Efficient Catalytic Ozonation over Reduced Graphene Oxide for p

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Efficient Catalytic Ozonation over Reduced Graphene Oxide for pHydroxylbenzoic Acid (PHBA) Destruction: Active Site and Mechanism Yuxian Wang, Yongbing Xie, Hongqi Sun, Jiadong Xiao, Hongbin Cao, and Shaobin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01175 • Publication Date (Web): 23 Mar 2016 Downloaded from http://pubs.acs.org on March 28, 2016

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ACS Applied Materials & Interfaces

Efficient Catalytic Ozonation over Reduced Graphene Oxide for p-Hydroxylbenzoic Acid (PHBA) Destruction: Active Site and Mechanism Yuxian Wang1,2, Yongbing Xie1, Hongqi Sun2, Jiadong Xiao1,3, Hongbin Cao1* and Shaobin Wang2*

1

Division of Environment Technology and Engineering, Beijing Engineering Research Center

of Process Pollution Control, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. 2

Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845,

Australia. 3

University of Chinese Academy of Sciences, Beijing 100049, China

*Corresponding Authors. Email: [email protected] (H.C); [email protected] (S.W)

1

ACS Paragon Plus Environment


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ABSTRACT

Nanocarbons have been demonstrated as promising environmentally-benign catalysts for advanced oxidation processes (AOPs) upgrading metal-based materials. In this study, reduced graphene oxide (rGO) with a low level of structural defects was synthesized via a scalable method for catalytic ozonation of p-hydroxylbenzoic acid (PHBA). Metal-free rGO materials were found to exhibit a superior activity in activating ozone for catalytic oxidation of organic phenolics. The electron-rich carbonyl groups were identified as the active sites for the catalytic reaction. Electron spin resonance (ESR) and radical competition tests revealed that superoxide radical (·O2-) and singlet oxygen (1O2) were the reactive oxygen species (ROS) for PHBA degradation. The intermediates and the degradation pathways were illustrated from mass spectroscopy. It was interesting to observe that addition of NaCl could enhance both ozonation and catalytic ozonation efficiencies and make •O2- as the dominant ROS. Stability of the catalysts was also evaluated by the successive tests. Loss of specific surface area and changes in the surface chemistry were suggested to be responsible for catalyst deactivation.

Keywords: Catalytic ozonation, reduced graphene oxide, surface carbonyl groups, p-hydroxylbenzoic acid, superoxide radical.

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1. INTRODUCTION Phenolic compounds are widely present in wastewaters discharged from various industrial processes. Among these contaminants, p-hydroxylbenzoic acid (PHBA) released from food processing industries is believed to be of high toxicity and biostability.1 Ozonation process is an eco-friendly technique for water remediation since it does not produce any secondary contaminants.

2-4

It is favorable for the decomposition of unsaturated aromatic and aliphatic

compounds, yet can hardly treat saturated organic compounds.5 For removal of persistent phenolic compounds, ozonation will be only effective in the first-step decomposition and incapable for intermediate removal, thus resulting in high chemical oxygen demand (COD) residuals. In the last two decades, the fast development in advanced oxidation processes (AOPs) has paved the road for treatment of these persistent organic pollutants. With reactive species, AOPs have demonstrated their complete degradation capability and exceptional mineralization efficiency for environmental remediation.6

The recently emerging catalytic ozonation demonstrated nonselective oxidation in water treatment. Introduction of catalysts would significantly promote the decomposition of ozone molecules to generate reactive oxygen species (ROS) such as hydroxyl radical (•OH), superoxide radical (•O2¯) and singlet oxygen (1O2) for destroying both saturated and unsaturated organics.7-8 However, the dominant ROS during the catalytic ozonation process is still controversial. In most of the previous studies, the high efficiency of catalytic ozonation was ascribed to the generated •OH with the highest oxidation potential.9 Some recent researches revealed that catalytic ozonation was not necessarily relied on the •OH.10-11 Studies also suggested that the dominant ROS were relevant with the target pollutants.12 Therefore, unveiling the dominant ROS in catalytic ozonation for PHBA removal is in urgent demand.

Metal-based

materials

have

demonstrated

their

excellent

activities

as

homogeneous/heterogeneous catalysts.13-14 However, secondary pollution brought by the metal-leaching has always incurred an issue for these metal-based catalysts. To circumvent 3



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this issue, catalytic ozonation using metal-free catalysts such as activated carbon, carbon black and carbon nanotubes (CNTs) has been investigated.15-18 It was revealed that the surface functional groups (acidic and basic groups) and the textural structures (defective structures and textural properties) could immensely influence the catalytic activities of the metal-free catalysts, yet insight into the contribution of catalyst properties has been rarely reported.5, 19-20 Moreover, in previous literatures, graphene/rGO was often employed as a catalyst support to facilitate target pollutant adsorption and to promote the electron transfer rate in the catalytic ozonation processes.21-22 To the best of our knowledge, metal-free graphene/rGO as the catalysts for catalytic ozonation has never been discovered.

In practical production, NaCl is widely presented in industrial wastewater at different concentrations. In tannery and dye manufacturing effluents, high concentrations of NaCl (> 50 g/L) can be frequently detected. NaCl at a lower concentration could be also observed from off-shore oil extraction wastewater.

23

Nevertheless, the presence of NaCl on the

organics degradation during wastewater treatment is not well explicated. Thus, evaluating the influence of chloride anion at different concentrations on catalytic ozonation efficiency as well as its impact on ROS generation are highly critical for both industrial practices and mechanism studies.

Herein, we reported a facile method for synthesis of rGO with a low defect/disorder level by thermal reduction of graphene oxide to minimize the influence of the defective structure for differentiating the contribution of oxygen-containing functional groups to catalytic ozonation. Reactive species responsible for PHBA degradation were identified by electron spin resonance (ESR) and quenching tests. Reaction intermediates were detected with mass spectrometry (MS). Meanwhile, the effect of different concentrations of NaCl in solution on the catalytic ozonation efficiency was investigated. Stability of the catalysts was also evaluated. From the variations of oxygen-containing surface functional groups on fresh and used catalysts, the main active sites for catalytic ozonation were discussed.

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2. EXPERIMENTAL METHODS 2.1. Synthesis of low defective reduced graphene oxide (rGO) Graphene oxide (GO) was prepared by a modified Hummers’ method

24

and used as the

precursor for synthesis of reduced graphene oxide (rGO). One rGO sample was synthesized under static air atmosphere. In the process, 1 g GO was transferred into a capped crucible and heated in a muffle furnace at 80 oC for 1 h, followed by heating at 300 oC for another 1 h. And the obtained sample was denoted as rGO-300. The other rGO sample was prepared under N2 atmosphere, in which 0.5 g GO was transferred into a quartz boat and annealed in a tube furnace under N2 atmosphere at 700 oC for 1 h. And the acquired sample was labelled as rGO-700. Prior to heating, the tube furnace was flushed with pure nitrogen for 3 h at a flow rate of 50 mL/min to remove the residual air.

2.2. Characterization X-ray diffraction (XRD) patterns were obtained from a X’Pert-PROMPD (PAN analytical B.V.), operating at an accelerated voltage of 40 kV and an emission current of 40 mA with Cu Kα radiation (λ = 1.5418 Å). N2 sorption isotherms at -196 oC were acquired from Autosorb-iQ,

Quantachrome.

The

Brunauer−Emmett−Teller

(BET)

equation

and

Barrett−Joyner−Halenda (BJH) method were employed to calculate the specific surface area (SSA) and pore size distribution of the carbon materials, respectively. Prior to testing, the samples were degassed at 120 oC for 12 h. Morphology of the samples were observed on a JSM-7001F field emission scanning electron microscope (FESEM) at the accelerating voltage of 15 kV. Transmission electron microscopy (TEM) images were obtained from a JEOL 2100F (UHR) TEM instrument. Surface composition of the carbon materials was characterized by X-ray photoelectron microscopy (XPS, Thermo Fisher Scientific ESCALAB 250Xi) under UHV condition with Al-Kα X-ray. Raman spectra of the samples were recorded by an ISA argon-laser Raman spectrometer (LabRAM-HR800, Horiba Jobin Yvon). Total organic carbon was measured on a Shimadzu TOC-vcph analyzer. Electron spin resonance (ESR) experiments for radical analysis were performed on a JEOL JES X310 spectrometer with 5,5-dimethyl-1-pyrroline (DMPO) and 2,2,6,6-tetramethyl-4-piperidinol (TEMP) as 5



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spin-trapping agents.

2.3. Catalytic ozonation process A semi-batch reactor containing 0.5 L of PHBA solution was employed for the catalytic ozonation process. Unless specified, the concentration of PHBA was 20 ppm. The reactor was submerged in a water bath with the temperature set at 25 oC and a stirring speed of 300 rpm. Ozone was generated by an Anseros Ozomat GM ozone generator from high purity oxygen (99.9%). The inlet flow rate of ozone was 100 mL/min. And unless further specified, the concentration of ozone was set to be 20 mg/L. In a typical test, 0.05 g catalyst was added into the PHBA solution and kept stirring for 30 min to achieve adsorption-desorption equilibrium and the initial pH of the solution was determined to be 3.5. Then the valve connecting to the ozone generator was opened to let ozone be fed into the bottom of the reactor through a porous glass-made diffuser. At certain time intervals, water samples were withdrawn from the reactor and filtered into a vial through a 0.22 µm PTFE filter. The concentrations of PHBA samples were analyzed by a high performance liquid chromatography (HPLC, Agilent Series 1200) set at a wavelength of 270 nm using a C-18 column. The mobile phase was made of 75% of dilute phosphate acid solution (1 wt%) and 25% of methanol under a flow rate of 0.25 mL/min. For the stability tests, the used sample was collected by vacuum filtration after each run and then washed with ultrapure water for 3 times.

3. RESULTS AND DISCUSSION 3.1. Characterization of as-synthesized GO and rGO samples

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(A)

rGO-300 rGO-700

Relative Intensity (a.u.)


GO rGO-300 rGO-700

20

25

30

2 Theta (deg)

10

20

30

40

50

60

70

2 Theta (deg)

(B )

D band


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


G band

GO I D /I G =0.86

rGO-300 rGO-700

I D /I G =0.84

I D /I G =0.98

500

1000

1500

2000

-1

2500

R am an S hift (cm )

Figure 1. (A) XRD patterns of GO and rGO (Inset: high resolution of graphite phase). (B) Raman spectra of GO, rGO-300 and rGO-700.

XRD patterns of GO and rGO samples are shown in Fig. 1(A). Diffraction plane (002) of graphene oxide at 2θ = 10.6o was observed for GO sample, revealing a well-ordered and layered structure. The two dimensional reflection (110) was found at 2θ =42.5o, suggesting a short ranged order in graphene oxide layers.25 According to Bragg’s law, the interlayer spacing of GO was calculated as 0.74 nm, which is much higher than the graphite (0.34 nm),26 revealing that oxygen atoms were bonded to the graphite surface in terms of oxygen-containing groups and that C=C double bonds were also formed during the oxidation of graphite.27 After reduction under heat treatment (in air or N2 atmosphere), the diffraction peak at 2θ = 10.6o disappeared and a broad peak centered at 2θ = 24.6o emerged, indicating 7



that the thermally reduced graphene oxides were disordered, yet not in a high level. Noted that when GO was annealed under N2 atmosphere, the broad peak was shifted to a higher 2θ value (25.1o), suggesting a better degree of reduction by removal of more surface functional groups. 28

The ratio of ID/IG (intensity of D band by G band) obtained from Raman spectrum is usually employed to reflect the defective graphitic degree of carbon materials.29 Shown in Fig. 1(B), after thermal reduction, little change occurred to the ID/IG ratios of rGO-300 and GO (0.84 vs. 0.86), demonstrating that low-temperature thermal reduction in air atmosphere did not create more defective sites on the graphene layers. While for rGO-700, ID/IG ratio increased from 0.86 to 0.98, suggesting more defective sites were formed due to the partially aggregated edges. Depending on the level of disorder, ID/IG ratios ranging from 1.1 to 1.5 were reported in the previous studies.28, 30-31 In this work, as-synthesized rGO samples demonstrated a much lower ID/IG ratio, indicating the rGO samples with less amount of defects were obtained.

(A)

284.5 eV 44.75% 285.6 eV 36.41% 286.8 eV 10.06% 288.4 eV 5.16% 290.8 eV 3.63%

Intensity

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C-C/C=C

C-O-C/C-OH

-C=O O-C=O π−π* shake-up

294

292

290

288

286

284

282

280

Binding Energy (eV)

8


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(B)

284.5 eV 62.02% 285.6 eV 20.51% 286.8 eV 10.06% 288.4 eV 5.16% 290.8 eV 3.63%

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C-C/C=C

C-O-C/C-OH

-C=O O-C=O π−π∗ shake-up

294

292

290

288

286

284

282

280

Binding Energy (eV)

Figure 2. XPS spectra on C 1s for rGO-300 (A) and rGO-700 (B). X-ray photoelectron spectroscopy (XPS) was employed to evaluate the surface chemical states of the as-prepared samples. Shown in full scan (Supplementary Data, Fig. S1), GO has the highest oxygen content (28.8%) presented in oxygen-containing functional groups because a large amount of oxygen atoms were introduced to the graphite layers during synthesis. Fig. S2 displayed that carbonyl group (C=O) was the major oxygen-containing group in the GO structure (41.6%). On the other hand, both air and N2 annealing treatments could significantly reduce the carbonyl group and transfer it into other oxygen-containing groups, especially hydroxyl (C-OH) and ester groups (C-O-C) (Fig. 2). As observed, the thermal reduction at low temperature in static air could reduce the loading of carbonyl group from 41.6% to around 10%. The high temperature N2 treatment could further reduce the carbonyl group to 8.42%. In addition, the total amount of oxygen-containing groups in rGO-700 was less than that in rGO-300, confirming more oxygen atoms were removed in the high temperature N2 treatment.

9



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(A)

(B)

(C)

(D)

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Figure 3. SEM images of rGO-300 (A and B) and rGO-700 (C and D). Observed in SEM images (Fig. 3), removal of surface oxygen-containing functional groups could also result in exfoliation of the graphene oxide nanosheets, which was also confirmed by TEM images (Fig. S3). Compared with the wrinkled and silk-like sheet structure of GO (Fig. S4), both rGO-300 and rGO-700 demonstrated highly exfoliated layers of graphene nanosheets. Fig. 3(A) and (B) clearly reveal the exfoliation of the thick layers of GO into numerous sub-layers under thermal reduction. For rGO-700 (Fig. 3(C) and (D)), the partially aggregated and crinkled structure observed at the end of the graphene nanosheets could be ascribed to the creation of more defective sites after the thermal treatment at a higher temperature.28

Exfoliation of graphene oxide nanosheets also resulted in the enlargement of the specific surface area. Fig. S5 described nitrogen sorption isotherms and pore size distributions of GO and rGO samples. After thermal reduction, wrinkled and silk-like sheet structure of GO exfoliated and the BET surface areas of rGO-300 and rGO-700 tremendously increased from 10


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40.3 to 305.4 and 265.4 m2/g, respectively, owing to the removal of surface functional groups. In addition, heat treatment also increased total pore volume and average pore diameter of the samples (Table 1). Table 1. Textural properties of GO, rGO-300 and rGO-700. Surface area (SBET m2/g) 40

Pore volume (cm3/g) 0.065

Average pore diameter (nm ) 6

rGO-300

305

1.62

21

rGO-700

265

1.86

28

rGO-300 used

127

0.39

12

rGO-300 used with NaCl

91

0.27

13

Catalyst GO

3.2 Catalytic ozonation employing as-synthesized GO and rGO samples Catalytic activities of the carbon materials were evaluated in catalytic ozonation of PHBA with an initial pH of 3.5. For these two graphene materials, the adsorption only led to negligible removal of PHBA, though they showed large specific surface areas (Fig. 4(A)). The point of zero charge (pHpzc) of rGO-300 and rGO-700 were 4.7 and 4.9, respectively. This indicated that they were positively charged in the solution at pH 3.5. With a pKa value of 4.85, PHBA mainly existed in a molecular form rather than ionic form, and thus leading to low physical adsorption. Literatures revealed that ozonation also resulted in changes in surface chemistry of nanocarbons, which would affect their adsorption capabilities.32-34 In order to investigate the changes in adsorption capacity arising from ozonation, rGO-300 was treated in solution by ozone for different time intervals and its adsorption abilities in PHBA were evaluated and compared (Fig. S6). PHBA adsorption on these ozone treated rGOs was still insignificant, though longer ozone treatment induced slightly higher adsorption abilities. The adsorption efficiencies of PHBA were 8.2%, 9.4% and 10.1% on the rGO treated by ozone for 15, 30, and 60 min, respectively.

To quantitatively determine the surface acidity/alkalinity of the rGO samples, Boehm titration method employing NaOC2H5 and HCl solutions was further performed (detailed procedures

11



of the Boehm titration are illustrated in SI). It was found after ozone treatment, the acidic surface groups of rGO samples increased, however, the alkalinity remained the same (Table S1). Additionally, pHpzc of ozone-treated rGO slightly decreased to 4.4 due to the increase of acidic groups.

PHBA Degradation (C/C0)

(A) 1.0 Ozonation only rGO-300 adsorption rGO-700 adsorption Catalytic ozonation using rGO-300 Catalytic ozonation using rGO-700

0.8

0.6

0.4

0.2

0.0 0

10

20

30

Time (min)

40

50

60

50

60

(B) 1.0 0.8

TOC Removal (C/C0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

0.4

0.2

Ozonation only Catalytic ozonation using rGO-300 Catalytic ozonation using rGO-700

0.0 0

10

20

30

40

Time (min)

Figure 4. Catalytic ozonation with various catalysts (A) and the corresponding TOC removal (B); Reaction conditions: [PHBA]0 = 20 mg/L, Catalyst loading = 0.1 g/L, Ozone flow rate: 100 mL/min, Ozone concentration: 20 mg/L, Temperature: 25 oC, Solution pH = 3.5.

PHBA degradation by ozonation provides a benchmark for catalytic ozonation. The ozonation resulted in 95% of PHBA decomposition and 25% of TOC mineralization after 1 h (Fig. 4(B)), indicating ozone molecules are favorable to attacking the aromatic compounds with 12


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unsaturated bonds, but inactive to the saturated organics. While for catalytic ozonation employing rGO catalysts, degradation and mineralization efficiencies were enormously promoted. As seen, rGO-300 and rGO-700 displayed similar catalytic activities. For both rGO materials, a complete PHBA removal was achieved within 30 min, and the TOC removal was notably enhanced from 25% to 95%, suggesting the rGO catalysts could effectively activate ozone molecules to generate reactive species. rGO-300 obtained a higher specific surface area than rGO-700 (305.4 m2/g vs. 265.4 m2/g), but its adsorption capability and catalytic activity did not exceed. This suggested that specific surface area is not the decisive factor to adsorption capability and catalytic activity. On the other hand, the higher ID/IG ratio indicates more defective sites were formed within rGO-700 than rGO-300 (Fig. 1(B)), but rGO-700 showed less amount of oxygen-containing groups than rGO-300 (Fig. S1). Previous studies revealed that the defective sites and oxygen-containing groups are the main active sites for catalytic reactions.28,31 The similar catalytic activities of rGO-300 and rGO-700 might be aroused from the combination effect of defective sites and the oxygen-containing groups on the rGO samples. It was also found GO was also active for catalytic ozonation of PHBA and complete decomposition occurred at 60 min with 70% of TOC removal, much lower than those of rGO samples (Fig. S7).

In this work, excellent catalytic activities of rGO/graphene materials for catalytic ozonation were observed. This significantly extends the application of nanocarbon catalysis for environmental remediation. To further investigate the reaction kinetics, a pseudo-first order reaction was applied to describe the as-mentioned reactions, which is shown below.

= −

Eq. 1

Linear relationships with high regression coefficients were observed from the kinetic studies (Fig. S8). And the reaction rate constants (k) for PHBA ozonation and catalytic PHBA degradation on acidic AC, rGO-300, rGO-700 and GO were calculated to be 0.056, 0.083, 0.19, 0.21 and 0.11 min-1, respectively. Compared with the commercial AC, the reaction rates of rGO materials were enhanced more than two times. In addition, GO displayed an inferior reaction rate to rGO materials. 13



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Since rGO-300 and rGO-700 demonstrated similar catalytic activities for PHBA degradation, rGO-300 requiring less energy input for synthesis was selected for subsequent studies. The influence of initial PHBA concentration on its degradation efficiency is shown in Fig. S9. Catalytic ozonation efficiency decreased with increase of the initial PHBA concentration. Complete decomposition was postponed to 45 min when 35 and 50 ppm PHBA solutions were tested. In terms of TOC removal, a higher initial concentration brought about an inferior mineralization capability.

The influence of ozone concentration on the degradation efficiency was also investigated (Fig. S10). At 50 mg/L ozone concentration, 100% degradation of PHBA can be obtained within 15 min. Nevertheless, 10 mg/L ozone concentration prolonged complete PHBA degradation to 45 min. TOC removal immensely improved from 40% to nearly 95%, when ozone concentration was increased from 10 to 20 mg/L. However, 50 mg/L ozone did not provide significant increase in TOC removal, and the TOC removal rate only increased by 5.6%. Thus, 20 mg/L of ozone concentration was a proper level for removal of organic contaminants. Further increase in ozone loading would lead to the excessive production of reactive species, but the effect to TOC removal is very limited.

3.3. Probing reactive species in catalytic ozonation using rGO as a catalyst The mechanisms of catalytic ozonation using metal-based catalysts have been well investigated and •OH or •O2- are suggested to be the major ROS. However, there has been no research on the generation of radicals for graphene activated ozonation process.9, 35-36 In order to probe this process, electron spin resonance (ESR) experiments were performed utilizing rGO-300 as the catalyst to directly identify the possibly generated ROS. To detect •OH or •O2-, 5,5-dimethyl-1-pyrroline (DMPO) was employed as the spin trapping agent in ESR experiments by measuring the signals of DMPO-OH adducts and DMPO-•O2- adducts, respectively.37-38 On the other hand, 2,2,6,6-tetramethyl-4-piperidinol (TEMP) was utilized as the spin trapping agent to detect 1O2.39

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ESR experiments were first carried out in ultrapure water for •OH detection. As seen in Fig. 5(A), rGO/O3 system produced no characteristic signals for DMPO-•OH adducts on the ESR spectrum, suggesting •OH did not form in this graphene-based catalytic ozonation, or its concentration was very low and below the detection limit. For the detection of •O2-, absolute ethanol was employed as the reaction medium instead of ultrapure water, not only to act as the scavenger for •OH, but also to prolong the half-life of •O2-. With the addition of DMPO, the sextet ESR signal assigning to DMPO-•O2- adducts was observed in the catalytic ozonation using rGO-300 as a catalyst.40 On the contrast, for the ozonation process without the catalyst, no ESR signals were detected except for background noise. DMPO only

(A)

(B)

TEMP only

Detection for hydroxyl radical

Ozonation with TEMP

Catalytic ozonation with rGO-300

Catalytic ozonation with TEMP

Detection for superoxide radical Catalytic ozonation with rGO-300

318.3

318.4

318.5

318.6

Magnetic field (mT)

318.7

321

322

323

324

325

Magnetic field (mT)

Figure 5. (A) ESR spectra using DMPO as a trapping agent. EPR operating conditions: Centerfield: 323.2 mT; sweep width: 0.5 mT; microwave frequency: 9.057 GHz; modulation frequency: 100 GHz; and power: 4 mW. Catalytic ozonation with DMPO was carried out in absolute ethanol; (B) ESR spectra using TEMP as a trapping agent. EPR operating conditions: Centerfield: 323.2 mT; sweep width: 5 mT; microwave frequency: 9.057 GHz; modulation frequency: 100 GHz; and power: 4 mW. Catalytic ozonation with TEMP was carried out in ultrapure water. Reaction conditions: [PHBA]0 = 5 mg/L, catalyst loading= 0.2 g/L, Ozone flow rate: 100 mL/min, Ozone concentration: 5 mg/L, Temperature: 25 oC.

Due to the rapid reaction with 1O2, TEMP is regarded as the sensitive probe for 1O2 detection. The formed TEMP-1O2 adducts (TEMPO) is quite stable and ready to be detected from its characteristic three-line ESR spectrum with equal intensities (aN=1.69 mT, g=2.0054). In this work, this three-line ESR spectrum was observed in the rGO-300/O3 system in Fig. 5(B), 15



suggesting

1

O2 was produced as the ROS during the catalytic ozonation process.

Comparatively, no characteristic signals were observed in the ozonation process without the addition of rGO-300, revealing singlet oxygen was not produced from ozone molecules without the catalyst. 1.0


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0.8 Ozonation with 12mM p-BQ and 12mM NaN3

0.6

Catalytic ozonation

0.4 Catalytic ozonation with 12mM TBA Catalytic ozonation with 12mM p-BQ Catalytic ozonation with 12mM NaN3 Catalytic ozonation with 12mM p-BQ and 12mM NaN3

0.2

0.0 0

10

20

30

40

50

60

Time (min) Figure 6. Competitive radical tests for catalytic ozonation process using rGO as a catalyst. Reaction conditions: [PHBA]0 = 20 mg/L, Catalyst loading = 0.1 g/L, Ozone flow rate: 100 mL/min, Ozone concentration: 20 mg/L, Temperature: 25 oC. TBA=12 mM, p-BQ=12 mM, NaN3=12 mM.

To further examine the contribution of the reactive species to the PHBA degradation, competitive radical quenching experiments employing the same rGO-300 catalyst were performed (Fig. 6). Owing to the instantaneous reaction with hydroxyl radicals (3.8×108 7.6×108 M-1s-1) and the stagnated reaction with ozone (3 ×10-5 M-1s-1), tert-butanol (t-BA) was suggested to be the effective scavenger for •OH in the catalytic ozonation process.41 Addition of 12 mM t-BA brought no inhibition effect to the catalytic ozonation reaction, indicating •OH were not the reactive species for this catalytic ozonation system, which confirms the ESR results.

The contribution of •O2- for PHBA degradation was determined using p-benzoquinone (p-BQ) as the quenching agent.42-43 p-BQ with various dosages (1, 3, 12 and 24 mM) was added to 16


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the catalytic reaction solution (Fig. S11). It was found that with the increase of p-BQ dosage, more •O2- was quenched and lead to lower PHBA degradation rate. However, when p-BQ dosage was above 12 mM, increase of the dosage did not result in further enhancement of quenching effect, revealing that addition of 12 mM of p-BQ was sufficient for scavenging the generated radicals. In this process, part of p-BQ was consumed by ozone, but the remained p-BQ was still sufficient for quenching free radicals. At the presence of 12 mM of p-BQ, PHBA degradation efficiency dramatically decreased and around 50% of PHBA still remained after 1 h, revealing that •O2- were responsible for PHBA decomposition.

Previous studies discovered that 1O2 was also one of the ROS generated from the catalytic ozonation process.44-45 Sodium azide was effective for quenching 1O2 (2×109 M-1), however, it also react rapidly with •OH (1.2×109 M-1)

42, 46

. Addition of 12 mM of sodium azide

suppressed the degradation efficiency and resulted in 80% PHBA degradation. Since •OH were proved not active for PHBA decomposition, 1O2 was believed to be responsible for this reduction in efficiency. The contribution of ozone molecules to PHBA degradation was further examined by addition 12 mM of p-BQ and 12 mM of NaN3 to the ozonation process. As illustrated, 30% of PHBA was removed after 1 h, suggesting the contribution of ozone molecules. We noticed that with the addition of 12 mM of p-BQ and 12 mM of NaN3, the degradation rates of PHBA in ozonation and catalytic ozonation were very close, which meant the radicals were effectively quenched in the catalytic ozonation process. To clarify the competition between O3, PHBA and p-BQ and NaN3, we carried out the ozonation tests for the degradation p-BQ and NaN3, respectively. The reaction rate constants for ozonation of p-BQ and NaN3 were calculated as 0.018 min-1 and 0.004 min-1, respectively, which were much lower than ozonation of PHBA (0.056 min-1). This meant the direct competition between the scavengers and PHBA to ozone molecular is relatively low, which indicated that the quenching experiments were credible.

Detection of DMPO-•O2- adducts and TEMP-1O2 adducts within the ESR spectra confirmed the formation of •O2- and 1O2 during the catalytic ozonation. Meanwhile, competitive radical tests demonstrated their contribution to PHBA decomposition. On the basis of the ESR and 17



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the scavenging tests, •OH were excluded in this rGO/O3 system. It is widely recognized as a very active species in catalytic ozonation, though its role in direct PHBA degradation is negligible, it might still contribute to the further oxidation of intermediates of PHBA oxidation. Therefore, •O2- and 1O2 were recognized as the dominant ROS generated in this catalytic rGO/O3 system.

3.4. Investigation of catalytic degradation pathway of PHBA Mass spectroscopy (MS) with electrospray ionization (ESI) in a negative mode was utilized for detecting the reaction intermediates and revealing the mineralization process (Fig. S12). MS suggested that during the catalytic ozonation process, a hydroxyl group was firstly added on the aromatic ring of PHBA to form dihydroxylbenzoic acid. With the reaction proceeding, the benzoic acid was further oxidized and subsequently, the aromatic ring was broken and small molecular organic acids such as maleic acid, oxalic acid and crotonic acid were generated. The intermediates evolution process was clearly reflected in the peak intensities of the corresponding chemicals and the degradation pathways of PMBA are proposed in Fig. 7. Moreover, the decrease in peak intensities confirmed TOC reduction, which was well in agreement with the results in Fig. 4(B).

18


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Figure 7. Proposed degradation pathways of PHBA by catalytic ozonation.

3.5. Effect of saline solution on catalytic ozonation efficiency

(A)1.0

Ozonation Ozonation with 1g/L NaCl Ozonation with 10g/L NaCl Ozonation with 50g/L NaCl Ozonation with 100g/L NaCl


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

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Catalytic ozonation Catalytic ozonation with 1g/L NaCl Catalytic ozonation with 10g/L NaCl Catalytic ozonation with 50g/L NaCl Catalytic ozonation with 100g/L NaCl

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0.8 Ozonation with 50g/L NaCl and 12mM p-BQ and 12mM NaN3

0.6

Catalytic ozonation with 50g/L NaCl Catalytic ozonation with 50g/L NaCl and 12mM TBA Catalytic ozonation with 50g/L NaCl and 12mM p-BQ Catalytic ozonation with 50g/L NaCl and 12mM NaN3

0.4

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Catalytic ozonation with 50g/L NaCl and 12mM p-BQ and 12mM NaN3

0.0 0

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Figure 8. (A) Ozonation on rGO-300 with addition of various amounts of NaCl; (B) Catalytic ozonation on rGO-300 with addition of various amounts of NaCl; (C) Competitive radical tests for catalytic ozonation process with addition of NaCl. Reaction conditions: [PHBA]0 = 20 mg/L, Ozone concentration: 20 mg/L, Ozone flow rate: 100 mL/min, Temperature: 25 oC, Solution pH = 3.5.

Effect of NaCl on ozonation/catalytic-processes was investigated (Fig. 8). It was found that NaCl in various dosages (from 1 g/L to 100 g/L) promoted ozonation with enhanced performances in PHBA degradation and TOC removal. For ozonation only, degradation efficiency (Fig. 8(A)) and TOC removal (Fig. S13(A)) were successively enhanced with the increase of NaCl dosage. At 100 g/L NaCl dosage, complete PHBA degradation was achieved within 20 min and the TOC removal was enhanced from 20% to 70%. The reactions between ozone and chloride anion were proposed in Eqs. 2-5.47 In an acidic solution, Cl2 and HOCl would be produced and partially word as the active species for the effective degradation of 20


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carboxylic acids such as oxalic acid and maleic acid.48 Our tests demonstrated that more ozone was consumed in solution with the addition of NaCl due to the reaction with chloride anions (Fig. S14). Therefore, considering the acidic pH of the reaction solution, the generated chlorine species are responsible for the improvement of the degradation efficiency and TOC removal and the enhanced solubility of ozone in salt solution can also benefit this process.

+ → +

Eq. 2

2 + + → + !

Eq. 3

+ → + +

Eq. 4

→ +

Eq. 5

For catalytic ozonation (Fig. 8(B)), low amount of NaCl dosage (1g/L or 10g/L) led to negative effect or negligible increases in degradation efficiency and TOC removal, while further increasing the dosage enhanced the catalytic ozonation efficiency. Addition of 50 g/L NaCl reduced the complete PHBA degradation time from 20 to 10 min. Moreover, TOC removal rate increased. Due to the excessive reactive species produced, further increase of NaCl dosage did not lead to a significant improvement in degradation efficiency. However, a different phenomenon was observed in the previous researches. It was suggested that the strong scavenging effect of chloride anion with •OH would suppress the catalytic ozonation efficiency, especially in acidic environment.49-50

To investigate the variations in the active species in the presence of NaCl, competitive radical tests were conducted at 50 g/L of NaCl in solution (Fig. 8(C)). With the presence of 50 g/L of NaCl, •OH was also not responsible for PHBA degradation, proven by addition of 12 mM of t-BA. However, when 12 mM p-BQ was added, only 20% of PHBA was decomposed after 1 h, while the removal rate was 50% without NaCl. This variation suggested •O2- played a more critical role in oxidation of PHBA with the presence of 50g/L of NaCl and was responsible for the improvement of degradation efficiency. It was then deduced that chloride anions might act as the initiator to stimulate the production of •O2- and these generated •O2overwhelmed 1O2 and become the single dominant reactive species for PHBA degradation. 21



On the other hand, chloride anions might facilitate the formation of chloride substitutes which are beneficial to attracting of nucleophilic reactive species and therefore making the aromatic rings more vulnerable being attacked. 51

When 50g/L of NaCl was present, addition of 12 mM of NaN3 prolonged the time of PHBA full removal from about 10 min to 45 min, indicating 1O2 also worked for PHBA removal in NaCl solution. When both 12 mM p-BA and 12 mM NaN3 were added as the quenching agents, the degradation curves in ozonation and catalytic ozonation was very similar to each other in Fig 8(C). This indicated that about 23% of PHBA was oxidized by species other than •O2- and 1O2., which could be ozone molecular and chlorine species.

3.6. Stability test for rGO and probing the active site for catalytic ozonation

1.0

(A) PHBA Degradation (C/C0)

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Ozonation st Catalytic ozonation 1 time with rGO-300 nd Catalytic ozonation 2 time with rGO-300 recycled from normal condition nd Catalytic ozonation 2 time with rGO-300 recycled from 50g/L NaCl added condition

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(B)

Intensity

284.5 eV 50.31% 285.8 eV 28.15% 286.9 eV 5.69% 288.4 eV 9.87% 290.8 eV 5.97%

C-C/C=C

C-O-C/C-OH

π−π∗ shake-up

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292

-C=O O-C=O

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(C)

284.4 eV 51.1% 285.6 eV 25.68% 286.8 eV 11.64% 288.4 eV 9.88% 290.8 eV 1.71%

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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C-O-C/C-OH -C=O O-C=O π−π* shake-up

294

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Figure 9. (A) Stability tests of rGO-300 with/without addition of NaCl. Reaction conditions: [PHBA]0 = 20 mg/L, catalyst loading = 0.1 g/L, Ozone flow rate: 100 mL/min, Ozone concentration: 20 mg/L, Temperature: 25 oC; (B) XPS spectra on C 1s for used rGO-300; (C) XPS spectra on C 1s for used rGO-300 regenerated from NaCl involved reaction.

Previous studies reported that carboncatalysts such as rGO and doped-rGO presented poor catalytic stabilities due to the changes of surface chemistry and coverage of active sites by reaction intermediates.6, 52 Compared with the fresh catalyst, complete PHBA decomposition time for the secondly used catalyst was prolonged from 20 min to 45 min, and TOC removal declined from 90% to 60% (Fig. S15). It is also worth noting that rGO-300 regenerated from saline catalytic ozonation showed minor deactivation; 100% degradation of PHBA was accomplished within 30 min, and TOC removal rate was around 80%. 23



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In order to evaluate the catalytic stability of rGO-300 in a long term run, the catalytic activities up to the fifth run was investigated (Fig. S16). It was found that the deactivation continued, which should be ascribed by the weight loss of the catalyst, further oxidation of the surface chemistry and possibly adsorption of degradation intermediates. We also found that the deactivated catalyst can be completely recovered by thermal treatment in a muffled furnace under air at 300 oC for 1h. This indicated that weight loss is not the determining factor to the catalyst deactivation. Surface chemistry change and possible intermediate adsorption should be responsible for the deactivation.

Decrease of the specific surface area might lead to the catalyst deactivation. The BET surface area of the used catalysts (Table 1) shrunk dramatically due to the strong interaction between the reaction intermediates and the sp2 hybridized carbon, as well as the surface re-oxidation by ozone, which led to not only the diminution of the total pore volume and average pore size, but also the reduction in the amount of active sites. However, rGO collected from 50 g/L of NaCl solution had a smaller BET surface area but a higher activity, indicating that the decrease of the BET surface area was not the only factor inducing the deactivation.

XPS studies on C1s region were carried out on the used catalysts to examine the changes in surface functional groups (Fig. 9(B) and (C)). The surface chemistry of the used rGO catalysts varied remarkably compared with the fresh one due to the oxidation by the reactive species and the reconstruction of carbon structure.52 The hydroxyl group (C-OH) and carbonyl group (C=O) on the fresh catalysts have been oxidized to carboxyl group (O-C=O). Moreover, the increase in carbon content might be ascribed to the carbon structural reconstruction during the oxidation process. Nevertheless, the changes of carbon content in used rGO-300 from saline solution demonstrated a different scenario. Some hydroxyl groups were directly oxidized to carboxyl groups, leading to the corresponding changes of the carbon contents. However, the presence of chloride anions inhibited the oxidation process of carbonyl groups and the content of carbonyl groups remained similarly to that on the fresh catalysts (11.6% vs. 10.6%). This high content of electron-rich carbonyl groups not only kept 24


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the minor deactivation of the catalyst, but also suggested carbonyl groups are the major active sites among the oxygen-containing functional groups.

For nanocarbon catalysis, it was reported that oxygen-containing functional groups, heteroatom and defective sites in the rGO structure (zigzag edges, nonhexagonal unit and vacancy defects) might be the active sites.

28, 31, 53

Electron-rich carbonyl groups (C=O) are

acknowledged to be the active sites among the oxygen-containing functional groups for catalytic reactions.54 For oxidative dehydrogenation (ODH) of light alkanes using carbon nanotubes as a catalyst, carbonyl groups rich in electrons are not only as Lewis basic sites for combing and scissoring the C-H bonds, but also as the active sites for water desorption and regeneration.55-56 In liquid phase catalysis, Duan et al. employed in situ electron paramagnetic resonance (EPR) for characterizing the hydroxyl radical signal for activation of persulfate (PS) using benzoquinone, 1,4-dicarboxybenzene and 1,2-dihydroxybenzene as homogeneous catalysts.52 EPR spectra indicated strong hydroxyl radical signals when benzoquinone was utilized as the catalyst, and thus confirming the critical role of carbonyl groups in PS activation. Besides, anomaly structures existing on the graphene edges are commonly recognized as the active sites for catalytic reactions owing to their delocalized electrons.28, 57-58

Doped heteroatoms with lonely electron pairs such as nitrogen and boron could disrupt

the chemical/electron inertness of the original sp2 carbon structure, generate new active sties and eventually alter the chemical and physical properties of rGO/graphene.29, 59-61

In this study, the low ID/IG ratio obtained from Raman spectra revealed the disorder and defect levels of rGO samples were low, indicating fewer amounts of defective sites. In addition, the effect of heteroatoms was also excluded. Therefore, carbonyl groups on the rGO/GO surface were suggested to be the active sites for catalytic degradation of PHBA. XPS study revealed a high content of oxygen (Fig. S3) in GO. However, it displayed much inferior TOC mineralization to the rGO materials with far low oxygen levels. Previous studies on GO/rGO catalysis for PMS/PS activation also reported the similar observations.52-53 It was suggested that the excess oxygen contents in GO, especially presented as the electron-rich carbonyl group, altered relative electronic potential between 25



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themselves and ozone, encumbered the electron transfer efficiency and thus affecting the redox cycle. To further improve the catalytic activity, development of rGO/graphene with an optimum oxygen content for electron transfer is highly imperative for the future studies. The proposed degradation mechanism on catalytic ozonation for PHBA degradation is described in Scheme 1.

Scheme 1. Proposed degradation mechanism of catalytic ozonation for PHBA degradation.

4. CONCLUSIONS In this study, reduced graphene oxides (rGOs) with low defect/disorder levels were synthesized by thermal reduction of GO under either static air or nitrogen and were utilized for catalytic ozonation of PHBA. The rGOs demonstrated much higher catalytic ozonation activities than commercial activated carbon and GO in both PHBA degradation and mineralization. Due to the low defect level, the oxygen-containing functional groups on the rGO surface were proposed to be the active sites for redox reactions. MS results revealed that PHBA was oxidized to produce small molecular carboxylic acids. Enhancements in both 26


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ozonation and catalytic ozonation efficiency were observed with the addition of NaCl. Mechanistic studies revealed that ozone molecules, •O2- and 1O2 were the main reactive species for catalytic ozonation. However, with NaCl addition, ·O2- became the dominant radical. Meanwhile, stability tests suggested that deactivation occurred due to the changes of the surface chemistry and textual structures. Carbonyl groups were suggested to be the main active sites within the oxygen-containing groups.

ASSOCIATED CONTENTS Supporting information Supporting information includes SEM images, N2 adsorption, XPS spectra and surface acidic/basic sites of the samples, variation of ozone concentration during the experiments, kinetic studies in effects of some reaction conditions and MS spectra of reaction intermediates.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (H.C) *Email: [email protected] (S.W);

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors greatly appreciate the financial supports from the National Natural Science Foundation of China (No. 21207133) and the National Science Fund for Distinguished Young Scholars of China (No. 51425405).

REFERENCES 27



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Deactivated

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Used rGO

Fresh rGO

+e

PHBA

O3 . 1

O2

O2


Efficient Catalytic Ozonation over Reduced Graphene Oxide for p

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