Photocatalysis as an advanced reduction process (ARP): The reduction of 4-nitrophenol using titania nanotubes-ferrite nanocomposites

doi:10.1016/j.jhazmat.2018.12.090

Journal of Hazardous Materials

Volume 372, 15 June 2019, Pages 37-44

https://doi.org/10.1016/j.jhazmat.2018.12.090 Get rights and content

Highlights

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Synthesis of TiO₂ nanotube-Ferrite (TCF) magnetic nanocomposites.
•
High activity of TCF for photocatalytic reduction of 4-nitrophenol under UV light.
•
Green synthesis of aminophenol high added value chemical through photocatalysis.

Abstract

TiO₂ photocatalysis is an advanced process, employed worldwide for the oxidation of organic compounds, that leads to significant technological applications in the fields of health and environment. The use of the photocatalytic approach in reduction reactions seems very promising and can open new horizons for green chemistry synthesis. For this purpose, titanium dioxide nanotubes (TNTs) were developed in autoclave conditions using TiO₂ P25 as a precursor material. Based on these nanotubular substrates, TiO₂/CoFe₂O₄ (TCF) nanocomposites were further obtained by wet impregnation method. The materials were thoroughly characterized and their structural, textural, vibrational, optoelectronic and magnetic properties were determined. The composite materials combine absorbance in the visible optical range and high BET surface area values (˜100 m²/g), showing extremely high yield in the photocatalytic reduction of 4-nitrophenol (4-NP), exceeding 94% within short illumination time (only 35 min). The developed nanocomposites were successfully reused in consecutive photocatalytic experiments and were easily removed from the reaction medium using magnets. Both remarkable recycling ability and high-performance stability in the photocatalytic reduction of nitrophenol were observed, thus justifying the significant economic potential and industrial perspectives for this advanced reduction process.

Graphical abstract

Introduction

Water reservoirs usually contain organics having toxic and carcinogenic character. Due to their high solubility and stability, aromatic nitro compounds, especially nitrophenols are among the major distinctively dangerous pollutants existing in wastewaters [1]. p-nitrophenol (4-NP or 4-hydroxynitrobenzene) is a hazardous substance, which possesses a significant potential threat to human health, causing irritation, eyes inflammation, skin allergies and respiratory problems. The LD50 in mice is 282 mg/kg and in rats is 202 mg/kg (p.o.) and its interaction with blood may potentially be responsible for cyanosis, confusion, and unconsciousness [2]. Therefore, its elimination in surface water and waste effluents is indispensable.

The 4-NP material can be reduced to 4-aminophenol (4-AmP), a valuable chemical usually employed in the synthesis of peptides, pharmaceuticals (including antipyretic and analgesic drugs) and corrosion protection compounds [3]. As a result, such a transformation benefits the chemical industry and can be used as an efficient and low-cost procedure to remove the 4-NP pollutant from the environment. This is implemented via photocatalysis, a green chemistry approach that has gained significant importance in many applications, including the production of fuels, green synthesis of added value products and water detoxification [4]. This advanced process makes use of nanostructured semiconductors and light to trigger the corresponding chemical reactions. In particular, TiO₂ and ZnO photocatalysts have been used for the photodegradation of a high number of organic pollutants under UV [[5], [6], [7], [8], [9], [10], [11]].

Titanium dioxide (TiO₂) is a photocatalyst able to catalyze the transformation (and/or complete mineralization) of a series of organic contaminants in water, under natural and artificial light. However, there are several limitations in the field of TiO₂ photocatalysis: the first one is the large band gap of the photocatalyst (eg. 3.2 eV for its anatase form and 3.0 eV for rutile), that limits the useful light in the narrow UV domain, below 400 nm; the second problem arises from considerable charge carriers (photogenerated electrons and holes) recombination, making difficult, and sometimes practically inefficient the photocatalytic reactions. A third drawback comes from the use of slurries, which requires an additional and difficult (time-consuming and costly) final separation step.

The efficiency of the photocatalytic process is strongly related to the transport of the photogenerated carriers (electrons and holes), that depends on the materials morphology and particles dimension. These characteristics also determine the amount of the pollutant that can be adsorbed on the semiconductor surface. Among different nanostructures TiO₂ nanotubes present a self-organized structure with a low number of defects, permitting vectorial electron transport across the tube axis and justifying a high interest in photoinduced processes, including their successful use in solar energy conversion devices. Their tubular morphology leads to high aspect to volume ratio, ensuring a large surface area with active sites able to accommodate a great number of pollutant molecules. In fact, titania nanotubes meet the requirement of the large surface area and the relatively short conduction path, while their tubular structure may provide sufficient active sites for reactions [12]. In addition, recently, iron and cobalt oxide magnetic nanoparticles have been combined in photocatalysis studies [13,14]. Such nanocomposites show magnificent photocatalytic and magnetic properties, enabling the separation of the photocatalysts from the reaction mixture by a magnetic bar [13]. Thus, the successful decomposition of acid red 88 organic dye in the presence of Co²⁺ and peroxomonosulphate ions has been reported [15].

TiO₂ photocatalysis is considered now as a well-established advanced oxidation process/technology (AOP-AOT). On the contrary, this is not the case of photocatalytically induced reduction reactions, where the existing data are very limited. In fact, TiO₂ photocatalysis can be also considered as an advanced reduction process-technology (ARP-ART), presented in numerous light-driven catalytic reductions reactions in various applications, including reduction of 4-NP [3,16]. Furthermore, titania–alginate polymer (TAP) hybrids decorated with copper (Cu) and copper oxide (Cu₂O) nanoparticles (TiO₂/TAP/Cu/Cu₂O) have been recently developed [17] and successfully used in the catalytic conversion of Cr (VI) into Cr (III) under both UV and visible irradiation. These nanocomposite photocatalysts overcome the disadvantage of pure TiO₂ that requires acidic aqueous solutions for the photocatalytic reduction of carcinogenic hexavalent chromium.

In this work, the concept of advanced reduction processes is expanded for the first time to green chemistry procedures. Thus we achieved to synthesize and characterize binary nanocomposites of TiO₂ nanotubes with CoFe₂O₄ ferrites, successfully followed by their use in the photocatalytic reduction of 4-nitrophenol.

Section snippets

Synthesis of titania nanotubes

Titanium dioxide nanotubes (TNTs) were synthesized in an autoclaved teflon (300 mL), where 3.0 g of TiO₂ (Aeroxide P25, Evonik) powder was mixed with 90 mL of NaOH (11.25 mol L⁻¹). The mixture, previously homogenized by stirring, was treated for one day at 150 °C. The resulting solid precipitate was washed twice with deionized water and 0.1 M HCl solution, until the pH of the solution became 6.5, and then dried at 80 °C, for 1 day. In the final step, the precipitate was calcinated at 400 °C,

Powder XRD analysis

Fig. 1 shows the X-ray powder diffraction patterns of the synthesized samples. TNTs diffraction peaks were observed at 2θangles of 25.2°, 37.8°, 48.1°, 53.8°, 55.1°, 62.6° and 68.7° for the materials, being characteristic of the (101), (004), (200), (105), (211), (204) and (116) hkl indices of the tetragonal anatase phase (JCPDS No. 21–1272) [27]. For CoFe₂O₄ all peaks observed in the diffraction patterns, are consistent with the cubic spinel structure of CoFe₂O₄ (Space group Fd-3m) (JCPDS No.

Conclusions

In summary, we have presented a facile method for the synthesis of novel TCF nanocomposite photocatalysts through the chemical impregnation method. TCF materials exhibited high efficiency for the photocatalytic reduction of 4-NP in the presence of NaBH₄ reducing agent (above 94% in 35 min), proceeding directly via the photoexcited electrons in the TiO₂ conduction band. Furthermore, the cobalt ferrite-nanotubular titania nanocomposites showed high stability during consecutive photocatalytic

Acknowledgements

P. Falaras acknowledges financial support by Prince Sultan Bin Abdulaziz International Prize for Water (PSIPW)-Alternative Water Resources Prize 2014. I. Ibrahim is financially supported by Science Achievement Scholarship of High Education Ministry of Egypt and the Hellenic Ministry of Foreign Affairs for his PhD work.

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Photocatalysis as an advanced reduction process (ARP): The reduction of 4-nitrophenol using titania nanotubes-ferrite nanocomposites

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis of titania nanotubes

Powder XRD analysis

Conclusions

Acknowledgements

Appl. Catal. B-Environ.

J. Mater. Res. Technol.-Jmr&T

J. Memb. Sci.

Appl. Catal. B

Chem. Eng. J.

Appl. Catal. B-Environ.

Chem. Eng. J.

J. Mol. Catal. a-Chem.

Electrochim. Acta

Mater. Lett.

Optik

J. Mol. Liq.

J. Alloys. Compd.

Appl. Catal. A-Gen.

J. Hazard. Mater.

Chem. Eng. J.

J. Magn. Magn. Mater.

Sep. Purif. Technol.

Appl. Catal. B-Environ.

J. Photochem. Photobiol. A-Chem.

Mater. Sci. Semicond. Process.