Superior performance of FeVO4@CeO2 uniform core-shell nanostructures in heterogeneous Fenton-sonophotocatalytic degradation of 4-nitrophenol

https://doi.org/10.1016/j.jhazmat.2019.121059Get rights and content

Highlights

  • FeVO4@CeO2 act as a commendable catalyst for sonophotocatalytic degradation of 4-NP.

  • Sonophotocatalytic system shows superiority over the individual US or UV.

  • FeVO4@CeO2 possesses high stability and durability after six consecutive cycles.

  • FeVO4@CeO2 core shell nanostructures play the major role in extending the pH.

Abstract

Porous FeVO4 nanorods decorated on CeO2 nanocubes (FeVO4@CeO2) were successfully prepared via a facile hydrothermal route and tested in the degradation of 4-nitrophenol (4-NP) for enhanced heterogeneous oxidation using ultrasonic (US), ultraviolet (UV), and binary irradiation US/UV, respectively. The nanostructure of the core-shell FeVO4@CeO2 was characterised using XRD, SEM, EDS elemental mapping, TEM, HRTEM, SAED, FTIR, Raman, BET, point of zero charge (PZC), XPS analysis and UV–vis DRS. The effect of various parameters, for examples, nanostructured core-shell amounts, hydrogen peroxide concentration, initial concentration, pH and irradiation time, on 4-NP degradation were investigated for the optimisation of the catalytic performance. The durability and stability of the core-shell nanostructured materials were also investigated and the obtained results revealed that the catalysts can endure the harsh sonophotocatalytic conditions even after six cycles. Mineralisation experiments were investigated using the optimised parameters. The core-shell nanostructured FeVO4@CeO2 has higher PZC than pure FeVO4 and CeO2, leading to excellent sonophotocatalytic activity even at high pH and stability for the degradation of 4-NP after six cycles. A possible mechanism over the FeVO4@CeO2 was proposed based on the special three-way Fenton-like mechanism and the dissociation of H2O2 with the experiments of active species trapping and calculated band gap energy.

Introduction

Nitro aromatics have been widely used in the manufacture of many chemical products like pharmaceuticals, dyes, solvents, explosives and fungicidal agents (Herrera-Melián et al., 2012; Yehia et al., 2016). Nevertheless, these nitro compounds have serious effects on the environment and also a highly toxic effect on living organisms (Lunhong and Jing, 2013). Among these compounds, 4-nitrophenol (4-NP) is used as a chemical intermediate for rubber, lumber preservatives, azo and others (Peretz, 2019). According to the USA Environmental Protection Agency (EPA), 4-NP has been classified as a priority toxic pollutant (Mishra and Gogate, 2011). Advanced Oxidation Processes (AOPs), are considered as the most promising approaches to the destruction of recalcitrant organic pollutants, and are based on the generation of hydroxyl radicals (Erick et al., 2004). Recently, sonolysis with an appropriate catalyst has received great attention as a promising system owing to the generation of more free radicals (Ricardo and Efraim, 2018; Slimane et al., 2010).

Nevertheless, it has been observed that a combination of different AOPs has been found to be more efficient and foster the degree of mineralisation of organic pollutants (Gogat, 2008; Bagal and Gogate, 2014). It has been reported that the simultaneous use of sonolysis and photocatalysis called sonophotocatalysis can be considered a promising technique with increasing the production of radical dotOH radicals (Chilukoti et al., 2011; ElShafei et al., 2018).

Nowadays, the core-shell nanostructure have great potential in catalysis because of their unique chemical and physical characteristics and have many advantages such as an increased number of surface-active sites for the core and promotion by the shell of catalytic activity (Zaleska et al., 2016; Mitsudome and Kaneda, 2013). Moreover, the reactions of all catalysts occur at the same time and the strength of the shell can protect the core and prevent assemblage of the particles (Zhang et al., 2014; Pham et al., 2018). In this context, many researchers have used various core shell nanostructures (Siadatnasab and Khataee, 2017; Shah et al., 2016; Pham and Kim, 2018; Khanchandani et al., 2016; Ding et al., 2016; Kanmani and Ramachandran, 2012; He et al., 2014; Habila et al., 2016; Zheng et al., 2018a; Huang et al., 2017; Hu et al., 2011; Li et al., 2015a; Khanchandani et al., 2014; Ghows and Entezari, 2012; Ijaz et al., 2016). Among the inorganic shell materials, cerium oxide (CeO2) has served as one of the most efficient photocatalysts for the degradation of organic pollutants because of its ability to transform between Ce+3 and Ce+4 states at oxygen vacancy sites, and to facilitate electron transfer and e/h+ pair diff ;usion between CeO2 and another semiconductor such as Cu2O (Chae et al., 2017), CdS (Zhang et al., 2017) and TiO2 (Chen et al., 2017; Cargnello et al., 2010). Accordingly, the preparation of core-shell materials with a metal as a core and ceria as a shell is of great significance. Iron vanadate FeVO4 is considered as a semiconducting, highly stable, selective as a photocatalyst accordingly to its unique and excellent performance (Hosseinpour-Mashkani et al., 2016).

The aim of the current work is to investigate the combination of heterogeneous Fenton-like and sonophotocatalysis which is considered as another possible way to increase the generation of free radicals in the system. Whereas the use of heterogeneous Fenton-like with sonophotocatalysis plays an effective role in enhancing the extent of degradation with adjusting the parameter conditions in order to get more hydroxyl radical and maximum beneficial effects.

To the best of our knowledge, successful synthesis of a FeVO4@CeO2 core shell nanostructure and its use as heterogenous Fenton-like catalyst in the degradation of 4-NP in the presence of ultrasonic (US), ultraviolet (UV), and binary irradiation US/UV, using H2O2 as an oxidant have not been reported. In this work, a FeVO4@CeO2 was synthesized and investigated with XRD, SEM, EDS, TEM, HRTEM, SAED, FTIR, Raman, N2-adsorption-desorption, DRS and XPS. The different degradation parameters, for instance, irradiation time, catalyst loading, pH and H2O2 initial concentration were optimized, and the mineralization and recyclability were examined. Moreover, the synergistic effect with reaction kinetics was investigated in detail and the possible mechanism was discussed.

Section snippets

Preparation of porous FeVO4 nanorods

FeCl3.6H2O (2 mmol) was dissolved in 10 mL of deionised water and NH4VO3 (2 mmol) was dissolved in another 10 mL of deionised water at 90 °C. Next, NH4VO3 solution was slowly added to FeCl3.6H2O solution under constant stirring. After 20 min of stirring, the obtained slurry was placed in a 50 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated at 180 °C for 3 h and then left to cool down naturally to the ambient temperature. The obtained precipitate was separated by

XRD

The crystalline phases and composition of pure FeVO4, CeO2 and FeVO4@ CeO2 were determined by XRD analysis Fig. 1. The porous FeVO4 nanorods present a similar XRD pattern to the triclinic FeVO4 (JCPDS No. 71- 1592). Further, CeO2 prepared ultrasonically shows all the diffraction peaks matched very well with the nanoceria of cubic fluorite structure (JCPDS No. 75-0390). The diffraction pattern of the FeVO4@CeO2 shows that all the peaks are in good agreement with the peaks of the triclinic

Conclusions

Core shell nanostructured FeVO4@CeO2 acted as effective Fenton-like achieve high performance in the sonophotocatalytic degradation of 4-NP at low concentrations of the catalyst and oxidant. The synergistic effect of FeVO4@CeO2 enlarges the photoactivity of FeVO4 and effectively inhibits the charge carrier recombination, ultimately improving the photocatalytic activity of FeVO4 with CeO2. The hydroxyl radicals (radical dotOH) and holes (h+) play a more important role than that of super oxide radicals (O2radical dot)

Acknowledgements

The authors wish to acknowledge the financial support of this research by Department of Green Chemistry, School of Engineering Science, LUT University, Mikkeli, Finland.

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