Synergistic effects of cupric and fluoride ions on photocatalytic degradation of phenol

https://doi.org/10.1016/j.jphotochem.2007.04.023Get rights and content

Abstract

Synergistic effects of cupric ions and fluoride ions were investigated on the photocatalytic degradation of phenol in TiO2 (Degussa P25) suspensions at pH 3 under UV irradiation. Under all tested conditions, the phenol photodegradation was observed to follow a pseudo-first-order reaction in kinetics. The rate constant k of the phenol photodegradation was evaluated as 0.010 min−1 in the absence of Cu2+ and F ions, and was increased up to 0.014 min−1 by adding 0.4 mmol L−1 Cu2+, and to 0.022 min−1 by adding 5 mmol L−1 F ions. The coexistence of both Cu2+ and F ions increased the k value further to 0.041 min−1, being 410% of that without any addition of Cu2+ and F ions. The much increased rate of phenol degradation induced by the coexistence of Cu2+ and F ions were also confirmed by the monitoring of degradation intermediates. With the aid of the determination of radical dotOH radicals and H2O2 generated during the photodegradation, the observed significant synergistic effects were attributed to the shielding effect of fluoride on the charge separation efficiency and the efficient trapping of the photogenerated electron by the Cu2+ adsorbed on TiO2 surface.

Introduction

TiO2 photocatalysis has been intensively investigated for its application to the destruction of environmental toxic pollutants [1], [2], [3]. The primary event occurring on the UV-illuminated TiO2 is the generation of photo-induced electron/hole (e–h+) pairs. These charge carriers can rapidly migrate to the surface where they are captured by a suitable electron donor and acceptor, initiating an oxidation and reduction reaction, and/or they are recombined, dissipating the input light energy onto heat. The strong oxidizing ability of TiO2 photocatalysts has been ascribed to highly oxidative valence band holes (+2.7 V versus NHE) and various oxygen-containing radical species (e.g., radical dotOH, O2radical dot, HO2radical dot). Among these species, holes and radical dotOH radicals play the most important roles in the photodegradation of organic pollutants. Thus, the overall quantum efficiency is expected to be decided by the competition between charge-carrier recombination, trapping, and interfacial charge transfer [4], [5]. It is well known that improving photocatalytic efficiency requires primarily a decreased e–h+ recombination rate, which is generally achieved by increase of the rate of photogenerated electrons transfer to the oxidant at the interface and/or the capture of holes via oxidation process.

Among various efforts for enhancing photocatalytic efficiency, the surface modification of TiO2 seems mostly interesting. The surface modification of TiO2 can be performed by depositing noble metal clusters on the surface of TiO2. For example, platinized-TiO2 photocatalysts show higher photoactivity by increasing the rate of electron transfer from TiO2 to O2 [1], [3], [5]. It can be also carried out in situ by adding appropriate compounds into solution during the photodegradation. It has been reported that the photocatalytic decomposition of organic contaminants is greatly accelerated by the addition of metal ions to TiO2 suspensions, such as Fe3+ [6], [7], [8], Ag+ [8], [9], [10], Cr6+ [10], [11], and Cu2+ ions [12], [13], [14], [15]. This positive effect has been attributed to photoelectron trapping by metal ions, which reduces the electron–hole recombination and results in an increased concentration of radical dotOH radicals. The enhancing effects of fluoride anions are also concerned intensely [16], [17], [18], [19], [20], [21]. Lv and Xu have found that addition of fluoride anions into TiO2 dispersions accelerates the photocatalytic degradation of organic dye X3B [17]. Minero et al. have observed that the degradation of phenol is enhanced by using surface fluorinated-TiO2 [18]. Choi and coworkers have reported similar enhancing effect of F ions for the degradation of phenol, acid orange 7 and tetramethylammonium [19], [20], [21]. The surface fluoride-enhanced effect is ascribed to accelerating generation of mobile radical dotOH radicals, due to enhanced holes availability for water oxidation through the displacement of ≡Ti–OH by ≡Ti–F.

Although it is known that both the adsorption of metal ions and fluoride ions can promote the photocatalytic degradation of organic pollutants, we have not found any report on the possible synergistic effects of metal ions and fluoride ions. The further enhancing of the photocatalytic efficiency of TiO2 photocatalysts with the aid of the synergistic effect of metal ions and fluoride ions is very interesting from the view points of both academic and technological aspects in the field of photocatalysis. Therefore, the main purpose of the present work is to investigate the synergistic effects of Cu2+ ions and F ions. To achieve it, phenol is taken as a model compound to be degraded, because it is an important toxic pollutants with the safety levels in the range of 0.1–1.0 mg L−1 and is considered to be an intermediate product in the photooxidation pathway of higher molecular weight aromatic hydrocarbons [22].

Section snippets

Reagents and materials

TiO2 powders (Degussa P25, ca. 80% anatase, 20% rutile; BET area, ca. 50 m2 g−1) were used as received. Horseradish peroxidase (POD, specific activity of 300 units mg−1, RZ  3) was purchased from Tianyuan Biologic Engineering Corp. (China). All other chemicals (N,N-diethyl-p-phenylenediamine sulfate (DPD, from Aldrich), sodium diethyldithiocarbamat (DDTC), CuCl2, NaF, and phenol) were of analytical reagent grade and used without further purification. Distilled water was used throughout. The pH of

Synergistic effect of Cu2+ and F on photocatalytic degradation of phenol

In control experiments, we confirmed that the disappearance of phenol was negligible when the phenol solution is not irradiated with UV light or no TiO2 catalyst was added into the solution. The photocatalytic degradation of phenol only occurred over TiO2 under UV light illumination. As partly shown in Fig. 1, the phenol photodegradation under all the tested conditions was observed to follow a pseudo-first-order reaction in kinetics, which could be expressed as ln(ct/c0) = kt + y. Here, c0 and ct

Conclusions

By using phenol as a model compound of organic pollutants to be degraded, the effects of dissolved Cu2+ and F ions were investigated on the photocatalytic degradation of organic pollutants in UV/TiO2 system. When an appropriate amount of Cu2+ or F alone was added into the TiO2 suspensions, the adsorbed Cu2+ ions function as an electron scavenger, or the adsorbed F ions played a role in increasing holes availability and consequently accelerating to yield more radical dotOH radicals and/or enhance the

Acknowledgements

Financial supports from the National Science Foundation of China (Grants Nos. 30571536 and 20677019) are gratefully acknowledged. The Analytical and Testing Center of Huazhong University of Science and Technology is thanked for its help in the identification of intermediate products of phenol photodegradation.

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