The photocatalytic dehalogenation of chlorophenols and bromophenols by cobalt doped nano TiO2

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Highlights

  • Co doped TiO2 nanoparticle was used as photocatalyst for halophenols degradation.

  • It is the first time to compare the degradation rate between CPs and BPs on TiO2/Co.

  • The degradation of both CPs and BPs obeys the pseudo-first-order kinetics.

  • BPs are easier to be photodegradated than CPs by TiO2/Co nano catalyst.

  • The degradation of 2,4,6-TCP and 2,4,6-TBP occurs via the dehalogenation pathway.

Abstract

Cobalt doped TiO2 nanoparticle prepared by ultrasonic-assisted hydrothermal method was used as the photocatalyst in the degradation of chlorophenols (CPs) and bromophenols (BPs). The doping content of cobalt in this catalyst was optimized as 0.5 wt%. The degradation rates of 2,4,6-trichlorophenol (2,4,6-TCP), 2,4-dichlorophenol (2,4-DCP), 2,6-dichlorophenol (2,6-DCP), 2-chlorophenol (2-CP), 4-chlorophenol (4-CP), 2,4,6-tribromophenol (2,4,6-TBP), 2,4-dibromophenol (2,4-DBP), 2,6-dibromophenol (2,6-DBP), 2-bromophenol (2-BP) and 4-bromophenol (4-BP) were estimated based on the quantitative determination of the degradation products. It was found that the degradation of CPs and BPs underwent dehalogenation process with pseudo-first-order kinetics. Moreover, the photodegradation kinetics was compared between CPs and BPs. The degradation rate of BPs was more quickly than CPs due to the difference in bond energy of carbon–halogen, and the degradation rates of BPs and CPs were mainly dependent on the number and the position of substituting halogen atom respectively. In addition, 2,4,6-TCP and 2,4,6-TBP can be effectively degraded by more than 95.1% and 93.5%, respectively, within 240 and 30 min. Tri-substituted phenols were firstly photodegraded to corresponding di-substituted ones, then to mono-substituted ones, and finally to phenol by TiO2/Co nano catalyst, although there were some differences in selectivity, rate and yield in the dehalogenation pathway of 2,4,6-TCP and 2,4,6-TBP.

Introduction

Chlorophenols (CPs) and bromophenols (BPs) are widely used in chemical industry and wood industry. They are often employed as chemical intermediates, wood preservatives, herbicides, insecticides and fungicides. In addition, various BPs are largely used as flame retardants in the past decades [1], [2], [3], [4]. Due to constant use, however, they have been detected in soils, water and environmental organisms [5], [6], [7]. CPs and BPs have adverse impacts on the ecosystem for their toxic, carcinogenic and teratogenic properties. In recent years, concerns have been raised because of their persistence and bioaccumulation both in animals and in humans [8], [9], [10]. Therefore, it is important to find innovative and effective ways to minimize the harm of halophenols in environment.

Photocatalytic degradation by semiconductor nanoparticles irradiated with UV light is a useful method for the degradation of phenolic compounds [11], [12]. TiO2 nanoparticle has been commonly exploited as a photocatalyst for its reactivity, stability, environmental friendliness and low cost [13]. However, the application of naked TiO2 was mainly limited due to the recombination of the generated photoholes and photoelectrons. Hence, many reformative processes such as use of transition metal [14], [15], [16], nonmetal [17], binary oxide [18] and dye sensitizer [19] have been adopted as means to reduce the rate of electron-hole recombination and improve the photocatalytic effect.

Several research groups have investigated the effects of metal or metal ion doping on the photocatalytic properties of TiO2. The incorporation of metal or metal ions into the TiO2 crystal lattice may alter the photoreactivity. Noble metals were frequently adopted as doping metals of TiO2. Oros-Ruiz et al. [14] studied the particle size and deposition method of Au/TiO2 on the photodegradation of 4-chlorophenol (4-CP). Rengaraj et al. [15] found that 20 mg/L 2,4,6-trichlorophenol (2,4,6-TCP) was effectively degraded in aqueous Ag–TiO2 suspension by more than 95% within 120 min and the degradation of 2,4,6-TCP occurred via chlorine-release pathways. Transition metals and alkaline earth metals are used as dopants in TiO2 because of the comparatively high effect at much less cost than noble ones. Vijayan et al. [11] found that the photocatalytic activity of TiO2 doped with 0.5 wt% Fe exceeded that of non-doped commercial and synthesized pure TiO2. Venkatachalam et al. [20] concluded that the photocatalytic activity in the degradation of 4-CP is higher for Mg2+ and Ba2+ doped nano TiO2 than those for both pure nano TiO2 and commercial TiO2 (DegussaP-25).

Cobalt is an abundant transition metal and it could be an attractive dopant for TiO2. Cobalt doped TiO2 (TiO2/Co) has shown high activity for degradation of 2-chlorophenol (2-CP) [21], 4-CP [22], Bisphenol A [22], acetaldehyde [23], [24], [25], acetonitrile [26], methyl orange [16], methylene blue [27], [28], rhodamine B [29] and azo fuchsine [30]. To the best of our knowledge, few reports have been published on TiO2/Co nanoparticles for efficient photodegradation of BPs and other CPs. The application of TiO2/Co to photocatalytic degradation of halophenols should be a profitable attempt. It is also interesting to explore the degradation mechanism of TiO2/Co to halophenols. Meanwhile, the wastewaters are not only polluted by only one type of phenolic compounds but also different substituted and isomers of phenols [13], so it is of important significance to investigate the difference of various halophenols during the photocatalytic degradation process.

In this present work, TiO2/Co nanoparticles were prepared by ultrasonic-assisted hydrothermal method. Well characterized TiO2/Co was used as catalyst for photodegradation of CPs and BPs. The degradation rates of all the halophenols were carefully estimated based on the reliable determination of the degradation products. Moreover, the dehalogenation behaviors of 2,4,6-TCP and 2,4,6-tribromophenol (2,4,6-TBP) were compared in the process of photocatalytic degradation. In addition, the doping content of cobalt in TiO2/Co nanoparticles were optimized for degradation of 2,4,6-TCP and 2,4,6-TBP.

Section snippets

Materials

Tetra-n-butyl titanate (Ti(OBu)4) and cobalt nitrate (Co(NO3)2·6H2O) of analytical grade were purchased from Shanghai Chemical Regent Company (Shanghai, China) and used as titanium and cobalt sources, respectively, for preparation of TiO2 and TiO2/Co photocatalysts. Acetic acid and ammonium acetate of analytical grade from Shanghai Chemical Regent Company (Shanghai, China) were used to adjust the pH of substrate solutions. 2,4,6-trichlorophenol (2,4,6-TCP), 2,4-dichlorophenol (2,4-DCP),

XRD analysis

The phase composition and the crystallite size of the catalysts were examined by X-ray diffraction analysis. Fig. 1 shows the powder XRD patterns of TiO2/Co with different cobalt contents (0, 0.3, 0.5, 0.7 and 0.9 wt%). The presence of three main peaks at 2θ = 25.4°, 37.8° and 48.1° match the standard XRD data cards of TiO2 crystal (JCPDS No. 21-1272). All the catalysts possessed anatase titania structure. The XRD peaks of cobalt and cobalt compounds were not observed in the diffraction diagrams.

Conclusion

The active TiO2/Co nanoparticle (15–25 nm) with Co doping level of 0.5 wt% prepared by ultrasonic-assisted hydrothermal method has been employed as the photocatalyst in the degradation of CPs and BPs. It is the first time to compare the degradation rate between CPs and BPs, and subsequently to elucidate the photocatalytic degradation mechanism of the halophenols on this catalyst. It was indicated that the photocatalytic degradation of CPs and BPs follows pseudo-first-order kinetics and that BPs

Acknowledgements

This work was supported by the National Basic Research Program of China (973 program, 2009CB421601, 2011CB911003), National Natural Science Foundation of China (21275069, 90913012, 21177061), National Science Funds for Creative Research Groups (21121091), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (11KJB150008), and Analysis & Test Fund of Nanjing University.

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