Mechanism of Photodecomposition of H2O2 on TiO2 Surfaces under

May 23, 2001 - Photodecomposition of hydrogen peroxide on TiO2 surfaces under visible irradiation (λ > 420 nm) was investigated, and an interesting r...
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Langmuir 2001, 17, 4118-4122

Mechanism of Photodecomposition of H2O2 on TiO2 Surfaces under Visible Light Irradiation Xiangzhong Li, Chuncheng Chen, and Jincai Zhao* Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100101, China Received January 8, 2001. In Final Form: March 29, 2001 Photodecomposition of hydrogen peroxide on TiO2 surfaces under visible irradiation (λ > 420 nm) was investigated, and an interesting result was found that small organic compounds such as salicylic acid were degraded in the presence of TiO2 and H2O2 under visible irradiation, although either TiO2 or H2O2 alone cannot absorb visible light to decompose H2O2 or small organic compounds. Furthermore, the effects of H2O2 concentration, pH, and the wavelength of irradiation light on the degradation of H2O2 were studied. The photodegradation of H2O2 occurred via a zero-order process, and •OH radicals were detected by electron paramagnetic resonance experiments in visible-light-irradiated aqueous TiO2 suspensions. A reaction mechanism for the decomposition of H2O2 on the TiO2 surfaces is proposed involving a photoinduced electron transfer of the surface complexes of tTiIV-OOH.

Introduction Hydrogen peroxide widely exists in natural water including clouds, rainwater, freshwater, and seawater.1-3 Study on the formation and decay of H2O2 in natural water is of considerable importance for understanding the circulation of the environmental substances and removing pollutants in the natural environment. In recent years, much interest has been attracted to investigating the decomposition of H2O2 in water, in particular, under illumination conditions. The processes relevant to the decay of H2O2 have been summarized as follows: (I) direct dissociation of H2O2 to •OH through absorbing ultraviolet light (λ < 320 nm);4 (II) Fenton or Fenton-like reactions with iron ion5 or other metal ions6 in the dark or under irradiation to generate active oxygen species such as •OH or •OOH; (III) catalytic decomposition on the surface of metal oxides (such as Fe2O3)7 or photocatalytic decomposition on the surface of semiconductors (such as ZnO, TiO2) under UV irradiation.8 Some studies on the formation and decomposition of H2O2 in an aqueous TiO2 dispersion under UV irradiation have been reported,9-12 and several mechanisms have been proposed to explain hydrogen peroxide formation and * To whom correspondence may be addressed. Present address: Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, China. Fax: +86-10-6487-9375. E-mail: jczhao@ ipc.ac.cn. (1) Zika, R. G.; Cooper, W. J. Photochemistry of Environmental Aquatic System; Zepp, R. G., Skurlatov, Y. I., Pierce, J. T., Eds.; American Chemical Society: Washington, DC, 1985; p 215. (2) Cooper, W. J.; Zika, R. G. Science 1983, 220, 171. (3) Heikes, B. G.; Kok, G. L.; Walega, J. L.; Lazrus, A. L. J. Geophys. Res., D: Atmos. 1987, 92, 915. (4) Herrera, F.; Kiwi, J.; Lopez, A.; Nadtochenko, V. Environ. Sci. Technol. 1999, 33, 3145. (5) Pignatello, J. J. Environ. Sci. Technol. 1992, 26, 944. (6) Meisel, D.; Levanon, H.; Czapski, G. J. Phys. Chem. 1974, 78, 779. (7) Lin, S.; Gurol, M. D. Environ. Sci. Technol. 1998, 32, 1417. (8) Ilisz, I.; Foglein, K.; Dombi, A. J. Mol. Catal. A: Chem. 1998, 135, 55. (9) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Environ. Sci. Technol. 1988, 22, 798. (10) Harbour, J. R.; Tromp, J.; Hair, M. L. Can. J. Chem. 1985, 63, 204. (11) Salvador, P.; Decker, F. J. Phys. Chem. 1984, 88, 6116. (12) Hoffmann, A. J.; Carraway, E. R.; Hoffmann, M. Environ. Sci. Technol. 1994, 28, 776.

decomposition. Additionally, investigation on the effect of H2O2 on the TiO2 photocatalytic reaction was also carried out under UV light irradiation, which focused mainly on attempting to enhance the efficiency in the removal of organic pollutants by adding H2O213,14 through reducing recombination of the conduction band electron and the valance band hole. The kinetic behaviors of H2O2 in UVirradiated aqueous TiO2 suspensions were studied and found to obey zero-order kinetics.15 However, the above studies on the formation and decomposition of hydrogen peroxide have been limited only to the UV light irradiation. It is well-known that there are valance-unfilled Ti(IV) ion centers and O(II) centers which form basic tTiOH and acidic tOH on the TiO2 particle surface in aqueous solution, respectively.16 In the presence of H2O2, the -OOH groups of H2O2 substitute for the -OH groups in the basic tTiOH forming yellow surface complexes. The diffuse reflectance absorption spectra of H2O2-adsorbed TiO2 proved the generation of the surface complexes and displayed evident red shift even extending to the visible region compared with TiO2.17 The more the H2O2 molecules adsorbed on the TiO2 surface, the larger the magnitude of the red shift was.17 Recently, our research group has observed the formation of H2O2 in aqueous dye/TiO2 dispersions under visible light irradiation and proposed a formation mechanism of H2O2 (see eqs 1-5).18,19 At the same time, we also first reported simply the decomposition of H2O2 in an aqueous TiO2 dispersion under visible irradiation.18 But the detailed decomposition mechanism of H2O2 has not been clear yet, and few studies on visible-light-induced interface photochemical reaction in H2O2/TiO2 dispersions have been reported. It is possible that some reactive radicals could be produced in the photodecomposition of hydrogen (13) Wei, T.; Wang, Y.; Wan, C. J. Photochem. Photobiol., A 1990, 55, 115. (14) Auguliaro, V.; Davi, E.; Palmisano, L.; Schiavello, M.; Sclafani, A. Appl. Catal. 1990, 65, 101. (15) Jenny, B.; Pichat, P. Langmuir 1991, 7, 947. (16) Regazzoni, A. E.; Mandelbaum, P.; Matsuyoshi, M.; Schiller, S.; Bilmes, S. A.; Blesa, M. A. Langmuir 1998, 14, 868. (17) Boonstra, H.; Mutsaers, C. H. A. J. Phys. Chem. 1975, 79, 1940. (18) Wu, T.; Liu, G.; Zhao, J.; Hidika, H.; Serpone, N. J. Phys. Chem. B 1999, 103, 4862. (19) Liu, G.; Wu, T.; Zhao, J.; Hidaka, H.; Serpone, N. Environ. Sci. Technol. 1999, 33, 2081.

10.1021/la010035s CCC: $20.00 © 2001 American Chemical Society Published on Web 05/23/2001

Photodecomposition of H2O2 on TiO2 Surfaces

Langmuir, Vol. 17, No. 13, 2001 4119

peroxide in an aqueous TiO2 dispersion under visible irradiation.

dye + hν f dye*

(1)

dye* + TiO2 f ecb-(TiO2) + dye•+

(2)

ecb- + O2ad f O2•-

(3)

O2•- + ecb- + 2H+ f H2O2

(4)

2O2•- + 2H+ f H2O2 + O2

(5)

In this paper we further investigate the photodecomposition of H2O2 on the TiO2 surface under visible irradiation to reveal the photodecomposition mechanism of H2O2 utilizing UV-vis, electron paramagnetic resonance (EPR), and SPS (surface photovoltage spectra) techniques. We also found that organic compounds (such as salicylic acid) can be effectively degraded concomitantly with the photodecomposition of H2O2 in the TiO2 dispersion under visible light irradiation. The results provide us with a better understanding of the TiO2 photocatalytic pathway and interface reactions of organic compounds on the TiO2 surface under visible irradiation. Experimental Section Materials. TiO2 (P25: 80% anatase, 20% rutile; 50 m2/g) was kindly supplied by Degussa Co. Hydrogen peroxide (30%, analytical grade) was purchased from Beijing Chemical Industry, Horeradish peroxidase (POD) was obtained from Huamei Bio. Co. (China), N,N-diethyl-p-phenylenediamine (DPD) from Merck (p.a.), and 5,5-dimethyl-1-pyrroline (DMPO) from Sigma. All other chemicals were of analytical grade. Deionized and double distilled water was used throughout all experiments. Photoreactor and Light Source. A 500-W halogen lamp (Institute of Electric Light Source, Beijing) as the visible light source was put in a cylindrical glass vessel with a recycling glass water jacket to avoid overheating. A cutoff filter was placed outside the water jacket to remove wavelengths below 420 nm to ensure irradiation completely by visible light. Additionally, irradiation at 416, 437, 450, 500, and 550 nm was carried out with a series of light filters ((5 nm bandwidth) made by Shigoma Kouki Co., Japan. Procedures and Analyses. An aqueous TiO2 suspension was prepared by adding TiO2 nanoparticulates to a 50 mL solution containing H2O2 and stirring in the dark for 30 min under constant air-equilibrated conditions to reach an adsorption/ desorption equilibrium. The concentration of H2O2 at various time intervals was determined by the DPD method20 (a photometric method) with a detection limit of 1 × 10-7 mol/L in which the DPD reagent is oxidized by H2O2 based on the peroxidasecatalyzed reaction (λmax ) 551 nm,  ) 21 000 M-1 cm-1). Prior to analysis, TiO2 particulates were removed by centrifuging and filtering through a Millipore filter (0.22 µm pore size). SPS measurements were carried out with a solid-junction photovoltaic cell of indium tin oxide (ITO)/sample/ITO using a light source monochromator lock-in detection technique from a 500-W xenon lamp through a double-prism momochromator (Hilger and Watts, model D300). An amplifier (Brookdeal, 9503SC), synchronized with a light chopper, was utilized to amplify the photovoltage signals. The diffuse reflectance absorption spectra were measured using a Hitachi U-3010 spectrophotometer with an integrating sphere. The samples for measuring SPS and diffused reflection absorption spectra were prepared as follows: after adding TiO2 to H2O2 solution and stirring for 30 min, the TiO2 powder was taken out and dried under vacuum conditions at 30 °C constant temperature. (20) Bader, H.; Sturzenegger, V.; Hoigne, J. Water Res. 1988, 22, 1109.

Figure 1. Photodecomposition of H2O2 at different initial concentrations under visible irradiation in aqueous TiO2 dispersions (0.5 g/L) at pH ) 3. EPR signals of radicals trapped by DMPO were detected with a Brucker ESP 300 E spectrometer using the same irradiation source as that in the photodecomposition experiments (λ > 420 nm). Photoefficiencies (ξ) of photodecomposition of H2O2 in an aqueous TiO2 dispersion under visible light irradiation with different wavelengths were determined at 416, 450, and 500 nm. The photon flow of the 500-W halogen lamp at 416, 450, and 500 nm was 1.7 × 10-9, 2.0 × 10-9, and 2.5 × 10-9 einstein s-1, respectively, determined by the Reinechate actinometry method.21

Results and Discussion 1. The Photodecomposition of H2O2. Figure 1 shows the decomposition of H2O2 in an aqueous TiO2 suspension at different initial H2O2 concentrations under visible light irradiation (λ > 420 nm). In control experiments H2O2 could not decompose under visible light irradiation without TiO2 or in the dark in the presence of TiO2. Under visible irradiation and in the presence of TiO2, the concentration of H2O2 decreased linearly with the reaction time, indicating that the decomposition of H2O2 followed a zeroorder kinetics (eq 6) similar to that under UV irradiation. The initial rate of H2O2 decomposition (r0 ) k0) at pH ) 3.0 was about (2.5-4.0) × 10-9 M s-1 under different initial H2O2 concentrations.

-

d[H2O2] ) k0 dt

(6)

However, neither TiO2 nor H2O2 alone can absorb visible light (λ > 420 nm) to initiate the decomposition of H2O2 directly. When TiO2 was replaced with SiO2 under visible irradiation, H2O2 could not decompose although the adsorption amount of H2O2 did not decrease, which precluded the possibility resulting from adsorption and proved that the semiconductor nature was necessary for the decomposition of H2O2. So the mechanism of H2O2 decomposition under visible light irradiation was different from that under UV irradiation in the presence of TiO2. A plausible explanation on the H2O2 decomposition under visible irradiation is that the interaction between H2O2 and TiO2 leads to forming surface complexes that extend the photoresponse of TiO2 and can make the system absorb visible light. The decomposition of H2O2 in an aqueous TiO2 dispersion at different pH values under visible irradiation was carried out. As shown in Table 1, it can be found evidently that the pH value influences both the adsorption amount (21) Wegner, E. E.; Adamson, A. W. J. Am. Chem. Soc. 1966, 88, 394.

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Figure 2. Diffuse reflectance absorption spectra of TiO2 powder in the absence (spectrum 1) or presence (spectrum 2) of H2O2 (5 × 10-3 mol/L). Inset: differential diffuse reflectance spectrum of surface titanium(IV) hydrogen peroxide complexes.

Figure 3. Surface photovoltage spectra (SPS) of pure TiO2 and TiO2 pretreated with H2O2. Inset: dotted line is the simulated spectra of SPS of TiO2 pretreated with H2O2.

Table 1. Adsorption and Initial Photodecomposition Rate of H2O2 at Different pH Values under Visible Irradiation

photovoltage threshold value can actually attain to about 375 nm responding to the band gap (3.31 eV) of TiO2. After TiO2 powder was treated with H2O2, the amount of the surface states on the TiO2 surface changed greatly and the surface photovoltage spectrum became broader to extend to the longer wavelength range. The inset of Figure 3 shows the simulation spectra (dot line) of the curve 2. Two photovoltaic responses at 334 and 370 nm could be identified, corresponding to the transition from the valance band to the conduction band of TiO2 and the transition from the surface complex to the conduction band of TiO2, respectively. Compared with the spectra of untreated TiO2, a new photoresponse peak at 370 nm appeared and the photovoltage threshold value exceeded 400 nm, which was attributed to the occurrence of the transition from the surface complex of H2O2 with TiO2 to the conduction band of TiO2. 3. The Effect of Different Light Wavelengths on the H2O2 Decomposition. To clarify the mechanism of H2O2 decomposition, the effect of different wavelength light on the H2O2 decomposition was examined. Although the initial rates of H2O2 decomposition were different under different wavelength light irradiation, the reaction kinetics was still in agreement with zero-order process. The photoefficiencies (ξ), as defined by ξ ) N/I0 (N, decomposed H2O2 molecules, molecules/s; I0, volumetric flux of photons, einstein/s), for the H2O2 decomposition under visible light irradiation at different wavelengths were measured, respectively. Figure 4 shows the initial rate (r0, dark cycle) and the photoefficiency (ξ, white cycle) of H2O2 decomposition under visible light irradiation at different wavelengths. The initial rate and the photoefficiency of H2O2 decomposition decreased with increasing irradiation light wavelength and tended to zero at λ g 550 nm. The surface complexes cannot generate photoresponse at λ g 550 nm irradiation light, and hence the H2O2 molecules do not decompose any more. The tendency of the photoefficiency for the H2O2 decomposition is well in accordance with the diffused reflection absorption spectra in Figure 2 which further suggests that the formation of surface complexes extends the photoresponse of TiO2 to the visible region. 4. The Photooxidation of Salicylic Acid in the H2O2/ TiO2 System under Visible Irradiation. Whether the oxidation/reduction reaction of organic compounds can occur under visible irradiation of the H2O2/TiO2 system is of great importance. Salicylic acid was selected as a target pollutant and its photooxidation behaviors in H2O2/ TiO2 dispersion were studied in this work. As illustrated

[H2O2]0 (M)

pH

adsorption (molecules/cm2)

initial rate, r0 (M s-1)

1.0 × 10-4 1.0 × 10-4 1.0 × 10-4

2.0 3.0 5.4

5.78 × 1012 17.58 × 1012 29.38 × 1012

1.08 × 10-9 3.01 × 10-9 4.16 × 10-9

of H2O2 on the TiO2 surface and the photodecomposition rate of H2O2. When the pH value was higher, the adsorption amount and the photodecomposition rate of H2O2 were higher. The greater the adsorption amount of H2O2 on the TiO2 surface, the easier the surface electron transfer occurred, which would result in the more rapid decomposition of H2O2. But when the pH value was too high, the stability of H2O2 (pKa ) 11.56, see eq 7) became lower. Considering the stability of H2O2, the pH value was adjusted at pH ) 3.0 in other experiments.

H2O2 h H+ + -OOH

(7)

2. The Formation of Surface Complexes on the TiO2 Surface. Diffuse reflectance absorption spectra and surface photovoltage spectra of TiO2 powder were measured before and after treating with hydrogen peroxide. As shown in Figure 2, spectrum 2 of TiO2 treated with H2O2 occurred red shift in which a tailing absorbance in the visible region 400-500 nm was observed compared with that (spectrum 1) of pure TiO2 powder, indicating the formation of a new intermediate between TiO2 and H2O2. The new intermediate should be the surface complexes resulting from the interaction between H2O2 molecule and valance-unfilled Ti(IV) of TiO2 surface. The inset in Figure 2 shows the absorption spectrum of surface complexes. It was found that the absorption band exhibited a maximum at 416 nm and showed a significant absorption in the visible region that can be assigned to the intramolecular ligand to metal charge-transfer transition within the surface titanium(IV)-hydrogen peroxide complexes. The surface photovoltage method is a well-established technique for the characterization of semiconductors, which relies on analyzing the changes in the photoinduced surface voltage. Figure 3 shows the surface photovoltage spectra (SPS) of TiO2 powder (curve 1) and pretreated TiO2 powder with H2O2 (curve 2). The photoresponse peak at 334 nm is the transition from the valance band to the conduction band of TiO2. Although the photovoltage response peak of untreated TiO2 is about 334 nm, the

Photodecomposition of H2O2 on TiO2 Surfaces

Figure 4. Initial rate (solid cycle) and photoefficiency (open cycle) of the H2O2 (5 × 10-5 mol/L) decomposition in an aqueous TiO2 dispersion under visible light irradiation at different wavelengths (416, 437, 450, 500, and 550 nm).

Figure 5. UV-vis spectra changes of salicylic acid (1.0 × 10-4 mol/L) in a TiO2 (1 g/L)/H2O2 (1.0 × 10-3 mol/L) dispersion by visible light. Insert: changes in the concentrations of salicylic acid and H2O2 with the irradiation time under the same conditions.

in Figure 5, the characteristic absorption band of salicylic acid at 297 nm decreased rapidly with increasing the irradiation time under visible irradiation (λ > 420 nm) in an aqueous TiO2/H2O2 dispersion and after irradiation for 7 h about 70% salicylic acid was degraded. By TOC (total organic carbon) analysis, the mineralization yield of salicylic acid was about 25% after irradiation for 4 h. The inset in Figure 5 shows the temporal concentration changes of salicylic acid and hydrogen peroxide in the photoreaction process. The initial decomposition of both salicylic acid and H2O2 was in agreement with zero-order processes and a possible relation between the decomposition of salicylic acid and the decomposition of H2O2 must exist (d[SA]/dt ) k d[H2O2]/dt, k ) 0.01). The control experiments showed that salicylic acid could decompose scarcely in the presence of H2O2 or TiO2 alone under visible irradiation, which precluded the photoadsorption possibility. This result proved that the presence of H2O2 played an important role in the photodegradation of salicylic acid in an aqueous TiO2 dispersion under visible irradiation and the decomposition of H2O2 on the TiO2 surface resulted in the degradation of salicylic acid under visible light irradiation. 5. The Formation of •OH in the H2O2/TiO2 System under Visible Irradiation. To further reveal the photodecomposition mechanism of H2O2 and the concomitant photooxidation path of organic compounds on the TiO2

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Figure 6. EPR spectra change with increasing the irradiation time: (a) H2O2 (1 × 10-4 mol/L)/TiO2 (0.5 g/L) suspensions in the dark; (b) and (c) under visible light irradiation (λ > 420 nm) for 15 and 30 min, respectively.

surface in the presence of H2O2 under visible irradiation, EPR experiments were carried out utilizing DMPO as the trapper of active radical species (see Figure 6). The radical signals consisting of four characteristic peaks of 1:2:2:1 of DMPO-•OH were detected successfully under visible irradiation in aqueous TiO2/H2O2 suspensions (curves b and c). In blank experiments, it was found that no radical signal was detectable without visible light irradiation in aqueous TiO2/H2O2 dispersion (curve a) or under visible light irradiation in the absence of H2O2. This experimental result indicated the formation of active •OH radicals in the decomposition of H2O2 which possess strong oxidation ability and may cause oxidation reaction of organic compounds adsorbed on the TiO2 surface (as shown in Figure 5). However, the generation of •OH radicals should not result from the direct dissociation of H2O2 or the direct excitation of TiO2 because neither TiO2 nor H2O2 can absorb visible light. The possible mechanism involves the surface electron transfer from the surface complex to the TiO2 conduction band. H2O2 can easily adsorb on the TiO2 surface and give rise to the surface complexes which extend the photoresponse of TiO2 to the visible region. The generation of •OH radicals proved the occurrence of the electron-transfer process between surface complexes and the TiO2 conduction band under visible light irradiation whereas no •OH radical was generated in the H2O2/TiO2 system under dark conditions. The decomposition of organic compounds such as salicylic acid contributed to the attack of •OH radical. According to above discussions, the reaction mechanism is proposed (Scheme 1) as follows: the H2O2 molecule is adsorbed on the TiO2 surface to form surface titanium(IV) hydrogen peroxide complexes (tTiIV-OOH) which extend photoresponse to visible light and can be excited by visible light. The excited surface complex (tTiIVOOH)* injects an electron to the conduction band of the semiconductor and generates the conduction band electron e-cb and tTiIV-•OOH, which may further give rise to t TiIV-OH and 1/2O2. The conduction band electron further reacts with the adsorbed H2O2 on the TiO2 surface to generate the •OH radical18,19 which possesses strong oxidation ability (2.4 V vs NHE) and may further oxidize other organic compounds. Conclusions Photodecomposition of hydrogen peroxide occurred and followed the zero-order kinetics process in an aqueous TiO2 suspension under visible light irradiation. The

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Scheme 1. A Possible Mechanism of the Photoinduced Electron Transfer and Interface Photoreaction of the Surface Complex of H2O2 on the TiO2 Surface

surface complexes of H2O2/TiO2 are formed which extend the photoresponse of TiO2 to the visible region and result in the visible-light-induced surface electron transfer from surface complexes to the conduction band of TiO2. The decomposition of H2O2 under visible light irradiation leads to the generation of •OH radicals which further results in the photooxidation of organic compounds (such as salicylic acid).

Acknowledgment. The financial support of this work from the National Science Foundation of China (Nos. 20077027, 29725715, 4001161947, and 29877026), the Foundation of the Chinese Academy of Sciences, and the China National Committee for Sciences and Technology is gratefully acknowledged. LA010035S