Synthesis and characterization of nanometal-ordered mesoporous carbon composites as heterogeneous catalysts for electrooxidation of aniline

Synthesis and characterization of nanometal-ordered mesoporous carbon composites as heterogeneous catalysts for electrooxidation of aniline

Accepted Manuscript Title: Synthesis and characterization of nanometal-ordered mesoporous carbon composites as heterogeneous catalysts for electrooxid...

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Accepted Manuscript Title: Synthesis and characterization of nanometal-ordered mesoporous carbon composites as heterogeneous catalysts for electrooxidation of aniline Authors: Xiaoyue Duan, Yawen Chen, Xinyue Liu, Limin Chang PII: DOI: Reference:

S0013-4686(17)31772-3 http://dx.doi.org/10.1016/j.electacta.2017.08.118 EA 30122

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

13-7-2017 18-8-2017 19-8-2017

Please cite this article as: Xiaoyue Duan, Yawen Chen, Xinyue Liu, Limin Chang, Synthesis and characterization of nanometal-ordered mesoporous carbon composites as heterogeneous catalysts for electrooxidation of aniline, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.08.118 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and characterization of nanometal-ordered mesoporous carbon composites as heterogeneous catalysts for electrooxidation of aniline Xiaoyue Duana,b, Yawen Chenc, Xinyue Liuc, Limin Changa,* a Key Laboratory of Preparation and Application of Environmental Friendly Materials, Ministry of Education, Jilin Normal University, Siping 136000, China

b Key Laboratory of Environmental Materials and Pollution Control, the Education Department of Jilin Province, Siping 136000, China

c School of Environmental Science and Engineering, Jilin Normal University, Siping 136000, China

Highlights 

NM-OMC catalysts were prepared for electrochemical oxidation of aniline.



The oxidation of aniline was studied with NM-OMC catalysts suspended in solution.



The Cu-OMC exhibited the highest catalytic activity for aniline degradation.



The mineralization current efficiency was improved by 2 times with Cu-OMC catalyst.



An electrochemical mineralization pathway of aniline was proposed.

Abstract: The Cu, Co and Ni nanometal embedded ordered mesoporous carbons (NM-OMCs) were fabricated by a soft-template method using phenol/formaldehyde as carbon source and triblock copolymer F127 as template agent. The morphology, structure, surface physicochemical properties and pore structure of the NM-OMCs were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption-desorption isotherms. Their potential application to the electrocatalytic degradation of aniline was investigated using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and •OH radicals generation test. Furthermore, the electrochemical oxidation 1

process of aniline was also investigated in the presence of the OMC-based catalyst particles suspended in a Na2SO4 solution using a PbO2 anode. Results revealed that the NM-OMCs inherited the ordered mesostructure of pristine OMC and the metal nanoparticles (Cu, Co or Ni) were embedded in the carbon framework. The Cu-OMC exhibited significantly higher catalytic activity than OMC and other NM-OMCs for the electrooxidation of aniline. In electrochemical oxidation process of aniline, nearly all of aniline could be degraded after 120 min of electrolysis with Cu-OMC particles as catalyst, while 89%, 92%, and 97% with OMC, Co-OMC and Ni-OMC catalysts, respectively, obviously higher than 76% of electrochemical oxidation without assistance of catalysts. After characterization of intermediates, a possible electrochemical degradation pathway of aniline was proposed.

Keywords: Ordered mesoporous carbon; Nanometal; Catalyst; Electro-catalytic oxidation; Aniline

1. Introduction

Aniline and its derivatives are important raw materials for production of various synthetic compounds, including dye, pharmaceutical agents, pesticides, explosives, perfume, rubber, and so on [1], which have been found to be widely distributed in water environment due to their widespread use and inappropriate treatment [2]. However, the aniline is a typical toxic and persistent organic pollutant [3]. It has been found to be carcinogenic and also react easily in the blood to convert hemoglobin into methaemoglobin, thereby preventing oxygen uptake [2]. It is therefore important to find a suitable alternative methodology/technology to remediate aniline-containing effluents.

Electrochemical oxidation technology is one of the most effective methods for the treatment of wastewater containing toxic organic pollutants due to its strong oxidation performance, easy implementation, environmental compatibility and low cost [4], where organic pollutants can be completely

2

mineralized into CO2, H2O, and inorganic compounds, or at least transformed into more biodegradable products [5]. The material of anode is crucial for the degradation efficiency of pollutants in electrochemical oxidation process [6]. Thus, various stable anodes with high electro-catalytic activity and long lifetime were studied by researchers [4,7,8]. However, the electrochemical systems with common two-dimension (2D) anodes have some weaknesses such as low current efficiency and high energy consumption due to their low surface areas [3]. Thus, three-dimension (3D) electrodes, such as granulated activated carbons (GAC) [9,10], granulated graphite [11] and carbon aerogel (CA) [12], emerged to improve the current efficiency of electrochemical oxidation. Their large specific surface area can provide more activity sites and accelerate the electro-generation of ·OH radicals. In addition, the catalysts were also introduced into the electrochemical oxidation systems to further improve the oxidation efficiency. Gu et al. [13] synthesized a novel catalyst of CuO-Co2O3-PO43– modified kaolin for catalytic oxidation of anionic surfactants. Its COD removal efficiency reached up to 90% in 60 min, much higher than those without assistance of catalysts. Chen et al. [14] enhanced the electrochemical degradation of dinitrotoluene wastewater by Sn–Sb–Agmodified ceramic particulates (SCP). The strong oxidizing agents were generated by SCP in the electrochemical system and contributed to the electrochemical degradation of dinitrotoluene wastewater. Our previous studies using Cu-rare earth/Al2O3 as catalysts for electrochemical oxidation of p-nitrophenol have also reported that p-nitrophenol removal was significantly enhanced by the Cu-rare earth/Al2O3 catalysts [15]. However, in these studies, the catalysts were all prepared using an impregnation method, and only small amount of metal oxides were loaded on the surface of supports, which probably limited the catalytic activity of particles.

Recently, as a kind of novel carbon nanomaterial, ordered mesoporous carbons (OMCs) have attracted much attention due to their high surface area (up to 2500 m2 g–1), large pore volume, uniform and 3

adjustable pore size (2.0-5.0 nm), regularly aligned pore architecture, chemical inertness and good electrical conducting property [16-18]. More importantly, the regular pore channels of OMCs can provide a confined space for the growth of nanomaterials, which helps developing unique nano-reactor with notable performance [19]. These outstanding features make them be applied in versatile processes such as hydrogen storage, catalysis, and electrochemistry devices fabrication. Joo et al. [20] synthesized highly order, rigid arrays of nanoporous carbon using ordered mesoporous silica as templates. The resulted material supported a high dispersion of platinum nanoparticles, exceeding those of other common microporous carbon materials. Hydrogen adsorption on OMCs incorporated with Pd, Pt, Ni and Ru was enhanced by a factor 2.7-5.4 times over the pure carbon at hydrogen pressure of 800 Torr [21].

Hence, the aim of the present work is to in situ synthesize metal nanoparticles (Cu, Co and Ni) embedded ordered mesoporous carbons (denoted NM-OMCs) using a soft-template method for enhancing electrocatalytic oxidation removal of aniline. Their structure, composition, morphology, and textural properties were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption-desorption isotherms. Their electrochemical performances were also investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Furthermore, their catalytic activity for degradation of aniline was also studied. The primary intermediate products of aniline were identified by high performance liquid chromatography (HPLC) and the main degradation pathways were elucidated.

2. Experimental

2.1. Materials

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Triblock copolymer F127 (EO106PO70EO106, Mav = 13,338 g mol–1) was purchased from Sigma Aldrich. All other chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd, China. All the chemicals were of analytical grade and were used as received without any further purification. All solutions were prepared using ultrapure water. 2.2. NM-OMC preparation OMC and NM-OMCs were prepared according to the reported methods with some modification [22,23]. Firstly, 6.1 g of phenol was melted at 42 °C in water bath, and then 1.3 g of NaOH solution (20 wt%) was added and stirred for 10 min. After that, 10.5 g of formaldehyde (37 wt%) was slowly added into the above solution and the solution was stirred at 72 °C for 60 min. After natural cooling, the pH value of solution was adjusted to 7.0 using 6 mol dm–3 HCl and then dried to a constant weight at 45 °C in a vacuum drying oven to form resol. The resulting resol was dissolved in ethanol to get resol ethanolic solution (20 wt%).

Then, NM-OMC composite materials were prepared by co-assembly of F127, resol precursor, and Cu, Co, Ni species through evaporation induced self-assembly strategy. The fabrication process of NM-OMCs is schematically described in Fig. 1. In a detail procedure, 1 g of F127 was firstly dissolved in 14 ml of ethanol at 40 °C to form solution A. 0.25 mM Cu(NO3)2, Co(NO3)2 or Ni(NO3)2 was dissolved in 6 ml of ethanol to form solution B. Then solution B was slowly added into solution A, and the mixed solution was vigorously stirred for 30 min. Then, 5 g of resol ethanolic solution (20 wt%) was added dropwise into the above mixture. After further stirring for 1 h, the homogenous solution was obtained and transferred into dishes to evaporate the solvents at room temperature for 12 h. Then, the dishes were transferred into an oven and heated up to 100 °C for 24 h in order to thermopolymerize the precursors. The formed soft films collected from dishes were placed into a temperature-programmed vacuum furnace for heat treatment in nitrogen atmosphere with a heating rate of 1 °C min–1 and were kept at 350 °C and 720 °C for 3 h and 2 h, 5

respectively. During calcinations, the triblock copolymer template F127 was removed and NM-OMC composites (Cu-OMC, Co-OMC and Ni-OMC) were obtained. For comparison, the pristine OMC was also obtained through above steps without adding nitrates.

Fig. 1

2.3. Characterization

XRD patterns of samples were acquired on a Rigaku D-max/3C diffractometer using Cu K radiation, operated at 45 kV and 30 mA. FTIR spectra were recorded on a Cary 630 spectrometer using the KBr pellet technology. TEM images were obtained on a TECCNAI F20 miscroscope operated at an accelerating voltage of 200 kV. SEM was carried out on a XL30ESEM-FEG model instrument. XPS studies were performed on an ESCALAB250XI spectrometer using Al Kα radiation for excitation. The binding energies obtained from Cu, Co, and Ni photoelectron peaks were corrected for charging effects by referencing them to the C 1s peak at 284.6 eV. The software of “XPS Peak Fit” was used to analyze chemical elements, referring to LaSurface database [24]. The porous structure of samples was probed by N2 adsorption-desorption isotherms using a Micromeritics 3H-2000PS1 instrument at 77 K. The Brunauer-Emmett-Teller (BET) and Barett-JoynerHalenda (BJH) methods were used to estimate the specific surface area and pore properties (pore volume and pore-size distribution), respectively, on the basis of adsorption-desorption data.

2.4. Electrochemical measurement

The electrochemical characterization of samples was performed on a VersaSTAT 3 electrochemical workstation with OMC or NM-OMCs film loaded on a glassy carbon (GC) electrode as working electrode, a platinum sheet as auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. The working electrode was prepared as follows: firstly, the glass carbon electrode (3 mm of diameter) was 6

polished with alumina slurry and washed with ultrapure water; then, 5 mg of the catalyst was ground and mixed with 1mL ethanol and 50 μL of 5 wt% Nafion solution under sonicating for 30 min and 10 μL suspension was dropped on the surface of the polished glassy carbon electrode and naturally dried. All the electrochemical experiments were done at ambient temperature without any inert gas purging the solution. EIS measurements were carried out in 0.5 M Na2SO4 solution in the frequency range from 100 kHz to 0.01 Hz with an applied sine wave of 5 mV amplitude at open circuit potential. CV measurements were conducted in 0.5 M Na2SO4 solution with and without 200 mg dm–-3 aniline.

2.5. Determination of ·OH radicals generation

The electrochemical formation rates of ·OH radicals in the electrochemical oxidation processes with and without catalysts were determined by a terephthalic acid (TA) probe method [25], in which highly fluorescent product of 2-hydroxyterephthalic acid (2-HA) was formed through the reaction between TA and electrochemically-generated ·OH radicals. A PbO2 electrode and a stainless steel sheet with a same size of 3 cm × 5 cm were used as the anode and cathode, respectively. They were positioned vertically and parallel to each other with a distance of 5 cm. Approximately 20 mg of developed catalyst was dispersed into the solution between the anode and cathode. The electrolyte was 200 mL of 0.5 mM TA solution with 0.5 g dm – 3

NaOH and 0.05 M Na2SO4. The electrolysis was performed at a constant current density of 30 mA cm–2 and

under stirring by a magnetic follower in a temperature-controlled water bath of 30 ºC. Samples were drawn from the reactor every 5 min and diluted 10 times with ultrapure water, then analyzed by a fluorescence spectrophotometer (Cary Eclipse). Fluorescence spectra were recorded in the range of 370-520 nm, using an excitation wavelength of 315 nm.

2.6. Electrochemical oxidation of aniline

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The electro-catalytic oxidation experiments were operated in a rectangular plexiglas reactor by batch process. A PbO2 electrode and a stainless steel sheet with a same size of 3 cm × 5 cm were used as the anode and cathode, respectively. They were positioned vertically and parallel to each other with a distance of 5 cm. Approximately 20 mg of developed catalyst was disperse into the solution between the anode and cathode. The volume of aqueous electrolyte was 200 mL with an initial concentration of 50 mg dm–3 aniline. 0.05 M Na2SO4 was used as the supporting electrolyte. All the experiments were conducted at a constant current density of 30 mA cm–2 and under stirring by a magnetic follower in a temperature-controlled water bath of 30 ºC. The degradation of aniline was monitored using high-performance liquid chromatography (HPLC, Shimadzu, Japan). The chromatographic separations were performed on an Agilent C-18 column at a column temperature of 30 ºC and at a flow rate of 1.0 mL min–1. The mobile phase composition was 50% methanol and 50% water (volume fraction). The detection wavelength was set at 265 nm. The total organic carbon (TOC) of samples was determined using a TOC analyzer. The mineralization current efficiency (MCE) for the degradation of aniline was calculated as [26]:

MCE



  TOC

 exp

  TOC

 theor

 100 %

(1)

where ∆(TOC)exp and ∆(TOC)theor are the experimental TOC removal and theoretically calculated TOC removal (mg dm–3) at the electrolysis time t (s), respectively. Considering that the applied electrical charge is only consumed in the mineralization process of aniline, which can be written as follows: C6H7N + 15 H2O → 6 CO2 + NO3– + 37H+ + 36 e–

(2)

The theoretical value is calculated by the following formula [27]:

8

It   TOC

 theor



n eF

n C M  10

3

(3) V

where I is the electrolysis current (0.45 A), t is the electrolysis time (s), F is the Faraday constant (96,485 C mol–1 electrons), V is the electrolyte volume (0.2 dm3), M is the atomic weight of C (12 g mol–1), nC is the C number of the organic compounds, ne is the number of electron transfers in the oxidation of the aniline, and nC and ne are 6 and 36, respectively.

3. Results and discussion

3.1 Material characterization

Fig. 2 shows the large-angle XRD patterns of catalysts. All the samples exhibited a broad diffraction peak located at 2θ = ~23°, which was indexed as the (002) diffraction plane of graphitic carbon [19,28]. Besides, the Cu-OMC, Co-OMC and Ni-OMC exhibited other different diffraction peaks. The diffraction peaks at 2θ value of 43.3° and 50.4° were observed for Cu-OMC catalyst, and these features were attributed to the facets (111) and (200) of cubic Cu0 (JCPDs card no. 04-0836). The diffraction peaks at 2θ value of 44.1° and 51.4° for Co-OMC catalyst corresponded to the facets (111) and (200) of cubic Co0 (JCPDs card no. 15-0806), and the diffraction peaks at 2θ value of 44.5° and 51.8° of Ni-OMC catalyst were assigned to the (111) and (200) characteristic reflections from face centered cubic Ni (JCPDS card no. 04-0850). These results show that pure metallic copper, cobalt and nickel existed in carbon materials. However, it is generally known that nitrate should be decomposed to metal oxide through thermal decomposition. This may be due to that the Cu2+, Co2+ and Ni2+ were almost completely reduced to metallic copper, cobalt and nickel by the formed carbons during the carbonization process at 700 °C [29].

Fig. 2 9

The FTIR spectra were measured to track the functional groups on the surface of carbon samples, as shown in Fig. 3. For OMC, adsorption bands were observed at approximately 3435, 2923, 2852 and 1627 cm–1, in addition to the broad bands between 1000 and 1300 cm–1. The strong and broad peak centered at 3435 cm–1 arose from the O-H stretching vibrations of hydroxylic groups [30,31]. The bands at 2852 and 2923 cm–1 could be ascribed to the vibrations of the C-H bonds of aliphatic species [16,32]. The weak bands near 1627 cm–1 could be attributed to the small amount of carbonyl groups [32]. The weak and broad bands between 1000 cm–1 and 1300 cm–1 could be assigned to the stretching vibrations of C-O groups [33]. After the introduction of Cu, Co or Ni, all the main bands for OMC were still observable. However, the band between 1000 cm–1 and 1300 cm–1 became weaker, indicating the Cu, Co and Ni particles decreased the proportion of C-O groups.

Fig. 3

In order to characterize the morphology and meso-structure of OMC and NM-OMCs, the typical TEM and SEM images are shown in Fig. 4 and Fig. 5. The well-ordered arrangement of mesopores was clearly observed for OMC (Fig. 4a), giving evidence for the presence of ordered hexagonal meso-structure, which is a replica of F 127 template. For the NM-OMCs (Fig. 4b-d), the ordered mesoporous structures were still maintained, which implied that the ordered mesoporous structure was not destroyed by the incorporation of Cu, Co and Ni. The dark spots were observed for Cu, Co and Ni nanoparticles. The Cu nanoparticles were highly dispersed in the carbon matrix with a small and uniform particle size of 4-14 nm, while a relatively poor dispersion and larger particle size for Co and Ni agglomerates. As depicted in Fig. 5a-d, a large number of three-dimensional channels were formed on the surface of all the carbon samples, and some particles presented on the surface of NM-OMCs. Consistent with the results of TEM, the Cu particles are evenly and equally distributed on the surface of Cu-OMC, whereas the Co and Ni agglomerates unevenly appeared on 10

the surface of Co-OMC and Ni-OMC. The Co and Ni agglomerates may lead to a lower electrocatalytic activity for Co-OMC and Ni-OMC than Cu-OMC. This could be explained as follows: on the one hand, the agglomeration of Co and Ni would decrease their exposure area to the pollutants, resulting in lower active surface area; on the other hand, the large size of Co and Ni agglomerates may destroy the mesoporous structure of OMC, and cannot form the nano-reactor with notable catalytic performance. The chemical compositions of the OMC and NM-OMCs were measured by energy dispersive X-ray spectroscopy (EDS), as shown in Fig. 5e-h. Its result further confirmed the existence of Cu, Co and Ni in Cu-OMC, Co-OMC and NiOMC, respectively. The presence of oxygen probably came from CO2 adsorbed on the surface of the sample or some metals on the surface of OMC oxidized by the air [34].

Fig. 4

Fig. 5

XPS is employed to further provide a detailed description for surface composition of OMC and NMOMCs. The survey scan XPS spectra of all samples (Fig. 6a) show that the OMC contains carbon and oxygen elements. As expected, the signals associated with Cu, Co and Ni from Cu-OMC, Co-OMC and Ni-OMC were very weak due to their low contents in OMC. The atomic percentage of Cu, Co and Ni in Cu-OMC, Co-OMC and Ni-OMC was calculated from the XPS survey spectrum to be around 2.6, 3.4 and 1.1%, respectively. Thereby, the XPS results further proved the successful incorporation of Cu, Co and Ni atoms in the carbon lattice.

Fig. 6b-e shows the fitted C 1s XPS spectra of all samples. The C 1s XPS spectra of OMC, Cu-OMC and CoOMC all were fitted by three component peaks at binding energies of about 284.3, 284.8 and 285.7 eV

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(marked I, II and III), while only peaks I and II for Ni-OMC. These peaks were assigned to graphitized carbon (284.3 eV), C-O (284.8 eV) and C-H (285.7 eV) species, respectively.

The Cu 2p, Co 2p and Ni 2p XPS spectra of Cu-OMC, Co-OMC and Ni-OMC are shown in Fig. 6f-h. The peaks Cu 2p at 932.8 eV (II), Co 2p at 778.2 eV (I) and Ni 2p at 852.1 eV (I) were assigned to metallic Cu0, Co0 and Ni0, indicating that metal ions (Cu2+, Co2+ and Ni2+) were reducted to pure metals by hot carbons during the pyrolysis processes, which was in good accordance with the results of the XRD spectra. However, the peaks with binding energies of 932.5 and 934 eV in Fig. 6f were attributed Cu2O and CuO, the peaks with binding energies of 779.2, 780.3 and 780.9 eV in Fig. 6g were attributed Co2O3 and Co3O4, and the peaks with binding energies of 853.5, 856.1 and 861.3 eV in Fig. 6h were attributed NiO and Ni2O3. This result confirmed the speculation of EDS that some metallic oxides might exist on the surface of OMC. This phenomenon also emerged in previous studies, and it was explained that the metal oxide may be attributed to oxygen chemisorptions at step and kink sites present on the metal surface [35].

Fig. 6

The pore texture properties of the OMC-based catalysts were investigated by N2 adsorption-desorption isotherms. Fig. 7 presents the typical N2 adsorption-desorption isotherms and BJH pore size distributions of OMC, Cu-OMC, Co-OMC and Ni-OMC. All N2 adsorption-desorption isotherms of samples exhibited type IV curves with a large uptake at low relative pressure and a pronounced hysteresis loops at high relative pressures of 0.4-1.0, indicating well developed meso- and microporosity formed through the removal of F127 template and burn-out of C, H, and O from the phenolic resin framework during pyrolysis [18,29,36]. The calculated BET specific surface areas and pore volumes are listed in Table 1. It can be found that the incorporation of metal nanoparticles significantly reduced the specific surface area and increased the pore

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volume of OMC. This was probably ascribed to the consumption of carbon during the reduction process of metal oxides/metal nitrates to Cu, Co and Ni by surrounding carbon, leading to the decrease of BET specific surface area and the increase of pore volume. Besides, the decrease of specific surface area was also related with the partial shrink of the carbon nanostructure caused by the embedding of metal nanoparticles. The degree of disruption to larger-size Co and Ni agglomerates to carbon nanostructure was more serious than Cu particles, consequently, the specific surface areas of Cu-OMC and OMC were proximate while those of Co-OMC and Ni-OMC were significantly lower. It also should be noted that CuOMC presented similar behaviour to the pure OMC in N2 adsorption-desorption isotherms, and their hysteresis loops can be classified as type H2, while Co-OMC and Ni-OMC more accorded with type H1. This also shows the effect of Cu particles on the mesoporous structure was much less than that of Co and Ni particles, which may be more beneficial to develop notable catalytic performance through its better nanoreactor for Cu-OMC composites. The pore size distribution calculated from the adsorption branch using the BJH method is shown in Fig. 7b and related parameters are listed in Table 1. As expected, the pore size distribution of OMC and Cu-OMC was centered at narrow wide of 3-5 nm, whereas Co-OMC and Ni-OMC materials presents a narrow peak at approximately 2.4-4 nm and a broad and low peak at 10-25 nm. This result may be explained by the result of TEM. The existence of large Co and Ni agglomerates in the OMC caused the formation of small part of large-sized mesopores (10-25 nm) in Co-OMC and Ni-OMC catalysts.

Fig. 7

3.2 Electrochemical measurement

Electrochemical characterization was firstly performed by EIS to investigate the interfacial behavior of OMC, Cu-OMC, Co-OMC and Ni-OMC catalysts at open circuit potential (E=248, 270, 289, 242 mV for OMC.

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Cu-OMC, Co-OMC, and Ni-OMC, respectively). As shown in Fig. 8, all the Nyquist plots of samples displayed an arc in the high frequencies and nearly 45° sloping line at low frequencies. The experimental impedance spectra were fitted using an equivalent circuit presented as an inset image in Fig. 8. In this Rs[(CPEdl||(RctZw)] circuit, Rs, the intersection of plots with the real axis at the high-frequency end, reflects the bulk resistance [37,38]; Rct, one arc appeared in the high frequency range, represents the chargetransfer resistance for the equilibrium reaction at the contact interface between electrode and electrolyte solution: at negative wave disturbance: O2 + 2e–→ 2OH– (4) at sine wave disturbance: 2OH– - 2e– → O2

(5)

Zw, a straight sloping line displaying in the low frequency, stands the Warburg impedance related to the diffusive resistance of the electrolyte in electrode and the proton diffusion in host materials [39]; the double layer capacitance, Cdl, was replaced by the constant phase elements CPEdl. The impedance of a CPEdl is given as [40-42]:

Z CPE 

1 Y ( j )

n

(6)

where Y is a capacitance parameter (in F cm–2 sn–1) and n is a dimensionless parameter with a value between 0 and 1. EIS simulation parameters are shown in Table 2. It can be found that the bulk resistance was relatively small (about 35 Ω) for all the plots. The Rct values of NM-OMC catalysts were much smaller than that of OMC, indicating that the incorporation of Cu, Co, and Ni metals could reduce the charge transfer resistance values of OMC due to the good electrical conductivity of Cu, Co, and Ni metals. It is also worth noting that the Rct value of Cu-OMC (6.236×103 Ω) was significantly smaller than those of Co-OMC

14

(1.618×104 Ω) and Ni-OMC (1.698×104 Ω) because of the lower resistivity of Cu (1.678 × 10–8 Ω·m) than Co (6.64 × 10–8 Ω·m) and Ni (6.84 × 10–8 Ω·m). In general, the Rct value is inversely proportional to the electrochemical reaction rate [4]. Therefore, the low Rct value of Cu-OMC indicates a faster kinetics in electrochemical reactions than OMC, Co-OMC and Ni-OMC catalysts.

The activity of a catalyst should also be related to its electrochemically accessible surface area. The Cdl is directly proportional to the electrochemically accessible surface area of the electrodes, so values of Cdl of OMC, Cu-OMC, Co-OMC and Ni-OMC were calculated using Brug et al. [43] equation:

C dl

   Y dl  

 1 1    R ct  Rs

   

 n 1 

   

1/ n

(7)

The calculated results of Cdl of OMC, Cu-OMC, Co-OMC and Ni-OMC were 1.353×10–5, 3.252×10–5, 3.618×10–6 and 6.263×10–6 mF cm–2, respectively. Remarkably, Cu-OMC manifested higher Cdl value than OMC, implying the Cu-OMC may offer more electrochemically accessible surface area in electrochemical reaction. However, the Cdl values of Co-OMC and Ni-OMC were lower than that of OMC, which may be related to their significantly lower specific surface area (Table 1).

Fig. 8

The direct oxidation of aniline at OMC, Cu-OMC, Co-OMC and Ni-OMC films was investigated by CV in 0.5 M Na2SO4 medium, in absence or presence of 200 mg dm–3 aniline with a scan rate of 50 mV s–1. As shown in Fig. 9, in blank Na2SO4 solution, no redox peak appeared within the scan range of 0-1.5 V vs SCE for all samples, while in presence of 200 mg dm–3 aniline, obvious oxidation peaks presented at the potential of about 0.85 V vs SCE, which probably involves oxidation reactions including deprotonation and polymerization reactions [44]. The oxidation peak current density for the OMC, Cu-OMC, Co-OMC and Ni15

OMC films was evaluated to be 0.278, 0.624, 0.238 and 0.351 mA cm–2, respectively. The Cu-OMC catalyst provided higher current density than other catalysts, revealing that the direct oxidation of aniline occurred more easily on Cu-OMC catalyst than OMC, and Ni or Co could not improve the direct oxidation ability of OMC significantly. It is well known that a large number of strong oxygen-based oxidizers (e.g., ·OH, ·ClxOy) could be produced in electro-catalytic process, leading to the indirect oxidation of organic pollutants [1]. Therefore, it could be concluded that the aniline could be degraded on as-prepared OMC-based catalysts through direct and indirect oxidation processes. To investigate the kinetics aspect of aniline oxidation on the OMC, Cu-OMC, Co-OMC and Ni-OMC catalysts, the cyclic voltammograms of 200 mg dm–3 aniline in 0.5 M Na2SO4 aqueous solution were measured at scan rates from 10 to 50 mV s–1. As shown in Fig. 10, the oxidation peak current increased with increasing the scan rate. The analysis of the peak current as a function of the scan rate was performed by plotting peak current as a function of the scan rate or the square root of scan rate. The result showed that the peak current presented a linear relationship with the scan rate (insets of Fig. 10). Thus, the electrochemical oxidation reaction of aniline on OMC, Cu-OMC, CoOMC and Ni-OMC was a typical adsorption-controlled electrochemical process within the tested scan range.

Fig. 9

Fig. 10

3.3 ·OH radicals generation

In order to investigate the effect of OMC-base catalysts on the ·OH radicals generation in electro-catalytic oxidation (EO) processes, the electrochemical formation rates of ·OH radicals in the EO processes with and without catalysts were determined by a TA probe method [25], in which highly fluorescent product of 2-HA

16

was formed through the reaction between the TA and electrochemically-generated ·OH radicals, and the fluorescence intensity of 2-HA can be used to represent the amount of generated ·OH radicals. Fig. S1 shows the fluorescence spectra obtained for the EO processes with and without catalysts at various electrolysis periods. It can be seen that the fluorescence intensity of 2-HA around 425 nm gradually increased with the increase of electrolysis time in all systems, indicating ·OH radicals were constantly generated during EO oxidation with or without catalysts. Fig. 11a shows the fluorescence intensity as a function of the duration of electrolysis. Since the fluorescence intensity increased linearly with the electrolysis time, the formation rate of 2-HA was calculated from this slope and is shown in Fig. 11b. It can be found that the formation rates of ·OH radicals increased in the following order under the same conditions: EO only (13.84) < EO/OMC (17.34) < EO/Co-OMC (23.10) < EO/Ni-OMC (25.64) < EO/Cu-OMC (29.18). Therefore, OMC-based catalysts dispersed in the solution could significantly accelerate the formation of ·OH radicals in EO processes, and the Cu-OMC had the highest catalytic activity in all the catalysts. The OMC-based catalysts could accelerate the ·OH radicals generation may be ascribed to the large number of H2 and O2 produced on the surface of cathode and anode, which reacted to generate H2O2 under the action of the catalysis of catalysts through Eq. (8), and the H2O2 was further catalytically transformed to ·OH radicals through Eq. (9) [45].

H2 + O2 → H2O2

H2O2 → 2·OH

(8)

(9)

Fig. 11

3.4 Electro-catalytic oxidation of aniline

17

The catalytic activities of OMC, Cu-OMC, Co-OMC and Ni-OMC were further determined by the degradation of aniline. Fig. 12a shows the aniline removal efficiency comparison in EO processes with various catalysts. To investigate the contribution of these catalysts for the removal of aniline, the degradation process without any catalyst and the adsorption processes of catalysts were also compared in Fig. 12a. It is clearly shown that the removal efficiency of aniline with the catalysts was significantly higher than that without any catalyst, and very low removal occurred in the adsorption processes, which confirms that the adsorption of as-prepared catalysts for removal of aniline can be negligible, and the enhancement of removal efficiency of aniline was largely ascribed to their catalytic activities for the electrochemical degradation of aniline. Comparing the removal efficiency with different catalysts, it can be observed that, around 76% aniline was removed within 120 min in the absence of any catalyst, but the presence of OMC, Co-OMC and Ni-OMC catalysts led to 89%, 92% and 97% removal of aniline within 120 min, respectively, while almost complete aniline removal took place within the same period of electrolysis with Cu-OMC as catalyst. In addition, degradation processes of aniline were plotted according to the pseudo-first-order model (ln(C0/C = kt)) (Fig. 12b). The results show that the electrochemical degradation of aniline obeyed first-order kinetic model with and without the catalysts. By fitting to the logarithmic expression, the degradation rate for as-prepared catalysts is EO/Cu-OMC (0.036 min–1) > EO/Ni-OMC (0.029 min–1) > EO/CoOMC (0.022 min–1) > EO/OMC (0.019 min–1) > EO only (0.012 min–1). The aniline removal rate for EO/CuOMC was nearly 2 times faster than that of EO/OMC and 3 times faster than that of EO only under the same conditions.

Fig. 12

In order to evaluate the mineralization ability of EO/OMC, EO/Cu-OMC, EO/Co-OMC, EO/Ni-OMC and EO only systems towards aniline, the TOC removal efficiency during the whole degradation processes was 18

measured and shown in Fig. 13a. Similarly to the aniline removal efficiency, the highest TOC removal efficiency was observed in the EO/Cu-OMC system, in which 59% of the TOC was removed after 120 min. In the EO/OMC, EO/Co-OMC, EO/Ni-OMC and EO only systems, 45%, 50%, 52% and 33% of the TOC were removed, respectively. Thus, EO/Cu-OMC system had stronger mineralization ability towards aniline than EO/OMC, EO/Co-OMC, EO/Ni-OMC and EO only systems, and most of aniline was finally transformed into CO2 and H2O in this degradation process. The MCE is another evaluation index for the degradation performance of the electrochemical oxidation process [27]. As shown in Fig. 13b, the MCE values for degradation processes gradually decayed, indicating a continuous fall in mineralization ability, which may be due to the progressive larger acceleration of non-oxidizing parasitic reactions (10)-(12) [46]. 2PbO2(·OH) → 2PbO2 + O2 + 2H+ + 2e–

(10)

2SO42– → S2O82– + 2e–

(11)

3H2O → O3 + 6H+ + 6e–

(12)

From Reaction (10), the hydroxyl radicals were consumed by the oxidation of PbO2(·OH) to O2. In addition, the PbO2(·OH) concentration at the anode surface also be diminished by the production of other weaker oxidants like S2O82– and ozone from Reaction (11) and (12). The highest MCE value was achieved by EO/CuOMC system (16%, 20 min), followed by the EO/Ni-OMC (14%, 20 min), EO/Co-OMC (13%, 20 min) and EO/OMC (10%, 20 min) systems, which were 2.29, 2, 1.86, and 1.43 times than that of EO only process (7%, 20 min), respectively. Therefore, the EO/Cu-OMC system had a higher efficiency in mineralizing organic pollutants.

All these results demonstrate that the Cu-OMC catalyst had higher catalytic activity than OMC, Co-OMC and Ni-OMC catalysts for electrooxidation of aniline. It is well known that the electrochemical activity of 19

electrodes for the electro-catalytic oxidation process is related to its real surface area and the number of active sites accessible to the electrolyte [6]. However, the BET specific surface area of Cu-OMC (422.89 m2 g–1) was smaller than that of OMC (445.03 m2 g–1). In addition, the Co-OMC (367.74 m2 g–1) and Ni-OMC (369.07 m2 g–1) had obvious lower specific surface areas than OMC, but they still presented higher catalytic activity than OMC. These results showed that Cu-OMC catalyst had the higher catalytic activity may be related with its high specific surface area, but mainly is ascribed its active sites accessible to the electrolyte, which lead to generation of more strong oxygen-based oxidizers (·OH) in EO process [47].

Fig. 13

3.5 Degradation mechanism of aniline

The evolution of the UV absorption spectra during aniline degradation in EO/Cu-OMC system is shown in Fig. 14. Two defined peaks at 229 and 280 nm were observed in the initial UV-Vis spectra of the wastewater, which are characteristic peaks of aniline. During the degradation process, the intensity of the peak at 229 nm reduced rapidly with the electrolysis time proceeded, indicating the rapid degradation of aniline. However, the intensity of the peak at 280 nm decreased slowly at the intial 30 min of electrolysis, and then decreased quickly. In addition, it also can be observed that a plateau appeared around 250-270 nm from 20 to 60 min of electrolysis, and it also then decreased with the degradation time proceeded. The slow decrease of peak intensity at 280 nm and the appearance of the plateau around 250-270 nm indicate that other byproducts were formed during the oxidation process of aniline. To understand the nature of the intermediates better, we compared these peaks with those previously reported for the destruction of aniline by the electrochemical oxidation. According to the literature [48,49], the appearance of the plateau around 250-270 nm could be related to the formation of benzoquinone, and the slow decrease of peak

20

intensity at 280 nm may be due to some byproducts with a chemical structure similar to aniline. Total disappearance of the all peaks after 120 min of electrolysis suggests that the aniline and all byproducts were degraded during the electrochemical oxidation process. These results can be proved by the color change of the degradation solution likely caused by the presence of byproducts (the inset of Fig. 14).

Fig. 14

In order to further analyze the role of the suspended OMC-based composite particles in the mechanism of aniline oxidation at the PbO2 anode, the intermediates produced in EO only and EO/Cu-OMC systems were identified using HPLC. All the intermediates were identified by comparing the retention time of standard compounds. Fig. 15a and b show the HPLC chromatograms for aniline degradation at different electrolysis time in EO only and EO/Cu-OMC systems, respectively. Comparing the intermediates produced in EO only and EO/Cu-OMC systems, it can be found that the main intermediates were almost the same for these two degradation processes, including p-aminophenol, nitrosobenzene, phenol, hydroquinone, pnitrophenol, p-benzoquinone, melaic acid, succinic acid, oxalic acid, and formic acid. However, concentrations variation of intermediates in two systems was obviously different according to the peak areas. In EO/Cu-OMC system, during the electrolysis period of 30-60 min, vast majority of the aniline was degraded and the concentrations of all the intermediates reached the highest values, then they decreased significantly, and disappeared basically at the electrolysis time of 120 min; while in EO only system, at the electrolysis time of 30 min, only small amounts of intermediates were formed, then the concentrations of intermediates increased with aniline degradation, but they still maintained high values after 120 min of electrolysis. These results demonstrate that the OMC-based catalysts might only accelerate the rate of ·OH radicals generation and degradation reaction, and not change the degradation pathway of aniline.

21

Fig. 15

According to the intermediates detected during the HPLC analysis, the aniline degradation pathway was proposed and shown in Fig. 16. As depicted in Fig. 16, the first step of aniline degradation took place by electrophilic ·OH radicals attracts at the amino group or para-position of the aniline, leading to the formation of nitrosobenzene and p-aminophenol. The para-position of nitrosobenzene was further attracted by ·OH radicals and formed the p-nitrophenol. Then, the –NH2 group of p-aminophenol and –NO2 group of p-nitrophenol were attracted by •OH radicals and yielded the aromatic products of phenol and hydroquinone. The hydroquinone was likely further converted to p-benzoquinone, followed by the open of benzene, which resulting short carboxylic of maleic acid. The maleic acid was further oxidized into succinic acid, oxalic acid and formic acid. Finally, the succinic acid, oxalic acid and formic acid were mineralized into carbon dioxide and water. This degradation pathway of aniline was consistent with the main degradation pathway of aniline during the electro-oxidation with boron doped diamond (BDD) anode [48]. However, polyanilines were also detected in the electro-oxidation with BDD anode [48], which was not detected in our study due to the lack of standard compounds. Even so, no other obvious peaks presented except identified compounds in the HPLC chromatograms (in Fig. 15), meaning the production of polyanilines was very little in our study, which may be due to that the polymerization of aniline might be reduced under the action of the catalysis of the Cu-OMC catalyst.

Fig. 16

4. Conclusions

OMC-based catalysts (OMC, Cu-OMC, Co-OMC and Ni-OMC) were successfully synthesized through a softtemplate method and fully characterized. Notably, compared to pristine OMC, Cu-OMC, Co-OMC and Ni-

22

OMC exhibited lower charge transfer resistance and higher catalytic activity, especially Cu-OMC catalyst. Heterogeneous electrooxidation of aniline was investigated in the presence of the OMC-based catalyst particles suspended in a Na2SO4 solution using a PbO2 anode, in which, the Cu-OMC catalyst showed the highest catalytic performance for the mineralization of aniline. In the EO/Cu-OMC system, nearly 100% of aniline was degraded with 59% TOC elimination after 120 min electrolysis, while 89%, 92% and 97% removal of aniline with 45%, 50% and 52% TOC elimination for EO/OMC, EO/Co-OMC and EO/Ni-OMC systems, respectively, which were greatly superior to the values (76% in aniline removal and 33% in TOC reduction) achieved with EO only system. In addition, nine intermediates of aniline were identified during the electro-catalytic oxidation processes, from which the main degradation pathways of aniline were proposed.

Acknowledgments This project was supported by the Young People Fund of Jilin Science and Technology Departmen (20150520079JH), the Science and technology Project of Jilin Education Bureau (No. 2016225), and the Open Funds of the State Key Laboratory of Rare Earth Resource Utilization (RERU2017010).

23

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Fig. 1. Schematic view of the procedure for preparing NM-OMC materials.







C  Cu  Co  Ni



Ni-OMC 





Co-OMC







Cu-OMC 

10

20

30

40

OMC 50

60

2/degree Fig. 2. XRD spectra of NM-OMC catalysts.

31

70

80

v(C-H)

v(O-H)

4000

3500

v(C-O)

v(C=O)

Transmittance/a.u.

OMC Cu-OMC Co-OMC Ni-OMC

3000

2500

2000

1500

Wavenumber/cm

1000

500

-1

Fig. 3. FTIR spectra of OMC and NM-OMCs.

Fig. 4. TEM images of OMC (a), Cu-OMC (b), Co-OMC (c), Ni-OMC (d). 32

33

Fig. 5. SEM images and EDS spectra of OMC (a and e), Cu-OMC (b and f), Co-OMC (c and g), Ni-OMC (d and h). C

a

b O

OMC C 1s

Ni-OMC

(C-O) (graphitized carbon)

(C-H)

Co-OMC

Cu-OMC

OMC

0

200

400

600

800

1000

283

1200

284

285

286

287

288

Binding Energy/eV

Binding Energy/eV

c

d Co-OMC

Cu-OMC (graphitized carbon)

C 1s

C 1s (graphitized carbon)

(C-O) (C-H)

283

284

285

286

(C-O)

287

288

283

284

Binding Energy/eV

(C-H)

285

286

287

288

Binding Energy/eV

e

f Cu-OMC Cu 2p

Ni-OMC 0

(Cu2O)

C 1s

(Cu )

(C-O)

(CuO)

(graphitized carbon)

283

284

285

286

287

288 930

Binding Energy/eV

931

932

933

934

935

Binding Energy/eV

34

936

937

938

g

(Ni2O3) Ni-OMC

h

Co-OMC Co 2p

(Co3O4)

IV(NiO)

Ni 2p (NiO)

(Co2O3)

IV(Co2O3) 0

(Ni ) 0

(Co )

776

777

778

779

780

781

782

783

784 850

852

854

856

858

860

862

864

Binding Energy/eV

Binding Energy/eV

Fig. 6 (a) The wide scan XPS spectra of OMC and NM-OMCs; (b-e) the C 1s XPS spectra of OMC, Cu-OMC, Co-OMC and Ni-OMC; (f) the Cu 2p XPS spectrum of Cu-OMC; (g) the Co 2p XPS spectrum of Co-OMC; (h) the Ni 2p XPS spectrum of Ni-OMC.

35

800

600

Ni-OMC

3

Volume adsorbed/cm g

-1

a

Co-OMC 400

Cu-OMC 200

OMC

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0) 0.4

b OMC Cu-OMC Co-OMC Ni-OMC

dV/dD

0.3

0.2

0.1 2

3

4

5

6

0.0 0

10

20

30

40

Pore size/nm

Fig. 7. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution curves for OMC and NM-OMCs

36

CPEdl

25000 Rs

20000 zw

-z''/ cm

2

Rct

15000 10000 OMC Cu-OMC Co-OMC Ni-OMC

5000 0 0

5000

10000

15000

z'/ cm

20000

25000

30000

2

Fig. 8. Nyquist plots of OMC, Cu-OMC, Co-OMC and Ni-OMC in 0.5 mol dm–3 Na2SO4 solution; points-experimental, lines-fitted lines. The inset image is the equivalent circuit.

a

0.0015

0.0010

b

0.0009

Na2SO4

OMC

Na2SO4

Cu-OMC

Na2SO4 + Aniline

0.0006 -2

0.0005

j/A cm

-2

Na2SO4 + Aniline

j/A cm

0.0012

0.0003 0.0000

0.0000

-0.0003 -0.0005

-0.0006 0.0

0.3

0.6

0.9

1.2

0.0

1.5

0.3

0.6

c

Na2SO4

0.0006

Na2SO4 + Aniline -2

0.0006

0.0003

j/A cm

-2

1.2

1.5

d 0.0009

0.0009 Co-OMC

j/A cm

0.9

E vs SCE/V

E vs SCE/V

Na2SO4

Ni-OMC

Na2SO4 + aniline

0.0003 0.0000

0.0000

-0.0003 -0.0003

-0.0006 0.0

0.3

0.6

0.9

1.2

0.0

1.5

0.3

0.6

0.9

E vs SCE/V

E vs SCE/V

37

1.2

1.5

Fig. 9. CV curves of OMC (a), Cu-OMC (b), Co-OMC (c) and Ni-OMC (d) in 0.5 mol dm–3 Na2SO4 solution without and with 200 mg dm–3 aniline at a scan rate of 50 mV s–1.

0.0016

0.00015 0.00010 0

0.0004

20 40 -1 Scan rate/mV s

60

-1 10 mV s Cu-OMC -1 20 mV s -1 30 mV s -1 40 mV s -1 50 mV s

-2

0.0012 0.0008

-2

0.00020

0.0016 0.0006

j/mA cm

j/mA cm

-2

0.0008

0.00025 -2

0.0012

b

-1

10 mV s OMC -1 20 mV s -1 30 mV s -1 40 mV s -1 50 mV s

0.0000

jPeak/mA cm

0.00030

jPeak/mA cm

a

0.0004 0.0002 0.0000

0

10 20 30 40 50 60 Scan rate/mV s

0.0004 0.0000

-0.0004

-0.0004

0.0

0.3

0.6

0.9

1.2

0.0

1.5

0.3

0.6

j/mA cm

0.00008 0.00000

0

0.0003

20

40

Scan rate/mV s

1.2

1.5

60 -1

-1

10 mV s Ni-OMC -1 20 mV s -1 30 mV s -1 40 mV s -1 50 mV s

0.0003 -2

-2

0.00016

0.0004

0.0008

jPeak/mA cm

0.0006

jPeak/mA cm

-2

0.0009

d 0.0012

j/mA cm

-1 10 mV s Co-OMC -1 20 mV s -1 30 mV s -1 40 mV s -1 50 mV s

0.00024

-2

0.0012

0.9

E vs SCE/V

E vs SCE/V

c

-1

0.0004

0.0002 0.0001 0

20 40 -1 Scan rate/mV s

60

0.0000

0.0000 -0.0004

-0.0003 0.0

0.3

0.6

0.9

1.2

1.5

0.0

E vs SCE/V

0.3

0.6

0.9

1.2

1.5

E vs SCE/V

Fig. 10. CV curves and the plot of peak currents vs. the scan rates (inserts) of OMC, Cu-OMC, CoOMC and Ni-OMC in 0.5 mol dm–3 Na2SO4 aqueous solution with 200 mg L–1 aniline under different scan rates (from inner to outer: 10, 20, 30, 40, and 50 mV s–1).

38

b EO/OMC EO/Cu-OMC EO/Co-OMC EO/Ni-OMC EO only

600

30 25 20

-1

800

k/a.u. min

Fluorescence intensity/a.u.

a 1000

400

15 10

200

5 0 0

5

10

15

20

25

0

30

EO/OMC

EO/Cu-OMC EO/Co-OMC EO/Ni-OMC

Time/min

EO only

System

Fig. 11. (a) Plots of fluorescence intensity against electrolysis time in EO processes with and without OMC-based catalysts; (b) Rate constants of 2-HA formation in all systems. a 1.0 EO/OMC EO/Cu-OMC EO/Co-OMC EO/Ni-OMC EO only OMC adsorption Cu-OMC adsorption Co-OMC adsorption Ni-OMC adsorption

0.8

C/C0

0.6

0.4

0.2

0.0 0

20

40

60

Time/min

39

80

100

120

b

5

2

EO/OMC y=0.019x+0.088 R =0.9936 2 EO/Cu-OMC y=0.036x-0.056 R =0.9961 2 EO/Co-OMC y=0.022x+0.163 R =0.9859 2 EO/Ni-OMC y=0.029x+0.039 R =0.9988 2 EO only y=0.012x+0.088 R =0.9914

4

ln(C0/C)

3

2

1

0 0

20

40

60

80

100

120

Time/min

Fig. 12. (a) Aniline removal profiles in various conditions. Reaction conditions: [aniline]0 = 50 mg dm– , volume = 200 cm–3, catalyst loading = 20 mg dm–3, current density = 30 mA cm–2, temperature: 30

3

°C. (b) Plots of ln(C0/C)–t for electro-catalytic oxidation processes with and without catalysts.

a

1.0 EO/OMC EO/Cu-OMC EO/Co-OMC EO/Ni-OMC EO only

0.9

TOCt/TOC0

0.8 0.7 0.6 0.5 0.4 0

20

40

60

Time/min

40

80

100

120

b

18 EO/OMC EO/Cu-OMC EO/Co-OMC EO/Ni-OMC EO only

15

MCE/%

12 9 6 3 0 20

40

60

80

100

120

Time/min

Fig. 13. TOC removal efficiency (a) and mineralization current efficiency (MCE) (b). Reaction conditions: [aniline]0 = 50 mg dm–3, volume = 200 cm3, catalyst loading = 20 mg L–3, current density = 30 mA cm–2, temperature: 30 °C

3.5 3.0 0 min 5 min 10 min 20 min 30 min 60 min 90 min120 min

Absorbance/a.u.

2.5 2.0 0 min 10 min 20 min 30 min 60 min 90 min 120 min

1.5 1.0 0.5 0.0 200

250

300

350

400

Wavelength/nm

Fig. 14. UV absorption spectra of aniline degradation solution at different electrolysis time in EO/CuOMC system. Inset: color evolution of degradation solution during electro-oxidation process.

41

b

3

a

2

hydroquinone p-benzoquinone oxalic acid

1

120 min

1 0 3

0 3 90 min

1

1

0 3 60 min

2

maleic acid

formic acid

1

nitrosobenzene

0 3

0 3

0 3 0 min

hydroquinone succinic acid nitrosobenzene phenol maleic acid

1

1

0

0 3

4

5

0 min

2

aniline

2

6

7

p-benzoquinone p-aminophenol p-nitrophenol

30 min

2

0 3

1

60 min

1

1

0

formic acid

2

1

2

oxalic acid

0 3

30 min

2

90 min

2

p-nitrophenol p-aminophenol succinic acid

Intensity/a.u.

2

Intensity/a.u.

3 2

120 min

0

8

1

aniline

2

3

4

5

6

7

8

Time/min

Time/min

Fig. 15. HPLC chromatograms for aniline degradation in EO only (a) and EO/Cu-OMC (b) systems.

NH2

NH2

aniline

OH

OH p-aminophenol

phenol

OH

OH hydroquinone

O

O

O p-benzoquinone

C

OH

C

OH

O maleic acid

O O

O N

ON+

O-

O

C

O OH

N+

+ C

OH

O succinic acid nitrosobenzene

OH p-nitrophenol

C

OH

C

OH

O oxalic acid

CO2 + H2O

Fig. 16 Proposed degradation pathway of aniline.

42

Table 1. BET surface areas (S), pore volumes (V) and pore sizes (D) for the obtained samples. SBET a

Vtotal b

Vmeso c

Vmeso/Vtotal

Dave d

(m2/g)

(cm3/g)

(cm3/g)

(%)

(nm)

OMC

445.03

0.39

0.33

84.15

3.79

Cu-OMC

422.89

0.41

0.36

87.80

4.00

Co-OMC

367.74

0.49

0.42

85.71

6.51

Ni-OMC

369.07

0.58

0.51

87.93

7.88

Samples

a

The specific surface area (SBET) was calculated by the BET method.

b

Vtotal represented the total pore volume.

c

Vmeso rerepresented the volume of mesopore calculated from the adsorption branch using BJH method. d

The pore diameter Dave is referred to the average pore size calculated by BJH method.

43

Table 2. EIS simulation parameters of OMC, Cu-OMC, Co-OMC and Ni-OMC.

Samples

Rs (Ω cm–2)

Ydl (F cm–2 sn–1)

ndl

Rct (Ω cm–2)

W (μMho)

OMC

33.76

1.169×10–4

0.7197

2.141×104

8.656×10–5

Cu-OMC

32.39

1.103×10–4

0.8220

6.236×103

1.517×10–4

Co-OMC

34.12

3.959×10–5

0.7342

1.618×104

1.948×10–4

Ni-OMC

35.08

7.216×10–5

0.7099

1.698×104

1.302×10–4

44