Different recycle behavior of Cu2+ and Fe3+ ions for phenol photodegradation over TiO2 and WO3

Journal of Hazardous Materials 262 (2013) 114–120

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Different recycle behavior of Cu2+ and Fe3+ ions for phenol photodegradation over TiO2 and WO3 Lianghui Wan, Jiayi Sheng, Haihang Chen, Yiming Xu ∗ State Key Laboratory of Silicon Materials and Department of Chemistry, Zhejiang University, Hangzhou 310027, China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Fe3+ is recyclable with all catalysts except WO3 .

except P25.

• Fe3+ is regenerated through H2 O2 photogenerated from TiO2 . • Cu2+ is regenerated through dissolved O2 in water.

Phenol C/C

• Cu2+ is recyclable with all catalysts

0.9

WO

0.6

Fe/WO

0.3 Fe/P25

P25

Cu/WO

Cu/P25

0.0 0

5

10

15

Irradiation time (h)

a r t i c l e

i n f o

Article history: Received 29 May 2013 Received in revised form 16 July 2013 Accepted 1 August 2013 Available online xxx Keywords: Anatase Rutile Tungsten trioxide Photocatalysis Transition metal ions

a b s t r a c t Photocatalytic degradation of organic pollutants on TiO2 and WO3 have been widely studied, but the effects of Cu2+ and Fe3+ ions still remain unclear. In this work, we have found that the recycle behavior of Cu2+ and Fe3+ are greatly dependent on the photocatalytic activity of metal oxide used. With TiO2 (P25, anatase, and rutile), all the time profiles of phenol degradation in water under UV light well fitted to the apparent first-order rate equation. On the addition of Cu2+ , phenol degradation on anatase, rutile and WO3 also followed the first-order kinetics. On the addition of Fe3+ , the initial rate of phenol degradation on each oxide was increased, but only the reactions on three TiO2 became to follow the first order kinetics after half an hour. The relevant rate constants for phenol degradation in the presence of Cu2+ or Fe3+ were larger than those in the absence of metal ions. Under visible light, phenol degradation on WO3 was also accelerated on the addition of Fe3+ or Cu2+ . Moreover, several influencing factors were examined, including the metal ion photolysis in solution. It becomes clear that as electron scavengers of TiO2 and WO3 , Fe3+ is better than Cu2+ , while they are better than O2 . We propose that Fe3+ recycle occurs through H2 O2 , photogenerated from TiO2 , not from WO3 , while Cu2+ regeneration on a moderate photocatalyst is through the dissolved O2 in water. © 2013 Published by Elsevier B.V.

1. Introduction Semiconductor photocatalysis for environmental remediation and pollutant destruction has been studied for over 30 years [1–4]. Among all the photocatalysts studied so far, TiO2 is the best one for environmental use. This is mainly because the conduction band electron and valence band hole of TiO2 can reduce O2 and oxidize

∗ Corresponding author. Tel.: +86 571 87952410; fax: +86 571 87951895. E-mail address: [email protected] (Y. Xu). 0304-3894/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jhazmat.2013.08.002

H2 O/OH− , respectively, through one-electron transfer pathway. However, TiO2 is not active under visible light. Therefore, great effort has been also made for exploring a visible-light-driven catalyst. Among those catalysts, WO3 is the mostly interesting. Not only it can absorb the visible light at wavelengths shorter than 490 nm, but also it has a valence hole capable of H2 O oxidation into • OH [5]. However, one obvious disadvantage of WO3 is that its conduction band electron is not able to reduce O2 into O2 −• . As a result, organic degradation on WO3 in air and in aerated aqueous solution is usually very slow, either under UV or visible light. Deposition of Pt and Pd noble metals onto WO3 [6,7], and/or addition of H2 O2 to

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the suspension of WO3 [5] can result into significant enhancement in the rate of organic photodegradation, but the composite systems are costly for practical application. In order to improve the efficiency of charge separation, transitional metal ions such as Fe3+ and Cu2+ have been studied as electron scavengers of TiO2 and WO3 . However, controversial results have been reported in the literature. On one hand, addition of Fe3+ or Cu2+ ions into the aerated aqueous suspension of TiO2 [8–19] or WO3 [20,21] can result into significant enhancement in the rate of organic degradation under UV or visible light. On the other hand, Fe3+ and Cu2+ ions are detrimental to some photoreactions on TiO2 [13,14,20], while the combined system of Fe3+ and TiO2 becomes even less photoactive than Fe3+ alone [10,12]. Note that in those studies, P25 TiO2 , a mixture of anatase and rutile, is used as photocatalyst. Moreover, as electron scavengers of TiO2 and WO3 , both the higher and lower reactivities of Cu2+ than Fe3+ have been claimed [13,14,20,21]. We speculate that these discrepancies may result from several factors. First of all, Fe3+ and Cu2+ ions in aqueous solution can undergo photolysis to generate • OH. Second, these metal ions may form a photoactive complex with the model organic substrates used, such as 1,4-dioxane [17] resorcinol [18], and formic acid [21], consequently leading to organic degradation [22] not through a semiconductor photocatalysis. Third, the crystal structures and surface properties of TiO2 and WO3 may have influence on the specific adsorption and photoreduction of metal ions on the oxides in aqueous solution. Forth, the complicated hydrolysis of Fe3+ and Cu2+ ions in aqueous solution may result to species that have different interaction with the photocatalyst, consequently influencing the interfacial charge transfer [10,11]. In fact, when TiO2 and WO3 are modified with CuO or Fe2 O3 through an impregnation method, both the increased and decreased photocatalytic activities for organic degradation in water have been reported in the literature [23–28]. Therefore, a further study of metal ion effect is needed, which might provide useful and first-hand information for the development of the CuO and Fe2 O3 modified photocatalysts. With those considerations in mind, we have re-investigated the effects of Cu2+ and Fe3+ ions on the photocatalytic activities of TiO2 and WO3 under carefully controlled conditions. Three different samples of TiO2 in the crystal forms of anatase, rutile and their mixture were used as photocatalysts. Their activities were measured by using phenol degradation as a model reaction. The effect of metal ions was examined in terms of their concentration, solution pH, and hydrolysis time. Possible formation of H2 O2 and the reduced Cu2+ species were measured by a colorimetric method, and X-ray photoelectron spectroscopy, respectively. Finally, the observed different effect of metal ions on the photocatalytic activity of each oxide is discussed. 2. Experimental 2.1. Materials Most of the chemicals in analytical grade were obtained from Shanghai Chemicals Inc., China, including CuCl2 ·2H2 O, anhydrous FeCl3 , phenol and WO3 . N,N-diethyl-p-phenylenediamine and horseradish peroxide were purchased from Sigma–Aldrich. Three TiO2 samples of P25 (Degussa), anatase and rutile (Sigma–Aldrich) were used as received. All solutions were freshly prepared at room temperature with a Milli-Q ultrapure water. 2.2. Characterization X-ray diffraction (XRD) pattern was recorded on a D/max2550/PC diffractometer (Rigaku). Diffuse reflectance spectra (DRS)

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were recorded on a Shimadzu UV-2550 using BaSO4 as a reference. The specific Brunauer–Emmett–Teller (BET) surface area was measured by N2 adsorption at 77 K on a Micromeritics ASAP2020 apparatus. X-ray photoelectron spectroscopy (XPS) data was recorded with a Kratos AXIS UItra DLD spectrometer, calibrated with C 1s at 284.8 eV. 2.3. Photoreaction and analysis Reactor (inner diameter 2.9 cm and height 9.1 cm) was made of a Pyrex-glass, and was attached with a water jacket thermostated at 25 ◦ C during the photoreaction process. Light source (Shanghai Yaming) was a 300 W high-pressure mercury lamp equipped with a 320 nm cut-off filter for UV light, and a 500 W halogen lamp equipped with a 420 nm cut-off filter for visible light, respectively. Unless stated otherwise, experiments were carried out under fixed conditions: 0.43 mM phenol, 2.00 g/L catalyst, 0.90 mM metal ions, and pH 3.0. Typically, an aqueous suspension containing necessary components was first stirred in the dark for 1 h, and then irradiated externally with UV or visible light. At given intervals, 2.0 mL of the suspension was withdrawn by a micro-syringe, and filtered through a membrane (pore size = 0.22 ␮m). The filtrate was immediately analyzed by the standard method of high performance liquid chromatography (HPLC) on a Dionex P680 (Apollo C18 reverse column, and 50% CH3 OH aqueous solution as an eluent). Absorption spectra for the solutions were taken with an Agilent 8453 UV–vis spectrophotometer. Cupric ions were measured at 435 nm through its complex with sodium diethyldithiocarbamatre [29]. Ferric ions was analyzed at 512 nm through a ferrous complex with 1,10-phenathroline, before which Fe(III) was reduced by hydroxylamine hydrochloride [30]. Hydrogen peroxide was quantified at 551 nm through the peroxidase-catalyzed oxidation of N,N-diethyl-1,4-phenolenediammonium [31]. 3. Results and discussion 3.1. Reactions under UV light Phenol degradation was carried out in aerated aqueous solution under UV light at wavelengths longer than 320 nm. With each TiO2 , phenol concentration (C/C0 ) in aqueous phase exponentially decreased with irradiation time (t), while the reaction on WO3 was very slow or negligible (Supplementary data, Fig. S1). Control tests in the dark or under UV light in the absence of catalyst showed negligible phenol degradation. This observation indicates that the observed phenol degradation is only due to semiconductor photocatalysis, and that the rate of phenol degradation can be taken as a measure of the relative photocatalytic activity among different catalysts. Fig. 1 shows the plots of ln(C/C0 ) vs. t for phenol degradation. In the absence of metal ions (Fig. 1A), all the time profiles of phenol degradation on three TiO2 were well fitted to the pseudo first-order rate equation. This reaction kinetics is often observed with TiO2 photocatalysis [1], and ascribed to the fact that any consumption of O2 for organic degradation is immediately supplied from air. The relevant rate constants for phenol degradation (kobs ) are tabulated in Table 1, which follows the increasing order of P25 > rutile > anatase > WO3 . In the presence of Cu2+ ions (Fig. 1B), the time profiles of phenol degradation on anatase, rutile and WO3 also well fitted to the firstorder rate equation, while the reaction on P25 did not follow the first-order kinetics at all. It implies that Cu2+ concentration remains constant during the phenol degradation on anatase, rutile and WO3 . The rate constants of phenol degradation measured in the presence of Cu2+ ions were all larger than those measured in the absence of metal ions (Table 1). Since phenol degradation in the homogeneous

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Fig. 2. Formation of H2 O2 in the aerated aqueous suspensions of (a) P25, (b) WO3 , (c) anatase, and (d) rutile under UV light.

(e)

0

20

Irradiation time t /min

0.0

-0.6

(b)

100

120

Irradiation time t /min Fig. 1. Time profiles for phenol degradation under UV light in the aerated aqueous suspensions of (A) metal oxide, (B) metal oxide + Cu2+ , and (C) metal oxide + Fe3+ . Metal oxides were (a) P25, (b) WO3 , (c) anatase, and (d) rutile, while solutions were (e) CuCl2 , and (f) FeCl3 .

solution of Cu2+ under UV light was negligible (curve e, Fig. 1B), these observations indicate than Cu2+ is probably a better electron scavenger than O2 for both TiO2 and WO3 . The detrimental effect of Cu2+ observed here with P25 is similar to that reported in the literature [13], which will be discussed below in Section 3.5. In the presence of Fe3+ ions (Fig. 1C), the kinetics of phenol degradation was very complicated. First of all, in the homogeneous solution of Fe3+ (curve f, Fig. 1C), phenol degradation was significant, ascribed to Fe3+ photolysis that generates • OH and Fe2+ [22]. However, after 30 min, phenol degradation nearly terminated. This observation implies that all Fe3+ ions have transformed to Fe2+ ions, which are difficulty reoxidized by O2 in acidic aqueous solution, as confirmed by a separate experiment (data not shown here). Second, with each oxide, phenol degradation in the presence of Fe3+ was faster than that in the absence of metal ions. Interestingly, after 30 min, phenol degradation over TiO2 (P25, anatase, and rutile) became to follow the first-order kinetics. The resulting rate constants of phenol degradation are all larger than those measured in the absence of metal ions (Table 1). These observations indicate that Fe2+ ions can be re-oxidized to Fe3+ by some reactive species generated from the irradiated TiO2 , not from the irradiated WO3 . Since metal oxide and Fe3+ are both photoactive, their contribution to the overall degradation of phenol are not distinguishable at the moment, which will be discussed below in the section of metal ion adsorption. It is highly possible that Fe3+ ions are regenerated through H2 O2 . On one hand, H2 O2 can be produced by TiO2 photocatalysis [32–36]. On the other hand, the Fenton reaction between Fe2+ and H2 O2 to give Fe3+ and • OH in acidic medium is well known [37]. With this concern, possible formation of H2 O2 was separately investigated, and result is shown in Fig. 2. In the presence of phenol, the formation of H2 O2 on TiO2 (P25, anatase and rutile) was very obvious, while such reaction was negligible with WO3 . This result gives a strong support of the above hypothesis that Fe3+ is regenerated through H2 O2 produced from the irradiated TiO2 ,

Table 1 Physical parameters and rate constants of phenol degradation under UV light.a Samples P25 Anatase Rutile WO3 a b c

SBET (m2 /g) 50 148 6.6 2.3

QFe (␮mol/g) 279 351 33 28

kobs (O2 ) (10−3 /min) 5.62 1.06 2.33 0.15b

SBET , specific surface area; QFe , the amount of Fe3+ adsorbed; kobs , rate constant. If fitted. Fitted after the first 30 min.

kobs (Cu2+ ) (10−3 /min) b

3.49 2.15 5.66 1.86

kobs (Fe3+ ) (10−3 /min) 15.27c 7.73c 3.12c 3.83b,c

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(d)

1.0

(b)

0.9

(c)

1.00

0.8

0.95

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0.90

C/C0

C/C0

(e)

(f) t /min 0

0.6

40

80

120 160

(a)

0.5 0

100

200

300

400

Irradiation time t /min Fig. 3. Phenol degradation under visible light over (a) WO3 + Fe3+ , (b) WO3 + Cu2+ , (c) P25 + Fe3+ , and (d) Fe3+ . Insert presents the results obtained under a stronger visible light (150 W Xenon lamp, cut-off at 420 nm) for phenol degradation over (e) WO3 and (f) WO3 + Cu2+ .

not from the irradiated WO3 . It is conceivable that once Fe3+ ions are all reduced to Fe2+ ions, the adsorbed O2 on TiO2 would capture the conduction band electron of TiO2 to form H2 O2 , followed by a quick regeneration of Fe3+ ions through the Fenton reaction. 3.2. Reactions under visible light Phenol degradation was carried out in aerated aqueous solution under visible light at wavelengths longer than 420 nm. Under such condition, only WO3 and Fe3+ in the reaction mixture can absorb the visible light (Supplementary data, Fig. S2), possibly resulting to phenol degradation. Fig. 3 shows the result of phenol degradation obtained with WO3 and P25. First of all, phenol degradation on WO3 in the presence of Fe3+ was much faster than that in the presence of Cu2+ . Control experiments with WO3 alone or in the homogeneous solution of Fe3+ or Cu2+ showed negligible phenol degradation. This observation clearly indicates that WO3 is also active under visible light, but the reaction is efficient only in the presence of Fe3+ and Cu2+ as electron scavengers. In this case, Cu2+ and Fe3+ ions are also recyclable and not recyclable, respectively, as observed under UV light. Second, phenol degradation on P25 under visible light was observed only in the presence of Fe3+ . However, the amount of phenol degraded at 4 h was only 3.6%, which was much less than that (45.6%) observed with WO3 under similar conditions. Since P25 is not the light absorbing species, the observed degradation of phenol in the presence of Fe3+ is mostly due to some reactions initiated by the photoactive ferric species formed and adsorbed on TiO2 in aqueous suspension. 3.3. Adsorption of metal ions Experiment was carried out in the dark under similar conditions as above, and result is shown in Table 1. In aqueous solution, Fe3+ ions highly adsorbed onto each metal oxide, but Cu2+ adsorption on the metal oxide was not measurable, even under a modified condition (0.30 mM CuCl2 , 4.00 g/L oxide, and pH 3.0). It is highly possible that the adsorption of Fe3+ and Cu2+ on the metal oxide in water follow different mechanisms, at least because Fe3+ hydrolysis in water is more complicated than that of Cu2+ (see below). Moreover, the amount of Fe3+ adsorbed on the oxide in water was in proportion to the specific surface area of the oxide measured by N2 adsorption (Table 1). Then, question arises how to evaluate the effect of metal ions on the photocatalytic reaction.

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In aqueous solution, Cu2+ adsorption on each oxide is negligible. Then, we can conclude that Cu2+ is a better electron scavenger than O2 for anatase, rutile and WO3 (Table 1). However, in the presence of Fe3+ , phenol degradation can occur through Fe3+ photolysis, semiconductor photocatalysis, and/or both. Due to Fe3+ adsorption (Table 1), the amount of free Fe3+ ions in the equilibrated suspension would decrease in the order of anatase < P25 < rutile < WO3 . Such trend will be followed for the contribution of Fe3+ photolysis to the overall degradation of phenol. Since all Fe3+ ions have been reduced to Fe2+ ions in the first 30 min (Fig. 1), and the Fenton reaction between Fe2+ and H2 O2 is fast, we speculate that once Fe3+ ions are regenerated by H2 O2 , they would adsorb onto the catalyst, without significant photolysis occurring in solution. Then, the observed phenol degradation could be ascribed to semiconductor photocatalysis. Since the rate constants of phenol degradation obtained in the presence of Fe3+ are larger than those in the absence of metal ions (Table 1), we can conclude that Fe3+ is also better than O2 for scavenging the conduction band electrons of TiO2 . As it will be proposed below, Cu2+ ions are regenerated through the reoxidation of the reduced copper by O2 . This rate of Cu2+ regeneration would be determined by the photocatalytic activity of metal oxide for the production of the reduced copper, and by the concentration of the dissolved O2 in water as well. Since the dissolved O2 in the aerated aqueous suspensions of different metal oxides would have similar concentration, the metal oxide that has a higher photocatalytic activity would show a larger rate enhancement of phenol degradation on the addition of Cu2+ , as observed by experiment (Table 1). On the contrary, Fe3+ regeneration occurs through Fe2+ reoxidation by H2 O2 . This process would be determined by the rate of H2 O2 production, not by the rate of Fe2+ production, because H2 O2 is generated only after Fe3+ ions have been reduced to Fe2+ . In other words, the rate enhancement of phenol degradation on the addition of Fe3+ would be determined by the rate of H2 O2 production, not by the photocatalytic activity of metal oxide observed for phenol degradation in aerated aqueous suspension. In fact, the same trends in the activity among the catalysts (P25 > anatase and rutile) were observed for the production of H2 O2 (Fig. 2), and for the rate enhancement of phenol degradation on the addition of Fe3+ (Table 1). Moreover, with anatase, the rate constant of phenol degradation in the presence of Fe3+ was larger than that measured in the presence of Cu2+ (Table 1). The larger effect of Fe3+ than Cu2+ was also observed with WO3 under visible light, where Fe3+ photolysis in solution was not involved (Fig. 3). These observations might be taken as evidence that Fe3+ is a better electron scavenger than Cu2+ . However, with rutile, the rate constant of phenol degradation in the presence of Fe3+ was smaller than that in the presence of Cu2+ (Table 1). This discrepancy may arise from the different mechanisms for Fe3+ and Cu2+ regeneration. Rutile has a higher photocatalytic activity [kobs (O2 )], but a lower rate of H2 O2 production (Fig. 2), as compared to anatase. As a result, for phenol degradation in the presence of Cu2+ , rutile is more active than anatase [kobs (Cu2+ )], while for phenol degradation in the presence of Fe3+ , rutile is less active than anatase [kobs (Fe3+ )]. 3.4. Effect of experimental variables Many studies have reported that the initial pHs, and initial concentrations (Cm ) of metal ions in solution have great effect on the photocatalytic reaction of TiO2 [8–19]. However, this study has not been made with WO3 photocatalysis. Fig. 4 shows the results of phenol degradation over WO3 under UV light. With the increase of initial pH or Cm , the yield of phenol degradation first increased, and then decreased. The maximum rate of phenol degradation was observed at initial pH 3.0 for both Cu2+ and Fe3+ , while it was

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24

Phenol degraded (%)

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3.5. Possible mechanisms

30

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0 10

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3+

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Hydrolysis time

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Phenol C/C0

in Fig. 4 is still informative. For example, both phenol and WO3 have redox potentials changing with pH in a variation of 59 mV/pH [23,38]. Then, the observed pH-dependent rate of phenol degradation (Fig. 4A) indicates that there is a strong effect of metal ion hydrolysis. In fact, when the aqueous solution of metal ions were pre-equilibrated in the dark for different times (th ), the rate of phenol degradation on WO3 in the presence of Fe3+ notably decreased with the increase of th , while such effect of th for the reaction in the presence of Cu2+ was relatively very small (Fig. 4C). In order to understand the above phenomena, the absorption spectra for metal ion solutions were recorded, and results are shown in Fig. S3 of the Supplementary data. With the increases of initial pH, Cm , and th , the absorption spectrum of Fe3+ solution was red-shifted, while this shift was very small with Cu2+ solutions. These observations indicate that Fe3+ more easily hydrolyzes in aqueous solution than Cu2+ . Moreover, with Fe3+ solution, there were two absorption peaks at 240 and 295 nm, assigned to Fe(H2 O)6 3+ and Fe(H2 O)5 (OH)2+ , respectively [39]. Interestingly, in all the solutions of Fe3+ at pH 3.0, Fe(H2 O)5 (OH)2+ was the main species, which was more (photo)active than Fe(H2 O)6 3+ (Fig. 4A). The observed decrease in the rate of phenol degradation with Cm and th is due to the increased hydrolysis of metal ions in aqueous solution. These results obtained here with WO3 are similar to those reported with TiO2 [8–19].

0.5 h 24 h 72 h

0.9 (b) 0.8 0.7 (a)

0.6 0

20

40

60

80

100

120

Irradiation time t /min Fig. 4. Effects of (A) initial pH, (B) initial concentration, and (C) hydrolysis time of metal ion solutions on the WO3 -photocatalyzed degradation of phenol under UV light in the presence of (a) Fe3+ , and (b) Cu2+ . Curve (c) presents the reaction in the homogeneous solution of Fe3+ . The percent of phenol degraded in the presence of Fe3+ and Cu2+ was calculated at 5 and 60 min, respectively.

observed with Cu2+ and Fe3+ at Cm = 9.0 and 1.2 mM, respectively. Note that the initial pH shown in Fig. 4 is the initial pH of water solvent used for preparation of the working solution, because WO3 and metal ions easily hydrolyze in aqueous solution. However, when the initial pH of water solvent was higher than 3.0, the final pHs for the Cu2+ - and Fe3+ -containing suspensions were always 3.0 and 4.3, respectively. Despite that problem in pH control, the result

In aqueous solution at pH 0, the conduction band potentials for WO3 , rutile and anatase TiO2 are 0.45, 0.08, and −0.12 V vs. NHE, respectively [1–3,23], while the standard redox potentials for the O2 /HO2 • , Fe3+ /Fe2+ , Cu2+ /Cu+ and Cu2+ /Cu couples are −0.05, 0.77, 0.16 and 0.34 V vs. NHE, respectively. Then, in thermodynamics, the reduction of Cu2+ by the conduction band electron of WO3 is not allowed. However, in practice, Cu2+ ions are surely recyclable either with TiO2 or with WO3 for phenol degradation in aerated aqueous suspension (Figs. 2, S1, and S4). In order to examine the role of O2 , a supplementary experiment was carried out, and the result in shown in Fig. 5A. In the presence of WO3 and Cu2+ , the rate of phenol degradation increased in the order of O2 > air > N2 . Moreover, the first-order kinetics of phenol degradation was followed only under O2 or air, while phenol degradation under N2 eased at 8 h, indicative of the complete reduction of Cu2+ . These observations clearly show that O2 is needed for Cu2+ recycle on WO3 . In order to identify the reduced Cu species, the irradiated samples were filtered, dried in a vacuum, and then analyzed by XPS (Fig. 5B). With the sample previously irradiated under N2 , the binding energy of Cu 2p was located at 932.7 eV, assigned to Cu+ or Cu0 [40]. However, with the sample previously irradiated under air, the signal due to Cu2+ at 935.7 eV was also observed. XPS analysis confirms that Cu2+ can be reduced on WO3 to form Cu+ or Cu, followed by Cu2+ regeneration through O2 . Similar result has been reported by Sayama and coworkers for the photocatalytic decomposition of formic acid on WO3 under N2 in the presence of Cu2+ . Through a colorimetric analysis, they have identified Cu+ as the reduced Cu2+ species, without interpretation [21]. Since phenol degradation under UV light (Fig. 1B) is much faster than that under visible light (Fig. 3), we propose that Cu2+ reduction on WO3 might occur through the high energy electrons populated in the upper conduction band of WO3 . The second question is that why Cu2+ ions are recyclable with all catalysts except P25. By using a stopped flow technique [41], Bahnemann and coworkers have recently reported that Cu2+ reduction by the stored electrons of anatase is a two-electron transfer process, followed by a quick regeneration in air. Through the photocatalytic reduction of Cu2+ on P25 under N2 in the presence of different organic substrates [19], Koval and coworkers have observed that

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(b)

activity lower than that of anatase for organic degradation in aerated aqueous solution [43,44]. This might explain why rutile is less active than anatase for the production of H2 O2 (Fig. 2). Note that the different catalysts may have different interaction with H2 O2 [32–36]. Therefore, the rate of H2 O2 production (Fig. 5), measured in solution, only serves as a reference for understanding Fe3+ regeneration.

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4. Conclusions

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As electron scavengers of TiO2 and WO3 , both Fe3+ and Cu2+ are better than O2 , while Fe3+ is better than Cu2+ . Since several influencing factors have been considered, this conclusion would be more relevant than those reported in the literature. Moreover, Fe3+ ions are recyclable with TiO2 , not with WO3 , while Cu2+ recycle is only achieved with a moderately active photocatalyst. We propose that Fe3+ regeneration occurs through H2 O2 produced in situ from TiO2 photocatalysis, while Cu2+ regeneration is through the dissolved by O2 in water. These results are new, and will be useful to practical application for water treatment.

XPS intensity

Acknowledgements

932.7

This work was supported by the 973 program of China (Nos. 2009CB825300, and 2011CB936003). Appendix A. Supplementary data

(a) Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jhazmat.2013.08.002.

955

950

945

940

935

930

Binding Energy /eV Fig. 5. (A) Phenol degradation over WO3 under UV light in the presence of Cu2+ . The suspension was (a) open to air, (b) under N2 , and (c) under O2 . (B) XPS spectra of Cu 2p recorded with the samples collected after the reactions in Fig. 1B.

not all of the colored samples (the reduced Cu) can return back to white (TiO2 ), with 100% of Cu2+ regeneration (Table 3, in Ref. [19]). We speculate that the rate of Cu oxidation by O2 is size-dependent. In the present study, P25 has the highest photocatalytic activity among the catalysts (Table 1). Then, it is highly possible that the Cu0 particles formed on P25 is too large to be oxidized by O2 , and/or the rate of Cu oxidation by O2 is slower than the rate of phenol degradation. Then, the colored Cu species would shield the incident photons reaching P25, consequently decreasing the rate of phenol degradation. However, in the irradiated P25 sample (Fig. 1B), the metallic Cu particles were not found by XRD, probably due to too low content of Cu (maximum at 3% by weight). We consider that Cu2+ regeneration by O2 only operates with a moderately active photocatalyst. In fact, the kinetics for phenol degradation on rutile in the presence of Cu2+ was also not really first order in phenol (Fig. 2B). Furthermore, the one-electron reduction of O2 over WO3 and rutile is also not thermodynamically allowed. However, phenol degradation on rutile well followed the first-order kinetics, while the reaction on WO3 could proceed slowly in the first 60 min (Fig. 1A). It is highly possible that O2 reduction occurs through a multi-electron transfer pathway [6,42]. Interestingly, rutile is even more active than anatase [kobs (O2 ) in Table 1]. In a previous study, we speculate that this rutile is produced at a higher temperature than anatase [43], so that the former is better crystallized, and thus more active than the latter. Since rutile has a weaker affinity to the dissolved O2 in water than, it usually shows a photocatalytic

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