Heterogeneous photo-Fenton processes using graphite carbon coating hollow CuFe₂O₄ spheres for the degradation of methylene blue

Highlights

• A novel graphite carbon coating hollow CuFe₂O₄ spheres was prepared.

• H-CFO@C-600 °C-0.15 SPs exhibit excellent catalytic activity in photo-Fenton-like degradation of methylene blue.

• The efficient separation and transfer of photoinduced electrons accelerated the enhancement of catalytic activity.

• High stability and great separability extend applied prospects.

Abstract

The novel graphite carbon coating hollow CuFe₂O₄ spheres were fabricated through solvothermal method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectra, etc. The catalytic performance of the graphite carbon coating hollow CuFe₂O₄ spheres was evaluated in photo-Fenton-like degradation of methylene blue (MB) using H₂O₂ as a green oxidant under light irradiation (λ > 400 nm). The results demonstrated that the hollow CuFe₂O₄ spheres with graphite carbon coating exhibited superior catalytic activity. In the preparation process of catalyst, the addition of glucose was very important to its catalytic performance. Photoresponse analysis of the typical samples proved that CuFe₂O₄@graphite carbon core-shell hollow spheres possessed excellent photocurrent response and lower electrochemical impedance. In addition, a possible mechanism for photocatalytic degradation of MB had been presumed. Moreover, after five regeneration cycles, the graphite carbon coating hollow CuFe₂O₄ spheres still exhibited better properties.

Introduction

In recent years, the rapid growth of the global economy greatly accelerates the development of chemical products. Inevitably, a good deal of wastewater containing organic contaminants was generated in production process [1]. These organic compounds in water has resulted in clean water crisis and public health problem since most of them are toxic, endocrine disrupting, mutagenic or potentially carcinogenic to humans, animal and aquatic life [2]. Furthermore, many organic pollutants are considered as micro quantities and uneasy mineralization in wastewater, which increase the difficulty of wastewater treatment. Conventional treatment methods such as biological processes are not effective due to the recalcitrant nature of the contaminants [3], [4], [5]. However, advanced oxidation processes (AOPs) are able to degrade those biorefractory organic compounds and considered as alternative wastewater treatment processes [6]. Among different AOPs, photo-Fenton-like reaction was widely concerned because it could produce more oxidative species like hydroxyl radicals (·OH) to accelerate the reaction [7]. Nevertheless, some drawbacks still existed in the catalysis processes, such as low catalytic efficiency and high cost. Thus, it is very necessary to obtain heterogeneous photosensitive Fenton catalysts with higher activity and better stability.

As compared common iron oxides and iron hydroxides (Fe₃O₄, α-Fe₂O₃ and α-FeOOH), ferrites were considered as novel and promising heterogeneous photosensitive Fenton catalysts due to their stable structure, low band gap energy and excellent magnetic properties [8]. However, the low efficiency of converting solar energy into chemical energy greatly restricts their applications in photo-Fenton-like process. Recently, CuFe₂O₄, with a relatively narrow band gap [7], [9], [10], has been widely used as heterogeneous photosensitive Fenton catalysts because of its excellent visible-light response, low cost and good photochemical stability. For example, Sharma et al. [11] reported that CuFe₂O₄ was the best catalyst as contrasted to CoFe₂O₄, NiFe₂O₄, and ZnFe₂O₄ in the optimum reaction conditions, which might be attributed to coupling between Fe²⁺/Fe³⁺ and Cu⁺/Cu²⁺ redox pairs thus generating more efficient Fenton reagent under illumination. However, a rapid recombination of electron-hole pairs, a high value of ion leaching and a low ratio of surface area to mass still affected the catalytic activity and applicability of CuFe₂O₄ in photo-Fenton-like reaction [8], [12], [13]. To change it, several attempts had been developed to reduce the recombination of photoinduced electron-hole pairs and increase the photocatalytic efficiency of copper ferrite by synthesizing various structures, doping with other elements, coupling with other semiconductors and supporting with other materials, such as nano-CuFe₂O₄ and meso-CuFe₂O₄ [8], Cu_0.5Mn_0.5Fe₂O₄-TiO₂ [14], CuFe₂O₄-MO₂ (M = Sn, Ce) [15], [16], core-shell CuFe₂O₄@C₃N₄ [7], [9], core-shell CuFe₂O₄@TiO₂ [17], CuFe₂O₄/AgBr [18]. Nevertheless, the fact that the states introduced act as combining centers for electron-hole pairs as well as the thermal instability associated with doping materials gave rise to some doubts about their performances [19]. Though appropriate semiconductor coating can enhance the efficiency of converting solar energy into chemical energy and prevent ions leaching on the surface of CuFe₂O₄, Fenton-like reaction process is also extinguished owing to the interruptive contact of heterogeneous Fenton catalyst and hydrogen peroxide. Recent researches showed that ultrathin graphite carbon coating as conjugated material not only possessed excellent visible light response due to the electronic coupling of π states of the graphite carbon and conduction band states of photocatalysts, but also had unique properties in electron or hole transport [19], [20], [21], [22], [23], [24]. Besides, nano-scale thin layer is helpful to the contact of heterogeneous Fenton catalyst and hydrogen peroxide and limits ions leaching on the photocatalyst surface. Therefore, the introduction of graphite carbon coating can be a promising attempt to improve catalyst activity for heterogeneous Fenton reaction.

In the present work, we predominantly focus on increasing contact area and enhancing energy transformation to improve the catalytic activity of copper ferrite in photo-Fenton-like reaction. In order to obtain a greater ratio of surface area to mass, the hollow spheres of copper ferrite had been successfully synthesized by a general method. Primarily, according to the aforementioned assumption, a thin carbon layer was covered on the surface of nude hollow spheres using glucose as raw material. Subsequently, the obtained products were calcinated at proper temperature under nitrogen atmosphere to achieve ultrathin graphite carbon coating on the catalyst surface. The obtained samples had been characterized by XRD, SEM, TEM, FT-IR, Raman spectra, VSM, BET, PL, and UV-DRS, etc. Meanwhile, the catalysis activity of samples in heterogeneous Fenton reaction was tested by the catalysis degradation of target contaminant, and a postulated mechanism for the enhancement of catalytic efficiency was also proposed.