Efficient and stable catalysis of hollow Cu9S5 nanospheres in the Fenton-like degradation of organic dyes

https://doi.org/10.1016/j.jhazmat.2020.122735Get rights and content

Highlights

  • HS-Cu9S5 nanospheres were fabricated by one-step anion exchanging strategy.

  • HS-Cu9S5 has a preferable catalytic activity in Fenton-like degradation of organic pollutants.

  • HS-Cu9S5 nanospheres have a wide pH application scope as 5.0–9.0 in the catalysis.

  • HS-Cu9S5 shows outstanding stability and low Cu2+ ion leaching rate in 15 catalytic cycles.

Abstract

The development of new heterogeneous catalysts with stable catalytic activity in a wide pH range to prevent polluting precipitation plays a vital role in large-scale wastewater treatment. Here, a facile anion exchange strategy was designed to fabricate hollow Cu9S5 nanospheres by using Cu2O nanospheres as hard-templates. The structural and compositional transformation from Cu2O nanospheres to hollow Cu9S5 nanospheres were investigated via X-ray diffraction, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The Fenton-like degradation of organic dyes was used to evaluate the catalytic performance of the obtained Cu-containing catalysts. Results reveal that the hollow Cu9S5 nanospheres have the best catalytic activity among five kinds of Cu-containing catalysts. Hollow Cu9S5 nanospheres can effectively accelerate the decomposition of H2O2 into hydroxyl radicals and superoxide radical, which have been proven to be mainly oxidative species in the Fenton-like degradation of organic pollutants. Hollow Cu9S5 nanospheres have a wide pH application range of 5.0–9.0, and their extremely stable activity can be maintained in at least 15 catalytic cycles with a Cu2+ ion leaching rate of less than 1.0 %. The outstanding catalytic performance of the Cu9S5 catalyst is expected to enhance the practical applications of copper sulfide catalysts in Fenton-like wastewater treatment.

Introduction

Fenton or Fenton-like reactions involve the catalytic generation of radical species for the exhaustive oxygenolysis of organic pollutants into non-toxic products, including H2O, CO2 and inorganic salts, and this method is developing into one of the most promising technologies in advanced oxidation processes (AOPs) of textile wastewater due to its operational flexibility and low hardware demand (Bethi et al., 2016; Collivignarelli et al., 2019; Miklos et al., 2018; Pavithra et al., 2019; Sharma et al., 2018; Sillanpää et al., 2018; Mirzaei et al., 2017). The classic Fenton reaction always involves the cracking of hydrogen peroxide (H2O2) catalyzed by iron ions to form strongly oxidizing hydroxyl radicals (radical dotOH) and superoxide radical (radical dotO2) via a series of elementary reactions (Eqs. 1 and 2). Some metal ions or ion pairs, including Cu+/Cu2+, Cr3+/Cr6+, Co2+/Co3+, Ce3+/Ce4+, Mnx+/Mnx+1 and Rux+/Rux+1 also exhibited catalytic activities in Fenton-like reactions due to their variable valances (Bokare and Choi, 2014; Lee et al., 2018; Peng et al., 2016, 2019; Pouran et al., 2014). However, metal ions have some disadvantages in homogeneous Fenton reactions in spite of their highly catalytic activities: (ⅰ) Soluble metal ions can be hardly recycled from treated water; (ⅱ) The catalytic activities of metal ions are highly dependent on the pH values of the Fenton system due to recession of catalysis and formation of metal hydroxide sludge occurring under unsuitable pH value; (ⅲ) Some transition metal ions, such as Cr6+ and Co2+/Co3+, are extremely hazardous to biological and environmental health. To conquer the above problems, a series of heterogeneous catalysts containing transition metal elements have been exploited and applied in the Fenton-like degradation of organic dyes so as to control metal ions release and facilitate catalyst recycling (Bokare and Choi, 2014; Nidheesh et al., 2018; Pouran et al., 2014; Zhu et al., 2019). Cu-containing solid catalysts have attracted extensive attention because they have the similar reaction mechanism as Fe-based catalysts but with superior catalytic activities and preferable pH-tolerance (Eqs. 3 and 4). In Fenton-like reactions, the complexes formed by Cu sites and organic intermediates are more unstable and easily decomposed by radical dotOH in comparison with that of Fe species, thereby achieving the complete mineralization of organic pollutants. Therefore, a number of Cu-containing catalysts, such as Cu–Ni, Cu–Fe, Cu–Co bimetallic oxides; N-doped carbon/CuO-Fe2O3; CuO-CeO2-CoOx; CuFe2O4; CuVOx; Cu–Fe oxides/SBA-15; Cu–SBA-15; Cu-γ-Al2O3; Cu layered double hydrotalcite; Cu-Fe hydroxide and TiCuAl-SiO2 nanospheres, have been developed and used in the Fenton-like degradation of organic pollutants due to their preferable catalytic activities and deeper mineralization in comparison with Fe-containing heterogeneous catalysts (Ghasemi et al., 2019; Karthikeyan et al., 2016; Liang et al., 2019; Lyu et al., 2015, 2016, 2017; Miao et al., 2019; Wang et al., 2018; Yan et al., 2019; Yu et al., 2018; Zhang et al., 2020). Nevertheless, Cu-containing heterogeneous catalysts always involve some unsatisfactory aspects in the Fenton-like degradation of organic pollutants. Firstly, the catalytic activities of Cu-containing solid catalysts are much lower than that of Cu2+ ions in a homogeneous catalytic system. Since the heterogeneous Fenton reactions always involve a complex process including adsorption, interfacial reactions and desorption, the catalytic activities of the solid catalysts are greatly limited by their physicochemical properties, such as pore structure, surface area, adsorption capacity, exposed catalytic sites and redox activity (Ivanets et al., 2018, 2019; Miklos et al., 2018; Pavithra et al., 2019; Sharma et al., 2018; Yi et al., 2019; Zhang et al., 2018, 2019). On the other hand, the reported Cu-containing solid catalysts had an unsatisfactory catalytic stability due to their structural collapse and Cu2+ ions leaching from the host matrix in the aqueous Fenton-like process. As shown in Table S1, the catalytic activities for most of them declined more than 2.0 % after five catalytic cycles (Kong et al., 2016; Liang et al., 2019; Ling et al., 2014; Lyu et al., 2015; Ren et al., 2019; Xia et al., 2011; Yan et al., 2019; Zhang et al., 2020). Therefore, it is significant and desirable to develop a new type of Cu-containing solid catalyst with special structure and preferable stability for the application in the AOPs of textile wastewater.Fe2+ + H2O2 → Fe3+ + radical dotOH + OH (k = 63–76 M–1·s−1)Fe3+ + H2O2 → Fe2+ + radical dotO2 + H2O (k = 0.001–0.01 M–1·s−1)Cu+ + H2O2 → Cu2+ + radical dotOH + OH (k = 1.0 × 104 M–1·s−1)Cu2+ + H2O2 → Cu+ + radical dotO2 + H2O (k = 4.6 × 102 M–1·s−1)

Transition metal sulfides have remarkable potential as persistent and active catalysts in the Fenton-like wastewater management due to their variable valences, abundant structures and low solubility in aqueous solutions. MoS2, WS2, Cr2S3, CoS2, PbS and ZnS, have been proved to be stable and effective co-catalysts to facilitate electron transfer in photo-assistant and homogeneous Fenton degradation of persistent pollutants (Dong et al., 2018; Xing et al., 2018). As a typical redox catalyst, Cu2O has been widely applied in Sonogashira coupling reaction, CO oxidation and CO2 photoreduction, but it seems to be unsuitable for the Fenton reactions due to its poor stability, especially in an oxidative aqueous system (Gusain et al., 2016; Kou et al., 2012; Leng et al., 2010; Luo et al., 2018). Owning to the inherent instability of Cu2O crystals, the effortless oxidation of Cu+ ions into soluble Cu2+ ions gives rise to collapse of the crystal lattice and uncontrolled leaching of Cu2+ ions. Differently, copper sulfide (Cu2-xS, 0<x<1) materials have richer composition and preferable stability in comparison with Cu2O, which makes them to be extensively applied in the field of photocatalysis, energy conversion and biomedicine (Coughlan et al., 2017; Liu et al., 2017). To date, researchers have discovered more than 40 types of Cu2-xS which can be mainly classified into three categories (hexagonal close packing, cubic close packing and an integration of hexagonal close packing with covalent bonding) in accordance with the packing mode of sulfide atoms in the crystal structures (Chen et al., 2018; Guo et al., 2014; Hosseinpour et al., 2015; Jiang et al., 2014; Sun et al., 2011, 2012, 2013, 2017 Wang et al., 2015). Among them, digenite (Cu1.80S or Cu9S5) with short Cu–S and Cu–Cu chemical bonds close to metallic Cu–Cu bond distance generates high electrical conductivity and the most stable lattice in thermodynamics. Therefore, Cu9S5 is very suitable for catalyzing the redox in some corrosive systems. By using Cu2O crystals as hard templates, Cu9S5 materials with diverse microstructures have been fabricated on the base of ion exchanging, Ostwald ripening or Kirkendall Effect, and they have been used as core components in photocatalysts, batteries, supercapacitors and sensors (Coughlan et al., 2017; Liu et al., 2017; Sun and Yang, 2014). Unfortunately, the applicable potential of Cu9S5 in the AOPs seems to be ignored by the people even if it owns highly active Cu species and inherent stability. So far, the relevant work by using Cu9S5 as primary catalyst in Fenton-like degradation of organic dyes is scarcely reported despite its widespread application in other fields.

Herein, hollow-structured (HS) Cu9S5 nanospheres synthesized via a facile anion exchange method were applied to Fenton-like degradation of organic dyes. The obtained HS-Cu9S5 nanospheres could effectively accelerate the decomposing of H2O2 into oxidative radical dotOH and radical dotO2. HS-Cu9S5 catalysts had a wide pH application range of 5.0–9.0 and exhibited splendid catalytic stability and low Cu2+ ion leaching rate in 15 catalytic cycles, which make them to be a kind of promising heterogeneous catalysts in the large-scale AOPs of textile wastewater.

Section snippets

Materials

Cupric acetate (Cu(AC)2, AR), cuprous bromide (CuBr, AR), sodium hydroxide (NaOH, AR), D-(+)-glucose (AR), ethylene glycol (EG, AR), sodium sulfide (Na2S, AR), hydrogen peroxide (H2O2, 30 wt.%, AR), methyl orange (MO, AR) and methylene blue (MB, AR) were purchased from Sinopharm Chemical Reagent Corporation. All reagents were used directly in the experiments.

Fabrication of Cu2O nanospheres

In a typical procedure, 2.0 mL of NaOH aqueous solution (1.0 mol·L−1) was added into 25 mL of Cu(Ac)2-EG solution (0.01 mol·L−1) preheated

Transformation of Cu2O nanospheres into CS-Cu2O@Cu9S5, YS-Cu2O@Cu9S5 and HS-Cu9S5

Fig. 1 depicts the XRD patterns of samples 1–4. As shown in Fig. 1(A), five diffraction peaks correspond well to (110), (111), (200), (220) and (311) lattice planes of face-centered cubic (fcc) Cu2O (PDF#05-0667) in sequence, indicating the successful synthesis of pure Cu2O. With increasing dosage of S2− ions in the sulfuration reaction, the five diffraction peaks belonging to the Cu2O crystal gradually weaken, and a new peak that can be indexed to the (110) lattice plane of digenite (Cu9S5)

Conclusion

A facile anion exchange strategy was used to transform Cu2O nanospheres into CS and YS structured Cu2O@Cu9S5 nanospheres, and finally into hollow Cu9S5 nanospheres by simply increasing the dosage of Na2S. The HS-Cu9S5 nanospheres exhibit excellent catalytic activity in the Fenton-like reaction for the decomposition of H2O2 into radical dotOH and radical dotO2, which play a dominant role in the oxidative degradation of organic pollutants, not just in discoloration. In the presence of HS-Cu9S5 nanospheres, the TOCs

CRediT authorship contribution statement

Xiaolin Luo: Conceptualization, Methodology, Project administration, Writing - original draft, Validation. Huanting Hu: Methodology, Data curation, Formal analysis. Zhe Pan: Methodology, Data curation, Formal analysis. Fei Pei: Data curation, Formal analysis. Huaming Qian: Data curation, Formal analysis. Kangkang Miao: Data curation, Formal analysis. Sifan Guo: Data curation, Formal analysis. Wei Wang: Data curation, Formal analysis. Guodong Feng: Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 21871007, 21801009); the Natural Science Foundation of Shaanxi Province (Nos. 2019JLM-15, 2018JM2006); and Research Foundation of Baoji University of Arts and Sciences (Nos. ZK2017025, ZK2018071).

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