Double pyramid stacked CoO nano-crystals induced by graphene at low temperatures as highly efficient Fenton-like catalysts

Abstract

Transition metal oxides are widely used as Fenton-like catalysts in the treatment of organic pollutants, but their synthesis usually requires a high temperature. Herein, an all-solid-state synthesis method controlled by graphene was used to prepare a double pyramid stacked CoO nano-crystal at a low temperature. The preparation temperature decreased by 200 °C (over 30% reduction) due to the introduction of graphene, largely reducing the reaction energy barrier. Interestingly, the corresponding degradation rate constants (k_obs) of this graphene-supported pyramid CoO nano-crystals for organic molecules after their adsorption were over 2.5 and 35 times higher than that before adsorption and that of free CoO, respectively. This high catalytic efficiency is attributed to the adsorption of pollutants at the surface by supporting graphene layers, while free radicals activated by CoO can directly and rapidly contact and degrade them. These findings provide a new strategy to prepare low carbon-consuming transition metal oxides for highly efficient Fenton-like catalysts.

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1. Introduction

Developing effective catalysts to replace precious metals has attracted widespread attention. Transition metals possess unoccupied valence d orbitals and numerous single electrons,^1–6 which facilitate the formation of steadfast metal ions or complexes, making them an ideal alternative to precious metal-based catalysts.^7–11 With the increasing population and the development of industrialization, persistent organic pollutants such as antibiotics and cyclic aromatic compounds have been detected in water samples from a wide range of living environments, and many efforts have been made to tackle this problem.^10,12–18 As an advanced oxidation technology, Fenton and Fenton-like processes employing transition metal oxide-based catalysts to address the continuous worsening of environmental pollution present promising potential for the development of various low-carbon and sustainable industrial production methods.^19–28

However, in these catalytic processes, the related radicals have only a very limited lifetime. For example, the free radical half-lives of ˙OH and ˙SO₄⁻ are only 1 μs and 30–40 μs, respectively.^29–31 This leads to insufficient time for free radicals in the solution to diffuse to organic pollutants in the solution, thus seriously affecting degradation efficiency. New strategies to produce transition metal oxides with low-energy technologies and to minimise exposure to impurities and free radicals remain highly desirable.

In this study, CoO nano-crystals supported on graphene, named CoO@graphene, were successfully synthesized at a lower temperature. The treated temperature decreased from 600 °C down to only 400 °C due to the introduction of graphene, which largely reduces the reaction energy barrier for phase conversion. Interestingly, its corresponding degradation rate constants (k_obs) for organic molecules with pre-adsorption by graphene are over 2.5 and 35 times higher than that of CoO@graphene and CoO without pre-adsorption, respectively. Supporting graphene layers adsorb pollutants on the surface, while free radicals activated by CoO can directly and rapidly contact and degrade them.

2. Experimental

2.1 Chemicals and materials

Graphene (purchased from Wuxi Huxin Testing Technology Co., Ltd), cobalt chloride hexahydrate (CoCl₂·6H₂O), peroxymonosulfate (PMS), sodium hydroxide (NaOH, ≥96.0%), sulfuric acid (H₂SO₄, ≥98.0%), rhodamine B (RhB, C₂₈H₃₁ClN₂O₃, ≥99.0 wt%), benzoquinone (BQ), furfuryl alcohol (FFA), dimethyl sulfoxide (DMSO) acetone, methanol (MeOH), 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,6,6-tetramethyl-4-piperidinyloxyl (TEMP). Acetonitrile (C₂H₃N, ≥99.0 wt%), methyl phenyl sulfoxide (PMSO), methyl phenyl sulfone (PMSO₂), sodium chloride (NaCl), sodium sulfate (Na₂SO₄) and sodium carbonate (Na₂CO₃). The aqueous solutions of all the above reagents were prepared with deionized water and distilled water.

2.2 Synthesis of CoO@graphene

Precisely, 10 mg of graphene powder and cobalt chloride hexahydrate were mixed in a mortar, and after the powdered mixture was completely ground, it was transferred to a crucible and calcined in a tube furnace at 100 °C, 200 °C and 400 °C for 1 hour each. The argon gas flow rate was 150 mL min⁻¹, and the heating rate was 10 °C min⁻¹.

2.3 Synthesis of CoO

10 mg of cobalt chloride hexahydrate was accurately weighed and thoroughly ground in a mortar. The powder was then transferred to a crucible and calcined in a tubular furnace at 600 °C for 1 hour. An argon gas flow rate of 150 mL min⁻¹ and a heating rate of 10 °C min⁻¹ were maintained.

2.4 Radical quenching experiments

The use of the radical quenching experiment is a prevalent means for identifying active substances in AOPs. It is both intuitive and operable, whereby the intended active substance is completely suppressed with an excessive quenching agent, such as methanol (CH₃OH/TBA). Consequently, the production and contribution of reactive substances can be evaluated by observing the influence of adding or not adding the quenching agent on the organic dye's degradation efficiency. Abbreviations of technical terms are explained upon their initial introduction to avoid confusion. 10 mg of the catalyst and 20 mL of RhB solution with a quencher (TBA/MeOH = 100 mM, FFA = 1 mm, BQ = 10 mm) were combined in a glass beaker and stirred mechanically. Afterward, 90 ppm of PMS was added to initiate the Fenton-like reaction. A 1 mL suspension was collected at specified intervals and promptly centrifuged at 6000 rpm for 5 minutes to eliminate the catalyst. Methyl phenyl sulfoxide (PMSO, 100 μM), and methyl phenyl sulfone (PMSO₂, 100 μM), 10 mg catalyst (0.5 g L⁻¹), and 20 mL RhB solution (10 mg L⁻¹) were mixed in a glass beaker and mechanically stirred through the whole experiment process.

2.5 Material characterization

The crystal structure of the synthesized samples was determined using X-ray powder diffraction (XRD) on a Smartlab instrument (Rigaku D/max-2500) with Cu Kα radiation (λ = 0.15405 nm). Sequential scans were conducted from 5° to 90° at a scanning rate of 20° min⁻¹. The field emission scanning electron microscopy (SEM, Gemini G300) was employed to characterize the morphologies. The surface properties were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha). The absorbance of the RhB solution was measured on an ultraviolet spectrophotometer (UV-1600). Electron paramagnetic resonance (EPR) spectra obtained using a JEOL FA200 were used to study the reactive radicals produced during PMS activation. These radicals were trapped by spin-trapping agents such as 5,5-dimethylpyrroline oxide (DMPO) and 2,2,6,6-tetramethyl-4-piperidinyloxyl (TEMP). High-performance liquid chromatography (HPLC, Thermo U3000) was used to determine the PMSO and PMSO₂ concentrations at fixed intervals during the decomposition process.

3. Results and discussion

Considering the cation–π interactions between metal cations and graphene,^15,35 CoO nano-crystals were prepared using an all-solid-state synthesis method controlled by graphene. The CoO nano-crystals supported by graphene, called CoO@graphene, were prepared by a thermal decomposition reaction of CoCl₂·6H₂O on graphene at a relatively low temperature. The catalytic process is illustrated in Fig. 1a. Graphene was prepared via vapor deposition. Then, 10 mg of graphene and 10 mg of CoCl₂·6H₂O were thoroughly mixed by grinding in a mortar. The resulting mixture was placed into a tube furnace and calcined at different temperatures (100, 200, 400 and 600 °C) for 60 minutes under an argon gas atmosphere. CoO was obtained at 400 °C in the presence of graphene, as displayed in Fig. 1b. In contrast, the CoO was obtained at 600 °C without graphene (Fig. 1b). The preparation temperature decreased by 200 °C (over 30% reduction), which can be ascribed to the introduction of graphene, largely reducing the reaction energy barrier for the phase conversion from the Co-based on the corresponding chloride salts.^32–34

Fig. 1 (a) Schematic illustration of the Fenton-like catalysis mechanism via CoO@graphene. (b) X-ray diffraction (XRD) patterns of CoO@graphene and CoCl₂·6H₂O after calcination at various temperatures up to 600 °C. (c) and (d) The scanning electron microscopy (SEM) image of CoO@graphene and the size of single CoO.

Using field emission scanning electron microscopy (FESEM), the crystal morphology of CoO@graphene was observed. Interestingly, we found many double pyramidal stacked CoO nano-crystals on the graphene surface (Fig. 1c). The elemental mapping analysis showed the uniform distribution of C, O and Co. The atom ratio of C, O and Co was 19 : 2.8 : 1 (Fig. S1, ESI†). The side length of the crystal is approximately 500 nm (Fig. 1d). The triangular surface of the crystal appeared frosted (Fig. 1c).

Such CoO@graphene nano-crystals showed high catalytic efficiency of the Fenton-like reaction. The sample was additionally washed and dried to remove unreacted soluble CoCl₂·6H₂O before use. 10 mg CoO@graphene nano-crystals were introduced into a 20 mL solution of rhodamine B (RhB) as an example of the organic pollutants at a concentration of 10 mg L⁻¹, followed by the initiation of the adsorption process in a 50 mL reactor for a duration of 2 hours. Then, 1 mL solution was withdrawn from the solution at time intervals of 0, 30, 60, 90 and 120 minutes to test the adsorption amount of RhB on the sample. The absorbance of the extracted solution was quantified using UV-visible absorption spectrometry. Finally, after 120 minutes of the adsorption process, a 100 μL solution of peroxymonosulfate (PMS) (18 g L⁻¹) was introduced to initiate the Fenton-like catalytic reaction. In only 5 minutes, the RhB degradation efficiency reached 100% (Fig. 2a). For comparison, controlled experiments of pure CoO and graphene at the same operating processes were performed. After 5 minutes, the RhB degradation efficiency reached only 12% and 67% for pure CoO and graphene, respectively (Fig. 2a).

Fig. 2 (a) TRhB degradation was catalyzed by CoO@graphene calcined at 400 °C, graphene and pure CoO. (b) and (c) Different degradation efficiencies and corresponding degradation rate constants (k_obs) for CoO@graphene catalysts with or without adsorption treatment. (d) and (e) The degradation performance at different conditions. Reaction condition: solution pH = 3, 5, 7, 9 and 11; PMS = 30 ppm, 60 ppm, 90 ppm and 120 ppm. (f) The reusability of CoO@graphene catalysis.

To illustrate the influence of adsorption on the catalytic efficiency of CoO@graphene nano-crystals on organic pollutants, we performed the RhB degradation experiment without the organic pollutant adsorption process. A 100 μL solution of PMS (18 g L⁻¹) was directly introduced to the mixture solution of RhB and CoO@graphene nano-crystals without the 120-minute adsorption process to initiate the Fenton-like catalytic reaction. After 5 minutes, the RhB degradation efficiency reached only 83% (Fig. 2b). The CoO@graphene nano-crystals clearly showed a higher catalytic efficiency of the Fenton-like reaction after adsorption of the organic pollutants. To quantitatively evaluate the increment of the reaction efficiency, the corresponding pseudo-first-order degradation rate constants (k_obs) were calculated. Fig. 2c shows that the values of k_obs for CoO@graphene with the adsorption treatment, without the adsorption treatment, and CoO were 0.770, 0.305 and 0.021 min⁻¹, respectively. Its corresponding k_obs values for the former were more than 2.5 and 35 times higher than those for the latter two, respectively. We considered that the CoO can be produced on the graphene due to the cation–π interaction between Co²⁺ and graphene flake. Besides, the contaminants and active species were also adsorbed on the CoO@graphene, overcoming the problem of the low utilization rate of active species because of their short half-lives. The high electron transport capacity of graphene accelerated the reaction process and improved the catalytic activity. In addition, we noted that the k_obs value was also (6.6/60) times that of the CoO from other studies.¹

CoO@graphene showed an exceptional degradation capacity within a wide pH range of 3 to 7. We performed the degradation experiments at different pH values from 3 to 11. Fig. 2d shows that the RhB degradation efficiency reached about 80% even if the pH value was reduced to 3, while it sharply decreased to only about 40% when the pH value increased to 9. It illustrated that the CoO@graphene showed a high degradation capacity under acid and neutral solutions. Furthermore, we also performed the influence of the dosages of the PMS added in the solution. Fig. 2e shows that the addition of PMS at 90 ppm completely degrades RhB.

Fig. 2f shows that the Fenton-like activity remained consistently high over the course of 5 cycles in the CoO@graphene/PMS system. In addition, the Fenton-like performance of the CoO@graphene/PMS system was also studied in the presence of 0.1 M NaCl, Na₂SO₄ and Na₂CO₃. As shown in Fig. S2 (ESI†), the degradation efficiency of Rhb in the presence of NaCl, Na₂SO₄ and Na₂CO₃ was higher than 98.0% within 5 min, indicating that the CoO@graphene/PMS system provides an effective way for saline organic wastewater treatment. Using X-ray photoelectron spectroscopy (XPS), we characterized the influence of Co in CoO@graphene before and after catalytic reactions. From a full-scale XPS survey of CoO@graphene, we observed the presence of C, Co and O elements (Fig. S3, ESI†). We also measured the changes in the chemical states of the Co elements on the surface of CoO@graphene. Fig. 3b shows the XPS spectrum of the CoO@graphene with the peaks of binding energies at 781.09 eV and 783.77 eV, evidencing the presence of Co(II); meanwhile, the two peaks at 797.78 eV and 796.46 eV correspond to Co(III).^36,37 We found, after the Fenton-like reaction, the mole ratio of Co(II) slightly decreased from 54.7% to 53.9%, and the corresponding amount of Co(III) slightly increased from 19.8% to 20.5%. This illustrated that the Co was stable in the catalytic process, indicating that CoO@graphene has a good catalytic cycle stability,^9,38 which further confirmed the cycle catalytic experimental results.

Fig. 3 (a) Pseudo-first order rate constants (k_obs) from quenching experiments. (b) High resolution spectra of Co 2p for fresh and used CoO@graphene. (c) and (d) EPR spectra of DMPO and TEMP adducts for CoO@graphene/PMS and pure PMS reaction systems.

To illustrate the molecular mechanism of the CoO@graphene/PMS catalytic system, we investigated the possible reactive species such as ˙OH, ˙SO₄⁻, ˙O₂⁻ and ¹O₂ by the free radical quenching experiments. Fig. 3a shows that the Fenton-like catalytic rate coefficients of methanol for ˙OH together with the ˙SO₄⁻ scavenger and phenol for the ˙OH scavenger in the CoO@graphene/PMS system was reduced from 0.770 min⁻¹ to 0.553 min⁻¹ and 0.275 min⁻¹, respectively. This indicated the presence of ˙OH. Furthermore, the reaction rate k_obs decreased from 0.770 to 0.431 min⁻¹, when benzoquinone (BQ) for ˙O₂⁻ inhibitor was incorporated. It indicated an important role of ˙O₂⁻ in the Fenton-like systems. Finally, the addition of furfuryl alcohol (FFA) significantly decreased the catalytic efficiency of RhB, and the value of k_obs was reduced from 0.770 to 0.201 min⁻¹. This demonstrates that ¹O₂ plays a dominant role in this Fenton-catalyzed RhB degradation reaction.

To further verify the active species in the Fenton-like reaction, the trapped electron paramagnetic resonance (EPR) of 5,5-dimethylpyrroline-oxide (DMPO) experiments were conducted. Fig. 3d shows that no characteristic signals were detected when PMS was added to the reaction system, indicating that PMS hardly generates radicals without the activation of catalysts. However, when the CoO@graphene sample was added into the system, the characteristic signals for ˙SO₄⁻ ˙OH and ˙O₂⁻ were detected, demonstrating that PMS can produce ˙SO₄⁻ and ˙OH by the activation of the catalysts. The quantitative information of 2,2,6,6-tetramethyl-4-piperidinyloxyl (TEMPX) adducts, resulting from the reaction between ¹O₂ and TEMP, can be observed. Double electron transfer was shown to form Co(IV) in cobalt-based catalysts/peroxides. The RhB degradation decreased significantly in the presence of DMSO associated with the decrease of k_obs from 0.770 to 0.415 min⁻¹ (Fig. 3a). The conversion efficiency from PMSO to PMSO₂ was 85.7% (Fig. S4, ESI†). These results suggested that Co(IV) was an important active species in the CoO@graphene/PMS system. Oxidation of micropollutants is typically dominated by peroxides, such as PMS and peracetic acid. The oxidation of micropollutants is typically dominated by peroxides such as PMS and peracetic acid in cobalt-based catalysts/peroxides.

Thus, we can describe the possible mechanism process for the CoO@graphene/PMS system^39–44 using the following equations:

Co²⁺ + HSO₅⁻ = Co³⁺ + ˙SO₄⁻ + OH⁻

˙SO₄⁻ + H₂O = SO₄²⁻ + ˙OH + H⁺

˙SO₄⁻ + OH⁻ = SO₄²⁻ + ˙OH

Co³⁺ + HSO₅⁻ = Co²⁺ + ˙SO₅⁻ + H⁺

The Co species can activate PMS to generate hydroxyl radicals and sulfate radicals. Part of the ˙SO₄⁻ produced will react with water in the solution to form ˙OH, while the remainder will be adsorbed onto the graphene surface to occupy the active site. The hydration of SO₅²⁻ to form H₂O₂, which is then regenerated to form ˙O₂⁻, can be described as the mechanism for the production of ˙O₂⁻. The generation of ¹O₂ was not only attributed to the autoxidation of peroxymonosulfate (PMS) but also resulted from the chemical interaction between the superoxide anion radical (˙O₂⁻) and hydroxyl radical (˙OH).^45,46 It can be described using the following equations:

SO₅²⁻ + H₂O = SO₄²⁻ + H₂O₂

˙HO₂ = H⁺ + ˙O₂⁻

HSO₅⁻ = ˙SO₅⁻ + H⁺ + e⁻

2˙SO₅⁻ = S₂O₈²⁻ + ¹O₂

˙OH + ˙O₂⁻ = OH⁻ + ¹O₂

4. Conclusions

In summary, CoO@graphene with pyramid CoO nano-crystals was prepared using a facile calcination approach at a relatively lower temperature. The treatment temperature could be reduced by 200 °C (over 30% reduction) because of the introduction of graphene, largely reducing the reaction energy barrier. Interestingly, its corresponding degradation rate constants (k_obs) for organic molecules with pre-adsorption by graphene are over 2.5 and 35 times higher than those of CoO@graphene and CoO without pre-adsorption, respectively. It can be ascribed to the adsorption of the supporting graphene layers for pollutants, while the free radicals activated by the CoO can directly make rapid contact and degrade them. Our findings provide a viable strategy to prepare transition metal oxides with lower-carbon consumption and graphene-supporting transition metal oxides for highly efficient Fenton-like catalysts.

Author contributions

Guosheng Shi and Minghong Wu designed the project. Kui Lu, Tao Ding, Mengxiang Zhu, Junjie Chen, Dongting Yue and Xing Liu performed the experiments. Kui Lu, Tao Ding, Xiaoqin Fang, Junfang Xia, Zhiyuan Qin and Guosheng Shi analyzed the data and co-wrote the paper. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (U1932123, 12105166), the National Science Fund for Outstanding Young Scholars (No. 11722548).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp00334a

‡ Kui Lu and Tao Ding have contributed equally.

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