Rate Enhancement and Rate Inhibition of Phenol Degradation over

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19024

J. Phys. Chem. C 2007, 111, 19024-19032

Rate Enhancement and Rate Inhibition of Phenol Degradation over Irradiated Anatase and Rutile TiO2 on the Addition of NaF: New Insight into the Mechanism Yiming Xu,*,† Kangle Lv,†,‡ Zhigang Xiong,† Wenhua Leng,† Weiping Du,† Ding Liu,† and Xiaojin Xue† Department of Chemistry, Zhejiang UniVersity, Hangzhou, Zhejiang 310027, China, Department of Chemistry and Materials, South-Central UniVersity for Nationalities, Wuhan 430073, China ReceiVed: August 8, 2007; In Final Form: October 7, 2007

Several studies have shown that addition of NaF into the aqueous dispersion of TiO2 (Degussa P25) can result in significant enhancement in the photocatalytic degradation (PCD) of organic pollutants, ascribed to the enhanced production of free OH radicals in solution as a result of fluoride displacement of surface hydroxyl groups. In this work, we have observed different results of NaF addition for the PCD of phenol over synthetic TiO2 in aqueous suspension under UV light irradiation (λ g 320 nm). Upon the addition of NaF, the rate of phenol PCD was only increased with anatase, but it was decreased with rutile under similar conditions. In the presence of AgNO3, however, the fluoride-induced rate enhancement of phenol PCD could be observed with both anatase and rutile, ascribed to the increased rate of scavenging the conduction band electrons. As the catalyst sintering temperature was increased, the amount of fluoride adsorption on TiO2 was decreased, but the degree of PCD rate enhancement due to NaF addition as observed with anatase was first increased and then decreased, the trend of which was similar to that in the absence of NaF. The result reveals that the excess fluoride ions present in the suspension play some positive role to the phenol PCD, which is hardly interpreted by previous mechanism of surface fluorination. Moreover, as initial concentration of fluoride and initial pH of suspension were increased, the degree of rate enhancement was increased and decreased, respectively, which also could not be ascribed solely to the change in fluoride adsorption. Possible interference from catechol and hydroquinone intermediates and the fluoride-induced enhancement in the production of OH radicals in solution are analyzed. A new mechanistic model is proposed, involving enhanced desorption of surface bound OH radicals from irradiated TiO2, by fluoride ions present in the Helmholtz layers, through a fluorine hydrogen bond.

1. Introduction Photocatalytic reactions of titanium dioxide have been widely studied since the 1980s for potential application in air purification and water treatment.1-3 In such an irradiated system, a wide range of organic pollutants in aqueous and vapor phases can be degraded and/or completely mineralized into CO2 and the corresponding inorganic ions. However, the detailed events occurring on the UV-excited TiO2 have not been completely elucidated,4 which limits further development of highly efficient photocatalyst. It is generally recognized that the band gap excitation of TiO2 results in the generation of conduction band electrons (ecb-) and valence band holes (hvb+), respectively. These charge carriers may recombine into heat without a net chemical reaction or migrate into the surface where they are trapped and eventually react with a suitable electron donor and acceptor. For instance, ecb- is captured by surface adsorbed O2 to form superoxide radicals (O2-•),4,5 whereas hvb+ is proposed to react with surface OH- groups or adsorbed H2O to produce surface-bound hydroxyl radicals (•OH) through an electrontransfer pathway (eq 1)1-3 or a Lewis-acid base reaction (eq 2)6,7 (the notation tTi-X represents the surface species). The •OH radicals found in the air phase have been recently proposed * Corresponding author. Mailing address: Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, China. E-mail: xuym@css. zju.edu.cn. Phone: +86-571-87952410. Fax: +86-571-87951895. † Zhejiang University. ‡ South-Central University for Nationalities.

as the photolysis product of the diffused H2O2 in gas phase8 or as the reduction product of the adsorbed H2O2 by ecb- on TiO2.7b Moreover, singlet oxygen (1O2) has been also found in situ and debated as the reoxidation product of the adsorbed O2-• by hvb+9 or as a result of energy transfer from UV-excited TiO2 to the ground triplet O2.10 These active species are agreed upon to be responsible for the observed organic degradation over irradiated TiO2. However, it still remains unclear which conditions favor generation of these reactive species. It is often observed that efficiency of organic degradation is a function of physical parameters of TiO2, such as the crystalline phase, crystallinity, particle size, surface area, and so on. However, their correlation with the production of reactive species is hardly evaluated, due to complex interplay among the variables.11-15 tTi-OH + hvb+ f tTi‚‚‚•OH

(1a)

tTi‚‚‚OH2 + hvb+ f tTi‚‚‚•OH + H+

(1b)

tTi-O-Tit + hvb+ + H2O f tTi-O•HO-Tit + H+

(2)

tTi-OH + H+ + F- f tTi-F + H2O

(3)

tTi-F + H2O + hvb+ f tTi-F + •OHfree + H+

(4)

Recently, several studies have shown that addition of NaF into the aqueous dispersions of TiO2 in acidic medium can result in

10.1021/jp076364w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/04/2007

Rate of Phenol Degradation significant enhancement in the rate of organic photocatalytic degradation (denoted PCD below), including phenol,16 benzoic acid,17,18 cyanuric acid,19 benzene, 4-chlorophenol, nitrosodimethylamine, tetramethylammonium,20 and several anionic azodyes.18,21-23 Minero and co-worker found it first16 and ascribed this to the enhanced generation of free •OH radicals in solution, since fluoride displacement of surface OH- groups forces the valence holes to oxidize solvent water (eqs 3 and 4). This enhanced production of •OH radicals due to NaF addition was confirmed latter by Mrowetz and Selli,18a using a DMPO-spin trap ESR technique. We have recently shown that the surface fluorination does result in the enhanced production of free •OH radicals in solution, whereas similar surface modification with polyoxometalate only gives the enhanced formation of bound •OH radicals on the catalyst surface.23 Interestingly, Choi and co-worker have demonstrated that for remote PCD of stearic acid and acetaldehyde in gaseous phase, the surface fluorination of TiO2 can also result in the enhanced PCD.24 They ascribed this to the enhanced generation of airborne •OH radicals but excluded the possibilities of H2O2 photolysis8 and/or tTi-F+• production.25 However, Macyk and co-worker have recently claimed that the surface modification of TiO2 with fluoride or silyl groups leads to the enhanced production of 1O2, instead of •OH radicals.10 The debate then arises whether the Minero mechanism of free •OH radicals is still operative. In fact, in the Minero mechanism, the issue still remains unclear why the valence holes, after surface fluorination, turn to oxidize solvent water into free •OH radicals (eq 4) instead of the surfaceadsorbed water into surface bound •OH (eq 1b).26 Furthermore, Degussa P25 TiO2 (denoted P25 below), a mixture of anatase and rutile, has been used in all related studies.16-24 Then the question may arise whether the outstanding effect of NaF is universal to any type of TiO2 such as anatase and rutile. The answer to these questions is not only essential to environmental photocatalysis but also important to water splitting over irradiated TiO2. In this work, we have examined effect of calcination temperature on the PCD of phenol with synthetic TiO2 in aqueous suspension in the absence and presence of NaF. It is known that as the sintering temperature is increased the number of surface OH- groups on TiO2 is decreased. According to the Minero’s mechanism, this would result in a reduced amount of fluoride adsorption and, thus, a decreased degree of the fluorideinduced rate enhancement of phenol PCD on TiO2. It is also known that as the sintering temperature is increased the photocatalytic activity of naked TiO2 is first increased and then decreased.15 Then, the optimal sintering temperature, at which the photocatalyst displays a maximum activity, would be lower in the presence of NaF than that in the absence of NaF. Moreover, the phase transition from anatase to rutile can be controlled by sintering temperature and/or duration time, so that a series of TiO2 samples with different phase content can be prepared for the study of crystalline structure effect. However, experimental evidence obtained in this work is not in agreement with the expectation. The fluoride-induced rate enhancement of phenol PCD in aerated aqueous suspension was only observed with anatase but not with rutile. Accordingly, we propose a new mechanistic model to account for this new evidence and all available data published in the literature. 2. Experimental Section All of the chemicals were of analytic grade and used as received, mostly from Shanghai Chemicals. Inc. Doubly distilled water was used throughout this study. The solution pH was

J. Phys. Chem. C, Vol. 111, No. 51, 2007 19025

Figure 1. Effect of calcination temperature on (a) anatase content and (b) crystallite size of AT, and on (c) crystallite size of RT. The data were obtained from XRD analysis (see the text).

adjusted by HClO4 and NaOH. The pH and fluoride measurement were made on a pHS-3C pH meter. Organic analysis was performed on a Dionex P680 HPLC (Apollo C18 reverse column, 60% methanol-water eluent set at 1.0 mL/min), and/ or an Agilent 8453 UV-vis spectrometer. Powder X-ray diffraction (XRD) patterns of catalysts were recorded on a D/max-2550/PC diffractometer (Rigaku), operated at 40 kV and 300 mA. Commercial TiO2 sample, denoted as P25, was Degussa P25. Two sets of synthetic TiO2 samples (AT and RT) were prepared according to literature procedures.27 Briefly, the AT sample was synthesized by the hydrolysis of TiCl4 (125 mL) in distilled water (720 mL) containing 0.1 M HCl and 3.3 M (NH4)2SO4 in an iced bath, followed by boiling for 2 h and then neutralization with ammonia at room temperature. The RT sample was prepared by the hydrolysis of TiCl4 (40 mL) in 1.2 M HCl (1000 mL) in an iced bath, followed by heating at 60 °C for 2 h, and then storage overnight at room temperature. After these white precipitates were filtered off and washed thoroughly with water until no chloride in the filtrate was detected by AgNO3, they were heated in air for 1 h at different temperatures (x ) 100-900 °C). The resulting AT or RT samples were then denoted as ATx or RTx, where x presents the sintering temperature (for example, AT900 means that the as-synthesized AT has been sintered at 900 °C for 1 h in air). The crystallite structures were verified by XRD. Figure 1 summaries the data obtained for both sets of AT and RT samples as a function of the temperature, where the phase content was estimated by the integrated intensities of anatase (101) and rutile (110),28 and the crystallite size was calculated using the Scherrer equation. Note that the diffraction intensity (not shown) also increased with the sintering temperature for both sets of AT and RT samples. The adsorption isotherm of fluoride on TiO2 in aqueous suspension at pH 3.0 was measured in the dark. The mixture containing 50.0 mg of TiO2 and 50.0 mL of NaF solution (C0 ) 0-2 mM) was first sonicated for 5 min and then was mechanically shaken overnight. After the particles were removed through a 0.45 µM membrane, the filtrate was analyzed on a PCI-1 selective electrode (Shanghai Kangning) for fluoride concentration Ce. The decreased concentration (C0 - Ce) was then used to calculate the amount of fluoride adsorption qe, in unit of moles per gram of TiO2. The adsorption of catechol on TiO2 in water at pH 3.0 in the absence and presence of NaF was measured similarly as above.

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Xu et al.

Figure 3. Maximal amount of fluoride adsorption (left axis) and adsorption constant (right axis) on (a) AT and (b) RT, obtained from Figure 2 by application of the Langmuir adsorption equation.

Figure 2. Adsorption isotherms of fluoride on (A) AT and (B) RT in aqueous suspensions at pH 3.0. The catalyst was calcined at (a) 100, (b) 200, (c) 400, and (d) 700 °C, respectively.

Photocatalytic reactions were carried out with a high-pressure mercury lamp (375 W, Shanghai Yamin) as light source, emitted mainly at 365 nm. The reactor (80 mL) was made of a Pyrex glass, and positioned at a fixed distance of ca. 10 cm from the lamp. During the photoreaction, the reactor was thermostated at 25 °C through a recycle system and stirred mechanically at a constant rate. Except stated otherwise, the experiments were all set at an initial concentration of 1.0 g/L TiO2, 0.43 mM phenol, and 0.10 mM NaF, and an initial pH of 3.0, respectively. Before light irradiation, the aqueous suspensions containing all necessary components were mechanically shaken in the dark overnight. At given intervals of irradiation, small aliquots were withdrawn by a syringe, and filtered through a membrane (pore size 0.45 µm). The organics remaining in the filtrate were then analyzed by HPLC. The change of phenol concentration with irradiation time was well fitted to the first-order rate equation. Since phenol does not readily adsorb on TiO2, the resulting apparent rate constant kapp obtained under similar conditions could be used for comparison of activity among different photocatalysts.29 3. Results and Discussion Fluoride Adsorption. Figure 2 shows the adsorption isotherm of fluoride on different samples in aqueous suspension at pH 3.0. On a given catalyst, the amount of fluoride adsorption, qe, increased with the fluoride concentration in solution at equilibrium, Ce. Among the different catalysts, the amount of fluoride adsorption decreased with the temperature at which the catalyst was previously sintered. This trend in fluoride adsorption is consistent with that in the number of surface OH- groups

on TiO2.30 By XPS analysis,24a Choi and co-worker have confirmed that the adsorbed fluoride is only located on the surface but not in the lattice of TiO2.28 All of the isotherms could be not fitted with the Freundlich adsorption equation, but they were well fitted to the Langmuir adsorption equation, qe/qmax ) KCe/(1 + KCe), where qmax and K are the maximal amount of adsorption and adsorption constant, respectively (reciprocal plots not shown). The resulting parameters are summarized in Figure 3. As the sintering temperature was increased, both the maximal amount of adsorption and the adsorption constant were decreased. This means that the strength of fluoride binding to TiO2 sintered at a higher temperature is weaker. Although rehydration of the dehydrated TiO2 in aqueous medium is expected,31 all of the data presented in this work are those only measured after the aqueous suspension had been equilibrated overnight. As compared to RT, the AT samples have a higher adsorption capacity toward fluoride but a weaker strength of binding to fluoride (at T e 400 °C). This difference between AT and RT in fluoride adsorption and binding strength will contribute to the differing effect of surface fluorination on the PCD of phenol, as will be shown below. Phenol PCD. Photocatalytic degradation of phenol in the aerated aqueous suspension of TiO2 at initial pH 3.0 was carried out with UV light cutoff at 320 nm. No direct photolysis of phenol was found under the present conditions. Figure 4 shows the results of phenol PCD in the absence and presence of NaF. With both AT and RT samples, the rate constant of phenol disappearance was a function of sintering temperature, similar to those reported for the PCD of many organic compounds.3,11,13,15 However, the addition of NaF into the suspension only increased the rate of phenol PCD on AT (Figure 4A, except AT900), but it decreased the rate with RT (Figure 4B). The former observation is similar to those reported with P25,16-23 but the latter has been not found in the literature. XRD analysis showed that all RT samples were in the form of pure rutile. It seems that the positive effect of NaF on the phenol PCD is only operative with anatase. This was further supported by the following evidence. First, among all AT catalysts, only AT900 was in the form of pure rutile, and thus this catalyst gave a decreased rate constant from 0.0084 to 0.0056 h-1 on the addition of NaF. Second, the phase content of anatase in the AT samples calcined at 700-800 °C decreased with the sintering temperature (Figure 1). This trend in anatase content as a function of temperature was in agreement with that in the degree of PCD rate enhancement on the addition of NaF

Rate of Phenol Degradation

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Figure 5. Amount of catechol adsorption in water at pH 3.0 on (a) AT and (b) RT as a function of TiO2 sintering temperature, measured in the absence of NaF (solid bars) and in the presence (open bars) of NaF. Conditions: catechol 200 µM, TiO2 1.0 g/L, and NaF 1.0 mM.

Figure 4. Apparent rate constants of phenol PCD on (A) AT and (B) RT, as a function of TiO2 calcination temperature, measured (a) in the absence of NaF and (b) in the presence of NaF.

(Figure 4A). Third, the as-received P25 is a mixture of anatase and rutile. The rate constant of phenol PCD on P25 was 0.345 and 2.35 h-1 in the absence and presence of NaF, respectively. However, after P25 was completely transformed into rutile at 800 °C, the relevant rate constant was decreased from 0.0083 to 0.0024 h-1 on the addition of NaF. These results clearly show that NaF present in the suspension has a detrimental effect on the phenol PCD with rutile crystallites. With AT samples, there appeared a maximum rate constant of phenol PCD (Figure 4A). However, the optimal temperature observed in the presence of NaF was nearly the same as that in the absence of NaF. This is not expected from the Minero’s mechanism, as reasoned in the Introduction section. Further evidence not in agreement with the previous mechanism was the degree of rate enhancement as a function of temperature, i.e., the curve b divided by the curve a, in Figure 4A. Before 700 °C, the degree of rate enhancement was reverse to the trend in fluoride surface adsorption (curve a, Figure 3). At 700800 °C, the amount of fluoride adsorption was very low, but the degree of rate enhancement was still very high. With RT samples, the degree of rate inhibition as a function of temperature (Figure 4B) was consistent with the trend in fluoride adsorption (curve b, Figure 3), but it was also consistent with the trend in the photoactivity of naked RT (curve a, Figure 4B). This means that the degree of rate enhancement with AT and the degree of rate inhibition with RT, as a result of NaF addition, are determined by the photoactivity of naked TiO2, not by the surface amount of the adsorbed fluoride. This is reasonable since the photocatalytic activity of TiO2 is the cause for the phenol PCD, and fluoride is only the effect. However, it is unlikely that a slightly adsorbed fluoride on TiO2 can result in a

significantly enhanced rate of phenol PCD. The excess fluoride ions present in the aqueous phase may also play some positive role during the process of phenol PCD. This result is hardly interpreted by the Minero’s mechanism (eqs 3 and 4), which will be further discussed below. Reaction Intermediates. Catechol (CA) and hydroquinone (HQ) were the major intermediates of phenol PCD, as detected by HPLC in the filtrate of the irradiated suspension. It has been argued that the adsorbed CA on TiO2 may act as recombination centers by consuming actives species in net null cycles,16a,32 i.e., CA + hvb+ f CA-•, and CA-• + ecb- f CA. If it is the case, any action that decreases CA adsorption would result in an increased rate of phenol PCD. In fact, the fluoride-induced decrease of CA adsorption on P25 in water at pH 3.5 has been reported by Minero and co-worker.16a However, such NaF effect on CA adsorption and consequently on phenol PCD with P25 has not been illustrated in detail. We confirmed that the CA adsorption on AT or on RT in water at pH 3.0 was decreased on the addition of NaF (Figure 5). However, as the sintering temperature was increased, the amount of CA adsorption was also decreased. Since the fluoride adsorption decreased as well with the temperature (Figure 3), the interpretation of NaF effect on the phenol PCD became somewhat complicated. Thus, it is necessary to examine the fates of the intermediates produced during the process of phenol PCD. It was noted that HQ adsorbed hardly on TiO2 under similar conditions (data not shown). Figure 6 shows the intermediate distribution as a function of irradiation time, measured with two representative catalysts, AT400 and AT700. First of all, the production of CA or HQ was faster in the presence of NaF than that in the absence of NaF, and also it was faster with AT700 than with AT400. This trend in intermediate production was consistent with that in phenol loss monitored at the same time (Figure 4A). Second, as the reaction proceeded, the intermediate concentration increased first and then decreased, indicating that the intermediates also underwent degradation during the PCD process. Comparatively, CA degradation was easier than HQ, whereas both CA and HQ degraded faster in the presence of NaF than in the absence of NaF. This result obtained from in situ assessment was similar to those obtained with CA or HQ alone for their PCD on P25, in the absence and presence of NaF.16a Third, the concentration ratio of CA to HQ, observed at 1 h with AT400, was much higher in the presence of NaF than that in the absence of NaF, whereas such ratio with AT700 was

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Figure 6. Distribution of catechol (circle), hydroquinone (square) and resorcinol (triangle) as a function irradiation time, detected in the filtrate of phenol PCD (Figure 4) with (A) AT400 and (B) AT700, in the absence of NaF (solid bar) and in the presence of NaF (open bar).

similar in the absence and presence of NaF. This means that both fluoride and CA adsorb highly on AT400 but very weakly on AT700 during the PCD process. Figure 7A shows the concentrations of CA and HQ produced at 1 h during the phenol PCD with AT (Figure 4A). The enhanced production of CA and HQ on the addition of NaF was observed with all of the AT catalysts, simply due to the enhanced oxidation of phenol. However, such enhancement in the intermediate formation was much lower (1-3.5 times) than that of phenol loss (1-14.9 times). This is ascribed to the fact that both CA and HQ undergo in situ degradation, with a rate faster in the presence of NaF than in the absence of NaF. Since CA adsorbs highly on TiO2, its real concentration should be higher than that only detected in solution phase, especially at low sintering temperature. Since HQ adsorbs hardly on TiO2, only the profiles presented by HQ could be taken as the real temperature effect on the intermediate formation which (Figure 7A) are essentially the same as those presented with phenol oxidation (Figure 4A). A similar explanation could be made with RT samples. The addition of NaF into the suspension inhibited the rate of phenol PCD (Figure 4B), thus decreasing the yield of intermediate production (Figure 7B). The enhanced generation of CA with RT100 on the addition of NaF is due to CA adsorption that is high on naked RT100 (Figure 5). Also, the temperature dependence of HQ production was similar to that of phenol disappearance. The adsorbed CA as a recombination center might be true for the AT samples calcined at 100-700 °C, since the CA adsorption on these catalysts is high, and it is decreased on the addition of NaF. However, the contribution from such decreased

Xu et al.

Figure 7. Formation of catechol (circle) and hydroquinone (square) in the filtrate at 1 h as a function of calcination temperature, during the PCD of phenol (Figure 4) on (A) AT and (B) RT, in the absence of NaF (solid bar) and in the presence of NaF (open bar).

adsorption of CA to the overall rate enhancement would be very small. First, CA alone can undergo efficient PCD in situ, as shown above. Second, as the sintering temperature is increased, the CA adsorption on naked AT is decreased (Figure 5a), whereas the rate of PCD after reaching a maximum is decreased (curve a, Figure 4A). In other words, the temperature dependence of phenol PCD is mainly due to the activity of photocatalyst itself,1,3 not due to the change in CA adsorption, even in the region of 100-700 °C. This is also illustrated with naked RT. As the temperature is increased, the CA adsorption is decreased (Figure 5b), but the rate of PCD is also decreased (Figures 4B). Third, upon addition of NaF, the CA adsorption is decreased (Figure 5). As the temperature is increased, the fluoride adsorption is also decreased (Figure 3). This would result in a decreased effect of NaF on CA adsorption and thus a decreased contribution from the adsorbed CA to the overall rate enhancement. However, a higher degree of rate enhancement at a higher temperature was observed in the PCD of phenol (Figure 4A) and in the production of HQ (Figure 7A). At T ) 700-800 °C, both fluoride and CA adsorption on AT are very low, but the rate enhancement upon the addition of NaF is still significant, as shown typically with AT700 (Figure 6B). Otherwise, with RT samples, a large decrease in CA adsorption due to NaF addition (Figure 5) would result in a large increase in the rate of PCD, if the adsorbed CA does act as a recombination center. However, the observed rates of phenol PCD and HQ production are both decreased on the addition of NaF (Figures 4B and 7B). Moreover, the fluoride-induced rate enhancement of acetaldehyde PCD on P25 in the gas phase has been reported,24b during which the nonaromatic intermediate

Rate of Phenol Degradation

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Figure 8. Effect of TBA (1.0% or 10% v/v) on the PCD of phenol on AT650 in the absence of NaF (solid bar) and in the presence of NaF (open bar).

Figure 9. PCD of phenol in aerated aqueous solution containing different additives of (a) AgNO3, (b) RT500, (c) RT500 + NaF, (d) RT500 + AgNO3, and (e) RT500 + AgNO3 + NaF. Conditions: RT500, 1.0 g/L; NaF, 1.0 mM; AgNO3, 0.43 mM; pH, 3.0.

as a recombination center is not expected. Therefore, the adsorbed CA that acts as a possible recombination center16,32 is not important in the present system. The observed rate enhancement of phenol PCD upon NaF addition is not due to the decrease in CA adsorption. Effect of tert-Butyl Alcohol (TBA). TBA is an efficient scavenger of •OH. Figure 8 shows the results of TBA effect on the PCD of phenol on AT650. In the absence of NaF, the rate constant of phenol PCD on the addition of TBA at 1 and 10% was decreased from 0.0815 to 0.0552 and 0.0216 h-1, respectively. In the presence of NaF, such a detrimental effect of TBA was even more efficient than that in the absence of NaF. The relevant rate constants obtained at 0, 1, and 10% TBA were 1.17, 0.102, and 0.0510 h-1, respectively. This result with synthetic TiO2 was similar to those obtained with P25 for the PCD of phenol,16,17 and other organic substrates.18,20-23 By elegant kinetic analysis, Minero and co-workers have shown that the PCD of phenol on naked TiO2 proceeds for ca. 90% via surface-bound •OH radicals, whereas the reaction in the presence of NaF occurs almost entirely via homogeneous •OH radicals.16 Thus, a similar conclusion can be made here that the addition of fluoride into the AT dispersion leads to enhancement in the production of free •OH radicals and thus in the rate of phenol degradation. Hoffmann and co-worker have reported that the rate of 4-chlorocatechol PCD on P25 is linearly proportional to the surface amount of the substrate.33 Since the amount of CA adsorption on TiO2 is decreased on the addition of NaF, the observed rate enhancement of CA PCD suggests that additional active species is produced in solution such as free •OH, which is more reactive than hvb+ or surface bound •OH for CA degradation. A similar conclusion can be made with HQ since the PCD is also enhanced on the addition of NaF, as shown above. Effect of AgNO3 Addition. The detrimental effect of NaF on the phenol PCD in an aerated aqueous suspension has been observed with all rutile catalysts. In order to understand this negative effect of NaF, the photoreactions were carried out in the presence of AgNO3. It is known that, as a scavenger of ecbon TiO2, silver ions are more efficient than molecular oxygen [E°(Ag+/Ag) ) 0.799 V, E°(O2/O2-•) ) -0.33 V vs NHE]. Figure 9 shows the effect of AgNO3 on the rate of phenol PCD with RT500, in the absence and presence of NaF under similar conditions. First, addition of AgNO3 into the aerated suspension did result in an increased rate of phenol PCD. However, the increased rate was only observed in the early stage, after which

TABLE 1: Initial Rate of Phenol PCD over TiO2 under Different Conditionsa catalysts

air

AgNO3

NaF

AgNO3 + NaF

RT30 RT400 RT500 RT600 AT500

3.09 0.87 0.94 0.86 4.22

2.76 2.30 7.78 9.14 7.29

0.89 0.27 0.65 0.20 16.3

3.11 10.34 11.24 14.28 18.09

a Initial rate (10-5 µM/h) was determined at 1 h of irradiation time. Conditions: TiO2 1.0 g/L, phenol 0.43 mM, AgNO3 0.43 mM, NaF 1.0 mM, and pH 3.0.

the rate became nearly the same as that in the absence of AgNO3. This is because all of the Ag(I) species present have been reduced to silver particles after about 2 h.34 The control experiment without TiO2 showed that the photoreaction in solution was negligible (curve a, Figure 9). Second, the rate of phenol PCD in the presence of both AgNO3 and NaF was faster than in the presence of AgNO3 alone. Again, such a rate enhancement appeared only at the early stage of about 2 h. A similar effect of AgNO3 on the PCD was also observed with other catalysts, and the relevant data are listed in Table 1. Third, in an aerated aqueous dispersion, RT was much less active than AT, but its activity became similar to AT in the presence of AgNO3, and/or in the presence of both AgNO3 and NaF. Moreover, as the sintering temperature was increased, the photoactivity of RT in the presence of AgNO3 was also increased, regardless of the presence or absence of NaF. This trend in photoactivity with rutile as a function of temperature was similar to that observed with anatase (Figure 4A). Park and Choi have reported that, upon surface fluorination, the over-potential of ecb- transfer on TiO2 (P25) is increased by about 31 mV, ascribed to the adsorbed fluoride that tightly holds the trapped electrons.21 Mrowetz and Selli have observed a decreased production of H2O2 over irradiated TiO2 upon surface fluorination and also ascribed this to the decreased rate of interfacial ecb- transfer to O2.18b Boonstra and Mutsaers have reported that the surface photoadsorption of O2 on anatase and rutile decreases with the number of surface OH- groups.35 These factors of O2 adsorption and ecb- overpotential would be very important to rutile, since RT has a stronger binding strength to fluoride, as compared to AT (Figure 3). However, these factors would be less critical to anatase, since anatase has the combined effects of lower binding strength to fluoride, higher adsorption capacity toward O2, and lower rate in charge carrier recombination, as compared to rutile.3,12 Since Ag(I) as an electron acceptor has a higher driving force than O2, the rate of

19030 J. Phys. Chem. C, Vol. 111, No. 51, 2007 recombination would be reduced, and the rate of water oxidation by hvb+ (eq 1) would be increased. As a result, the rate enhancement of phenol oxidation on the addition of AgNO3 was observed with both anatase and rutile, either in the absence or presence of NaF. Experimental evidence clearly shows that, as long as the ecb- on TiO2 is efficiently removed, the rate enhancement of phenol PCD on the addition of NaF can be achieved with both anatase and rutile crystallites. Mechanism. As shown above, the rate enhancement of phenol PCD due to NaF addition does not always require the fluoride to be preadsorbed on the surface of TiO2. This principal result is hardly interpreted by the Minero mechanism that the NaF-enhanced generation of free OH radical in solution is the result of surface fluoride adsorption (eqs 3 and 4).16 In fact, similar evidence can be found in the early Minero work (Figure 3A, in ref 16a). In the aerated aqueous suspension of P25 (0.1 g/L) at pH 3.6, the rate enhancement of phenol PCD continuously increases with the concentration of NaF up to 10 mM, which is a much greater excess than the 0.03 mM required for full surface coverage of P25 by fluoride (qmax ) 0.27 mmol/g at pH 3.023). The excess fluoride ions dissolved in the solution phase may also make a significant contribution to the observed enhancement in the rate of phenol PCD at high NaF loading. Park and Choi have proposed that the fluoride adsorption on TiO2 is enhanced under UV illumination,21 on the basis of two pieces of experimental evidence. The first was the enhanced rate of phenol PCD over P25 (0.5 g/L) in an alkaline solution at pH 9 on the addition of NaF (10 mM), even though under such conditions the dark adsorption of fluoride on TiO2 was totally absent.21 The second was the frequency of the surface O-H stretching vibration that is higher in tTiIIIO-H (deeply trapped ecb-) than in tTiIVO-H by 69 cm-1.36 Since tTiIIIOH is less deprotonated than tTiIV-OH, they proposed that the irradiated tTiIV-OH would be easier replaced by fluoride to form tTiIII-F, as compared to the dark tTiIV-OH to produce tTiIV-F. However, the adsorbed fluoride on TiO2 has been found to be not stable even at neutral pH. After the fluoride-modified TiO2 was washed with water, the positive effect of fluoride on the PCD of phenol21 and cyanide37 completely disappeared. Moreover, the surface tTiIII-F, if formed, would be less stable than tTiIV-F. Therefore, the enhanced PCD of phenol due to the enhanced fluoride adsorption on irradiated TiO2 is in our opinion almost unlikely. Macyk and co-workers have recently proposed that the surface modification of P25 TiO2 with NaF or silyl groups results in enhancement in the production of singlet oxygen, not free •OH radicals, for the PCD of cyanuric acid.10 Although phenol can react with 1O2 to form benzoquinone in acidic aqueous solution, the reaction is very slow (k ) 2.6 × 106 M-1 s-1),38 as compared to the phenol reaction with •OH (k ) 1.4 × 1010 M-1 s-1).16b In the present study, no benzoquinone was found by HPLC during the phenol PCD with all catalysts at pH 3-9, in the absence or presence of NaF. Moreover, one azo-dye X3B hardly reacts with 1O2, but its PCD on P25 in water at pH 3.0 was significantly enhanced on the addition of NaF.23 The enhanced formation of 1O2 due to surface fluorination is possible, but it is not the main factor responsible for the observed rate enhancement of phenol and X3B PCD upon the addition of NaF. Moreover, in all related studies with P25,16b,18a,20b,21-23 the PCD quenching with alcohols is more efficient in the presence of NaF than in the absence of NaF, quite different from those made only with cyanuric acid.10,19,25 Kubo and Tatsuma have recently shown that the airborne, •OH radicals are produced by remote photolysis of H O (O -• 2 2 2

Xu et al. SCHEME 1: Possible Mechanism for the Fluoride-Induced Enhancement in the Production of Free •OH Radicals in Bulk Solution from Irradiated TiO 2

+ 2H+ + ecb- f H2O2).8 For this concern, Park and Choi have examined this possibility and conclude that the NaF-induced enhancement in the production of •OH radicals is not the result of H2O2 photolysis at λ g 320 nm.24a Mrowetz and Selli have shown that both the production and decomposition of H2O2 on UV-excited TiO2 are slower in the presence of NaF than those in the absence of NaF.18b However, very recently, Nosaka and co-workers have proposed that H2O2 is produced from the Lewis acid-base reaction between water and hvb+ (eq 2, followed by the equation tTi-O•HO-Tit + H2O + hvb+ f tTi-OTit + H2O2 + H+), and this kind of H2O2 is then reduced by ecb- to produce the airborne •OH.7 If this is true, the rate of •OH production on the addition of NaF would be decreased, since the reduction of H2O2 by ecb- into •OH radicals on TiO239 is slower in the presence of NaF than that in the absence of NaF.18b,40 However, the enhanced production of •OH radicals on the addition of NaF is observed.16,18b In this regard, the production of H2O2 from a Lewis acid-base reaction (eq 2) is not reasonable. Hoffmann and co-worker have shown that the production of H2O2 on irradiated ZnO is from O2 reduction by ecb-.41 Nakato and co-workers have also shown that H2O2 is produced from O2 reduction by ecb- on rutile TiO2.5b Thus, we can conclude that neither H2O2 photolysis nor H2O2 reduction by ecb- is responsible for the enhanced production of •OH on the addition of NaF. The production of •OH radicals mostly likely occurs via electron transfer from surface OH-/H2O to trapped hvb+ (eq 1), since the nucleophilic attack model for •OH generation (eq 2) still remains unclear.6,7 It is generally accepted that the •OH radicals produced by eq 1 is strongly surface-bound.1,7a,16,26 Then, a mechanism for the observed enhancement in the production of free •OH radicals in solution upon addition of fluoride into the suspension is needed. We propose that the fluoride ions present in the Helmholtz layer are able to promote the desorption of surface-bound •OH radicals into solution from irradiated TiO2, through a fluorine hydrogen bond (Scheme 1). As fluoride concentration is increased, the number of fluoride in the Helmholtz layer is increased, and the rate of desorption of surface-bound •OH is promoted, thus accelerating the PCD of phenol in solution. Since the production of surface-bound •OH radicals is determined by the intrinsic photoactivity of TiO2, the rate enhancement approaches a limit at 10 mM NaF.16a As the solution pH is increased, the amount of fluoride in the Helmholtz layer is decreased, and thus the degree of rate enhancement is decreased, as observed by Park and Choi for phenol PCD with P25 (0.5 g/L, plus 10 mM NaCl) and NaF (10 mM) at initial pHs 4 and 9.21 However, Minero and co-workers mentioned that at pH 10, the rate of phenol PCD on P25 (0.1 g/L) was slightly decreased on the addition of NaF (10 mM).16a To clarify this, the PCD of phenol with AT700 and P25 at different pHs was carried out (Table 2). The degree of rate enhancement did decrease with

Rate of Phenol Degradation

J. Phys. Chem. C, Vol. 111, No. 51, 2007 19031

TABLE 2: Apparent Rate Constants (h-1) for Phenol PCD over TiO2 at Different Initial pHsa catalysts

pH 3.0

pH 6.5

pH 9.0

AT700 AT700 + NaF P25 TiO2 P25 TiO2 + NaF

0.0917 1.03 0.345 2.35

0.242 0.601 0.488 0.637

0.405 0.444 0.576 0.459

a Conditions: TiO 1.0 g/L, phenol 0.43 mM, and NaF 1.0 mM. 2 P25 TiO2 was used as received.

the initial pH. The ratio of rate constant in the presence of NaF to that in the absence of NaF, measured at initial pH ) 3.0, 6.5, and 9.0, was 11.2, 2.48 and 1.10 with AT700, and 6.81, 1.31 and 0.80 with P25, respectively. The rate of phenol PCD with P25 at pH 9 decreased on the addition of NaF, which can be ascribed to the fluoride-induced aggregation of P25 particles in an alkaline solution.16a Such particle aggregation was less efficient with AT700, since AT700 was less dispersed in water, as compared to P25. The data in Table 2 also shows that the rate enhancement due to NaF is more profound with AT700 than that with P25. This might relate to the fact that AT700 is purely anatase, whereas P25 is a mixture of anatase and rutile (8:2). However, the negative effect of NaF on the PCD with rutile is relatively weak, as compared to the positive effect with anatase (Figure 4). On the other hand, AT700 is less dehydrated than the asreceived P25. Then, the difference between AT700 and P25 might suggest that the fluoride present in the outer Helmholtz layer is more efficient than the surface adsorbed fluoride, for the desorption of surface-bound •OH into bulk solution. This is reasonable since the adsorbed fluoride on or bound to the surface Ti(IV) sites would be less efficient to form a fluorine hydrogen bond with the surface-bound •OH, as compared to the free fluoride ions (Scheme 1). This reasoning is consistent with the trend of rate enhancement with AT as a function of temperature (Figure 4A). The desorption of surface-bound •OH from irradiated TiO2 into solution via a fluorine hydrogen bond is thermodynamically possible. The surface-adsorbed •OH on TiO2, as reported first by Serpone42 and latter by Majima,43 has a redox potential of about 1.5-1.7 V vs NHE at neutral pH,42,43 whereas the redox potential of free •OH in solution at pH 7 is ca. 2.3 V vs NHE.44 If this surface-adsorbed •OH is taken as the surface-bound •OH, the desorption of surface-bound •OH from TiO into 2 solution would need a driving force of ca. 0.6-0.8 eV. This energy is approximately the same as the strength of the fluorine hydrogen bond in the form of F-H‚‚‚F-H and F‚‚‚H‚‚‚F-, which is 0.31 and 1.75 eV, respectively (HF has pKa ) 3.15 in water).45 A maximum rate enhancement at initial pH 4.5 for the phenol PCD over P25 on the addition of NaF has been reported.16a A decrease in rate enhancement at high pH (Table 2) is due to the reduced amount of fluoride in the Helmholtz layer (Scheme 1). Moreover, the surface-bound •OH (trapped hvb+) may lie at various levels above the valence band;4a,45 that is, it is not only limited to 1.5-1.7 V vs NHE. This would further allow the desorption process to occur spontaneously. 4. Conclusions This work has confirmed that the addition of NaF into the TiO2 dispersions can result in enhancement in the PCD of phenol.16-23 However, such rate enhancement is only operative with anatase but not with rutile, except that the conduction band electrons are efficiently removed by appropriate electron acceptor such as Ag(I). Moreover, such rate enhancement, as

demonstrated by both anatase and rutile TiO2, does not always require the fluoride preadsorbed on the catalyst surface, which cannot be interpreted by the early Minero proposal (eqs 3 and 4).16a Therefore, we have proposed a double layer model that both the fluoride ions in the inner and outer Helmholtz layers are able to facilitate the desorption of surface bound •OH radicals into solution from irradiated TiO2. Since the desorbed and free •OH radicals are more reactive than the surface-bound •OH, the rate enhancement of organic PCD upon NaF addition has been observed. Although formation of such fluorine hydrogen bond needs further proof, this model can account for all the evidence presented in this work and reported so far in the literature. Acknowledgment. Support of this work by the National Natural Science Foundation of China (Nos. 20273060, 20477038, and 20525724) is gratefully acknowledged. We also thank the reviewers for helpful comments. References and Notes (1) (a) Ollis, D. F.; Al-Ekabi, H., Eds. Photocatalytic Purification and Trteament of Water and Air; Elsevier: Amsterdam, The Netherlands, 1993. (b) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69-96. (2) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., A: Chem. 2000, 1, 1-21. (3) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33-177. (4) (a) Thompson, T. L.; Yates, J. T., Jr. Chem. ReV. 2006, 106, 44284453. (b) Serpone, N. J. Phys. Chem. B 2006, 110, 24287-24293. (5) (a) Peiro, A. M.; Colombo, C.; Doyle, G.; Nelson, J.; Mills, A.; Durrant, J. R. J. Phys. Chem. B 2006, 110, 23255-23263. (b) Nakamura, R.; Imanishi, A.; Murakoshi, K.; Nakato, Y. J. Am. Chem. Soc. 2003, 125, 7443-7450. (c) Szczepankiewicz, S. H.; Moss, J. A.; Hoffmann, M. R. J. Phys. Chem. B 2002, 106, 7654-7658. (6) (a) Nakamura, R.; Nakato, Y. J. Am. Chem. Soc. 2004, 126, 12901298. (b) Nakamura, R.; Tanaka, T.; Nakato, Y. J. Phys. Chem. B 2004, 108, 10617-10620. (c) Nakamura, R.; Okamura, T.; Ohashi, N.; Imanishi, A.; Nakato, Y. J. Am. Chem. Soc. 2005, 127, 12975-12983. (7) (a) Murakami, Y.; Kenji, E.; Nosaka, A. Y.; Nosaka, Y. J. Phys. Chem. B 2006, 110, 16808-16811. (b) Murakami, Y.; Endo, K.; Ohta, I.; Nosaka, A. Y.; Nosaka, Y. J. Phys. Chem. C 2007, 111, 11339-11346. (8) Kubo, W.; Tatsuma, T. J. Am. Chem. Soc. 2006, 128, 1603416035. (9) (a) Nosaka, Y.; Daimon, T.; Nosaka, A. Y.; Murakami, Y. Phys. Chem. Chem. Phys. 2004, 6, 2917-2918. (b) Daimon, T.; Nosaka, Y. J. Phys. Chem. C 2007, 111, 4420-4424. (10) Janczyk, A.; Krakowska, E.; Stochel, G.; Macyk, W. J. Am. Chem. Soc. 2006, 128, 15574-15575. (11) Agrios, A. G.; Pichat, P. J. Photochem. Photobiol. A 2006, 180, 130-135. (12) Hurum, D. C.; Gray, K. A.; Raji, T.; Thurnauer, M. C. J. Phys. Chem. B 2005, 109, 977-980. (13) Kominami, H.; Murakami, S.; Kato, J.; Kera, Y.; Ohtani, B. J. Phys. Chem. B 2002, 106, 10501-10507. (14) Emeline, A.; Salinaro, A.; Serpone, N. J. Phys. Chem. B 2000, 104, 11202-11210. (15) Tryba, B.; Toyoda, M.; Morawski, A. W.; Nonaka, R.; Inagaki. Appl. Catal. B 2006, 71, 163-168. (16) (a) Minero, C.; Mariella, G.; Maurino, V.; Pelizzetti, E. Langmuir 2000, 16, 2632-2641. (b) Minero, C.; Mariella, G.; Maurino, V.; Vione, D.; Pelizzetti, E. Langmuir 2000, 16, 8964-8972. (17) Vione, D.; Minero, C.; Maurino, V.; Carlottib, M. E.; Picatonottoa, T.; Pelizzetti, E. Appl. Catal. B 2005, 58, 79-88. (18) (a) Mrowetz, M.; Selli, E. Phys. Chem. Chem. Phys. 2005, 7, 11001102. (b) Mrowetz, M.; Selli, E. New J. Chem. 2006, 30, 108-114. (19) Oh, Y.; Jenks, W. S. J. Photochem. Photobiol. A: Chem. 2004, 162, 323-328. (20) (a) Kim, S.; Park, H.; Choi, W. J. Phys. Chem. B 2004, 108, 64026411. (b) Vohra, M. S.; Kim, S.; Choi, W. J. Photochem. Photobiol. A: Chem. 2003, 160, 55-60. (c) Lee, J.; Choi, W.; Yoon, J. EnViron. Sci. Technol. 2005, 39, 6800-6807. (d) Park, H.; Choi, W. Catal. Today 2005, 101, 291-297. (21) Park, H.; Choi, W. J. Phys. Chem. B 2004, 108, 4086-4093. (22) Yang, S.; Lou, L.; Wang, K.; Chen, Y. J. Photochem. Photobiol. A: Chem. 2006, 301, 152-157.

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