4-Phenoxyphenol-Functionalized Reduced Graphene Oxide

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4-Phenoxyphenol-Functionalized Reduced Graphene Oxide Nanosheets: A Metal-Free Fenton-Like Catalyst for Pollutant Destruction Lai Lyu, Guangfei Yu, Lili Zhang, Chun Hu, and Yong Sun Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04865 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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Environmental Science & Technology

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4-Phenoxyphenol-Functionalized Reduced Graphene Oxide Nanosheets: A

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Metal-Free Fenton-Like Catalyst for Pollutant Destruction

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Lai Lyuabc, Guangfei Yubc, Lili Zhangb, Chun Hu*,abcand Yong Sund

4 5 6

a

Research Institute of Environmental Studies at Greater Bay, Guangzhou University, Guangzhou, 510006, China

7 b

8 9

Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China c

10 11 12

Key Laboratory of Drinking Water Science and Technology, Research Center for

d

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

College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin 150001, China

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*Corresponding author Tel: +86-10-62849628; fax: +86-10-62923541;

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e-mail: [email protected] / [email protected]

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1

ACS Paragon Plus Environment


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ABSTRACT

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Metal-containing Fenton catalysts have been investigated widely. Here, we

19

report for the first time a highly effective stable metal-free Fenton-like catalyst with

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dual reaction centers consisting of 4-phenoxyphenol-functionalized reduced graphene

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oxide nanosheets (POP-rGO NSs) prepared through surface complexation and

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copolymerization. Experimental and theoretical studies verified that dual reaction

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centers are formed on the C-O-C bridge of POP-rGO NSs. The electron-rich center

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around O is responsible for the efficient reduction of H2O2 to •OH, while the

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electron-poor center around C captures electrons from the adsorbed pollutants and

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diverts them to the electron-rich area via the C-O-C bridge. By these processes,

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pollutants are degraded and mineralized quickly in a wide pH range, and a higher

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H2O2 utilization efficiency is achieved. Our findings address the problems of the

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classical Fenton reaction and are useful for the development of efficient Fenton-like

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catalysts using organic polymers for different fields.

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2


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INTRODUCTION

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Increasing amounts of non-biodegradable and persistent organic compounds are

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released into water and soil environments and adversely affect human health through

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entry into the food chain or via environmental cycles.1,

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development of efficient technologies for removing these pollutants is an urgent issue.

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The use of metal-containing Fenton catalysts is an alternative technique for this

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purpose and has been studied for a long time because they produce

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high-oxidation-potential hydroxyl radicals (•OH),3 which is an aggressive species

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used for water treatment,4, 5 soil remediation,6 material synthesis,7 polishing,8 and

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cancer therapy.9 However, whether homogeneous or heterogeneous Fenton process is

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used, the reaction always depends on the redox of the metal ions in the single-metal

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center. This intrinsic property of metal-containing Fenton catalysts often results in

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some problems, including the need for acidic reaction conditions (pH = 2–4),10

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occurrence of a rate-limiting step due to the low reaction rate constant for the

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reduction of M(n+m)+ to Mn+,11, 12 excessive consumption of H2O2,13, 14 and secondary

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pollution owing to the production of iron-containing sludge15 or metal leaching,2, 16

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which narrow the application of Fenton reactions for environmental remediation.

2

Consequently, the

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Recently, our research16, 17 has revealed that constructing dual reaction centers

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(electron-rich and electron-poor centers) in a catalyst is essential for overcoming the

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limitations of the classical Fenton reaction for environmental remediation and other

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applications. In the Ti, Cu and Al lattice-doped dandelion-like silica nanospheres

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(d-TiCuAl-SiO2 Ns),16 electron-rich and electron-poor centers are formed around the

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lattice Cu and Ti/Al, respectively, owing to the negative-charge non-uniformity of the

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lattice

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electronegativities.

O caused

by lattice

doping with three

Furthermore,

in

the

metals

hydroxylated

3


having

different

carbon-doped


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g-C3N4/CuCo-Al2O3 nanocomposite (OH-CCN/CuCo-Al2O3),17 electron-rich and

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electron-poor centers are formed around Cu and N, respectively, owing to Cu-π

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electron transfer because of surface complexation of organic g-C3N4 with the surface

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Cu. In dual-reaction-center Fenton-like systems, H2O2 does not react directly with the

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metal species, but instead traps excess electrons in electron-rich areas to generate •OH.

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These systems exhibit ~10-times higher efficiencies than those of classical Fenton

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systems for pollutant degradation. This presents the possibility of using metal-free

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materials as Fenton catalysts through the construction of dual reaction centers by

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polarizing the distribution of electrons.

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4-Phenoxyphenol (POP, p-hydroxydiphenyl ether) consists of two benzene rings

67

linked by a ether C-O-C bond, one with a para hydroxyl group (Figure S1,

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Supporting Information (SI)), and is a molecular material that can self-assemble

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into a porous channel structure under various solvent crystallization conditions. The

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structure of POP shows an unusual flexibility not only in direction, as influenced by

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weak hydrogen bonds, but also in its moderately hydrogen bonded O−H···OR66(12)

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rings, which can breathe to accommodate different solvent molecules, such as ethanol,

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into the channels.18 Thus, POP can be used as an excellent organic ligand.

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Graphene is a fascinating two-dimensional carbon material owing to its unique

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properties, such as high surface area, excellent electrical conductivity, and high

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mechanical strength.19, 20 Owing to strong sp2-conjugated bonds in the carbon lattice,

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electrons can move ballistically in graphene layers at room temperature with large

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intrinsic mobilities exceeding 15,000 cm2 V-1 s-1 without scattering.21 These

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characteristics have inspired research into using graphene as a component of

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Fenton-like catalysts.

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Herein, a metal-free Fenton catalyst consisting of POP-functionalized reduced 4


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graphene oxide nanosheets (POP-rGO NSs) is developed via a surface complexation

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and copolymerization process for the first time. POP-rGO NSs exhibits excellent

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Fenton-like activity, good stability, and high H2O2 utilization efficiency for the

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degradation of pollutants in a wide pH range, demonstrated using bisphenol A (BPA)

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and 2-chlorophenol (2-CP) as model pollutants. X-ray photoelectron spectroscopy

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(XPS) and Fourier-transform infrared spectroscopy (FTIR) analyses revealed that the

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connection between POP and graphene is achieved through C-O-C bonding, resulting

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from the deprotonated phenolic OH group of POP bonding with the C atoms in the

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graphitic rings. Copious electron-rich and electron-poor centers (i.e., dual reaction

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centers) form around the O and C atoms, respectively, in the C-O-C linkages of

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POP-rGO NSs, as confirmed by electron paramagnetic resonance (EPR) analysis and

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density functional theory (DFT) calculations. A preliminary effort to identify a

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correlation between the surface electron properties of POP-rGO NSs and its

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functionality has been undertaken, and a dual-reaction-center mechanism for the

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metal-free Fenton-like reaction has been proposed.

97 98

EXPERIMENTAL SECTION

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Synthesis of rGO and POP-rGO NSs. Graphene oxide (GO) was prepared by a

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modified Hummers method through oxidation of graphite powder,22, 23 the details of

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which are given in the SI. For the synthesis of POP-rGO NSs, finely grounded GO

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(0.5 g) and POP (0.2 g) were dissolved in 25 mL ethanol, stirred for 1.0 h, and

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sonicated for 30 min to form a well-dispersed solution. The obtained solution was

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maintained 70 °C for 4 h then dried by evaporating in a water bath at 90 °C to obtain

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the solid precursors. The precursors were ground into fine particles, placed in a

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semi-closed alumina crucible, and heated in a muffle furnace to 350 °C at a heating 5



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rate of 5 °C min-1, which was then maintained for 1 h, for annealing copolymerization.

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The obtained powder was washed with deionized water and ethanol several times and

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dried in an oven overnight to yield the final POP-rGO NSs sample. Reduced graphene

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oxide (rGO) was also prepared without the addition of the POP precursor for

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comparative purposes.

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Fenton catalyst performance measurement. 2-CP and BPA, as typical refractory

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organic pollutants, were selected to evaluate the performances of the catalysts. Their

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molecular structures are shown in Figure S1. The initial concentration of 2-CP or

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BPA was 10 mg L-1. The optimal dosages of the catalyst powder (0.4 g L-1, Figure

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S2a) and H2O2 (10 mM, Figure S2b) were determined according to the best activity

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for 10 mg L-1 2-CP degradation. These dosages were used in all experiments unless

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otherwise specified. Typically, the aqueous pollutant solution (50 mL) and the

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catalyst powder (0.02 g) were placed in a beaker (the natural pH value of the

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suspension was 6.5, and other pH values were obtained using HCl solution). The

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suspension was magnetically stirred for 30 min to establish the adsorption/desorption

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equilibrium between the catalyst and the organic pollutant. Then, H2O2 was added to

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the suspension under magnetic stirring, which was maintained throughout the

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experiment. At given time intervals, 1 mL aliquots were collected and filtered through

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a Millipore filter (pore size 0.22 µm) for analysis. The pollutants were immediately

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determined using a 1200 series HPLC (Agilent, U.S.A.) equipped with a UV detector

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and a ZORBAX Eclipse XDB-C18 column (4.6 × 150 mm, 5 µm). The mobile phase

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consisted of a mixture of methanol/water at a flow rate of 1 mL min-1. The total

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organic carbon (TOC) was determined with a Shimadzu TOC-VCPH analyzer using

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high-temperature combustion. The H2O2 concentration was determined using the

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N,N-diethyl-p-phenylenediamine (DPD) method.24 The reusability of the POP-rGO 6


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NSs catalyst was assessed by recovery using filtration. Typically, after one Fenton

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reaction, the catalyst was filtered out using a 0.22-µm Millipore filter. The solid was

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then washed with deionized water five times under neutral conditions. Then, the solid

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sample was dried at 70 °C for 6 h and reused in the following cycle. Each experiment

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was run in triplicate. The reported data are the arithmetic mean of three measured

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values. The chemicals and reagents, characterization, detection of •OH, O2•− EPR

138

signals, and density functional theory (DFT) calculations are presented in the SI.

139 140

RESULTS AND DISCUSSION

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Characterization of catalysts. Figure S3a,b shows low-magnification transmission

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electron microscopy (TEM) images of POP-rGO NSs. Large graphene nanosheets are

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situated on the top of the grid, where they resemble crumpled silk veil waves. This

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scrolling and corrugation is intrinsic to graphene nanosheets, and occurs to maximize

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the thermodynamic stability of the 2D membrane via microscopic bending or

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buckling.25 These nanosheets are transparent and exhibit stability under the electron

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beam. POP-rGO NSs mainly exhibits a disordered structure in the high magnification

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TEM images (Figure S3c,d). However, some ordered graphite lattices are also clearly

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visible, indicating that POP-rGO NSs is partially restored to its ordered crystal

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structure.21 The well-defined diffraction spot rings in the selected area electron

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diffraction (SAED) patterns can be assigned to the hexagonal structure of the

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honeycomb carbon lattice and the graphite planes.

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Figure S4 shows the powder X-ray diffraction (XRD) patterns of the prepared

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samples. GO shows a sharp (001) diffraction peak at 2θ = 11.8°, corresponding to an

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interlayer d-spacing of 0.746 nm between the stacked sheets, suggesting the complete

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exfoliation of the graphite.26 After reduction with thermal annealing, the sharp (001) 7



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diffraction peak disappears, indicating the removal of the oxygen functional groups.

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POP-rGO NSs exhibits diffraction peaks at 2θ = 25.6° with an interlayer d-spacing of

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0.348 nm and at 2θ = 43.5° with an interlayer d-spacing of 0.208 nm, which are quite

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different from those of the crystal phase of rGO and the condensation polymer of pure

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POP (POP-CP). This result indicates that the copolymerization of rGO and POP leads

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to the formation of a completely new hybrid, i.e., POP-rGO NSs.

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The formation of chemical bonds in the materials was analyzed using XPS

164

measurements. Compared with GO (Figure S5a,b), the surface oxygen functional

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groups of rGO (Figure S5c,d) are largely removed with some C-OH groups residual,

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indicating that GO is effectively reduced to rGO in the current study. The O1s XPS

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spectrum of POP-rGO NSs (Figure S6a) shows strong signals in the range 530−536

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eV, which can be deconvoluted into four peaks at 530.9 (O=C-OH), 531.5 (C=O),

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532.1 (C-O-C) and 533.6 eV (C-OH).27 Compared with those of rGO (Figure S5c),

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the intensities of the peaks for O=C-OH, C=O, and C-OH are weakened, but the

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intensity of the peak for C-O-C is increased, revealing that the oxygen species from

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POP are incorporated into the molecular structure of POP-rGO NSs upon

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copolymerization and recombination. This is confirmed by the increase in the oxygen

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content of POP-rGO NSs (16.3 wt%) in comparison with that of rGO (14.6 wt%). The

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C1s XPS spectrum for POP-rGO NSs (Figure 1a) can also be deconvoluted into four

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peaks at 284.6 (C-C/C=C), 285.7 (C-O), 288.4 (C=O), and 291.1 eV (O=C-OH).28-30

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The binding energy of the C-O bond is 0.3 eV lower than that of rGO (286.0 eV,

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Figure S5d), suggesting that the C-O-C bridges introduced in POP-rGO NSs are

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different from the surface ether C-O-C bridges of rGO. In addition, the subequal

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binding energies for C-O-C in the O1s XPS spectra of POP-CP (Figure S6b) and that

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of POP-rGO NSs confirm that the O in the C-O-C bonds of POP-rGO NSs mainly 8


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come from POP polymerization. However, the C-O binding energy in the C1s XPS

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spectrum of POP-rGO NSs (285.7 eV) is very different from that of POP-CP (286.2

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eV, Figure S6c). These results indicated that the O atoms in the C-O-C bridges of

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POP-rGO NSs originate from the deprotonated phenolic OH group of POP, while the

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C atoms in the C-O-C bridges partly come from the POP molecules and partly come

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from the rGO structure, revealing that the C-O-C bridges in POP-rGO NSs consist of

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the deprotonated phenolic OH groups of POP connecting with the C atoms in the

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graphitic ring through the copolymerization and dehydration processes.

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Figure 1b shows the FTIR spectra of the GO, rGO, and POP-rGO NSs samples.

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Compared with GO, both POP-rGO NSs and rGO show similarly low absorption peak

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intensities for the oxygen functional groups at 1500−3400 cm-1. However, the C-O-C

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stretching for POP-rGO NSs is significantly different from that in rGO. Firstly, the

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intensity of the C-O-C stretching in POP-rGO NSs is markedly increased in

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comparison with that of rGO, suggesting that the molecular-level recombination of

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the planar hexagonal structure with POP molecules introduces more C-O-C bonds.

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Moreover, the marked red-shift (11.6 cm-1) of the C-O-C stretching (1217.0 cm-1) in

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POP-rGO NSs from that of rGO (1228.6 cm-1) indicates that the introduced C-O-C is

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different from the surface ether C-O-C of rGO (Figure S7a). This result is consistent

200

with the XPS analysis, confirming that the extra C-O-C bonds are internal (Figure

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S7b). The FTIR spectra of POP and the condensation polymer of POP (POP-CP) was

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shown in Figure S8. From the spectra, we can found that the formed C-O-C (1217.0

203

cm-1) in POP-rGO NSs is similar to the C-O-C (1218.9 cm-1) between the two

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benzene rings in the POP molecule, but different from the C-O-C bridge (1211.2 cm-1)

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formed upon the self-polymerization of the POP molecule. All the results above

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indicate that the introduced C-O-C bridges include both the intrinsic C-O-C from POP 9



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and those formed by the deprotonated phenolic OH groups of POP connecting with C

208

atoms in the graphitic ring upon copolymerization and dehydration.

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Figure 1c shows the Raman spectra of POP-rGO NSs, rGO, and GO. All the

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samples exhibit two characteristic peaks for D and G bands at ~1340 and ~1590 cm-1,

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which are related to disorder and graphitic order, respectively.31 The D band

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corresponds to carbon atoms with sp3 hybridization that occur owing to defects or

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distortion of the crystal lattice. The G band corresponds to the active E2g vibration of

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sp2-hybridized carbon atoms.29 The intensity ratio of the D band to the G band (ID/IG)

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is usually a measure of the defects/disorder in graphene. A larger ID/IG indicates more

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sp3 defects or disorder and smaller average size (or larger amount) of in-plane

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graphitic crystallite sp2 domains.29, 32 Compared with GO (0.948), rGO and POP-rGO

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NSs show higher ID/IG ratios, indicating that rGO and POP-rGO NSs contain more

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defects. However, The ID/IG value of POP-rGO NSs (1.000) is almost the same as that

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of rGO (1.021), indicating that the copolymerization process between rGO and POP

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molecules actually occurs on the surface of the rGO through the deprotonated

222

phenolic OH groups of POP connecting with the C atoms in the graphitic rings and

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does not destroy the bulk structure of the rGO, so the intrinsic defects are not

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introduced to the rGO substrate in POP-rGO NSs. In addition, the Raman intensities

225

of POP-rGO NSs are enhanced compared to those of pure rGO, which could be

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attributed to the enhanced local electromagnetic field induced

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functionalization.33, 34

by POP

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The presence of single electrons in the as-formed samples was investigated by

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EPR spectroscopy (Figure 1d). GO shows a very sharp and symmetrical signal at g =

230

1.994, which may be assigned to the single electrons induced by the numerous surface

231

oxygen functional groups, such as -COOH, -OH, C=O and C-O-C groups, as revealed 10


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by XPS and FTIR.35 In rGO, the sharp EPR signal disappears, which is due to the

233

obvious decrease of the single electrons upon the marked reduction of the surface

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oxygen functional groups. In addition, a broader EPR signal at g = 1.996 is observed,

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which is closer to the g value (~2.0023) of the free electrons from the defects of the

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carbon framework.36 POP-rGO NSs also shows a similar EPR signal with the same g

237

value, and this signal is significantly enhanced in POP-rGO NSs, which is due to the

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bonding of the negatively-charged O and edge carbon atoms carrying π-electronic

239

spins in the formed C-O-C bridges.37 In contrast, no EPR signal is observed in the

240

corresponding range for pure POP. These results indicate that the C-O-C bridges

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formed by the deprotonated phenolic OH groups of POP connecting with the rGO

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substrate draw a large number of single electrons around the introduced O, forming

243

electron-rich centers.

244

Therefore, an efficient synthetic route for POP-rGO NSs is confirmed by the

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characterization analysis above, and is presented in Figure S9. Graphite consists of

246

stacking layers of flat carbon atoms in a honeycomb-like arrangement. GO is prepared

247

by the modified Hummers method through oxidation of graphite powder. rGO is

248

obtained by GO reduction via an annealing process. The functionalization of GO with

249

POP molecules is completed in an ethanol environment, accompanied by ultrasonic,

250

thermal, and evaporation procedures, resulting in formation of an intermediate state

251

(GO-POP). The POP molecules are complexed on the surface of the GO through

252

hydrogen bonds,18 which are of the O−H···O type between the hydroxyl groups of

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GO and the POP molecules and are of moderate strength.18 During annealing at

254

350 °C, the GO-POP intermediate undergoes dehydration, polymerization,

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recombination, and reduction, and eventually forms POP-rGO NSs. The POP

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molecule is eventually grafted on the graphene lattice via both C-O-C bridges and π-π 11



257

interaction, which result from the deprotonated phenolic OH groups of POP bonding

258

with the C atoms in the graphitic rings of the infinitely extended structure upon

259

copolymerization and dehydration. The formation of these special C-O-C bonding

260

bridges affects the electron distribution of the rGO substrate and draws a large number

261

of single electrons around the introduced O, which changes the electronic properties

262

of the catalyst.

263 264

DFT calculations for the electronic properties of catalysts. Distributions of

265

electrostatic potentials (ESPs) and electric charges can provide useful information for

266

reaction center analysis. Negative and positive areas are expected to be promising

267

reactive sites for reduction and oxidation reactions, respectively.38, 39 Therefore, we

268

investigated the distribution of the ESP on the graphene fragments for different

269

positions of the O atom and different electric charges for the O and C atoms of

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POP-rGO NSs through DFT calculations (Figure 2). Different colors in the

271

distribution maps are used to represent different values for the electrostatic potential,

272

where red and blue represent the most negative potential (electron-rich) and the most

273

positive potential (electron-poor) regions,40 respectively. Figure 2a shows an

274

optimized pure graphene fragment without any O groups, in which the ESP is evenly

275

distributed for each element. After introducing an -OH group on the surface of the

276

graphene fragment (Figure 2b), which actually represents GO or rGO (with -OH), the

277

ESP distribution becomes uneven. The negative potential is mainly located around the

278

O atom of the surface -OH group, which is consistent with the EPR result. The

279

electron density of the hexatomic ring (π electrons) that bonds with the surface -OH

280

group decreases, while the others away from the -OH group are not significantly

281

affected. Figure 2c shows an optimized structure fragment of POP-rGO NSs. The 12


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ESP map suggests that the most electronegative part (red) of the POP-rGO NSs

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fragment is the area around the O atom in the C-O-C bonding bridge connecting the

284

rGO and the POP molecule. In addition, the O atom that links the two benzene rings

285

in POP also shows a high electronegativity. The bluest parts are the areas around the

286

edge H atoms that are added to improve the computability of the model and do not

287

exist in the actual POP-rGO NSs structure, and are therefore not considered. C and O

288

sites are the main targets in the structure of POP-rGO NSs. Figure S10 shows that the

289

electric charge of the O atom connecting the rGO and POP molecule is -0.595, which

290

is the most negative for all the atoms of POP-rGO NSs, confirming the electron-rich

291

center of POP-rGO NSs located around the introduced O atom. The electric charges

292

of the two C atoms bonded with the center O atom are 0.159 and 0.345, much higher

293

than those of other C atoms in their respective benzene rings. Therefore, the

294

electron-poor centers of POP-rGO NSs are located around the C atoms that bond with

295

the electron-rich O atom.

296

These results are consistent with the EPR spectroscopy results, which revealed

297

that the surface complexation of POP with rGO via C-O-C bridges enables the

298

non-uniform distribution of electrons on the surface of the catalyst, producing the dual

299

reaction centers in the C-O-C bonding bridges of POP-rGO NSs. Around the O atoms,

300

the electron-rich centers appear, and at the edge C atoms, the electron-poor centers

301

appear. Owing to the special C-O-C connecting mode by the deprotonated phenolic

302

OH group of POP with the C of the rGO substrate, the free electrons are actively

303

attracted around the electron-rich O centers from the electron-poor C centers.

304

The chemical reactivity of POP-rGO NSs was theoretically analyzed using DFT.

305

The energy separation between the highest occupied molecular orbital (HOMO) and

306

the lowest unoccupied molecular orbital (LUMO) is often used as an indicator of 13



307

kinetic stability and chemical reactivity.41 The HOMO–LUMO gaps for the POP

308

molecule, graphene (with an -OH group), and POP-rGO NSs were calculated and are

309

shown in Table S1. The gap for the POP molecule is 5.60 eV, implying a high kinetic

310

stability and a low chemical reactivity.41 For GO (with the -OH group), the

311

HOMO–LUMO gaps for the α and β electrons are 3.87 and 3.88 eV, respectively.

312

However, the gaps for the α and β electrons decrease to 3.54 and 3.22 eV, respectively,

313

for POP-rGO NSs, which is energetically favorable for extracting electrons from the

314

low-lying HOMO and adding electrons to the high-lying LUMO, forming an activated

315

complex for the potential reaction.42 These results indicate that the chemical reactivity

316

of POP-rGO NSs is significantly higher than that of graphene and pure POP

317

molecules owing to the formation of dual reaction centers.

318 319

Interaction of POP-rGO NSs with organic pollutants and H2O2 in water. The

320

FTIR spectra of the POP-rGO NSs samples before and after adsorption of the

321

pollutants (BPA and 2-CP) are shown in Figure 3a. The spectrum of the fresh

322

POP-rGO NSs sample shows a broad absorption band at 3446.6 cm-1, ascribed to the

323

-OH stretching vibrations of H2O [ν(H2O)] adsorbed on the catalyst surface.23 After

324

adsorption of 2-CP and BPA, the ν(H2O) bands shift to 3442.7 and 3429.3 cm-1,

325

respectively, indicating deprotonation of the phenolic OH groups of 2-CP and BPA,43

326

verifying that the pollutants are adsorbed onto the surface of POP-rGO NSs, replacing

327

H2O. Of the other bands, only the C-O-C stretching shifts noticeably upon 2-CP and

328

BPA adsorption, suggesting that the interaction of pollutants with the catalyst is

329

strongly related to the C-O-C bond rather than the C=O or C=C bonds. Figure 3b

330

shows the 13C solid state NMR spectra of GO, rGO, POP-rGO NSs and the POP-rGO

331

NSs adsorbing BPA (POP-rGO NSs/BPA). The GO spectrum shows several peaks at 14


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67 (epoxide C-O-C), 79 (C-OH) and 137 (sp2 C) ppm,44 while the spectrum of rGO

333

shows almost complete elimination of the oxygen functionalities due to the successful

334

reduction of GO. For POP-rGO NSs, in addition to the evident the graphitic sp2

335

carbon peak, we also observed a peak at ~60 ppm corresponding to C-O-C.45 It is

336

worth noting that this peak have now exhibit an obvious upfield shift as compared to

337

that of the epoxide C-O-C (67 ppm) of GO, confirming the polymerization and

338

formation of framework C-O-C bridges between rGO and POP. After adsorbing BPA

339

on the surface of POP-rGO NSs, the

340

concealed and several peaks appeared in the range of -25 - 0 ppm corresponding to

341

the -CH3 on BPA, indicating BPA adsorbing on the C sites of C-O-C bridges on

342

POP-rGO NSs. According the characterizations and DFT calculations, the adsorbed

343

H2O molecules or pollutants are mainly complexed with the electron-poor C sites in

344

the C-O-C bridges (Figure S11). In the absence of pollutants, the electron-poor C

345

sites are complexed with adsorbed H2O through C···O-H. In the presence of

346

pollutants, the C sites tend to be complexed with the more electron-rich organic

347

pollutants by C···O-C.

13

C NMR spectra of C-O-C bridges was

348

H2O2 adsorption on the dual reaction centers of POP-rGO NSs was theoretically

349

modeled using DFT calculations. The results show that, at the electron-poor centers

350

(Figure 3c), H2O2 cannot directly absorb on the C atom, but connect to the H of OH

351

in the C···O-H (2.876 Å) produced by the adsorbed H2O with the O atom of H2O2

352

through a hydrogen bond (1.897 Å). However, in the presence of pollutants, this

353

phenomenon does not happen because, as indicated by the FTIR results (Figure 3a),

354

the C sites prefer to be occupied by organic compounds rather than H2O, hindering

355

the adsorption and oxidation of H2O2. In contrast, at the electron-rich centers (Figure

356

3d), H2O2 can directly absorb on the O sites through a H-O-O-H···O hydrogen bond 15



357

(1.898 Å), and this is not affected by the pollutants. We also calculated the H2O2

358

adsorption on the surface of graphene (with surface ether C-O-C) mode (Figure S12).

359

Very differently, H2O2 are more inclined to adsorb on the predominant C sites through

360

H-O-O-H···C bond (2.467 Å), rather than on the O sites of the few surface ether

361

C-O-C. Thus, the electron transfer from POP-rGO NSs to H2O2 is more easily than

362

that from graphene (with surface ether C-O-C) to H2O2 due to the H···O hydrogen

363

bond length (1.898 Å) much shorter than the H···C bond length (2.467 Å). The O-O

364

bond length of the adsorbed H2O2 on the electron-rich O center of POP-rGO NSs

365

increased comparing with that on the C site of graphene indicates that H2O2 on the

366

former is more likely to split via getting electrons from POP-rGO NSs and producing

367

active radicals.46, 47

368 369

Conversion of H2O2 on the catalyst surface. The conversion paths of H2O2 on

370

different material surfaces were studied using the EPR spin-trapping technique with

371

5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO) as a trapping agent.

372

The BMPO-•OH species were detected in water and the BMPO-HO2•/O2•− species

373

were detected in methanolic media due to the instability of the HO2•/O2•− radicals in

374

water. As shown in Figure 4a,b, in the absence of pollutants, no significant EPR

375

signals for •OH and HO2•/O2•− are observed in the GO suspension, indicating the

376

absence of usable free electrons on GO. Conversely, the rGO and POP-rGO NSs

377

suspensions exhibit evident •OH and HO2•/O2•− signals. The former presents

378

weaker •OH signals and the strongest HO2•/O2•− signals, while the latter presents the

379

greatest intensity for the •OH signals and a smaller intensity for the HO2•/O2•− signals,

380

indicating that the conversion paths of H2O2 are different on the surfaces of rGO and

381

POP-rGO NSs. 16


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382

Evidently, the rGO/H2O2 system is essentially a classical Fenton system owing to

383

the small amount of phenolic -OH groups remaining on the surface of the rGO.

384

According to a previous report,48 hydroquinone/quinone (-OH/=O) subunits on rGO

385

with adequate redox potential can act as the active sites for H2O2 activation. Thus, the

386

generation of HO2•/O2•− and •OH may result from the oxidation-reduction of H2O2 in

387

a single reaction center (-OH or =O state) of rGO. In such a system, each reaction

388

cycle would consume two H2O2 molecules, generating one •OH and accompanying a

389

HO2•/O2•− production.

390

In contrast, the dual reaction centers are responsible for the conversion of H2O2

391

on the POP-rGO NSs surface. At the electron-rich centers, a large amount of H2O2 is

392

absorbed on the O sites through the H-O-O-H···O hydrogen bonds and is rapidly

393

reduced to •OH. At the electron-poor centers, H2O occupies the C sites of POP-rGO

394

NSs through C···O-H interactions. The electrons of H2O2 cannot be directly adsorbed

395

at the C centers, but are indirectly adsorbed at the C centers via hydrogen bonds with

396

H2O, which prevent the rapid oxidation of H2O2 to generate HO2•/O2•−. Thus,

397

more •OH and less HO2•/O2•− are detected in the POP-rGO NSs suspension.

398

The single electrons of rGO and POP-rGO NSs remaining after reaction with

399

H2O2 were measured through EPR. The signal intensity of rGO visibly diminishes

400

after reacting with H2O2 (Figure S13a), suggesting that the free electrons on the

401

single active sites are delivered to H2O2 producing •OH, while the resulting quinone

402

state cannot be reduced in time by H2O2. The weak electron cycle capability of rGO

403

indicates that the classical Fenton process has a rate-limiting step that cannot be

404

overcome even in the metal-free rGO system. In contrast, the EPR signal intensities

405

for POP-rGO NSs before and after reaction are the same (Figure S13b), indicating

406

that in the electron-rich centers, the electrons are supplemented by H2O2 capture, 17



407

although at the electron-poor site, the rate of H2O2 electron donation is slow due to the

408

barrier of the OH-group in which H2O2 is absorbed by the hydrogen bond. This result

409

was contributed by the special connecting mode of C-O-C that produced by the

410

deprotonated phenolic OH groups of POP with the C atoms of the rGO substrate on

411

the surface of POP-rGO NSs, so that the electrons donated by H2O2 are quickly

412

diverted to the electron-rich centers from the electron-poor centers.

413

In the presence of organic pollutants (2-CP or BPA), the •OH signal intensities

414

for rGO (Figure S14a) and POP-rGO NSs (Figure 4c) are not significantly changed,

415

indicating that the presence of the organic pollutants does not affect the reduction of

416

H2O2. A slight decrease in the signals for POP-rGO NSs (Figure 4c, red and blue

417

lines) suggests that the generated •OH radicals are consumed by pollutants. However,

418

the addition of pollutants significantly affects the production of HO2•/O2•− in the

419

POP-rGO NSs and rGO suspensions. After adding 2-CP or BPA, the HO2•/O2•−

420

signals are enhanced in the rGO suspension (Figure S14b). This is because the

421

degradation of pollutants consumes •OH, producing more surface quinone state,

422

resulting in more H2O2 being oxidized to HO2•/O2•− or O2 by the conversion of

423

surface quinone to surface hydroquinone, which is the limiting step in the whole

424

reaction process. In general, some additional energy input, such as light, electricity,

425

ultrasound, or chelating agents are used to improve this step in the classical Fenton

426

reaction system.49, 50 However, the HO2•/O2•− signals are significantly weakened in the

427

POP-rGO NSs suspensions after adding 2-CP or BPA (Figure 4d). This is because the

428

electron-poor C sites tend to be complexed with the electron-rich organic compounds

429

by C···O-C- in the presence of the pollutants. Thus, the organic pollutants can also act

430

as electron donors at the electron-poor sites, causing free electrons to circulate back to

431

the electron-rich centers more quickly than that at the C sites where H2O2 is absorbed 18


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432

by C···O-H···O-, and avoiding the oxidation of partial H2O2 into HO2•/O2•−.

433

The greatest difference between the POP-rGO NSs/H2O2 system and the classical

434

Fenton system is that the reduction and oxidation reactions occur at two different

435

centers (electron-rich and electron-poor) on POP-rGO NSs. These dual reaction

436

centers can allow the selective reduction of H2O2 and the oxidative degradation of

437

pollutants at different sites, avoiding energy waste and requiring no additional energy.

438 439

Catalytic performance for refractory pollutant degradation. In order to confirm

440

the high Fenton-like catalytic performance at the formed dual reaction centers, the

441

refractory pollutants 2-CP and BPA were chosen as degradation targets in the presence

442

of H2O2 under the mild conditions. As shown in Figure 5a, no significant 2-CP

443

degradation is observed in the graphite suspension. The degradation of 2-CP in the

444

GO suspension is only 8.1% within 120 min. This value increases to 41.9% in the

445

rGO suspension. Astonishingly, the degradation rate of 2-CP in the POP-rGO NSs

446

suspension reaches 88.7% within 120 min, which is 35.8-, 25.6-, and 3.5-times higher

447

than that in the graphite, GO, and rGO suspensions, respectively (Figure S15). Along

448

with the degradation of 2-CP, the TOC removal rate reaches 70.0% in the POP-rGO

449

NSs suspension (Figure S16), which is significantly higher than that in the

450

suspensions of graphite (3.5%), GO (6.5%), and rGO (31.6%). The degradation order

451

of 2-CP is in accordance with the generation of •OH in the different suspensions with

452

H2O2 from the EPR results. These results suggest that the formation of the dual

453

reaction centers in the metal-free catalyst POP-rGO NSs greatly promotes its catalytic

454

activity for pollutant degradation. Similarly, BPA degradation in different suspensions

455

(Figure 5b) for 120 min also follows the order graphite (1.1%) < GO (4.0%) < rGO

456

(19.0%)

4-Phenoxyphenol-Functionalized Reduced Graphene Oxide

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