanism of single atom Fe sites (FT-IR spectra, TG and

421

DTG curves), characterization of SBA-15, SAFe-SBA and AGFe-SBA samples (XRD, N2 adsorption-

422

desorption isotherms, TEM, EDX, EXAFS fitting curves), DFT simulations, properties of bulk Fe2O3

423

(XRD, XPS, N2 adsorption-desorption isotherms, UV-vis DRS) and 1.0SAFe-SBA-15 after catalysis

424

(XRD, XPS, N2 adsorption-desorption isotherms), changes of H2O2 concentration, the H2O2

425

consumption index (XH2O2), HBA degradation on 1.0SAFe-SBA after repeated uses, HBA and phenol

426

degradation on iron ions (Figures S1-S13, Tables S1-S4).

427 428

AUTHOR INFORMATION

429

Corresponding Authors.

430

*Email: [email protected] (H. Sun);

431

*Email: [email protected] (S. Wang).

23



432 433

Page 24 of 31

Notes There are no conflicts to declare.

434 435

ACKNOWLEDGMENTS

436

The author (Sun) would like to express his thanks for the support from Vice-Chancellor's

437

Professorial Research Fellowship. This work was also financially supported by the National Natural

438

Science Foundation of China (No. 51602133, 21607029 and 21777033), Natural Science Foundation

439

of Jiangsu Province (No. BK20160555), Science and Technology Program of Guangdong Province

440

(No. 2017B020216003). The authors acknowledge as well the help of Martin Saunders and Alexandra

441

Suvorova from the Centre for Microscopy, Characterization and Analysis (CMCA) of the University

442

of Western Australia.

443 444

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Boosting Fenton-Like Reactions via Single Atom Fe Catalysis

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