Visible Light Photocatalysts Based on Homogeneous and

The surface atomic composition of hetero-SnP was determined by X-ray photoelectron spectroscopy (XPS) (Kratos XSAM 800pci) using the Mg Kα line (1253...
0 downloads 0 Views 457KB Size
Download PDF
J. Phys. Chem. C 2008, 112, 491-499

491

Visible Light Photocatalysts Based on Homogeneous and Heterogenized Tin Porphyrins Wooyul Kim,† Jihee Park,† Hwa Jin Jo,‡ Hee-Joon Kim,‡ and Wonyong Choi*,† School of EnVironmental Science and Engineering, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea, and Department of Applied Chemistry, Kumoh National Institute of Technology, 1 Yangho-dong, Gumi 730-701, Korea ReceiVed: June 18, 2007; In Final Form: September 19, 2007

Visible light photocatalysis using water-soluble tin porphyrin (s-SnP, [Sn(OH2)2(TPyHP)](NO3)6) and waterinsoluble tin porphyrin Sn(OH)2(TPP) (ins-SnP) immobilized on SiO2 (hetero-SnP) was investigated. The visible light photocatalytic activities of s-SnP and hetero-SnP were demonstrated successfully for the degradation of 4-chlorophenol (4-CP) and acid orange 7 (AO7) in water. The visible light activity of hetero-SnP increased with the ins-SnP loading and was saturated above 77 mg/g-SiO2, which corresponded to the homogeneous concentration of [ins-SnP] ) 50 µM. It is the Q band of SnP (500-650 nm) that is photocatalytically active under visible light, not the Soret band (420-430 nm) whose absorption intensity is much higher. When applied to the degradation of 4-CP and AO7, hetero-SnP was particularly stable and could be used repeatedly without losing the activity whereas the activity of s-SnP was reduced gradually with repeated uses. The photocatalytic degradation reactions of 4-CP, AO7, and other organic substrates were studied systematically to show that the operating mechanisms are very different depending on the kind of substrates. The properties and activities of s-SnP and hetero-SnP as visible light photocatalysts were investigated in various ways and discussed in detail.

Introduction Most photocatalysts are heterogeneous oxide semiconductors among which pure and modified TiO2 is the most popular.1,2 Because the photocatalysts based on metal oxides lack visible light activity, alternative visible light photocatalysts are being sought. Visible light absorbing metal-organic complexes are frequently employed for this purpose.3-6 Porphyrins are particularly of interest because they are the synthetic structural analogues of chlorophyll in plant photosynthesis. Porphyrins are excellent photosensitizers because of the small singlet-triplet splitting, the high quantum yield for intersystem crossing, and the long triplet state lifetimes. Porphyrins readily coordinate metal ions in the central cavity, which makes the photoresponse of porphyrin stronger and broader in the visible light region.7,8 The metalloporphyrins such as iron (e.g., heme), magnesium (e.g., chlorophyll), zinc, antimony, and tin porphyrins have been widely investigated for their photoactivity in hydrogen evolution and degradation of organic compounds.8-11 Metalloporphyrins can be oxidized or reduced at three discrete sites: the central metal, the axial ligands, and the porphyrin ring. The porphyrin ring oxidation and reduction are feasible because of the delocalized π electron system and are influenced by the electronic interaction between the π electron system and the central metal.12 Tin(IV) porphyrins (SnP) are usually sixcoordinated with trans-diaxial ligands and stable against the acid-induced demetallation. The high charge on Sn(IV) makes SnP the most easily ring-reduced among all metalloporphyrins.13 Therefore, the excited state of tin porphyrin (SnP*) has a high affinity for electrons to initiate photooxidative reactions, which is exactly opposite to the behavior of well-known zinc(II) * Corresponding author. E-mail: [email protected]; fax: +82-54279-8299. † Pohang Univ. Sci. Technol. ‡ Kumoh Natl. Inst. Technol.

porphyrins. The photooxidative capacity makes SnP an attractive environmental photocatalyst.14-17 However, the homogeneous SnP catalysts need to be immobilized on supports such as silica and zeolite for practical applications.18 By developing and utilizing the heterogenized SnP catalysts, the recovery of the catalyst from water is easier and the unwanted self-reactions of SnP can be inhibited. In addition, the heterogenized catalyst may behave differently from their homogeneous counterpart and need to be studied in detail for practical usage. In this study, we have utilized water-soluble [SnIV(OH2)2TPyHP]6+ (s-SnP) and water-insoluble SnIV(OH)2TPP (ins-SnP) immobilized on SiO2 (hetero-SnP) as illustrated in Scheme 1. The homogeneous s-SnP and heterogenized hetero-SnP were compared for the photocatalytic oxidation of several organic substrates in water under visible light. Although both s-SnP and hetero-SnP are visible-light active, hetero-SnP was far more stable than s-SnP. The properties and the operation mechanisms of SnP photocatalysts are discussed in detail. Experimental Section Materials and Chemicals. The following chemicals were used as test substrates for the photocatalytic oxidation: 4-chlorophenol (4-CP, Sigma), dichloroacetate (DCA,CHCl2CO2Na, Aldrich), dichloromethane (CH2Cl2, Sigma-Aldrich), acid orange 7 (AO7, Aldich), nitrosodimethylamine (NDMA, Aldrich), and 2,4-dichlorophenoxyacetic acid (2,4-D, Aldrich). Other materials and reagents used in this study were silica (SiO2, Aldrich), silver nitrate (AgNO3, Aldrich), 2,5-dimethylfuran (DMF, Aldrich), methanol (J.T.baker), and superoxide dismutase (SOD, Sigma, manganese-containing enzyme, lyophilized), N,N-diethyl-pphenylenediamine (DPD, Aldrich), all of which were of reagent grade and used as received. The BET surface area of SiO2 used as a support was measured to be 192 m2/g. Deionized water

10.1021/jp0747151 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/20/2007

492 J. Phys. Chem. C, Vol. 112, No. 2, 2008

Kim et al.

SCHEME 1: Structures of Water-Soluble Sn Porphyrin (s-SnP), Water-Insoluble Sn Porphyrin (ins-SnP), and Immobilized ins-SnP on SiO2 (hetero-SnP)

used was ultrapure (18 MΩ·cm) and prepared by a Barnstead purification system. Synthesis and Characterization of SnP Photocatalysts. trans-Dihydroxo[5,10,15,20-tetraphenylporphyrinato]tin(IV), Sn(OH)2(TPP), ins-SnP, and trans-dihydroxo[5,10,15,20-tetrakis(4-pyridyl)porphyrinato]tin(IV), Sn(OH)2(TPyP) were prepared according to the reported procedure.19,20 [Sn(OH2)2(TPyHP)](NO3)6, s-SnP, was obtained from Sn(OH)2(TPyP). Sn(OH)2(TPyP) (385.0 mg, 0.50 mmol) was dissolved in 5 mL of 1% nitric acid aqueous solution and acetone (50 mL) was layered over for slow diffusion. After the solution was allowed to stand for 3 days, the crystals were obtained by filtration. Yield: 92%. 1H NMR (200 MHz, D2O) δ 9.88 (s, 8H, Hpyrrole), 9.49 (d, J ) 2.2 Hz, 8H, Hpy-R), 9.26 (d, J ) 4.1 Hz, 8H, Hpy-β). UV-vis (H2O, nm): λmax 407, 519, 550, 590. MS (ESI): m/z 129.1 [M/6+ requires 129.36]. Anal. Calcd. for C40H32N14O20Sn: C, 41.87; H, 2.81; N, 17.09. Found: C, 41.53; H, 2.78; N, 16.91. ins-SnP was immobilized on silica with the following method. ins-SnP (5.7 mg) and silica (0.075 g) were added into CH2Cl2 (15 mL) and shaken for 1 h. This mixture was left for 24 h at room temperature and then in a water bath for 6 h at 60 °C. The typical loading of ins-SnP on SiO2 was 77 mg/g-SiO2. insSnP/silica (hetero-SnP) powder could be well suspended in water at 0.5 g/L whereas ins-SnP itself could neither be dissolved nor suspended in water at all. Immobilized ins-SnP was very stable and not detached from the silica surface into the solution even after continuous stirring for 24 h. The optical absorption spectra of s-SnP (in water) and insSnP (in CH2Cl2) were recorded with a UV-vis spectropho-

tometer (UV2401-PC, Shimadzu). The UV-visible spectra of hetero-SnP powder were obtained with the spectrophotometer equipped with a diffuse reflectance attachment (Shimadzu ISR2200). The surface atomic composition of hetero-SnP was determined by X-ray photoelectron spectroscopy (XPS) (Kratos XSAM 800pci) using the Mg KR line (1253.6 eV) as the excitation source. The binding energies were referenced to the Si 2p line (103.3 eV) in SiO2 and checked against the C 1s line (284.6 eV) originating from surface impurity carbons. The reduction waves of ins-SnP in THF solvent was observed at -0.87 and -1.30 V (vs NHE) from cyclic voltammetry.21 Photolysis and Analysis. The typical concentrations of s-SnP, hetero-SnP, and substrates were 50 µM, 0.5 g/L (77 mg/g-SiO2) and 100 µM, respectively. The aqueous suspension was stirred for 20 min to allow the equilibrium adsorption of substrates on hetero-SnP. A 450-W Xe arc lamp (Oriel) combined with a 10-cm IR water filter and a cutoff filter (λ > 420 nm for visible light irradiation) was used as a light source. A typical incident light intensity was measured using a power meter (Newport 1830-C) and determined to be about 100 mW/cm2 in the wavelength range of 420-550 nm. When the wavelengthdependent photocatalytic activities were investigated, a series of long-pass cutoff filters (λ > 300, 420, 495, 550, and 645 nm) were used. A 30-mL Pyrex reactor was open to the ambient air (air-equilibrated condition) or closed with a rubber septum under continuous purging with nitrogen gas (N2-saturated condition), and stirred magnetically during irradiation. Sample aliquots in the s-SnP and hetero-SnP systems were withdrawn by a 1-mL syringe intermittently during the photoreaction and filtered through a 0.45-µm PTFE filter (Millipore) to remove

Visible Light Photocatalysts

J. Phys. Chem. C, Vol. 112, No. 2, 2008 493

Figure 1. (a) UV-visible absorption spectra of s-SnP (in water) and ins-SnP (in CH2Cl2) and (b) the diffuse reflectance UV-visible spectrum of hetero-SnP (powder) in the region of Soret and Q bands. The intensity of the Soret band was so strong that it was recorded at a much lower concentration (0.5 µM) than the Q band (50 µM). The loading of ins-SnP on silica support was 77 mg/g-SiO2. The ordinate scale in the diffuse reflectance spectrum is expressed in Kubelka-Munk units.

suspended hetero-SnP particles. At least duplicate experiments were carried out under identical conditions to confirm the reproducibility. When the multicycles of photolyses were done to check the stability of SnP photocatalysts, a larger reactor (100 mL) was used to minimize the loss of SnP due to frequent samplings. The analysis of DCA was performed using an ion chromatograph (IC, Dionex DX-120), which was equipped with a Dionex IonPac AS14 (4 × 250 mm2) column and a conductivity detector. The eluent solution was 3.5 mM Na2CO3/1 mM NaHCO3. The concentrations of 4-CP, NDMA, and 2,4-D were quantified using a high-performance liquid chromatograph (HPLC, Agilent 1100 series) equipped with a diode array detector and a ZORBAX 300SB C18 column (4.6 × 150 mm2). The eluent consisted of 0.1% phosphoric acid solution and acetonitrile (80:20 by volume) for 4-CP and NDMA analyses or 0.1% acetic acid solution and acetonitrile (60:40 by volume) for 2,4-D analysis, respectively. The concentration of AO7 was determined spectrophotometrically by monitoring the absorbance of 485 nm. Dissolved organic carbon contents were quantified using a total organic carbon analyzer (TOC, Shimadzu TOCVCSH). Results and Discussion Characterization of s-SnP and hetero-SnP. The absorbance of s-SnP, ins-SnP, and hetero-SnP are compared in Figure 1. The main absorption bands closely coincide among them. The absorption bands of ins-SnP in CH2Cl2 were shown at 420 nm (Soret band) and 519, 559, 600, and 627 nm (Q bands) in agreement with the literature.16,22 The absorption bands of s-SnP in water were at 407 nm (Soret band) and 519, 550, 590 nm (Q bands). The Q bands of ins-SnP in CH2Cl2 and those of heteroSnP powder are very similar, which indicates that ins-SnP was

successfully loaded on SiO2 support with maintaining the porphyrin framework as illustrated in Scheme 1. The slight mismatch in the Q band’s position between ins-SnP and heteroSnP might be due to the porphyrin-porphyrin (π-π) or porphyrin-silica interaction. The absorption bands of s-SnP in water were little affected by pH (1.5-7.0), which indicates that the aggregation of s-SnP is prevented because of the presence of the axial ligands. Planar porphyrins typically form strong π-π aggregates in aqueous environments.23 The Sn content in hetero-SnP was estimated to be 1.85 atom % from the XPS analysis. Figure 2 compares the XPS spectra of hetero-SnP and bare SiO2 in the Sn 3d and Si 2p bands. The Sn 3d5/2 band of hetero-SnP was positioned at 486.9 eV, which is clearly different from Sn(0) (484.9 eV)24 but very close to Sn(II, IV) (486.87 eV).25 Sn(II) and Sn(IV) species differ little in the binding energy,26 but Sn(II) porphyrin is highly airsensitive and unstable in the ambient conditions. The intensity of the Si 2p band in hetero-SnP was reduced slightly (Figure 2b) because of the surface coverage by the ins-SnP layer, but the Si 2p binding energy (103.3 eV)27 in hetero-SnP was not changed compared with bare SiO2. Visible Light Activity of s-SnP and hetero-SnP. The photocatalytic degradation (PCD) rates of 4CP and AO7 with hetero-SnP were measured as a function of the loading of insSnP on SiO2 as shown in Figure 3. The visible light activity of hetero-SnP increased with the ins-SnP loading and was saturated above 77 mg/g-SiO2, which corresponded to the homogeneous SnP concentration of 50 µM. The surface coverage of ins-SnP on SiO2 is estimated to be 0.95 at 77 mg/g-SiO2 on the basis of the SiO2 surface area of 192 m2/g and the molecular crosssection of 3.0 nm2/ins-SnP. This indicates that the silica surface is almost completely covered by ins-SnP at 77 mg/g-SiO2. Therefore, the further loading above this level did not increase

494 J. Phys. Chem. C, Vol. 112, No. 2, 2008

Kim et al.

Figure 4. Photocatalytic degradation of several organic substrates in the presence of (a) s-SnP and (b) hetero-SnP under visible light. The experimental conditions were [s-SnP] ) 50 µM and [hetero-SnP] ) 77 mg/g-SiO2 (equivalent to 50 µM ins-SnP in the homogeneous system), [substrate]0 ) 100 µM for all substrates.

Figure 2. XPS spectra of hetero-SnP and bare silica powder in the (a) Sn 3d and (b) Si 2p band regions. The surface concentration of Sn on hetero-SnP was 1.85 atom %.

Figure 3. Initial removal rates of 4-CP and AO7 under visible light as a function of ins-SnP loading on SiO2. The experimental conditions were [SiO2] ) 0.5 g/L, [AO7]0 ) [4-CP]0 ) 100 µM, pHi ) 5.7 (4CP), and pHi ) 6.0 (AO7).

the activity as shown in Figure 3. The photocatalytic activities of s-SnP and hetero-SnP were also tested for the degradation of other organic compounds. Figure 4 compares the degradation

of 4-CP, AO7, NDMA, DCA, and 2,4-D between the two photocatalytic systems under visible irradiation. Both s-SnP and hetero-SnP systems contain the same number of SnP molecules in the reactor. No substrates were degraded under dark conditions. 4-CP and AO7 were degraded in both homogeneous and heterogenized systems, while NDMA and DCA were not degraded in either. The TOC removal in the hetero-SnP suspension with 4-CP was about 30% in 4 h irradiation, but that of AO7 was negligible. As for AO7, the chromophoric group is selectively destructed but its mineralization cannot be achieved. The photocatalytic activity of SnP seems to be very substrate-specific. The TOC analysis could not be done for the s-SnP reaction because the dissolved s-SnP itself contributes to TOC. It is interesting that the relative order of the degradation rate among the tested substrates is different between the homogeneous and heterogenized systems. In particular, the degradation of 4-CP was markedly retarded with hetero-SnP while that of AO7 was similar in both systems. As a result, the relative PCD order between 4-CP and AO7 was reversed. 2,4-D could be degraded slowly with s-SnP but not at all with heteroSnP. Such a complex behavior seems to be related with the catalyst-substrate interaction. s-SnP and ins-SnP are very different in the molecular charge and the substrate-SnP interaction should be influenced by the different electrostatic force. The presence of silica surface in hetero-SnP may significantly influence the reaction mechanism as well. SnP has a series of visible absorption bands in the 400-650 nm region. To identify which band is photocatalytically active,

Visible Light Photocatalysts

J. Phys. Chem. C, Vol. 112, No. 2, 2008 495 were also investigated by changing the cutoff filters, and results similar to Figure 5 were obtained (data not shown). The photoexcitation of SnP and the subsequent reactions are known to proceed as follows.17,18 Scheme 2 illustrates the reaction path as well.

SnP + hV f 1SnP* (excitation) 1

ISC

SnP* 98 3SnP* (intersystem crossing: ISC)

(2)

SnP* + O2 f SnP + 1O2 (energy transfer)

(3)

3

3

(1)

SnP* + D f SnP- + D‚+ (oxidative electron transfer) (4)

SnP- + O2 f SnP + O2‚- (reductive electron transfer) (5) Because the reduction potential for the (SnP/SnP-) couple is -0.87 VNHE for ins-SnP, the reduction potential of the excited ins-SnP (SnP*/SnP-) is estimated to be 1.10 VNHE on the basis of the following equation

E0(SnP*/SnP-) ) E0(SnP/SnP-) + ∆E(HOMO-LUMO)/q

Figure 5. Photocatalytic degradation of (a) 4-CP and (b) AO7 with hetero-SnP under different irradiation wavelengths (controlled by a set of long-pass filters transmitting λ > 300, 420, 495, 550, or 645 nm). (c) The diffuse reflectance spectrum of hetero-SnP is compared with the transmittance profiles of the cutoff filters. The experimental conditions were [hetero-SnP] ) 77 mg/g-SiO2, [SiO2] ) 0.5 g/L, [4CP]0 ) [AO7]0 ) 100 µM, pHi ) 5.7 (4-CP), and pHi ) 6.0 (AO7).

we compared the PCDs of 4-CP and AO7 with hetero-SnP under different irradiation wavelengths that were controlled by changing the cutoff filters (see Figure 5). When the Soret band excitation was inhibited by blocking the wavelengths below 495 nm, the PCD activity was only moderately (or little) reduced. This indicates that the Soret band is not mainly responsible for the photocatalytic activity of SnP. However, the PCD activity was completely quenched when the activation of the Q bands was blocked under the irradiation of λ > 645 nm. This concludes that the photocatalytic activity of hetero-SnP is initiated mainly by the Q-band excitation. The PCDs of 4-CP or AO7 with s-SnP

(6)

where ∆E(HOMO-LUMO) refers to the lowest energy gap between the ground and excited ins-SnP. The value of ∆E(HOMO-LUMO) corresponding to the excitation of the lowest-energy Q band is determined to be 1.97 eV from the intersection point between the normalized excitation and emission spectrum of ins-SnP in THF. However, it should be kept in mind that the above reduction potentials of the ground and excited SnP are estimated in THF solvent and that the corresponding potentials in water could be quite different. The energetics of reactions 4 and 5 is also illustrated in Scheme 2, which shows that the oxidizing power of SnP* is strong enough to abstract an electron directly from 4-CP [E0(4-CP‚+/4-CP) ) 0.8 VNHE]28 and the reduced SnP (SnP-) can be reoxidized by O2. The SnP catalyst is recycled through reactions 1-5 in the presence of O2, but the recycling should be hindered in deaerated conditions (i.e., paths 3 and 5 blocked in Scheme 2). Stability of s-SnP and hetero-SnP. To test the photostability of s-SnP and hetero-SnP, we repeated the PCDs of 4-CP and AO7 under visible light using the same catalyst as shown in Figure 6. In both cases, hetero-SnP is much more stable than s-SnP. The visible light activity of hetero-SnP was maintained without marked deactivation up to 5-10 repeated cycles, whereas that of s-SnP was reduced gradually with each cycle repeated. To monitor the stability of s-SnP, we measured the Q-band absorbance at 550 nm as a function of the irradiation time. Figure 7a shows that the photostability of s-SnP sensitively depends on the reaction conditions. In the absence of 4-CP, s-SnP in air-equilibrated solution was not degraded at all under visible light but it was degraded in N2-saturated solution. With 4-CP present, the Q-band reduction was markedly enhanced. After photoexcitation, 3SnP* can be efficiently quenched by O2 (reaction 3) but it may undergo a self-destructive reaction (3SnP* + SnP f degradation: similar to reaction 4) in the absence of O2. The porphyrin framework should be susceptible to the oxidative degradation. Such a self-destructive reaction explains why the Q-band absorbance was reduced gradually in N2-saturated solution of s-SnP and the photocatalytic activity of s-SnP was reduced gradually with repeated uses (Figure 6). When 4-CP is present as a substrate, the reaction 4 path is available with accompanying the conversion of SnP into SnP-. The observation that the shape of the Q band was changed after the visible light irradiation in the presence of 4-CP (see Figure

496 J. Phys. Chem. C, Vol. 112, No. 2, 2008

Kim et al.

Figure 6. Repeated cycles of 4-CP degradation with (a) s-SnP and (b) hetero-SnP; AO7 degradation with (c) s-SnP and (d) hetero-SnP under visible light. The experimental conditions were [4-CP]0 ) 50 µM, [s-SnP] ) 10 µM, [hetero-SnP] ) 77 mg/g-SiO2 for a and b; [AO7]0 ) 50 µM, [s-SnP] ) 10 µM, and [hetero-SnP] ) 15 mg/g-SiO2 for c and d. A 100-mL Pyrex reactor was used in this experiment.

SCHEME 2: Major Photosensitization Pathways and the Relative Energy States for ins-SnP

7b) indicates that the mechanism of Q-band reduction is different between the two cases (with and without 4CP). Alternatively, ins-SnP immobilized on silica is less likely to undergo the bimolecular self-reaction, which explains why the photocatalytic activity of hetero-SnP was not reduced with repeated uses. The diffuse reflectance UV-visible spectrum of hetero-SnP did not show any notable change before and after the photolysis with 4-CP (data not shown). Photodegradation Mechanisms of 4-CP and AO7 with hetero-SnP. As mentioned above, the excited SnP may undergo a series of energy-transfer and electron-transfer reactions (reactions 1-5), generating reactive oxygen species (ROS) such as singlet oxygen and superoxide. The degradation of organic

compounds can be initiated indirectly by ROS or directly by 3SnP*. The PCD mechanisms of 4-CP and AO7 were investigated by carrying out the reaction in the presence of various probing reagents. Figure 8 shows that the PCD behaviors of 4-CP and AO7 respond very differently to the change of the reaction conditions. First of all, the effects of dissolved O2 are very different. The absence of O2 (under N2 saturation condition) only moderately reduced the PCD of 4-CP but seriously inhibited the degradation of AO7. This implies that the degradation of 4-CP is initiated by the direct electron transfer (reaction 4) while that of AO7 is mediated by ROS. Such different oxidation mechanisms between 4-CP and AO7 may explain why the degradation of 4-CP with hetero-SnP was

Visible Light Photocatalysts

J. Phys. Chem. C, Vol. 112, No. 2, 2008 497

Figure 7. (a) Reduction of the Q-band absorbance (550 nm) of s-SnP as a function of the visible lithe irradiation time. The irradiated s-SnP solutions with or without 4-CP and O2 are compared. (b) The absorption spectra of s-SnP solutions irradiated for 60 min under the condition of a are compared. The experimental conditions were [4CP]0 ) 100 µM and [s-SnP] ) 50 µM.

markedly slower while that of AO7 was similar in both homogeneous and heterogenized systems (see Figure 4). That is, the photooxidation of 4-CP requires direct contact with the surface and should be diffusion-limited whereas that of AO7 is initiated by ROS off the surface. Although reaction 4 does not involve O2, the dissolved oxygen is essentially needed to recycle SnP (reaction 5). Figure 9 shows that the cycles of 4-CP PCD can be repeated without losing the activity in the presence of O2 but they cannot without O2. In the absence of O2, SnPaccumulates as the reaction proceeds and the overall activity should decrease gradually. Therefore, when Ag+ ions were added as an alternative electron acceptor (E0(Ag+/Ag) ) 0.8 VNHE) in the N2-saturated suspension of hetero-SnP, the PCD rate of 4-CP was markedly enhanced (see Figure 8). This is because the fast recycling of SnP is enabled by reaction 7.

SnP- + Ag+ f SnP + Ag0

(7)

The silver deposition on silica changed the color of hetero-SnP after the photoreaction. The Ag+-enhanced effect was also observed for the anoxic degradation of AO7. Because ROS cannot be generated in the absence of dissolved O2, the degradation of AO7 in the N2/Ag+ condition seems to follow a different pathway. The fact that AO7 can be degraded in the

Figure 8. Effect of various reagents on the visible light-induced degradation of (a) 4-CP and (b) AO7 with hetero-SnP under visible light. The experimental conditions were (a) [4-CP]0 ) 100 µM, (b) [AO7]0 ) 100 µM, [hetero-SnP] ) 77 mg/g-SiO2, and [SiO2] ) 0.5 g/L.

presence of Ag+ ions in the visible light-illuminated solution was reported previously: the excited dye reduces the silver ions with generating silver particles (reaction 8).29

AO7* + Ag+ f AO7•+ + Ag0

(8)

Therefore, the role of Ag+ ions in the anoxic degradation of AO7 appears to be different from that in the degradation of 4-CP. The possible roles of ROS such as singlet oxygen, superoxide, and hydroxyl radical were examined by using appropriate scavengers of these species. 2,5-dimethylfuran (DMF), superoxide dismutase (SOD), and methanol were used as the scavengers of singlet oxygen (k(DMF + 1O2*) ) 6.8 × 108 M-1s-1),30,31 superoxide, and hydroxyl radical (k(MeOH + OH) ) 9.8 × 108 M-1s-1),32,33 respectively. Their effects on the PCDs of 4-CP and AO7 are also shown in Figure 8. Because each scavenger may interfere with other reactions, the interpretation of the result may not be straightforward. First, the addition of methanol reduced the PCD rate of 4-CP, which is ascribed to either scavenging of OH radicals by methanol or the competition with 4-CP for the electron transfer (reaction 4). However, the generation of OH radicals is not possible because the oxidizing power of SnP* (E0(SnP*/SnP-) ) 1.1 VNHE) is not strong enough to generate them (E0(OH‚/OH-) ) 1.9 VNHE). In addition, the fact that the activity of hetero-SnP was maintained with the repeated uses indicates the absence of

498 J. Phys. Chem. C, Vol. 112, No. 2, 2008

Kim et al.

Figure 10. In situ production of H2O2 during the photoreaction of hetero-SnP with or without 4-CP. The experimental conditions were [4-CP]0 ) 100 µM, [hetero-SnP] ) 77 mg/g-SiO2, and [SiO2] ) 0.5 g/L.

Figure 9. Repeated cycles of 4-CP degradation with hetero-SnP in (a) air-saturated and (b) N2-saturated suspension under visible light. The experimental conditions were [4-CP]0 ) 100 µM, [hetero-SnP] ) 77 mg/g-SiO2 and [SiO2] ) 0.5 g/L. A 30-mL Pyrex reactor was used in this experiment while a 100 mL reactor was used in the test of Figure 6.

OH radicals, which would have degraded SnP itself during the photoreaction. Therefore, the methanol effect in the hetero-SnP /4-CP system should be related to its role as a competing electron donor that efficiently reacts with 3SnP*. Alternatively, the addition of methanol had no effect on the degradation of AO7, which reassures that the degradation of AO7 is initiated by ROS such as O2‚- and 1O2, not by the direct electron transfer (reaction 4). Second, adding excess DMF retarded the PCDs of both 4-CP and AO7, and its role could be either the scavenger of the singlet oxygen or the competing electron donor like methanol. The role of DMF as a competing electron donor (3SnP* + DMF: reaction 4) should be insignificant in AO7 degradation because methanol (electron donor) had no effect. The role of the singlet oxygen seems to be important. When [DMF] increased from 5 to 10 mM, the degradation of AO7 was further inhibited. Third, when SOD was added, the PCD rates of 4-CP and AO7 also decreased by about 25-30% (not shown in Figure 8), which might be related to the superoxide scavenging. However, SOD may also interfere directly with 3SnP* and the SOD-induced inhibition cannot be related solely to superoxide scavenging. Because the generation of superoxides is followed by the production of H2O2 (reaction 9)

2O2‚- + 2H+ f H2O2 + O2

(9)

the in situ production of H2O2 was monitored by the DPD method34 during the photoreaction of hetero-SnP. Figure 10 clearly shows that H2O2 was generated under the visible light irradiation of hetero-SnP suspension and its production was highly enhanced in the presence of 4-CP. This observation strongly supports that the PCD of 4-CP follows the pathways of reactions 4 and 5. Overall, the photocatalytic reaction mechanism of SnP seems to depend sensitively on the kind of substrates. Because the generation of OH radicals (nonselective oxidant) cannot be induced, the SnP-mediated photocatalytic oxidation is selective because of the moderate oxidation power and the level of mineralization is limited. Whether a substrate follows the 4CPtype (direct) pathway or the AO7-type (indirect) pathway should depend on many factors such as the redox potential of the substrate, the electrostatic/molecular interaction between the substrate and SnP, and the reactivity of the substrate with a specific ROS involved in the reaction. Both the direct and indirect pathways may proceed simultaneously. Conclusions A photocatalyst that is active under visible light is of paramount importance as an essential element of solar photoenergy utilization. Diverse approaches are being made in search of visible light active photocatalysts. In this study, SnP with strong visible light absorption was explored as an efficient visible light photocatalyst for oxidative degradation. We tested the photocatalytic activities of water-soluble homogeneous SnP (s-SnP) and the heterogenized SnP (hetero-SnP) and compared them for the degradation of several organic compounds in water. Although their visible light photocatalytic activities were demonstrated successfully in both homogeneous and heterogenized systems, hetero-SnP was far more stable than s-SnP. The stability of hetero-SnP is ascribed to the fact that the selfdestructive reaction between SnP molecules is inhibited when they are immobilized on the surface of silica. As for the photoactivity, it is Q bands (not Soret bands) that are responsible for the visible light activity. The photocatalytic reaction of the SnP photocatalyst is initiated by not only the direct electron transfer between 3SnP* and organic substrates but also the ROS generated from 3SnP*. For example, this study showed that 4-CP was degraded via the former path and AO7 via the latter path. Both singlet oxygen and superoxide seem to be involved in the

Visible Light Photocatalysts oxidation mechanism. The photooxidative degradation induced by SnP photocatalysts is limited and selective because hydroxyl radicals cannot be generated. The operating mechanism of SnP photocatalysts seems to be unique in many aspects in comparison with semiconductor photocatalysts and other sensitized photocatalysts. The development of porphyrin-based photocatalysts provides an alternative approach in harnessing solar visible light. More studies on porphyrin photocatalysts are needed for practical applications. Acknowledgment. This work was supported by the KOSEF Nano R&D program (Grant No. 2005-02234), the SRC/ERC program of MOST/KOSEF (Grant No. R11-2000-070-060040), and BK21 program. H.-J.K. and H.J.J. appreciate the financial support from the Korea Research Foundation (KRF2005-202-C00184) and the Program for the Training of Graduate Students in Regional Innovation conducted by the Korean Ministry of Commerce, Industry and Energy. References and Notes (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (2) Choi, W. Catal. SurV. Asia 2006, 10, 16. (3) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (4) Gust, D. M.; Moore, T. A. In The Porphyrin Handbook; Kardish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 8, p 153. (5) Cho, Y.; Choi, W.; Lee, C.-H.; Hyeon, T.; Lee, H.-I. EnViron. Sci. Technol. 2001, 35, 966. (6) Bae, E.; Choi, W. J. Phys. Chem, B 2006, 110, 14792. (7) Hopf, F. R. W., D. G. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: New York, 1975; Chapter 16. (8) Shiragami, T.; Matsumoto, J.; Inoue, H.; Yasuda, M. J. Photochem. Photobiol., C 2005, 6, 227. (9) Bhugun, I.; Lexa, D.; Save´ant, J.-M. J. Am. Chem. Soc. 1996, 3982. (10) Shiragami, T.; Shimizu, Y.; Hinoue, K.; Fueta, Y.; Nouhara, K.; Akazaki, I.; Yasuda, M. J. Photochem. Photobiol., A 2003, 156, 115. (11) Sakamoto, M.; Kamachi, T.; Okura, I.; Ueno, A.; Mihara, H. Biopolymers 2001, 59, 103.

J. Phys. Chem. C, Vol. 112, No. 2, 2008 499 (12) Lemke, F. R.; Lorenz, C. R. Recent Res. DeV. Electroanal. Chem. 1999, 1, 73. (13) Arnold, D. P.; Blok, J. Coord. Chem. ReV. 2004, 248, 299. (14) DeRosa, M. C.; Crutchley, R. J. Coord. Chem. ReV. 2002, 233234, 351. (15) Wang, S.; Tabata, I.; Hisada, K.; Hori, T. Dyes Pigments 2002, 55, 27. (16) Wang, S.; Tabata, I.; Hisada, K.; Hori, T. J. Porphyrins Phthalocyanines 2003, 7, 199. (17) Monteiro, C. J. P.; Pereira, M. M.; Azenha, M. E.; Burrows, H. D.; Serpa, C.; Arnaut, L. G.; Tapia, M. J.; Sarakha, M.; Wong-Wah-Chung, P.; Navaratnam, S. Photochem. Photobiol. Sci. 2005, 4, 617. (18) Guern, F. L.; Bied-Charreton, C.; Faure, J. Bull. Soc. Chim. Fr. 1993, 130, 753. (19) Crossley, M. J.; Thordarson, P.; Wu, R. A.-S. J. Chem. Soc., Perkin Trans. 2001, 1, 2294. (20) Jo, H. J.; Jung, S. H.; Kim, H.-J. Bull. Korean Chem. Soc. 2004, 25, 1869. (21) Kim, H. J.; Jeon, W. S.; Lim, J. H.; Hong, C., S.; Kim, H.-J. Polyhedron 2007, 26, 2517. (22) O’Rourke, M.; Curran, C. J. Am. Chem. Soc. 1970, 92, 1501. (23) Song, X. Z.; Jia, S. L.; Miura, M.; Ma, J. G.; Shelnutt, J. A. J. Photochem. Photobiol., A 1998, 113, 233. (24) Lin, A. W. C.; Armstrong, N. R.; Kuwana, T. Anal. Chem. 1977, 49, 1288. (25) Bancroft, G. M.; Adams, I.; Lampe, H.; Sham, T. K. J. Electron Spectrosc. Relat. Phenom. 1976, 9, 191. (26) Baronetti, G. T.; de Miguel, S. R.; Scelza, O. A.; Castro, A. A. Appl. Catal. 1986, 24, 109. (27) Moulder, J. F. S., W. F.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corperation: Eden Prairie, MN, 1992; p 57. (28) Stafford, U.; Gray, K. A.; Kamat, P. V. J. Phys. Chem. 1994, 98, 6343. (29) Kyung, H.; Lee, J.; Choi, W. EnViron. Sci. Technol. 2005, 39, 2376. (30) Zepp, R. G.; Schlotzhauer, P. F.; Sink, R. M. EnViron. Sci. Technol. 1985, 19, 74. (31) Gollnick, K.; Knutzen-Mies, K. J. Org. Chem. 1991, 56, 4017. (32) Young, R. H. B., D.R. In Singlet Oxygen, Reactions with Organic Compounds and Polymers; Ranby, B., Rabek, J. F., Eds.; John Wiley & Sons: New York, 1976; p 36-47. (33) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513. (34) Bader, H.; Sturzenegger, V.; Hoigne, J. Water Res. 1988, 22, 1109.