Visible Light-Induced Degradation of Carbon Tetrachloride on Dye

Environ. Sci. Technol. 2001, 35, 966-970

Visible Light-Induced Degradation of Carbon Tetrachloride on Dye-Sensitized TiO2

SCHEME 1. Photosensitized Degradation of CCl4 on Sensitized TiO2 Particlesa

YOUNGMIN CHO AND WONYONG CHOI* School of Environmental Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea CHUNG-HAK LEE, TAEGHWAN HYEON, AND HO-IN LEE School of Chemical Engineering, Seoul National University, Seoul 151-742, Korea

This study investigated an application of TiO2 photocatalyst sensitized with tris(4,4′-dicarboxy-2,2′-bipyridyl)ruthenium(II) complex to CCl4 degradation under visible light irradiation. By injecting electrons from the photoexcited sensitizer to the conduction band, the sensitized TiO2 degraded CCl4 under the irradiation of λ > 420 nm. The quantum yield of CCl4 dechlorination was about 10-3. The dechlorination rate of CCl4 was reduced in the presence of dissolved O2 due to its competition for conduction band electrons. The photolysis rate was dependent on pH due to the strong pH dependence of the sensitizer adsorption on TiO2 surface with a maximum degradation rate achieved at pH ∼3. A two-site Langmurian model successfully described the adsorption of the sensitizer on TiO2 particles. The monolayer coverage was achieved at the added sensitizer concentration of 10 µM at [TiO2] ) 0.5 g/L. However, the photolysis rate of CCl4 showed a maximum at a sensitizer surface coverage of 0.3 monolayer. Since the photoinduced electron injection gradually depleted active sensitizer molecules on TiO2, sacrificial electron donors to regenerate the sensitizer were sought. 2-Propanol as an electron donor was efficient in the present RuIIL3/TiO2/CCl4 system, which showed no sign of deceleration in the dechlorination rate up to 6 h of irradiation.

Introduction The photocatalytic degradation of organic pollutants using TiO2 has demonstrated successful performance in various remediation systems of polluted water and air (1-5). The pollutants that have been degraded or remediated using TiO2 photocatalysis include almost any kind of organic compounds, various inorganic and metal ions, and even biological pathogens such as bacteria and viruses. Although TiO2 is very popular as a photocatalyst, it suffers from the lack of visible light absorption. Photosensitization is widely used to extend the photoresponse of TiO2 into the visible region. The principle of photosensitization of TiO2 is illustrated in Scheme 1 that indicates the primary electron pathways. The visible light excites the sensitizer molecules adsorbed on TiO2 and subsequently inject electrons to conduction band (CB) of TiO2. While the CB acts as a mediator for transferring electrons * Corresponding author phone: +82-54-279-2283; fax: +82-54279-8299; e-mail: [email protected]. 966

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a S stands for sensitizer, RuIIL ; S*, electronically excited sensitizer, 3 (RuIIL3)*; S+, oxidized sensitizer, RuIIIL3; D, sacrificial electron donor for regenerating the sensitizer. The number represents the primary electronic pathways in photosensitization: 1, excitation of S; 2, fluorescent decay of S*; 3, electron injection into CB; 4, back electron transfer to S+; 5, electron migration within the lattice onto the surface; 6, electron transfer to CCl4; 7, sensitizer regeneration by electron donors.

from the sensitizer to substrate electron acceptors on TiO2 surface, the valence band (VB) remains unaffected in a typical photosensitization. Although the basic processes of TiO2 photosensitization have been extensively studied using ruthenium-based complexes (6) and various inorganic/ organic dyes (7-13) as a sensitizer in relation to photoelectrochemical cells and water splitting systems, the studies of photosensitization applied to the degradation of pollutants using visible light are few. Photosensitized degradation of organic dyes has been carried out on TiO2 where the organic dye serves as both a sensitizer and a substrate to be degraded (14-19). Although the nonregenerative organic dye/TiO2/visible light system where the sensitizer itself degrades can be a variable option for treating dye pollutants in wastewater effluents, an efficient regenerative sensitized TiO2 system with general applicability needs to be developed for water treatment using visible light. Recently, Bendig and co-workers have studied the photodegradation of an herbicide on sensitized TiO2 under visible light irradiation in a nonregenerative mode (20, 21). In particular, TiO2 particles sensitized by carboxylated ruthenium bipyridyl complexes have demonstrated good performance (21). In this work, we investigated the photoreductive decomposition of CCl4 using visible light on TiO2 sensitized by tris(4,4′-dicarboxy-2,2′-bipyridyl)ruthenium(II) complexes. On sensitized TiO2, the electrons injected from the excited sensitizer can directly transfer to CCl4 as illustrated in Scheme 1. The resulting ‚CCl3 radicals degrade through a series of complex reactions (22-24). The characteristics and performance of the ruthenium sensitizer in the photosensitized degradation of CCl4 under visible light irradiation are quantitatively described. A regenerative photosensitization system using electron donors is also discussed.

Experimental Section Materials. Tris(4,4′-dicarboxy-2,2′-bipyridyl)ruthenium(II) chloride ([RuII(bpy-(COOH)2)3]Cl2, abbreviated as RuIIL3) was synthesized according to the method described in the literature (21). The TiO2 (Degussa P25) suspension concentration was 0.5 g/L. The suspension was dispersed by simultaneous sonication and shaking in an ultrasonic cleaning bath (Branson 3210). CCl4 (J. T. Baker) was purified by distillation. All other chemicals used were of reagent grade and used without further treatments. 10.1021/es001245e CCC: $20.00

 2001 American Chemical Society Published on Web 01/17/2001

FIGURE 1. UV-vis diffuse reflectance spectra of pure TiO2 and sensitized TiO2 powder and absorption spectrum of RuII(bpy(COOH)2)32+ in aqueous solution (3 µM) at pH 3. The ordinate scale in the diffuse reflectance spectra is expressed in Kubelka-Munk unit. Optical Measurements and Characterization. The optical absorption spectra of RuIIL3 solutions and RuIIL3-coated TiO2 powder were recorded with an UV-visible spectrophotometer (Shimadzu UV-2401PC) with a diffuse reflectance attachment (Shimadzu ISR-2200). The RuIIL3-coated TiO2 powder sample was prepared by evaporating the aqueous suspension of the sensitized TiO2 particles (TiO2 suspension at pH 3 with 6 mg of RuIIL3Cl2/g TiO2) to dryness. Figure 1 compares the diffuse reflectance spectrum of the sensitized TiO2 with that of pure TiO2 powder. It shows the absorption spectrum of 3 µM RuIIL3 solution at pH 3 as well. The absorption maximum of the metal-to-ligand charge transfer (MLCT, d f π* transition) band of the sensitizer was at λmax ) 467 nm. The molar absorptivity,  at 467 nm, was determined to be 2.1 × 104 M-1 cm-1 by measuring the absorbance of the sensitizer standard solution and used in determining the concentration of RuIIL3 solution. This value agrees with a previously reported one (25). Preparation of Sensitized TiO2 Suspensions. The aqueous stock solution (50-100 µM) of the sensitizer was prepared at pH > 10 for the complete dissolution of the sensitizer. A typical sample preparation procedure is as follows. A 100 mL of sensitizer solution with a desired concentration (usually 3 µM) was made by dilution. A portion of 20 mL was reserved for the measurement of the initial absorbance of the sensitizer solution (Ai). A calculated amount of TiO2 was added to the remaining 80 mL of the solution to give a 0.5 TiO2 g/L suspension that was then left for 20 min under magnetic stirring for adsorption equilibration. The pH of both fractions (20 mL and 80 mL) was adjusted to 3.0 in a typical experiment or to a desired value with 0.1 N HClO4 or 0.1 N NaOH solutions. An aliquot (∼1 mL) of the sensitized TiO2 suspension was filtered through a 0.45-µm PTFE syringe filter (Millipore) and was measured for the absorbance at 467 nm (Aeq). The amount of the sensitizer adsorbed on TiO2 was calculated from the absorbance difference ∆A () Ai - Aeq). The main portion of the sensitized TiO2 suspension was used for photolysis experiments. Photolysis and Analysis. Photolyses were performed with a 450 W Xe-arc lamp (Oriel). Light passed through a 10-cm IR water filter and an UV cutoff filter (λ > 420 nm). The filtered light was focused onto a 30-mL quartz reactor with minimized headspace and a rubber septum. Light intensity was measured by chemical actinometry using (E)-R-(2,5dimethyl-3-furylethylidene)(isopropylidene) succinic anhydride (Aberchrome 540) (26). The actinometry monitored the photobleaching kinetics of the colored Aberchrome 540 solution in toluene spectrophotometrically. A typical incident light intensity (Ii) through the UV cutoff filter was about 6 ×

FIGURE 2. Production of Cl- from the photosensitized degradation of CCl4 as a function of irradiation time. The suspensions were sparged with O2, N2, or air before photolysis. The experimental conditions were: [TiO2] ) 0.5 g/L, [RuIIL3]i ) 3 µM, [CCl4] ) 1.0 mM, pHi ) 3, Ii (420 < λ < 550 nm) ≈ 6 × 10-3 Einstein L-1 min-1. 10-3 Einstein L-1 min-1 in the wavelength range 420-550 nm. The suspension was bubbled with O2 or N2 for 30 min before adding CCl4. A calculated amount of CCl4 was injected directly into the reactor cell with a GC syringe through a rubber septum. A typical concentration of CCl4 was 1 mM. For the complete dissolution of CCl4 into the solution, the suspension was stirred magnetically for 30 min before photolysis began. Sample aliquots of 1 mL were obtained from the illuminated reactor with a 2-mL syringe, filtered through a 0.45-µm PTFE filter, and injected into a 2-mL glass vial. The chloride production from the CCl4 dechlorination was followed with an ion chromatograph (IC). The IC system was a Dionex DX-120 with a conductivity detector and a Dionex IonPac AS-14 wide bore column (4 mm × 250 mm).

Results and Discussion Photosensitized Degradation of CCl4. The irradiation of the sensitized TiO2 suspension with visible light (λ > 420 nm) led to the decomposition of CCl4. Figure 2 shows the photolysis results. The initial photonic efficiency of CCl4 dechlorination, (d[Cl-]/dt)/Ii, which does not take the light scattering loss into account, is 1 × 10-4 for the N2-saturated system. Since the sensitized TiO2 suspension absorbed only a fraction of the incident light, the photonic efficiency, should be taken as a lower limit of the true quantum yield. When either the sensitizer or TiO2 was absent, CCl4 did not degrade. This proves that there is no direct electron transfer between an excited sensitizer and a CCl4 molecule in the homogeneous solution and that the degradation of CCl4 proceeds through a sensitized photocatalysis on TiO2. The CB of TiO2 seems to play an essential role of a mediator of electron transfer to CCl4. Table 1 lists elementary reaction steps for the photoreductive dechlorination of CCl4 on RuIIL3-sensitized TiO2 under visible light irradiation. When the sensitizer molecule is excited in the solution, it quickly relaxes into the ground state through fluorescence (reaction T11) with little chance of electron transfer to a CCl4 molecule (reaction T13). However, in the presence of TiO2, the electron injection from the excited sensitizer to the CB of TiO2 is faster than the direct relaxation to the ground state. A quantum yield of 0.6 ( 0.1 was reported for the electron injection process in the RuIIL3/TiO2 system (27). In the present RuIIL3/TiO2 system at pH 3, the estimated driving force for the electron injection (E(RuIIL3*/RuIIIL3) - ECB) is about 0.45 V (21), while the driving force sufficient for electron injection is as small as about 0.1 V (6). Since the rate of electron injection VOL. 35, NO. 5, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Initial Elementary Reaction Steps in the RuII-TiO2/ CCl4/Visible Light Systema

tion leaves fewer CB electrons available for CCl4 molecules, which results in lower dechlorination rates. Kamat and coworkers (30) demonstrated that the trapped CB electrons that were injected from the excited sensitizer were immediately scavenged by O2 when exposed to air. The O2saturated and air-equilibrated suspensions show only a minor difference in the dechlorination rate, which implies that the oxygen adsorption on TiO2 is nearly saturated under the airequilibration condition. Kinetic Analysis of CCl4 Dechlorination. The following kinetic analysis quantitatively expresses the dechlorination rate of CCl4 in the RuIIL3/TiO2 system. Let us assume that CB electrons transfer to the preadsorbed substrates. From reaction T9

d[Cl-] ) kT9[eCB][CCl4]ad dt

(1)

The CB electron concentration at steady state can be obtained from

d[eCB] ) kT7[RuII* - TiO2] - kT8[RuIII - TiO2][eCB] dt kT9[eCB][CCl4]ad - kT10[eCB][O2]ad ≈ 0 d[RuII* - TiO2] ) Iabs - kT7[RuII* - TiO2] ≈ 0 dt which yields

[eCB] )

Iabs kT9[CCl4]ad + kT10[O2]ad + kT8[RuIII - TiO2]

(2)

Substituting eq 2 into eq 1 gives a

RuII stands for a RuII(bpy-(COOH)2)32+ complex.

(reaction T7, kinj ) 3.2 × 107 s-1) in the RuIIL3/TiO2 system is 80 times faster than the back electron transfer (reaction T8, kb ) 4 × 105 s-1) (27) which leads to a null reaction, the electrons in the CB can transfer to a substrate (e.g., CCl4) on TiO2 surface. Since the above reaction sequence leads to the irreversible change of the active sensitizer molecules (RuIIL3 f RuIIIL3) as a result of electron transfer, the surface concentration of RuIIL3 on TiO2 decreases as the photolysis proceeds. As shown in Figure 2, the depleting RuIIL3 concentration on TiO2 gradually decelerates the rate of CCl4 dechlorination. Dissolved Oxygen Effect. Figure 2 also shows the effect of dissolved gas on the photolysis rate of CCl4. The initial dechlorination rates of CCl4 decrease in the order of N2 > air > O2-saturated system. The presence of O2 in the suspension lowers photosensitization efficiency by two ways: direct quenching of the excited sensitizer and scavenging CB electrons. Dissolved O2 is an efficient quencher of the excited ruthenium bipyridine complex. The quenching process of an excited tris(2,2′-bipyridyl)ruthenium(II), (RuII(bpy)32+), by O2 is diffuse-limited (kq ) 3.3 × 109 M-1 s-1) (28). Sutin et al. (29) suggested an electron transfer mechanism that yields initially RuIII(bpy)33+ and O2- which react back to give RuII(bpy)32+ and a singlet oxygen. The quenching rate of the excited sensitizer on TiO2 by O2 (reaction T5) is estimated to be 3 × 106 s-1 in oxygen-saturated suspension ([O2]sat ≈ 10-3 M), which is 10 times slower than the electron injection (reaction T7). The main effect of dissolved O2 seems to be a scavenging of CB electrons. As the reaction steps in Table 1 show, the dissolved oxygen competes for CB electrons with CCl4 molecules. The increasing dissolved oxygen concentra968

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d[Cl-] ) dt

Iabs 1+

kT10[O2]ad + kT8[RuIII - TiO2]

(3)

kT9[CCl4]ad

In N2-saturated suspension where [O2]ad ≈ 0

d[Cl-] ) dt

Iabs 1+

kT8[RuIII - TiO2]

(4)

kT9[CCl4]ad

then the dechlorination quantum yield is

ΦCl- )

d[Cl-]/dt ) Iabs

1+

1 kT8[RuIII - TiO2]

(5)

kT9[CCl4]ad

If we assume that 10% of the incident light (Ii) is absorbed by the suspension (see Figure 1) and substitute the measured initial dechlorination rate from Figure 2 into eq 5, we get the dechlorination quantum yield, ΦCl- ≈ 10-3. This indicates that the back electron transfer (T8) is 1000 times faster than the interfacial electron transfer to CCl4 (T9) in the present system. Adsorption of the Sensitizer and the Photoreactivity. Figure 3 shows the adsorption behavior of the sensitizer on TiO2. The following two-site model successfully describesthe adsorption isotherm:

[RuIIL3]ad [RuIIL3]mono

)

K1[RuIIL3]eq 1 + K1[RuIIL3]eq

+

K2[RuIIL3]eq 1 + K2[RuIIL3]eq

(6)

FIGURE 3. Adsorption isotherm of the sensitizer, RuIIL3 on TiO2. The solid line is a fit to the two-site model (eq 6).

FIGURE 4. Initial photoreductive dechlorination rates (b) of CCl4 as a function of the added sensitizer concentration, [RuIIL3]i are compared with the sensitizer adsorption (O) on TiO2. Experimental conditions were [TiO2] ) 0.5 g/L, pH ) 3, [CCl4] ) 1 mM, N2-saturated. where [RuIIL3]ad is the concentration of the adsorbed sensitizer on TiO2; [RuIIL3]eq, the equilibrium concentration of the sensitizer remaining in the solution; [RuIIL3]mono, the surface concentration of the adsorbed sensitizer on TiO2 at a monolayer coverage; and K1 and K2, the equilibrium adsorption constant of the sensitizer for each site. The values of the fitted parameters are [RuIIL3]mono ) 25.7 µmol/g TiO2, K1 ) 9.8 × 106 M-1, and K2 ) 7.3 × 104 M-1. The fitted results are shown in solid line along with its two components in dotted lines in Figure 3. The site 1 represents the strong first layer adsorption through a bidentate carboxylate linkage that is frequently assumed for the structure of the adsorbed sensitizer (31). The site 2 corresponds to loosely bound complexes through a monodentate carboxylate linkage or weak outer-sphere electrostatic interactions with multilayer deposition. While the stronger adsorption component quickly reaches a monolayer coverage, the weaker adsorption component increases slowly with [RuIIL3]eq. Assuming a specific surface area of 50 m2/g for the TiO2 suspension, we calculate [RuIIL3]mono to be 0.31 molecule/nm2, which corresponds to an average area of 3.2 nm2 occupied by a single RuIIL3 molecule. The sensitizer molecule’s geometric crosssection is 2 nm2 (molecular radius 8 Å) (27). In Figure 4, we also investigated the effect of the added sensitizer concentration, [RuIIL3]i, on the dechlorination rate. While the adsorption data shown together indicate that the coverage of the sensitizer starts to be saturated above [RuIIL3]i ≈ 15 µM, the photoreductive dechlorination rate shows a maximum at [RuIIL3]i ≈ 3 µM, which corresponds to 0.3 monolayer of RuIIL3 on TiO2. Since the light absorption by the sensitized TiO2 particles is proportional to [RuIIL3]ad (32,

FIGURE 5. The pH dependences of the sensitizer adsorption on TiO2 (b) and the photoreductive dechlorination rate of CCl4 (O) are compared. Experimental conditions were [TiO2] ) 0.5 g/L, [RuIIL3]i ) 3 µM, [CCl4] ) 1 mM, N2-saturated. 33), the fact that there exists an optimal surface concentration at a submonolayer coverage implies that other competing processes are affecting the photolytic degradation of CCl4. Competition for the bare TiO2 surface sites between the sensitizer and CCl4 molecules (reactions T1 vs T2) seems to be the most plausible explanation. While the light absorption increases with increasing [RuIIL3]ad, less bare surface sites become available for CCl4 molecules. As the photoreductive dechlorination of CCl4 requires the access of both CCl4 and the sensitizer onto the TiO2 surface, their competition for the surface sites should be compromised at a submonolayer coverage for CCl4 and the sensitizer, respectively. Their adsorbed concentrations, [RuIIL3]ad and [CCl4]ad, should be comparable at a typical experimental condition ([RuIIL3]i ) 3 µM; [CCl4] ) 1 mM) with KRuL3 ∼ 107 M-1 and KCCl4 ∼ 103 M-1 (vide infra). We confirmed their competitive adsorption on TiO2 by measuring [CCl4]eq as a function of [RuIIL3]i: when the added sensitizer concentration increased from 3 to 17 µM, the equilibrated CCl4 concentration in the solution increased by 10-20% due to desorption. A Langmuir-Hinshelwood equation successfully describes the dependence of [CCl4] on the initial dechlorination rates (data not shown)

d[Cl-] kCCl4KCCl4[CCl4] ) dt 1 + KCCl [CCl4]

(7)

4

where kCCl4 is the photoinduced dechlorination rate constant of CCl4 and KCCl4 is the adsorption constant of CCl4 on TiO2. The corresponding constants are kCCl4 ) 1.9 µM min-1 and KCCl4 ) 595 M-1. Hsiao et al. (34) reported a value of KCCl4 ) 1370 M-1 from their photocatalytic degradation study of the CCl4/UV/TiO2 system. pH Effect. The pH dependence of the sensitizer adsorption on TiO2 is largely of the electrostatic nature. A simple electrostatic model is frequently employed to explain the adsorption of charged substrates on metal oxides (35). The surface of TiO2 in aqueous suspensions takes positive charges at pH < 6 and negative charges at pH > 6 (i.e., pHzpc ) 6). The RuII(bpy-(COOH)2)32+ sensitizer used in this study has pKa values of 1-2 for the first and second deprotonation step (36). Therefore, the net charge of the sensitizer molecule is negative under normal pH (> 3) conditions. The pH dependence of [RuIIL3]ad shown in Figure 5 can be explained by the electrostatic interactions between the negatively charged sensitizer molecule and a charged TiO2 surface. The positively charged TiO2 surface at acidic conditions strongly attracts negatively charged sensitizer molecules, while the negatively charged TiO2 surface at basic conditions repels VOL. 35, NO. 5, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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with the back electron transfer. Further studies should be directed into resolving the problem of the slow interfacial electron transfer.

Acknowledgments We appreciate the financial supports from Korea Science and Engineering Foundation (KOSEF Grant No. 98-050203-01-3) and Pohang University of Science and Technology (POSTECH).

Literature Cited

FIGURE 6. The photoreductive dechlorination of CCl4 as a function of the irradiation time in the absence vs the presence of electron donors. them. A similar adsorption behavior of RuIIL3 on TiO2 has been reported (32). Figure 5 also shows the pH dependence of the CCl4 photolysis rate, which exhibits a qualitatively similar behavior to that of the dark adsorption. The dechlorination rate increases with lowering pH to acidic conditions. This correlation between the dark adsorption of the sensitizer and the photoreductive dechlorination rate reassures the fact that the direct contact of the sensitizer with TiO2 surface is the prerequisite condition for the efficient electron injection (21). Sensitizer Regeneration by Electron Donor. The RuIIsensitizer is sacrificial because it is oxidized to RuIII-species after the electron transfer. The regeneration of the sensitizer in the presence of suitable electron donors is a prerequisite for the development of the practical photosensitization system. We tried to use various alcohols and acids as a sacrificial electron donor to regenerate the sensitizer. With no electron donor added, the dechlorination rate decreased due to the depletion of the RuII-species as the reaction proceeded. On the other hand, when 2-propanol was present, the dechlorination rate remained constant for 6 h of irradiation without showing any signs of deceleration (Figure 6). Methanol and acetate were less effective as an electron donor. When humic acid and citrate were added, we observed that most of RuIIL3 desorbed from the TiO2 surface. They competed with RuIIL3 for TiO2 surface sites. Electron donors that have stronger surface affinity than the sensitizer should be avoided or used with caution. 2-Propanol seems to be an efficient electron donor to reduce the RuIIIL3 species in the present system. It was previously reported that 2-propanol was oxidized to acetone by RuIII(bpy)33+ in a photoilluminated RuII(bpy)32+/2-propanol/O2 aqueous solution (37). The following mechanism that involves the formation of R-hydroxyalkyl radical is proposed in order to explain the observed effect of 2-propanol.

RuIIIL3 + (CH3)2CHOH f RuIIL3 + (CH3)2C˙ OH + H+ (8) RuIIIL3 + (CH3)2C˙ OH f RuIIL3 + (CH3)2CO + H+

(9)

An efficient solar detoxification system using visible light depends on the development of durable sensitized photocatalysts. Although the present study demonstrated the potential use of sensitized TiO2 in visible light-induced degradation of pollutants, it also showed that the rate of CB electron transfer to substrates was not fast enough to compete 970

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Received for review May 10, 2000. Revised manuscript received October 26, 2000. Accepted November 2, 2000. ES001245E