Incorporative Effect of Pt and Na2CO3 on TiO2-Photocatalyzed

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Incorporative Effect of Pt and NaCO on TiOPhotocatalyzed Degradation of Phenol in Water Xianqiang Xiong, Xiao Zhang, and Yiming Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07951 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on October 27, 2016

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The Journal of Physical Chemistry

Incorporative Effect of Pt and Na2CO3 on TiO2Photocatalyzed Degradation of Phenol in Water Xianqiang Xiong, Xiao Zhang and Yiming Xu* State Key Laboratory of Silicon Materials and Department of Chemistry, Zhejiang University, Hangzhou 310027, China. * The corresponding author: E-mail: [email protected]. Tel: +86-571-87952410.

Abstract. Carbonate anions are often present in aqueous solution, but their effect on the semiconductorphotocatalyzed reaction has been rarely studied. In this work, we report a positive effect of Na2CO3 on the TiO2-photocatalyzed degradation of phenol, 2,4−dichlorophenol, and H2O2 in an aerated aqueous suspension at initial pH 8.0. The rate of phenol degradation, upon the addition of 2.0 mM Na2CO3, 0.52 wt% Pt, and 2.0 mM Na2CO3 plus 0.52 wt% Pt, was increased by 1.78, 3.38, 6.63 times, respectively. Such positive effect of carbonate was also observed from a TiO2 and Pt/TiO2 film electrode for the photoelectrochemical oxidation of phenol, but not water. However, the rates of phenol degradation over TiO2 and Pt/TiO2 became decreased as carbonate concentration exceeded 5.0, and 2.0 mM, respectively. It is proposed that CO3•− radicals are formed mainly from the hole oxidation of dicarbonate adsorbed on TiO2, followed by phenol degradation. At a high concentration, the CO3•− radicals would recombine to a peroxocarbonate that easily decomposes into CO2 and O2. The carbonate-mediated hole transfer from TiO2 to phenol would incorporate with the Pt-mediated electron transfer from TiO2 to O2, consequently resulting into a great improvement in the efficiency of charge separation for the reactions at interface. 1

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1. INTRODUCTION The photogenerated electrons and holes of TiO2 under UV light are reactive toward the reduction of O2 and the oxidation of various organics, respectively.1−3 Because of that, the TiO2 photocatalysis has been widely studied as a potential new technology for environmental remediation. However, the system efficiency is still not high enough to enable practical application. There are many influencing factors, among which the photocatalytic activity of TiO2 itself is the most critical one. It is often observed that the apparent photocatalytic activity of TiO2 for organic degradation in aqueous solution is greatly dependent of its physical parameters, including the crystal structure, surface area, and particle size.2 Recently, we have found that the intrinsic photocatalytic activity of TiO2 exponentially increases with its synthesis temperature, regardless of the solid structures in the crystal forms of anatase, rutile, brookite, and/or their mixtures.4−7 Such synthesis temperature effect is primarily due to the formation of a surface defect on TiO2, and secondarily due to the growth of TiO2 particles.8 A high intrinsic photocatalytic activity of TiO2 means a large number of electrons and holes that has been photogenerated, and reached the oxide surface. If these charge carriers are not immediately used for a chemical reactions, they would quickly recombine to heat. As a result, a low apparent photocatalytic activity of TiO2 is observed. In other words, the widely observed apparent photocatalytic activity of TiO2 that greatly changes from one oxide to another is the consequence of the combined effect between the synthesis temperature of TiO2 and its surface properties toward the uptake of reactants in aqueous solution. For example, anatase TiO2 have a stronger affinity to O2 in aqueous solution, and thus it shows a higher apparent photocatalytic activity for organic degradation, as compared to rutile TiO2 prepared at the same temperature.4 To achieve a high quantum efficiency of organic degradation over TiO2, therefore, the main challenge becomes how to speed up the interfacial charge transfer from TiO2 to O2, organics, and/or both.

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In fact, many studies have been made in the past years on the surface modification of TiO2 with a cocatalyst or sorbent.9 For example, the platinization of TiO2 (denoted as Pt/TiO2) can result into a great improvement in the photocatalytic activity for organic degradation in aqueous suspensions.10 The observed higher activity of Pt/TiO2 than that of TiO2 is mainly ascribed to the metallic Pt that serves as an electron sink for the multi-electron reduction of O2 to H2O2.11,12 Such Pt-catalyzed reduction of O2 would retard fast charge recombination of TiO2, and consequently promote the hole oxidation of organics at interfaces. Interestingly, several inorganic anions (F−, PO43−, SO42−, and B4O72−) are also beneficial to the TiO2-photocatalyzed degradation of many organics in aqueous suspensions.13−20 The observed positive effect of these anions are mainly attributed to enhancement in the hole transfer,21 for the formation of •OH radicals in solution,12−18 and/or for the formation of a borate radical.12,19 In all cases, these anions are claimed to be recyclable during the photocatalytic process. However, the effect of carbonate anions has been little studied. In one case, carbonate anions have a positive effect on the photocatalytic degradation of aniline in aqueous suspension, due to the increased adsorption of aniline on TiO2, through a hydrogen bond with a surface-adsorbed carbonate species.22 In the other cases, carbonate anions are detrimental to the photocatalytic degradation of methyl orange dye (MO) and dichloroethane (DCE) over TiO2 in aqueous suspensions, ascribed to the formation of CO3•− radicals that are less reactive than •OH radicals toward MO oxidation,23 and/or ascribed to the decreased adsorption of DCE (MO) on TiO2.24 Basically, these observations are in qualitative agreement with the fact that the initial rate of organic degradation is linearly proportional to the initial amount of organic adsorption over TiO2 in aqueous soluiton.25 However, the observed carbonate effect may also result from changes in the formation of reactive species over the irradiated TiO2. To examine whether carbonate has an effect on the TiO2-photocatalyzed reaction, it is better to use a model substrate that hardly absorbs onto TiO2 from

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aqueous solution. Since carbonate and dicarbonate anions are commonly present in an air-exposed aqueous solution, their effect on the TiO2-photocatalyzed reactions is worthy of being further studied. In this work, we report a positive effect of Na2CO3 on the photocatalytic degradation of phenol over TiO2 and Pt/TiO2 in an aqueous solution. Photoreactions were carried out under UV light at wavelengths longer than 320 nm. Under such conditions, phenol photolysis in aqueous solution and its dark adsorption on TiO2 were both negligible. This would ensure determination of the catalyst activity, without the interference from organic photolysis, organic adsorption, and dye sensitization. The deposition of Pt particles onto TiO2 was made through a photochemical impregnation method. The use of Pt/TiO2 as a reference photocatalyst was to examine whether or not carbonate anions are capable of mediating the hole transfer of TiO2. Interestingly, the rate of phenol degradation over TiO2, obtained in the presence of both Pt and Na2CO3, was much higher than the sum of the rates, measured in the presence of individual Pt and Na2CO3. To understand the observed carbonate effect, the influencing factors of carbonate concentration and solution pH were examined, whereas the formation of •OH and H2O2 and the interfacial charge transfer between carbonate and TiO2 were measured with a fluorescence spectroscopy, colorimetric method, (photo)electrochemical measurement, respectively. Furthermore, a possible mechanism responsible for the observed synergistic effect of Pt and carbonate is discussed. 2. EXPERIMENTAL SECTION Materials. Anatase TiO2, horseradish peroxide (POD), N,N−diethyl-p-phenylenediamine (DPD), chloroplatinic acid and coumarin were purchased from Sigma−Aldrich. Other chemicals in analytical grade were purchased from Sinopharm Chemical Reagent Co., Ltd., including Na2CO3, phenol, 2,4dichlorophenol (DCP), H2O2 and polyvinylalcohol (PVA). Milli-Q ultrapure water was used throughout this study. The solution pH was adjusted with a dilute solution of HClO4 or NaOH.

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The Pt-loaded TiO2 was prepared by a photochemical deposition method. An aqueous suspension (50 ml) containing 0.9 g of TiO2, 4 mL of CH3OH and 120 µL of H2PtCl6 stock solution was stirred in the dark for 30 min, followed by irradiation with a 300 W mercury lamp for 3 h. The solid was filtered, washed thoroughly with distilled water, and dried overnight in an oven at 80 oC. The amount of H2PtCl6 remaining in the filtrate was measured by an inductively coupled plasma mass spectroscopy, from which the amount of Pt in Pt/TiO2 was calculated to be 0.52 wt%.

Table 1. Physical Parameters of the Catalystsa Samples

dXRD (nm)

Asp (m2/g)

Vp (cm3/g)

dp (nm)

Eg (eV)

TiO2

13.6

140

0.312

81

3.20

Pt/TiO2

13.5

141

0.353

87

3.20

a

dXRD, crystallite size estimated by XRD from the (101) anatase; Asp, Brunauer−Emmett−Teller (BET) surface area measured by N2 adsorption; Vp, total pore volume; dp, average pore size; Eg, band gap energy estimated from the absorption spectra through a Tauc plot.

Characterization. Solids were characterized by X−ray diffraction (XRD), UV−vis diffuse reflectance spectroscopes, N2 adsorption, and fluorescence spectroscopy, and the results are shown in Figure S1 of the Supporting Materials. In brief, the XRD patterns of TiO2 and Pt/TiO2 well matched those of standard anatase TiO2 (PDF #65−5714). No diffractions of metallic Pt particles in Pt/TiO2 were observed, due to low content of Pt (0.52 wt%). However, the presence of Pt in Pt/TiO2 was inferred from a color change from white to gray, and from an absorption spectrum whose base line shifted upward in the visible light region. Furthermore, the band gap emission of Pt/TiO2 was weaker than that of TiO2, presumably indicative of that there is an interfacial electron transfer from TiO2 to Pt. Moreover, the TiO2 phases in TiO2 and Pt/TiO2 were similar in terms of the crystal structure, crystallite size, and band gap energy

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(Table 1). But Pt/TiO2 had a surface area, total pore volume, and average pore size slightly larger than those of TiO2, mostly ascribed to the fine Pt particles formed in the sample. Photocatalysis and Analysis. Reactions were carried out at 25 oC in a Pyrex−glass reactor. The aqueous suspension (50 mL) containing necessary components (0.43 mM phenol, 0.30 mM 2,4−DCP, 2.0 mM Na2CO3, 1.00 g/L catalyst, initial pH 8.0) was stirred in the dark for 1 h, and then irradiated with 300 W high pressure mercury lamp (Shanghai Yamin). The light intensity reaching the external surface of the reactor was 3.50 mW/cm2, as measured with an irradiance meter (Instruments of Beijing Normal University). At given intervals, 2.0 mL of the suspension was withdrawn, and filtered through a 0.22 µm membrane. The filtrate was analyzed by HPLC (high performance liquid chromatography) on a Dionex P680 (Apollo C18 reverse column). The eluent was a mixture of CH3OH and H2O at a volume ratio of 5:5 for phenol, and 7:3 for 2,4-DCP, followed by addition of 0.1 % acetic acid. H2O2 was measured at 553 nm on an Agilent 8453 UV–visible spectrophotometer through the POD−catalyzed oxidation of DPD.26 Hydroxyl radical was measured on a Shimadzu F−2500 spectrofluorometer through reaction with coumarin to form a fluorescent 7−hydroxycoumarin.27 Electrode Fabrication and Measurement. The TiO2 film electrode was prepared by the doctor blade method. A gel containing 0.8 wt% TiO2 and 2.9 wt% PVA was coated on indium−doped tin oxide (ITO) substrate, and then sintered in air at 500 °C for 3 h. After that, the ITO substrate was cut into pieces. Each piece had an exposed area of 1 × 1 cm2, and the other part was sealed by an epoxy resin. The Pt/TiO2 film electrode was prepared as below. Firstly, the prepared TiO2 film electrode was dipped in an aqueous solution containing 0.52 wt% H2PtCl6 and 2 M CH3OH for 30 min. Then the electrode was taken out from the solution, and irradiated with UV light for 10 min. These coated ITO glass were used as working electrodes. Measurement was carried out on a CHI660E Electrochemical Station (Chenghua, Shanghai), using a saturated calomel electrode (SCE) as the reference electrode, a platinum gauze as the

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counter electrode. The supporting electrolyte was 0.5 M NaClO4. The working electrode was illuminated through a quartz window with a 500 W Xe lamp from the electrode/electrolyte side. 0.45

(A) 0.40

(a)

0.35

(b)

0.30

(c)

(B)

0.30

2,4-DCP (mM)

0.25

Phenol (mM)

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0.25 (d)

0.20

(a)

0.20

(b)

0.15 (c)

0.10 0.05

0.15

(d)

0.00

-30

0

10

20

30

40

50

60

-30

0

Irradiation time (min)

10

20

30

40

50

60


Figure 1. Photocatalytic degradation of (A) phenol, and (B) DCP in aqueous solution at initial pH 8.0, measured under the conditions of (a) TiO2, (b) TiO2 + 2.0 mM Na2CO3, (c) Pt/TiO2, and (d) Pt/TiO2 + 2.0 mM Na2CO3.

3. RESULTS AND DISCUSSION Phenol Degradation. Figure 1A shows the results of phenol degradation over TiO2 and Pt/TiO2 in an aqueous suspension at initial pH 8.0, measured in the absence and presence of 2.0 mM Na2CO3. As the irradiation time increased, the concentration of phenol in aqueous phase decreased. All the time profiles for phenol degradation satisfactorily fit the first-order rate equation, and the resulting rate constants (kobs) are listed in Table 2. First, the rate of phenol degradation over Pt/TiO2 was about 3.38 times higher than that over TiO2. This observation is in agreement with the literature report that Pt has a positive effect on the TiO2-photocatalyzed degradation of organics in an aerated aqueous solution.10,11 Second, upon the addition of 2.0 mM carbonate, the rates of phenol degradation over TiO2 and Pt/TiO2 were increased by 1.79 and 1.96 times, respectively. Control experiments in the dark or under UV light without catalyst showed a negligible change of phenol concentration with time, either in the absence or presence of 7



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carbonate. Then, the observed rate increase of phenol degradation after the carbonate addition is indicative of a positive carbonate effect on the photocatalytic degradation of phenol over TiO2 and Pt/TiO2. Third, the rate of phenol degradation over TiO2, obtained in the presence of both Pt and carbonate, was higher than the sums of the rates, measured in the presence of individual Pt and carbonate. It implies that there is a synergistic effect between Pt and carbonate on the TiO2photocatalyzed degradation of phenol. Furthermore, hydroquinone and benzoquinone were identified as the major intermediates of phenol degradation in aqueous phase (Figure S2). The formation rates of these intermediates increased in the order of Pt/TiO2 + CO32− > Pt/TiO2 > TiO2 + CO32− > TiO2, the trend similar to that observed from phenol degradation. In all cases, the total amount of these intermediates formed at given time was always much lower than that of phenol disappeared. These observations indicate that the degradation pathways of phenol remain unchanged upon the addition of carbonate.

Table 2. Apparent Rate Constants (kobs, 10−3 min−1) of Organic Degradationa Catalyst

a

kobs (phenol)

kobs (DCP)

TiO2

2.41

6.67

TiO2 + Na2CO3

4.31

9.17

Pt/TiO2

8.15

11.78

Pt/TiO2 + Na2CO3

16.0

35.83

The corresponding results are from Figure 1.

Similar results were also observed from the photocatalytic degradation of DCP in an aqueous suspension at initial pH 8.0 (Figure 1B). In this case, DCP in aqueous solution notably adsorbed onto TiO2 and Pt/TiO2. Upon the addition of 2.0 mM Na2CO3, the amount of DCP adsorption on those catalysts were decreased, indicative of the competition between carbonate and DCP for the surface sites

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of TiO2. However, Upon the addition of 2.0 mM Na2CO3, the rates of DCP degradation over TiO2 and Pt/TiO2 were still increased by 1.37 and 3.04 times, respectively (Table 2). Once again, the rate of DCP degradation over TiO2, obtained in the presence of both Pt and carbonate, was also larger than the sum of the rates obtained in the presence of individual carbonate and Pt. These observations confirm that Pt and carbonate do have a cooperative effect on the TiO2-photocatalyzed reactions in aqueous solution.

16

(A)

15

(b)

(a)

4 3

-1

9

10

-3

kobs (10 min )

12

-3

-1

(B)

14

12

kobs(10 min )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8 6 4 2 0

2 0

2

4

6

8

10 18

20

Carbonate concentration (mM)

6

7

8

9

10

11

Initial pH of the suspension

Figure 2. Apparent rate constants of phenol degradation in aqueous suspension. (A) Effect of Na2CO3 concentration at initial pH 8.0, in the presence of (a) TiO2, and (b) Pt/TiO2. (B) Effect of the solution pH, measured with Pt/TiO2 in the presence of 3.0 mM Na2CO3.

Effect of Variables. Figure 2A shows the results of phenol degradation over TiO2 and Pt/TiO2 in aqueous suspension at a fixed initial pH 8.0. As the concentration of carbonate increased, the rate of phenol degradation increased, and then decreased. A maximum rate of phenol degradation was observed from TiO2 at 5.0 mM Na2CO3, and from Pt/TiO2 at 2.0 mM Na2CO3, respectively. At such optimum carbonate loading, the rates of phenol degradation over TiO2 and Pt/TiO2 were all increased by 1.96 times. These observations indicate that carbonate has a positive effect at low concentration, but a negative effect at high concentration. Moreover, the rate raise and decay of phenol degradation with carbonate concentration, observed from Pt/TiO2, was larger than that observed from TiO2. Assume that 9



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CO3•− radicals are produced from the oxidation of CO32− anions by the photogenerated holes of TiO2, and that CO3•− radicals are reactive toward phenol oxidation in aqueous solution (rate constant k = 2 × 107 M−1 s−1).28 Then the increase of CO32− concentration would lead to increase in the rate of CO3•− formation, and consequently in the rate of phenol degradation. Moreover, the rate of CO3•− formation would also increase with the photocatalytic activity of TiO2. Since Pt/TiO2 is more active than TiO2, the former shows a larger increase in the rate of phenol degradation with CO32− concentration than does the latter. However, at a high concentration, CO3•− radicals would recombine to a peroxocarbonate species (C2O62−), which easily decompose into CO2 and H2O2 (k = 5 × 107 M−1 s−1).28−30 Such self-recombination of CO3•− radicals would occur with a rate, that increases not only with the concentration of CO32−, but also with the photocatalytic activity of TiO2. On one hand, this would result into an optimum concentration of CO32−, at which a maximum rate of phenol degradation is reached. On the other hand, a catalyst that has a high photocatalytic activity would show a low optimum concentration of CO32−, and/or a large decrease in the rate of phenol degradation with CO32− concentration, as observed from Pt/TiO2 and TiO2. It is also speculated that the self-recombination of CO3•− radicals would occur once their concentration reach a certain level. This may serve as the reason why TiO2 and Pt/TiO2 exhibit a similar activity enhancement (1.96 times) upon the addition of 5.0 and 2.0 mM Na2CO3, respectively. Figure 2B shows the results of phenol degradation over Pt/TiO2 in aqueous suspensions at a fixed concentration of Na2CO3 (3.0 mM). As the initial pH of the suspension increased, the rate of phenol degradation increased, and then decreased. A maximum rate of phenol degradation was observed at initial pH 8.0. Understanding the observed pH effect is not straightforward. In an aqueous solution of 3.0 mM Na2CO3 at pH 8.0 and 10.5, the carbonate species would nearly exist in the forms of HCO3− and CO32−, respectively, as estimated from the first equilibrium constant of carbonate hydrolysis (pKb = 3.75). If the proposed pathway of CO3•− is hold, it follows that HCO3− is more efficient than CO32− in

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capturing the photogenerated holes of TiO2. However, in scavenging the •OH radicals in aqueous solution, HCO3− is less reactive than CO32− (the rate constants for HCO3− and CO32− are 8.5 × 106, and 3.9 × 108 M−1 s−1, respectively).31 This discrepancy may result from carbonate adsorption on the oxide. Unfortunately, it is difficult to measure the amount of carbonate adsorption on solid in aqueous solution. It is known that the pH value for TiO2 at the zero point charge in water is about 6.5. Then in an aqueous solution at pH > 6.5, the surface of TiO2 would be negatively charged, not favorable to anion adsorption. Since HCO3− is less negatively charged than CO32−, it is highly possible that HCO3− has a larger amount of adsorption on TiO2, and consequently a higher rate of CO3•− formation, as compared with CO32− in an alkaline aqueous solution. These observations and reasoning suggest the importance of carbonate adsorption on the TiO2-photocatalyzed reaction in aqueous solution. 8

100

(A)

(B)

(d)

80 6

In the dark

(c)

H2O2 (µM)

H2O2 (µM)

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4 (a)

60

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40

(b) (a)

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20 (d) (b)

0

0 0

10

20

30

40

50

60

-60

-50

-40


-30

-20

-10

0

10

Time (min)

Figure 3. (A) Photocatalytic formation of H2O2 in the presence of 0.43 mM phenol. (B) Dark adsorption and photocatalytic decomposition of 0.10 mM H2O2 without addition of phenol. All experiments were carried out in an aerated aqueous suspension at initial pH 8.0, under the conditions of (a) TiO2, (b) TiO2 + Na2CO3, (c) Pt/TiO2, and (d) Pt/TiO2 + Na2CO3.

Formation of Oxygen Reactive Species. It is generally recognized that organic substrate (S) is degraded by various reactive species (Ox) generated from the irradiated TiO2, including O2−•, and •OH.

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Under a fixed condition, the concentration of Ox would be constant. As a result, the rate of organic degradation is of the first order in S at low [S], and zero-order in S at high [S].32,33 In the present case, an increase in the apparent rate constant of phenol degradation upon addition of Pt and carbonate implies an increase in the formation rate of Ox. Then, it is necessary to examine the effect of carbonate on the generation of Ox. Figure 3A shows the result of H2O2 production over TiO2 and Pt/TiO2 in aqueous suspension. In this case, phenol was used as the hole scavenger of TiO2, so that the observed formation of H2O2 would be due to the electron reduction of O2 over TiO2. In the absence of carbonate, the initial rate of H2O2 generation from Pt/TiO2 was larger than that from TiO2. This observation is in agreement with the literature report that Pt particles act as a thermal catalyst for the multi-electron reduction of O2.10−12 Upon the addition of 2.0 mM Na2CO3, the initial rate of H2O2 formation over TiO2 was greatly decreased, but the initial rate of H2O2 formation over Pt/TiO2 was slightly increased. It is highly possible that the formation and decomposition of H2O2 occur at the same time. In fact, the concentration of H2O2 after reaching a maximum became to decrease with the irradiation time. To practice this hypothesis, a control experiment with H2O2 as a reacting substrate was performed, and the result is shown in Figure 3B. In the dark, H2O2 in aqueous solution greatly adsorbed onto TiO2, due to the formation of a surface complex with the Ti(VI) sites of TiO2. Upon the addition of carbonate, the amount of H2O2 adsorption on TiO2 was slightly decreased, due to the competitive adsorption of carbonate species on the oxide. Under UV illumination, the concentration of H2O2 gradually decreased with time. Interestingly, the rate of H2O2 decomposition in the presence of carbonate was higher than that in the absence of carbonate. Note that the photolysis of H2O2 in the absence of TiO2 was very slow. These observations indicate that carbonate also has a positive effect on the photocatalytic decomposition of H2O2 over TiO2. However, the experiment with Pt/TiO2 was not successful, because the change of H2O2 concentration with time was very fast, even in the dark. Since Pt/TiO2 contained only 0.5 wt% of Pt, the observed decrease of

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H2O2 concentration in the dark is largely ascribed to the catalytic effect of Pt on the decomposition of H2O2. Therefore, the observed negative effect of carbonate on the photocatalytic formation of H2O2 over TiO2 and Pt/TiO2 is more or less ascribed to the carbonate-caused acceleration of H2O2 decomposition. 50k

40k (d)

Emission Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30k (c) 20k (b)

10k

(a) 0

0

5

10

15

20


Figure 4. Emission of 7-hydroxycoumarin at 457 nm, measured in aqueous suspensions at initial pH 8.0, under the conditions of (a) TiO2, (b) TiO2 + Na2CO3, (c) Pt/TiO2, and (d) Pt/TiO2 + Na2CO3.

On the other hand, •OH radicals might be involved in the photocatalytic process. The detection of •

OH is usually made through an indirect method, such as a spin trapping electron paramagnetic

resonance spectroscopy.34 In this study, coumarin (CM) was used as a probe of •OH, because its reaction with OH gave a fluorescent 7−hydroxycoumarin (HCM).27 Figure 4 shows the results of HCM formation over TiO2 and Pt/TiO2 in aqueous suspension, measured in the absence and presence of 2.0 mM Na2CO3. In all cases, the emission intensity of HCM at 457 nm increased with the irradiation time. Among the different catalysts, the rates of HCM formation increased in the order of Pt/TiO2 + CO32− > Pt/TiO2 > TiO2 + CO32− > TiO2. This trend in the rate of HCM formation is in good agreement with that in the rate of phenol degradation (Figure 1A). Accordingly, one may conclude that the observed positive effect of Pt and carbonate on the TiO2-photocatalyzed degradation of phenol is due to increase in the formation of

13



•

OH radicals. However, HCM may result also from the hole oxidation of CM, followed by hydrolysis.35

In other words, this fluorescent study is not confident enough to verify the involvement of •OH radicals in the TiO2-photocatalzyed process. This issue needs to be addressed with other technique. 30

60 2

20

Current density (µA/cm )

(b)

2

Current density (µA/cm )

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(a)

10

(a) (b) 0 -10 -20 (A)

-30

50

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Figure 5. Linear sweep voltammetry for (A) TiO2 and (B) Pt/TiO2 film electrodes, recorded in the dark (dotted lines), and under UV light (solid lines), at a scan rate of 10 mV/s. Experiments were under N2 in 0.5 M NaClO4 at pH 8.0, containing (a) 0.40 mM phenol, and (b) 0.40 mM phenol + 2.0 mM Na2CO3.

(Photo)electrochemical Measurement. Possible carbonate-mediated hole transfer of TiO2 was then examined through a (photo)electrochemical method. Figure 5 shows the result of phenol oxidation over the TiO2 and Pt/TiO2 film electrodes under N2 in 0.5 M NaClO4. In the dark, the electrode current was very weak, either in the absence or presence of 2.0 mM Na2CO3. Under UV illumination, an anodic current was notably observed, indicative of the occurrence of phenol oxidation. As the applied external potential increased, the photocurrent continued to increase, due to improvement in the efficiency of charge separation. Importantly, the photocurrent at given potential became increased upon the addition of carbonate. Similar result was also observed from the Pt/TiO2 film electrode. The photocurrent due to phenol oxidation was not only observed, but also became increased upon the addition of carbonate into the electrolyte. Since the photogenerated electrons of TiO2 are removed through an external bias, the 14


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carbonate-caused increase of the photocurrent is surely due to the carbonate-mediated hole transfer from TiO2 to phenol. A control experiment with the TiO2 and Pt/TiO2 film electrodes in the absence of phenol showed no enhancement in the photocurrent upon the addition of carbonate (Figure S3). In other words, the carbonate anions had no effect on the photoelectrochemical oxidation of water. These observations are similar to those reported by Kazuhiro et.al.30 They found that the improvement of water splitting into H2 and O2 over Pt/TiO2 occurred only in a concentrated aqueous solution of Na2CO3 (0.29−2.18 M), ascribed to the self-recombination of CO3•− radicals to a peroxocarbonate (C2O62−), followed by the hole oxidation to release O2 and CO2. In the present case, the positive effect of carbonate on the TiO2photocatalyzed degradation of phenol has been observed in a dilute solution of carbonate (1−5 mM, Figure 2A). In other words, the CO3•− radicals, produced from the hole oxidation of carbonate at a low concentration, are reactive only to phenol, but not to water. Possible Mechanism. In an aqueous solution at pH 7, the one-electron potentials for the couples of CO3•−/CO32−, phenol+•/phenol, and •OH/H2O are 1.78, 1.03, and 2.40 V versus normal hydrogen electrode (NHE), respectively.36−40 All of these potentials are less positive than the valence band edge potential for anatase TiO2 (2.67 V vs NHE).2 In thermodynamics, all of CO32−, phenol and H2O are oxidizable by the photogenerated holes of TiO2. In the absence of carbonate, phenol degradation can occur, either through the photogenerated holes of TiO2, or through the •OH radicals formed from the hole oxidation of H2O. However, the possible hole oxidation of H2O and OH− to generate •OH over TiO2 has been rejected by Nakato, Salvador, and coworkers.41,42 If this is hold, phenol would be oxidized to phenol+• by the holes of TiO2, followed by hydrolysis to produce hydroquinone and benzoquinone as the intermediates of phenol degradation (Figure S2). Upon the addition of carbonate, the photocatalytic and photoelectrochemical oxidation of phenol were notably accelerated (Figures 1 and 5). This implies an enhancement in the hole transfer from the irradiated TiO2 to phenol. Since the electrons and holes of

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TiO2 are photogenerated in a pair, such carbonate-mediated hole transfer would make the electrons live longer. As a result, the rate of O2 reduction is increased, and the efficiency of charge separation is improved. However, the TiO2-photocatalyzed reduction of O2 to H2O2 upon the addition of carbonate is inhibited (Figure 3A), which might be ascribed to the carbonate-assisted photocatalytic decomposition of H2O2 over TiO2 (Figure 3B). On the other hand, the Pt-catalyzed electron reduction of O2 to H2O2 would make the holes live longer, and accelerate the hole oxidation of phenol. During the photocatalytic reactions, the Pt-mediated electron transfer from TiO2 to O2, and the carbonate-mediated hole transfer from TiO2 to phenol would promote each other, consequently resulting into a great enhancement in the rate of phenol degradation, as outlined in Scheme 1.

Scheme 1. Possible mechanism for the synergistic effect of Pt and carbonate on the TiO2-photocatalyzed degradation of phenol in an aerated aqueous suspension.

Nevertheless, phenol degradation may occur through •OH radicals. However, if it is hold, the reaction rate would be decreased upon the addition of carbonate. First, •OH radicals would be scavenged by CO32− and HCO3− to form CO3•−.31 Second, CO3•− is much less reactive than •OH toward phenol oxidation (the rate constants for CO3•− and •OH are 2 × 107, and 7 × 109 M−1 s−1, respectively).28,31 Under the present conditions, the scavenging of •OH radicals by carbonate was confirmed. Under UV light at

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wavelengths longer than 240 nm, the rate of phenol degradation in the homogeneous solution of 1.0 H2O2 at initial pH 8.0 was notably decreased upon the addition of 10−50 mM Na2CO3 (Figure S4). Therefore, the observed positive effect of carbonate on the TiO2-photocatalyzed reactions is ascribed to the hole oxidation of CO32− to CO3•−, followed by the substrate oxidation (phenol, DCP, CM, and H2O2).

4. CONCLUSIONS In this work, we have showed that carbonate anions are beneficial to the TiO2-photocatalyzed degradation of phenol, 2,4−DCP, and H2O2 in an alkaline aqueous solution. The observed positive effect of carbonate is ascribed to the formation of CO3•−, mainly from the hole oxidation of HCO3− adsorbed on TiO2, followed by phenol oxidation to regenerate carbonate. This pathway of CO3•− radicals is supported by the experiment with Pt/TiO2. There is an obvious incorporation between the carbonatemediated hole transfer from TiO2 to phenol, and the Pt-mediated electron transfer from TiO2 to O2. However, carbonate anions at high concentration are detrimental to the TiO2-photocatalyzed reactions, due to formation of a peroxocarbonate which easily decomposes into O2 and CO2. Since carbonate species are often dissolved in aqueous solution, their effect on the semiconductor photocatalysis and photoelectrochemistry should be taken into account. Furthermore, as the additive or co-catalyst of TiO2 photocatalysis for water treatment, carbonate anions would be much less expensive than other anions.

Supporting Information Available. XRD patterns, diffuse reflectance spectra, N2 adsorption–desorption isotherms, Photoluminescence spectra, Absorption spectra of carbonate, the generation of intermediate, current–voltage curves for water oxidation on TiO2 and Pt/TiO2 film electrode, Homogeneous degradation of phenol with H2O2 and/or carbonate. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No. 21377110), and by the Funds for Creative Research Group of NSFC (No. 21621005).

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Incorporative Effect of Pt and Na2CO3 on TiO2-Photocatalyzed

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