Journal of Photochemistry and Photobiology A: Chemistry
Synergistic effects of cupric and fluoride ions on photocatalytic degradation of phenol
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., OH, O2−, HO2). Among these species, holes and OH 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 OH 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 OH 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 OH 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.
References (34)
- et al.
J. Mol. Catal. A: Chem.
(1995) - et al.
Catal. Today
(2001) - et al.
Appl. Catal. A: Gen.
(1999) - et al.
Photobiol. A: Chem.
(2001) - et al.
Appl. Catal. B: Environ.
(2005) - et al.
J. Mol. Catal. A: Chem.
(2002) - et al.
Appl. Catal. B: Environ.
(2004) - et al.
Photobiol. A: Chem.
(2003) - et al.
Appl. Catal. B: Environ.
(2002) - et al.
Anal. Chim. Acta
(2006)
Electrochem. Commun.
Water Res.
Appl. Catal. B: Environ.
Appl. Catal. B: Environ.
Langmuir
Environ. Sci. Technol.
Langmuir
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2018, Separation and Purification TechnologyCitation Excerpt :Rate constants of phenol photodegradation with three types of TiO2 (Degussa P25, TiO2 of anatase and rutile phases from Sigma–Aldrich) under UV or visible-light irradiations in the presence of Cu2+ ions showed higher than those of the Cu2+-absence solutions [35]. The co-addition of Cu2+ and F− has been reported synergistically and significantly to improve the degradation rate constant of phenol relative to that obtained by adding one of the two ions [36]. The reversible photoreductive deposition and oxidative dissolution of Cu2+ in TiO2 aqueous suspensions has also been reported [37].