Catalytic degradation of Acid Orange 7 with hydrogen peroxide using Co_xO_y-N/GAC catalysts in a bicarbonate aqueous solution

Abstract

The cobalt-based heterogeneous catalysts Co_xO_y-N/GAC were prepared by pyrolysis of a cobalt–phenanthroline complex on granular active carbon (GAC) in a nitrogen atmosphere, and tested for the degradation of Acid Orange 7 using hydrogen peroxide as a benign oxidant in a bicarbonate aqueous solution. Characterization by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and electron spin resonance spectroscopy revealed the formation of a cobalt oxide nanoparticle shell with a metallic Co core on the surface of GAC; and the nitrogen substituted the selected carbon atoms during the pyrolysis process and bonded to cobalt. The catalysts were active for dye decolorization in an aqueous solution containing 10 mM H₂O₂ and 5 mM NaHCO₃ at room temperature. They also presented good stability with nearly no loss of cobalt ions after the reaction, in comparison with the high leaching of Co (0.25 mg L⁻¹) from the Co_xO_y/GAC catalyst without nitrogen. The production of intermediates, the formation of reactive radicals and the effect of HCO₃⁻ were also investigated to further explore the efficiency of the catalyst. This study can provide a promising way for the activation of the green oxidant H₂O₂ in a bicarbonate aqueous solution by heterogeneous cobalt catalysts for environmental remediation.

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1. Introduction

It is estimated that around 100 000 commercially available dyes with over 7 × 10⁵ tones of dyestuff are produced annually.¹ As they represent a serious danger for the aquatic environment, many approaches such as adsorption/precipitation,² electro-coagulation,^3,4 and biochemical degradation,⁵ have been employed for the treatment of wastewater containing dyes. But most of them proved to be limited in terms of cost and effective application. Recently, advanced oxidation processes (AOPs) have become increasingly important in the removal of organic dyes.^6–10 Among the approaches, the conventional Fenton reagent composed of hydrogen peroxide (H₂O₂) and ferric ions (Fe²⁺) has long been applied for the production of HO˙ and pollutant removal.⁷ However, several drawbacks of the current Fenton process limit the scale-up of its application, including the narrow pH range for reaction (pH = 2.5–3.5) and the accumulation of iron-containing sludge, which is regarded as a secondary pollution and the loss of catalyst. In order to overcome these drawbacks, many Fenton-like catalysts including heterogeneous metal oxides and homogeneous metal complexes have been developed.^11–16 For example, Taran et al. studied the catalytic behavior of perovskite-like oxides LaBO₃ (B = Cu, Fe, Co, Ni, Mn) in wet peroxide oxidation of phenol, and found that the catalytic activity to the reaction was only observed with Fe- and Cu-containing perovskites and LaCuO₃ was more active than LaFeO₃.¹⁷ At the same time, LaFeO₃, in addition to its high catalytic activity, was rather stable to leaching of the active component during forty cycles of catalytic tests.

In our recent published works, the simple Co²⁺ ions have also been demonstrated to exhibit high catalytic capacity to destroy organic dyes in the presence of H₂O₂ in bicarbonate aqueous solution.^18–22 For example, complete decolorization of Acid Orange 7 could be observed with only 5 μM Co²⁺ ions and 4 mM H₂O₂ in 10 mM NaHCO₃ aqueous solution within 10 min at pH 8.3, which was much faster than that of Fenton reagent with 50 μM Fe²⁺ ions at pH 3.0.²¹ The system produces hydroxyl radicals as the main reactive species from H₂O₂ by the in situ formed complexes between Co²⁺ and HCO₃⁻, and can even activate dioxygen to the radicals at room temperature in the presence of o-aminophenol and p-aminophenol.^23,24 However, in order to overcome the secondary pollution of cobalt ions, the development of heterogeneous cobalt catalysts with high activity and stability against serious leaching of active species in the operation should be performed. But until now few works have been done in H₂O₂ activation and pollutants degradation in bicarbonate aqueous solution using heterogeneous cobalt catalysts. Jawad et al. synthesized Co–Mg–Al layered double hydrotalcite and tested its activity and stability for phenols degradation using bicarbonate-activated H₂O₂ oxidation system in the presence of the supported catalysts.²⁵ However, the leached cobalt still reached to about 0.2 mg L⁻¹, which was very close to the amount of Co²⁺ ions in the homogeneous Co²⁺–HCO₃⁻ system.

Solid carbon and N-doped solid carbon have been investigated as supports in catalysis e.g. in the Suzuki–Miyaura reaction,²⁶ styrene oxidation,²⁷ and the Fischer–Tropsch process.²⁸ However, while the cobalt based catalysts supported on carbon–nitrogen materials have been previously synthesized and applied in selective oxidation of organic compounds,^29,30 their use as catalyst in bicarbonate–H₂O₂ oxidation system has not been reported. In this study, the Co_xO_y-N/C catalysts were prepared by wet impregnation of a combination of cobalt(II) acetate and nitrogen containing ligands on commercial activate carbon. Subsequent pyrolysis at 800 °C led mainly to metal cobalt and cobalt oxide in N₂ atmosphere. The catalysts were active for the dye decolorization in aqueous solution containing H₂O₂ and NaHCO₃ at room temperature. It also presented a good stability with nearly no loss of cobalt ions after reaction.

2. Experimental

2.1. Chemicals and reagents

Acid Orange 7 (AO7) was purchased from Sigma Chemical Co and used without further purification (>85%) for all experiments. Granular active carbon (GAC, 20–50 mesh, AR) was obtained from Aladdin Industrial Corporation. Hydrogen peroxide (30% w/w) was obtained from Sinapharm Chemical reagent Co., Ltd. Cobalt acetate, 1,10-phenanthroline and other chemicals were of analytical grade if not noticed otherwise. The sample solutions were prepared using deionized water throughout the experiments.

2.2. Preparation of Co_xO_y-N/GAC catalysts

Co_xO_y-N/GAC was synthesized by a pyrolysis method similar to that reported in literature.³⁰ The typical procedure was described as follows: a mixture of Co(OAc)₂·4H₂O and 1,10-phenanthroline (Co : phenanthroline = 1 : 2 mol ratio) in 50 mL ethanol was stirred for 20–30 minutes at room temperature. Then, GAC was added into the solution and stirred for further 30 min. The whole reaction mixture was dried at 80 °C until ethanol was removed. Then the solid mixtures was pyrolyzed at 800 °C for 2 hours in nitrogen atmosphere and then cooled to room temperature. The same procedure was applied for the preparation of carbon-supported Co catalysts (Co_xO_y/GAC) but without adding 1,10-phenanthroline.

2.3. Characterization

X-ray powder diffraction (XRD) pattern was obtained on a Bruker D8 powder. The surface morphology was characterized on a field-emission transmission electron microscope (FETEM, Tecnai G2 F20). The BET specific surface areas of the samples were carried out with a Quantachrome Autosorb-1. The amount of cobalt coating on the GAC surface was obtained by extracting the cobalt in a boiling, concentrated (10%) HNO₃ solution for 12 h and then measured by flame atomic absorption spectrophotometry. The surface compositions of the Co_xO_y-N/GAC catalysts were investigated by X-ray photoelectron spectra (XPS) on a VG Multilab 2000 spectrometer (Thermo Electron Corporation). Electron spin resonance (ESR) spectra were recorded at room temperature using a Bruker ESR A-300 spectrometer.

2.4. Catalytic degradation experiment

Degradation reactions were performed in a 100 mL open flask at ambient temperature. The flask was putted in a shaker at a stirring speed of 150 rpm. After the desired amount of organic dye in 50 mL of the aqueous solution was added into the reactor, the reaction was initialized by addition of the catalyst, H₂O₂ and NaHCO₃. For the recycling experiment, the catalyst was separated without any treatment after each recycle, and then the next reaction was started by adding a fresh solution of AO7, H₂O₂ and NaHCO₃.

2.5. Analysis

To monitor the degradation process of organic dyes, solution samples were taken out at given time intervals with solid removal and measured immediately on a Varian Cary 50 Scan UV-Vis spectrophotometer. All the measurements were conducted in triplicate with a mean deviation lower than 0.01 to ensure the reproducibility of experimental results. The maximum absorption wavelengths of the dyes AO7 is 484 nm. Initial rates of AO7 degradation were determined by the slope of kinetics curves at the beginning of reaction. The extinction coefficient of AO7 is 2.1 L (mol⁻¹ cm⁻¹), and its molar concentration was calculated by the Lambert–Beer's law. The Co ion leaching was monitored by atomic absorption spectroscopy (AAS, Analyst 300, P.E. Inc.). For identification of degradation products, the samples were analyzed by mass spectrometry, and the experiments were performed on an Esquire LC-ion trap mass spectrometer (Bruker Daltonics, Bremen, Germany) equipped with an orthogonal geometry ESI source. Nitrogen was used as the drying (3 L min⁻¹) and nebulizing (6 psi) gas at 300 °C. The spray shield was set to 4.0 kV and the capillary cap was set to 4.5 kV. Scanning was performed from m/z 70 to 800 in the standard resolution mode at a scan rate of 13 kDa s⁻¹. Before analysis, each sample was diluted ten times. Total organic carbon (TOC) was determined by an Apollo 9000 TOC analyzer.

3. Results and discussion

3.1. Catalytic characterization

The amount of cobalt on Co_xO_y-N/GAC catalyst surfaces was approximately 11 mg Co/g GAC by AAS. The BET surface areas for GAC and Co_xO_y-N/GAC were approximately 661 and 633 m² g⁻¹, respectively. The slightly lower surface area of Co_xO_y-N/GAC was also an indicator of the coating of cobalt on the GAC surface. Several technologies including XRD, EPR, TEM, EDS, BET and XPS were then used to characterize the prepared Co_xO_y-N/GAC catalyst. The X-ray diffraction pattern is shown in Fig. 1(A). It can be seen that the catalyst showed a broad peak around 44.5°, implying the presence of metallic cobalt in the catalyst.³⁰ The peak is very weak, indicating the high distribution of cobalt particles in GAC surface due to the low Co loading and high specific area of GAC. The characteristic peaks of CoO at 36.89°, 42.47° and 61.52°, and of Co₃O₄ at 36.35° are hardly seen, suggesting that the cobalt oxides phases were much fewer. In the carbon-thermal process, the activated carbon not only served as a support but also acted as a reducing agent, upon which cobalt acetate was reduced to cobalt metal at the cobalt–carbon interface at a high temperature of 800 °C, with some of the carbon being simultaneously oxidized to CO_x. The EPR spectrum of the catalyst given in Fig. 1(B) is characterized by a very broad signal around 3000 G, which can be assigned to metallic Co,³¹ in agreement with XRD data.

Fig. 1 (A) XRD and (B) ESR spectrum of Co_xO_y-N/GAC catalyst.

The morphology of Co_xO_y-N/GAC catalyst was examined by TEM studies. As shown in Fig. 2(A), the images reveal that the cobalt particles are uniformly distributed in the surface of GAC, mainly with a size range of 10–30 nm. The particles are rarely aggregated, indicating a good dispersion in the carbon structure. From the high-resolution TEM image focusing on single Co nanoparticles (NPs), the incorporation of some particles into the carbon layers was also identified. Such a structure of small sized Co particles uniformly embedded in the carbon matrix could effectively prevent the cobalt from agglomerating, and might cause fast electron transport between the carbon matrix and cobalt and thus the efficient chemical performance. Fig. 2(B) displays the EDS plot for Co_xO_y-N/GAC. As illustrated, the composition of the catalysts was confirmed by the presence of C, N, O and Co elements. The weight contents of the Co_xO_y-N/GAC were evaluated to 65.68, 12.49, 21.07 and 0.76% for C, N, O and Co, respectively.

Fig. 2 HRTEM (A) and EDS (B) images of Co_xO_y-N/GAC.

X-ray photoelectron spectroscopy (XPS) was used to investigate the elemental composition of Co_xO_y-N/GAC. As shown in Fig. 3(A), a set of peaks corresponding to C 1s (284.6 eV), N 1s (401.0 eV), O 1s (531.6 eV) and Co 2p (780.0 eV) is observed. The high-resolution spectra of Co 2p is shown in Fig. 3(B), in which the typical Co 2p 3/2 and Co 2p 1/2 at 780.8 eV and 796.9 eV of cobalt oxides (Co_xO_y) are found. The satellite peaks at 787.4 eV and 802.6 eV are characteristic of the cobalt(II).³⁰ The dominant existence of Co²⁺ oxides in the XPS analysis indicates that the Co NPs on the surface of the Co_xO_y-N/GAC composite are severely oxidized. This is well-explained by the fact that the exposure of Co NPs to ambient air could cause the formation of a thin CoO shell since the cobalt(0) nanoclusters are sensitive to aerobic atmosphere. Thus, those particles observed in TEM spectra might consist of a Co core and a CoO and/or Co₃O₄ shell. During the pyrolysis process the nitrogen ligand might form graphene-type layers, in which selected carbon atoms are substituted by nitrogen. XPS analysis showed three distinct peaks in the N 1s spectra of Co_xO_y-N/GAC with electron binding energies of 398.8, 400.3, and 403.4 eV, respectively. The lowest binding energy peak is attributed to pyridine-type nitrogen, which is bound to a metal ion.³² The electron binding energy of 400.8 eV is characteristic for pyrrole-type nitrogen contributing two electrons to the carbon matrix. Some of the nitrogen atoms are also bound to a hydrogen atom. Such types of nitrogen are often found after the carbonization of nitrogen-containing organic materials.³³ Finally, the small peak at 403.4 eV is typical for quaternary amine species (NR₄⁺).³⁴ It has been reported that the N species such as graphitic N and pyridinic N had positive effect on the chemical performance of N-doped carbon materials.³⁵ The ratio between all Co atoms and all N atoms in the near surface region of Co_xO_y-N/GAC is 1 : 2.3. Thus, the enriched N-doped carbon in this work might enhance the performance of the catalyst.

Fig. 3 (A) Wide survey XPS spectra and (B) Co 2p and (C) N 1s XPS envelop of Co_xO_y-N/GAC catalyst.

3.2. Catalytic activity

The performance of different catalytic systems for AO7 degradation is shown in Fig. 4. From Fig. 4(A), it can be seen that Co_xO_y-N/GAC showed a high efficiency for AO7 degradation with a removal of 94.4% after 20 min in the presence of H₂O₂ and NaHCO₃; while for the NaHCO₃/H₂O₂ system in the absence of the catalyst only about 2.3% of AO7 removal was observed. For the experiment with only Co_xO_y-N/GAC and NaHCO₃, the removal of AO7 was 14.7%, probably due to the adsorption of AO7 on the surface of the catalyst. When using GAC instead of Co_xO_y-N/GAC or using NaOH in place of NaHCO₃, the degradation of AO7 is less than 17%. Representative UV-Vis spectra changes observed during AO7 degradation with Co_xO_y-N/GAC + NaHCO₃/H₂O₂ system are depicted in Fig. 4(B). Before reaction it shows a main absorption bands at 484 nm, corresponding to the π–π* transition of the azo form, and another two bands at 230 nm and 310 nm in ultraviolet region, which is attributed to the π–π* transition of the benzoic and naphthalene ring respectively.²² The addition of H₂O₂ into the aqueous solution caused the decrease of the absorption bands of the dye in the visible region with time and finally disappeared, indicating the destruction of its chromophoric structure in the vicinity of the azo-linkage. At the same time, the decrease of the two bands in ultraviolet region was observed, due to the opening of the benzene and naphthalene ring. In addition, the absorbance at 256 nm increased at the first 5 min and then descended gradually in the late reaction stage should not be ignored. The change can be attributed to the formation and degradation of anaphthalene type intermediates.²²

Fig. 4 (A) Degradation of AO7 with different systems: (1) Co_xO_y-N/GAC + NaHCO₃/H₂O₂, (2) Co_xO_y-N/GAC + NaHCO₃, (3) GAC + NaHCO₃/H₂O₂, (4) Co_xO_y-N/GAC + NaOH/H₂O₂ (pH 8.6), (5) NaHCO₃/H₂O₂; and (B) UV-Vis spectra changes for AO7 degradation with Co_xO_y-N/GAC + NaHCO₃/H₂O₂ system. Conditions: Co_xO_y-N/GAC 1 g L⁻¹, GAC 1 g L⁻¹, NaHCO₃ 5 mmol L⁻¹, H₂O₂ 10 mmol L⁻¹, AO7 50 μmol L⁻¹, 25 °C.

As observed in above Fig. 4, HCO₃⁻ was absolutely necessary in the heterogeneous cobalt catalyst–bicarbonate system; then the effect of HCO₃⁻ concentration on AO7 degradation was further studied with the results shown in Fig. 5. It can be seen that with the increase of HCO₃⁻ concentration from 0 to 50 mM, the degradation efficiency first increased and then decreased. The maximum rate was obtained at 10 mM, but even at a low concentration of 2.5 mM, a fast degradation of the dye was still observed. The initial enhancement of AO7 degradation efficiency was attributed to the increase of active complexes formed between cobalt and HCO₃⁻ and then the production of active oxygen radicals from the reaction with H₂O₂. However at higher HCO₃⁻ concentrations, the number of HCO₃⁻ complexed to Co species increased and there were no equatorial positions available to bind the dye and H₂O₂, leading the efficiency to decrease. In addition, the free HCO₃⁻ in solution at higher concentrations may act as an ˙OH radical scavenger.

Fig. 5 Effect of NaHCO₃ concentration on AO7 degradation. Conditions: Co_xO_y-N/GAC 1 g L⁻¹, H₂O₂ 10 mmol L⁻¹, AO7 50 μmol L⁻¹, 25 °C.

3.3. Intermediate products analysis

To further evaluate the efficiency of the catalytic Co_xO_y-N/GAC + NaHCO₃/H₂O₂ system, the ESI-MS behavior of AO7 and its degradation products at different incubation times was performed. The results at negative ion mode are displayed in Fig. 6(A). At the beginning of the reaction, an intense ion of m/z 327 corresponding to AO7 [M − Na]⁻ was observed as expected. During oxidative treatment, the intensity of AO7 at m/z 327 decreased significantly, indicating that the dye was degraded into some intermediate products. Meanwhile, the peak at m/z 173 emerged with regularly increasing of the intensity. According to the values of m/z and the suggested structures, this product can be regarded as p-phenolsulfonic acid (p-PSA), which can be found as a common product that comes from the destruction of AO7 in the reported literature.³⁶ In addition, no other products such as 4-aminobenzenesulfonate and 4-nitrosobenzenesulfonate were found. All these implied that p-PSA was the only product from the broken C–N bond between the benzene ring and the azo bond. Positive ionization mode was also tested to detect positively charged species, and the results are shown in Fig. 6(B). At the beginning of the reaction, an intense ion of m/z 373 was corresponded to AO7 [M + Na]⁺. After 30 min of incubation, new peaks at m/z 197, 215 and 253 were observed. They could be attributed to 1,2-naphthaquinone and its further oxidized compounds. The toxicity of naphthaquinone from AO7 degradation is even higher than that of the azo dye;³⁷ fortunately, the compound can be easily degraded by the Co_xO_y-N/GAC + NaHCO₃/H₂O₂ system.

Fig. 6 (A) ESI (−) and (B) ESI (+) mass spectra of AO7 solution during degradation with the Co_xO_y-N/GAC + NaHCO₃/H₂O₂ system. Conditions: Co_xO_y-N/GAC 1 g L⁻¹, HCO₃⁻ 5 mmol L⁻¹, H₂O₂ 10 mmol L⁻¹, AO7 50 μmol L⁻¹, 25 °C.

The smaller acid products such as formate, acetate, and oxalate can be further detected by ion chromatography (shown in Fig. 7). These products, known as ultimate organic products from the aromatic ring-opening, are nontoxic and biodegradable. Some inorganic ions such as sulfate and nitrate were also observed in the figure. However, the TOC removal was only about 13%, suggesting the difficulty of complete oxidation of these products to CO₂ or CO in the reaction system.

Fig. 7 Ion chromatography after AO7 degradation with Co_xO_y-N/GAC + NaHCO₃/H₂O₂ system (1: CH₃COO⁻; 2: HCOO⁻; 3: Cl⁻; 4: SO₄²⁻; 5: SO₃²⁻; 6: C₂O₄²⁻; 7: NO₃⁻). Conditions: Co_xO_y-N/GAC 1 g L⁻¹, HCO₃⁻ 5 mmol L⁻¹, H₂O₂ 10 mmol L⁻¹, AO7 50 μmol L⁻¹, 25 °C.

Based on above analysis, a schematic degradation pathway of AO7 by the Co_xO_y-N/GAC + NaHCO₃/H₂O₂ system was proposed and is given in Scheme 1. During oxidation, AO7 was attacked by the active species and then decomposed to and 1,2-naphthaquinone and p-phenolsulfonic acid. Afterwards, the 1,2-naphthaquinone was further attracted to hydroxylated or polyhydroxylated derivatives, and finally to carboxylic acids by ring opening.

Scheme 1 Proposed pathways of AO7 degradation by Co_xO_y-N/GAC + NaHCO₃/H₂O₂ system.

3.4. Radicals determination

H₂O₂ is a common oxidizing agent and is often converted to free radicals with UV irradiation or the redox reaction of transition-metal ions. In the Co_xO_y-N/GAC–HCO₃⁻/H₂O₂ system, not only HO˙ can be produced during the oxidation of low valence cobalt species by H₂O₂, but also O₂⁻˙ can be formed by reduction of cobalt with high oxidation states. In order to identify the reactive oxygen species during AO7 degradation with the system, disodium salt of terephthalic acid (NaTA) photoluminescence probing technology and radical scavenging measurements were carried out. TA could react with ˙OH to give 2-hydroxyterephthalic acid (HTA), which exhibits a bright stable fluorescence.¹⁹ This reaction is unaffected by the presence of other reactive species such as H₂O₂, HO₂˙ and O₂⁻˙, so it could be used as a sensitive probe in detecting ˙OH radicals.³⁸ Fig. 8(A) shows the fluorescence spectra of the solution containing Co_xO_y-N/GAC, HCO₃⁻, H₂O₂ and NaTA. It can be seen that the fluorescence intensity increases sharply to 9 within 20 min, implying that ˙OH radicals were indeed generated in the system. Another way to detect radicals is adding different free radical inhibitor. MeOH and TBA can inhibit ˙OH radicals,¹⁹ while superoxide dismutase (SOD) can react with O₂˙⁻ radicals with a reaction rate constant of 2 × 10⁹ (mol L⁻¹)⁻¹ s⁻¹.³⁹ As shown in Fig. 8(B), in the presence of 1 mL MeOH or TBA, the catalytic activity was inhibited; and the rate of AO7 degradation also decreased with 600 U mL⁻¹ SOD. These results implied that ˙OH and O₂˙⁻ radicals were the main active species controlling the oxidation reaction in this system.

Fig. 8 (A) Variations of photoluminescence spectra of AO7 solution during reaction in the Co_xO_y-N/GAC + HCO₃⁻/H₂O₂ system and (B) inhibited effects of scavengers on AO7 degradation by Co_xO_y-N/GAC catalyst. Conditions: Co_xO_y-N/GAC 1 g L⁻¹, HCO₃⁻ 5 mmol L⁻¹, H₂O₂ 10 mmol L⁻¹, AO7 50 μmol L⁻¹, TA 1 mmol L⁻¹, 25 °C.

According to these analyses, a possible mechanism of H₂O₂ activation by Co_xO_y-N/GAC catalyst in HCO₃⁻ solution was proposed as follows (Scheme 2): H₂O₂ and HCO₃⁻ were first coordinated with the bonded metal cobalt; and then ˙OH radical was produced via electron transfer from the metal to H₂O₂. While metal cobalt was oxidized to Co(II). The Co(II) species or the formed Co(II) species during catalyst preparation can be further transformed to Co(III) by H₂O₂ with the formation of another ˙OH radical. The Co(III) species would be reduced to be Co(II) by H₂O₂ with the formation of O₂⁻˙ radicals, which also partly contributed to AO7 degradation.

Scheme 2 Mechanism of H₂O₂ activation and AO7 degradation by Co_xO_y-N/GAC in HCO₃⁻ solution.

3.5. Stability of catalyst

To test the stability and recyclability of Co_xO_y-N/GAC, the catalyst was collected by filtration after reaction, and the decolorization reaction was reinitiated by adding a fresh solution of AO7, H₂O₂ and NaHCO₃. As shown in Fig. 9, the efficiency obviously decreased during the next runs. After the fifth run, the removal of AO7 decreased to 32.8%. The deactivation of the catalyst could be attributed to intermediate deposition on the surface, as the activity of Co_xO_y-N/GAC was fully recovered after heat treatment at 350 °C for 1 h under N₂, and complete degradation of AO7 can be achieved within 20 min as the same as the first use.

Fig. 9 Degradation of AO7 with the used Co_xO_y-N/GAC catalyst (6^th*: the catalyst was thermal treated at 350 °C under N₂ for 1 h after the 5^th run). Conditions: Co_xO_y-N/GAC 1 g L⁻¹, HCO₃⁻ 5 mmol L⁻¹, H₂O₂ 10 mmol L⁻¹, AO7 50 μmol L⁻¹, 25 °C.

The deactivation of the catalyst might also be caused by the leaching of cobalt. To further evaluate the stability of Co_xO_y-N/GAC catalyst, the leaching of cobalt was measured in the time course of AO7 decolorization. The results are shown in Fig. 10. For comparison, the cobalt leaching from Co_xO_y/GAC catalyst was also given. It can be found that under the conditions, there was no any cobalt ions detected in the solution within 10 min; and after the time, the cobalt concentration increased with the reaction, but it was only 0.02 mg L⁻¹ after 20 min. The value was much lower than that reported in literature,^12,25 dealed with the phenol degradation by the solid cobalt catalysts–bicarbonate system, indicating the excellent stability of Co_xO_y-N/GAC catalyst against leaching. Another experiment was established to check whether the dissolved Co ions are responsible for the observed catalytic activity. After the reaction of 5 min, the catalyst was filtered and the rate of AO7 decolorization in the filtrate was measured. In this case, no reaction proceeded within 20 min. These results indicated that leaching of cobalt into the liquid phase was negligible and the observed catalysis was intrinsically heterogeneous. For the Co_xO_y/GAC catalyst, though the completely decolorization of AO7 was observed within 10 min, the concentration of cobalt ions found in the solution increased with the increasing of reaction time. After 20 min, the value was up to 0.25 mg L⁻¹, suggesting that the leaching of cobalt species from the catalyst is much easier in comparison with the Co_xO_y-N/GAC catalyst. In our previous works, it has been shown that AO7 can be nearly complete decolorized in 10 min with 0.295 mg L⁻¹ Co²⁺ in bicarbonate aqueous solution, and the efficiency of the Co_xO_y/GAC catalyst could be mainly attributed to the homogeneous reaction. Indeed, when the solid catalyst was separated after 2.5 min of reaction, the further decolorization of AO7 was still observed. From XPS result it could be found that there was nearly no nitrogen species in the Co_xO_y/GAC catalyst surface. Thus the high stability of cobalt species in Co_xO_y-N/GAC catalyst might come from the interaction between cobalt and nitrogen, as described in Fig. 3.

Fig. 10 (A) Comparison of AO7 degradation with different supported cobalt catalysts and (B) leaching of Co from the different supported cobalt catalysts. Conditions: Cat 1 g L⁻¹, HCO₃⁻ 5 mmol L⁻¹, H₂O₂ 10 mmol L⁻¹, AO7 50 μmol L⁻¹, 25 °C.

4. Conclusions

In summary, Co_xO_y-N/GAC is an efficient heterogeneous Fenton-like catalyst that is capable of oxidizing highly stable organic dye AO7 with the breaking of azo bonds and opening of naphthaquinone under mild reaction conditions. The HO˙ radicals and O₂⁻˙ ions were produced as the main reactive species. The catalyst has a great stability against cobalt leaching, due to the strong interaction between nitrogen and cobalt. Though the reusability of Co_xO_y-N/GAC is not very good, its activity can be fully recovered after heat treatment under N₂ atmosphere. However, the detailed mechanisms are still not well understood and should be studied in future work.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21207105).

References

1. C. Pearce, J. Lloyd and J. Guthrie, Dyes Pigm., 2003, 58, 179.
2. M. Zhu, L. Lee, H. Wang and Z. Wang, J. Hazard. Mater., 2007, 149, 735.
3. E. Neyens and J. Baeyens, J. Hazard. Mater., 2003, 98, 33.
4. B. Merzouk, B. Gourich, A. Sekki, K. Madani, C. Vial and M. Barkaoui, Chem. Eng. J., 2009, 149, 207.
5. K. Kumar, S. S. Devi, K. Krishnamurthi, S. Gampawar, N. Mishra, G. H. Pandya and T. Chakrabarti, Bioresour. Technol., 2006, 97, 407.
6. S. Kouraichi, M. E. Samar, M. Abbessi, H. Boudouh and A. Balaska, Catal. Sci. Technol., 2015, 5, 1052.
7. X. Yang, X. Xu, J. Xu and Y. Han, J. Am. Chem. Soc., 2013, 135, 16058.
8. J. Gong, J. Liu, X. Chen, Z. Jiang, X. Wen, E. Mijowska and T. Tang, J. Mater. Chem. A, 2014, 2, 7461.
9. Y. Tao, Q. Ni, M. Wei, D. Xia, X. Li and A. Xu, RSC Adv., 2015, 5, 44128.
10. X. Li, L. Yang, C. Qin, F. Liu, L. Zhao, K. Shao and Z. Su, RSC Adv., 2015, 5, 59093.
11. S. Navalon, R. Martin, M. Alvaro and H. Garcia, Angew. Chem., Int. Ed., 2010, 49, 8403.
12. L. Zhou, W. Song, Z. Chen and G. Yin, Environ. Sci. Technol., 2013, 47, 3833.
13. J. Wu, H. Zhang and J. Qiu, J. Hazard. Mater., 2012, 215, 138.
14. J. H. Ramirez, C. A. Costa, L. M. Madeira, G. Mata, M. A. Vicente, M. L. Rojas-Cervantes, A. J. Lopez-Peinado and R. M. Martin-Aranda, Appl. Catal., B, 2007, 71, 44.
15. W. Huang, M. Brigante, F. Wu, C. Mousty, K. Hanna and G. Mailhot, Environ. Sci. Technol., 2013, 47, 1952.
16. A. N. Pham, G. W. Xing, C. J. Miller and T. D. Waite, J. Catal., 2013, 301, 54.
17. O. P. Taran, A. B. Ayusheev, O. L. Ogorodnikova, I. P. Prosvirin, L. A. Isupova and V. N. Parmon, Appl. Catal., B, 2015, 180, 86.
18. A. Xu, X. Li, S. Ye, G. Yin and Q. Zeng, Appl. Catal., B, 2011, 102, 37.
19. X. Li, Z. Xiong, X. Ruan, D. Xia, Q. Zeng and A. Xu, Appl. Catal., A, 2012, 411–412, 24.
20. Z. Yang, H. Wang, M. Chen, M. Luo, D. Xia, A. Xu and Q. Zeng, Ind. Eng. Chem. Res., 2012, 51, 11104.
21. X. Long, Z. Yang, H. Wang, M. Chen, K. Peng, Q. Zeng and A. Xu, Ind. Eng. Chem. Res., 2012, 51, 11998.
22. M. Luo, L. Lv, G. Deng, W. Yao, Y. Ruan, X. Li and A. Xu, Appl. Catal., A, 2014, 469, 198.
23. X. Li, W. Shi, Q. Cheng, L. Huang, M. Wei, L. Cheng, Q. Zeng and A. Xu, Appl. Catal., A, 2014, 475, 297.
24. W. Shi, Q. Cheng, L. Duan, Y. Ding, Z. Xiong, X. Li and A. Xu, New J. Chem., 2014, 38, 4038.
25. A. Jawad, X. Lu, Z. Chen and G. Yin, J. Phys. Chem. A, 2014, 118, 10028.
26. A. A. Deshmukh, R. U. Islam, M. J. Witcomb, W. A. L. van Otterlo and N. J. Coville, ChemCatChem, 2010, 2, 51.
27. C. B. Putta and S. Ghosh, Adv. Synth. Catal., 2011, 353, 1889.
28. H. Xiong, M. Moyo, M. K. Rayner, L. L. Jewell, D. G. Billing and N. J. Coville, ChemCatChem, 2010, 2, 514.
29. R. V. Jagadeesh, H. Junge, M.-M. Pohl, J. Radnik, A. Bruckner and M. Beller, J. Am. Chem. Soc., 2013, 135, 10776.
30. D. Banerjee, R. V. Jagadeesh, K. Junge, M.-M. Pohl, J. Radnik, A. Buckner and M. Beller, Angew. Chem., Int. Ed., 2014, 53, 4359.
31. T. Stemmler, F. A. Westerhaus, A.-E. Surkus, M.-M. Pohl, K. Junge and M. Beller, Green Chem., 2014, 16, 4535.
32. J. Casanovas, J. M. Ricart, J. Rubio, E. Illas and J. M. Jiménez-Mateos, J. Am. Chem. Soc., 1996, 118, 8071.
33. J. R. Pels, F. Kapteijn, J. A. Moulijn, Q. Zhu and K. M. Thomas, Carbon, 1995, 33, 1641.
34. W. Grünert, R. Feldhaus, K. Anders, S. E. Shipiro, G. V. Antoshin and K. M. Minachev, J. Electron Spectrosc. Relat. Phenom., 1986, 40, 187.
35. Y. Su, Y. Zhu, H. Jiang, J. Shen, X. Yang, W. Zou, J. Chen and C. Li, Nanoscale, 2014, 6, 15080.
36. S. Rismayani, M. Fukushima, H. Ichikawa and K. Tatsumi, J. Hazard. Mater., 2004, 114, 175.
37. D. Mendez-Paz, F. Omil and J. M. Lema, Enzyme Microb. Technol., 2005, 36, 264.
38. A. Xu, X. Li, H. Xiong and G. Yin, Chemosphere, 2011, 82, 1190.
39. H. J. Forman and I. Fridovich, Arch. Biochem. Biophys., 1973, 168, 396.

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