Exploring the interparticle electron transfer process in the photocatalytic oxidation of 4-chlorophenol

Journal of Hazardous Materials B137 (2006) 1362–1370

Exploring the interparticle electron transfer process in the

photocatalytic oxidation of 4-chlorophenol

Hsin-Hung Ou

, Shang-Lien Lo

, Chung-Hsin Wu

a_{Research Center for Environmental Pollution Prevention and Control Technology, Graduate Institute of Environmental Engineering,} National Taiwan University, 71 Chou-Shan Rd., Taipei 106, Taiwan, ROC

b_{Department of Environmental Engineering, Da-Yeh University, 112 Shan-Jiau Rd., Da-Tsuen, Chang-Hua 515, Taiwan, ROC} Received 6 October 2005; received in revised form 15 March 2006; accepted 15 March 2006

Available online 16 May 2006

Abstract

This work aimed to investigate the interparticle electron transfer (IPET) process within the coupled-photocatalyst systems on the basis of the degradation of 4-chlorophenol (4-CP). TiO2, ZnO and SnO2 are used as the model photocatalysts owing to their increasing energy levels

which correspond to the IPET concept. In the single-photocatalyst tests, ZnO tests are associated with the highest degradation rate constants (0.347± 0.083 h−1at pH 7 and 0.453± 0.011 h−1at pH 11) and a better DOC reduction than in other single catalyst tests under given conditions. ZnO/SnO2coupled tests have constants of 0.612± 0.068 and 0.948 ± 0.069 h−1at pH 7 and 11, respectively. Additionally, the 4-CP prefers the

breakdown of chloride group in TiO2system while proceeding hydroxylation reaction in ZnO systems. Meanwhile, a phenomenonlogical model

coupled with the IPET effect was developed to explore the separation of photo-electrons and photo-holes within catalysts. Based on the model parameters, the recombination rate of photo-electrons and photo-holes in TiO2/SnO2and ZnO/SnO2systems is 20–45% lower than that obtained

by a respective single catalyst. Thus, coupled-photocatalyst tests, TiO2/SnO2and ZnO/SnO2efficiently suppress the recombination, particularly

for ZnO/SnO2tests at pH 11.

© 2006 Elsevier B.V. All rights reserved.

Keywords: Photodegradation; Interparticle electron transfer process; Coupled-photocatalyst

1. Introduction

The problem of pollutants, particularly the serious environ-mental impact of their residues and the relatively low efficiency of the current remediation process, has led to several efforts to elucidate more efficient degradation alternatives. Many works have attempted to degrade numerous pollutants by advanced oxidation processes (AOPs)[1,2], rather than by other treatment processes. Heterogeneous photocatalysis has been emerged as an efficient method for purifying water and air [3,4]. In most cases, the most effective materials for photocatalytic applica-tions are nano-sized semiconductor oxides, which have been proven as excellent catalysts because of their highly reactive surfaces[5]. Such oxides include TiO2, ZnO and CdS, among

others. TiO2and ZnO have been extensively examined as

het-erogeneous semiconductor photocatalysts, primarily because of

∗_{Corresponding author. Fax: +886 2 23928830.} E-mail address:sllo@ntu.edu.tw(S.-L. Lo).

their high capacity for degrading toxic and recalcitrant chemical species via relatively simple and low-cost procedures[6,7].

However, the use of semiconductors as photocatalysts is mainly limited in the recombination of the generated photo-holes and photo-electrons. Photocatalysis involves the oxidation of a chemical by photo-holes from the semiconductor, so every recombination event involves the loss of holes that might oth-erwise have promoted degradation. Thus, the vectorial transfer of photogenerated electrons and holes between the valence and conduction bands of semiconductors is an important process in photocatalysis. Accordingly, a major focus of current photo-catalysis research is to improve the separation characteristics, to improve photo-efficiency. Attempts are made to solve the prob-lem of increasing the photo-efficiency, one of these methods is interparticle electron transfer (IPET) process.

The concept of IPET process is to exploit the distinction of the band gap between semiconductors with different energy poten-tials to drive the electrons transfer. That means the coupled semi-conductors with their corresponding conduction and valence bands can be used to achieve such phenomenon, increasing the

0304-3894/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2006.03.023

H.-H. Ou et al. / Journal of Hazardous Materials B137 (2006) 1362–1370 1363

Fig. 1. Diagram depicting the redox potentials of the valence and conduc-tion bands and the band gap energies for various semiconductor particulates [8,30–33].

lifetime of charge carries and the efficiency of the interfacial charge transfer to the adsorbed substrate and ultimately enhanc-ing their photocatalytic performance considerably[8–11]. How-ever, an ambiguous point still needed to be clarified is the promotion in retarding recombination within IPET process. Therefore, it is essential to expound the IPET effect on the basis of some kinetic parameters, and which is the major concern in this study.

This study elucidate the IPET effect under three buffer con-ditions, pH 4, 7 and 11, using TiO2, ZnO and SnO2as model

photocatalysts because of the increase in their energy levels (Fig. 1). Attempts are made to compare the photocatalytic mechanism in the TiO2 and ZnO systems, by exploiting the

reduction of dissolved organic carbon and the ratios of released chloride ions to 4-CP concentration. Additionally, a notional model proposed in the study is used to ascertain whether IPET effect promote the separation of electrons and photo-holes.

2. Materials and methods

2.1. Reagent and chemicals

Three semiconductor powders TiO2 (Degussa P25), ZnO

(Fluka) and SnO2 (RDH) were used as model photocatalysts.

4-CP (99%) was purchased from Aldrich and was of the best grade available; these agents were used without further treat-ment. Water was deionized and doubly distilled with Milli-Q.

2.2. Experimental apparatus and procedures

All photodegradation experiments were conducted in a batch reactor. The reaction mixtures were illuminated under a UV lamp (8 W Xenon lamp, Philips), which was placed approxi-mately 35 cm below the bottom of the glass plate at a controlled reaction temperature of 25◦C during the experimental period. The intensity of UV lamp is 4.32 mW cm−2measured by UV radiometer (MS-100, UVP) at the sampling position.

Prior to the photocatalytic experiments, the suspensions were prepared by mixing definite volumes (3 L) of solutions of the desired concentration of 4-CP (2× 10−4M) with various

sin-Table 1

Characteristic property of TiO2, ZnO and SnO2 Catalyst Maximum absorption

wavelength (nm)

Band gap (eV) BET (m2_g−1₎

TiO2 391.4 3.17 54.18

ZnO 424.9 2.92 4.56

SnO2 300.4 4.13 4.78

gle and coupled-photocatalysts, TiO2, ZnO, SnO2, TiO2/SnO2,

ZnO/SnO2and TiO2/ZnO (1.2 g L−1). Meanwhile, the coupled

catalysts are of physical mixing in the solution without any pretreatments. The portion of SnO2 is 50 wt.% for respective

coupled catalyst. That means both the concentrations of TiO2

and SnO2in the coupled catalyst (TiO2/SnO2) are 0.6 g L−1, the

proportion of SnO2in ZnO/SnO2coupled catalyst is the same

with the aforementioned description. The slurry was stirred mag-netically for 30 min in the dark to achieve adsorption equilibrium for the substrate on the photocatalytic system. Hydrochloric acid or sodium hydroxide was added to maintain the desired buffered conditions. When the photocatalytic experiment was initiated, the samples were withdrawn at different times. After they were centrifuged at 3000 rpm with a centrifuge (KUBOTA KN-70) and then filtered through a 0.45␮m filter membrane (Millipore), the samples proceeded to a series of analysis, the remaining of 4-CP (HPLC, Millipore Waters 600E with a Waters 486 detector), the quantification of released chloride ions (IC, Dionex model DX-120) and dissolved organic carbon (TOC, 1020A,OI Ana-lytical). And all the physical characteristics of the photocatalysts performed in the study were presented inTable 1.

3. Model development

Although the production of by-products such as hydro-quinone (HQ), benzohydro-quinone (BQ) and 4-chlorocatechol (4-CC) can potentially influence the degradation behavior of 4-CP, their impacts have been inclusive of the model development. The model primarily concerns the type of coupled-photocatalystic systems and the reaction conditions to survey the IPET effect. The simplified notional mechanism of photocatalysis is described as follows:

TiO2, ZnO + hv → e−+ h+ (1) e−+ SnO2→ e−·SnO2 (2) e−+ h+→ heat (3) e−+ O2,ads→ O2− (4) h++ H2Oads→ OH• + H+ (5) h++ Subads→ R• + H2O (6)

Eq. (1) shows the formation of holes and photo-electrons in the surface of TiO2 or ZnO on irradiation with

the valid UV light energy which is significantly affected by the optical and physical properties of irradiated catalysts. Eq.

(2) refers to the vectorial transfer of photo-electrons between two catalysts owing to the distinct energy potential within

H.-H. Ou et al. / Journal of Hazardous Materials B137 (2006) 1362–1370 1369

decrease in the recombination rate is around 25± 3%. In contrast to TiO2and TiO2/SnO2tests under the same condition,

in which the recombination performance is almost 21± 0.3% lower. Therefore, the ZnO/SnO2tests provide a more reliable

channel to retard the recombination reaction. The same ten-dency can be observed at pH 7. These results reflect that the larger energy gap causes the more efficient separation of pairs of photo-electrons and photo-holes (Fig. 1). Notably, the variance of β values also remarkably reveals that the recombination rate is directly related to pH which seems to be responsible for the suppression of the recombination. However, the mechanism remains as a matter to be discussed further. Briefly, the results reveal that TiO2/SnO2, ZnO/SnO2 and TiO2/ZnO can

facilitate the separation of photo-electrons and photo-holes. The promotion extent of the IPET effect follows the order: ZnO/SnO2> TiO2/SnO2> TiO2/ZnO, which corresponds to the

corresponding energy gap. In summary, the efficiency of the photocatalysis is strongly attributed to the suppression of the recombination of photo-electrons and photo-holes, rather than the amount of the generated photons. Additionally, the results demonstrated herein constitute reliable evidences that the IPET effect apparently provide an effective means to improve photocatalysis.

5. Conclusions

In the single-photocatalyst test, ZnO exhibited the high-est catalytic activity and a better degree of reduction of DOC than other single catalyst tests at pH 7 and 11. The observed rate constants of the coupled-catalyst systems, TiO2/SnO2and

ZnO/SnO2, are higher than that of TiO2 and ZnO

approxi-mately by a factor of 1.5–3. Also, the reductions of DOC for TiO2/SnO2 and ZnO/SnO2 tests are 8–20% higher than that

of the single-photocatalyst system. One recommended course of action related to this study is to assess the recombina-tion rate within IPET process. A comparison of the model parameters revealed significant trends that can be qualitatively related to the experimental conditions and test systems. Sub-stantially, this study concludes that the IPET effect on the photocatalysis dose deliver the anticipated advantages based on the disappearance rate of 4-CP. The results demonstrate that the recombination rate is prone to decline in systems with a large energy gap, which dose correspond to the concept of IPET process.

Acknowledgement

The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 93-2622-E-264-004-CC3.

References

[1] M. Lewandowski, D.F. Ollis, A two-site model simulating apparent deac-tivation during photocatalytic oxidation of aromatics on titanium dioxide (TiO2), Appl. Catal. B Environ. 43 (2003) 309–327.

[2] A. Sobczynski, L. Duczmal, W. Zmudzinski, Phenol destruction by pho-tocatalysis on TiO2: an attempt to solve the reaction mechanism, J. Mol. Catal. A Chem. 213 (2004) 225–230.

[3] K. Chiang, R. Amal, T. Tran, Photocatalytic oxidation oxidation of cyanide: kinetic and mechanistic studies, J. Mol. Catal. A Chem. 193 (2003) 285–297.

[4] P.B. Amama, K. Itoh, M. Murabayashi, Photocatalytic degradation of trichloroethylene in dry and humid atmospheres: role of gas-phase reac-tions, J. Mol. Catal. A Chem. 217 (2004) 109–115.

[5] C.B. Almquist, P. Biswas, Role of synthesis method and particle size of nanostructured TiO2 on its photoactivity, J. Catal. 212 (2002) 145– 156.

[6] J.C. Lee, M.S. Kim, B.W. Kim, Removal of paraquat dissolved in a pho-toreactor with TiO2immobilized on the glass-tubes of UV lamps, Water Res. 36 (2002) 1776–1782.

[7] A.D. Paola, E. Garcia-Lopez, S. Ikeda, G. Marci, B. Ohtani, L. Palmisano, Photocatalytic degradation of organic compounds in aqueous system by transition metal doped poly crystalline TiO2, Catal. Today 78 (2002) 87–93.

[8] N. Serpone, P. Maruthamuthu, P. Pichat, E. Pelizzatti, H. Hidaka, Exploiting the interpartical electron transfer process in the photocatalysed oxidation of phenol, 2-chlorophenol and pentachlorophenol: chemical evidence for electron and hole transfer between coupled semiconductors, J. Photochem. Photobiol. A Chem. 85 (1995) 247–255.

[9] C. Wang, J. Zhao, X. Wang, B. Mai, G. Sheng, P. Peng, J. Fu, Preparation, characterization and photocata;ytic activity of nano-sized ZnO/SnO2 coupled-photocatalysts, Appl. Catal. B Environ. 39 (2002) 269–279.

[10] S.C. Lo, C.F. Lin, C.H. Wu, P.H. Hsieh, Capability of coupled CdSe/TiO2 for photocatalytic degradation of 4-chlorophenol, J. Hazard. Mater. B114 (2004) 183–190.

[11] C.H. Wu, Comparison of azo dye degradation efficiency using UV/single semiconductor and UV/coupled semiconductor systems, Chemosphere 57 (2004) 601–608.

[12] M.R. Hoffmann, S.T. Martin, W. Choi, W. Bahnemann, Environmen-tal applications of semiconductor photocaEnvironmen-talysis, Chem. Rev. 95 (1995) 9–96.

[13] R.B. Draper, M.A. Fox, Titanium dioxide photosensitized reactions stud-ied by diffuse reflectance flash photolysis in aqueous suspensions of TiO2 power, Langmuir 6 (1990) 1396–1402.

[14] E.R. Carraway, A.J. Hoffman, M.R. Hoffman, Photocatalytic oxidation of organic acids on quantum-sized semiconductor colloids, Environ. Sci. Technol. 28 (1994) 1994.

[15] K.I. Ishibashi, A. Fujishima, T. Watanabe, K. Hashimoto, Quantum yields of active oxidative species formed on TiO2photocatalyst, J. Photochem. Photobiol. A Chem. 134 (2000) 139–142.

[16] L. Sun, J.R. Bolton, Determination of quantum yield for the photochemical generation of hydroxyl radicals in TiO2suspensions, J. Phys. Chem. 100 (1996) 4127–4134.

[17] H. Gerischer, Photocatalysis in aqueous solution with small TiO2particles and the dependence of the quantum yield on particle size and light intensity, Electrochim. Acta 40 (1995) 1277–1281.

[18] C.B. Almquist, P. Biswas, A mechanistic approach to modeling the effect of dissolved oxygen in photo-oxidation reactions on titanium dioxide in aqueous systems, Chem. Eng. Sci. 56 (2001) 3421–3430.

[19] Y. Meng, X. Huang, Y. Wu, X. Wang, Y. Qian, Kinetic study and model-ing on photocatalytic degradation of para-chlorobenzoate at different light intensities, Environ. Pollut. 117 (2002) 307–313.

[20] R.A. Doong, C.H. Chen, R.A. Maithreela, S.M. Chang, The influence of Ph and Cadmium sulfide on the sulfide on the photocatalytic degradation of 2-chlorophenol in titanium dioxide suspensions, Water Res. 35 (2001) 2873–2880.

[21] Y. Ku, I.L. Jung, Photocatalytic reduction of Cr(VI) in aqueous solutions by irradiation with the presence of titanium dioxide, Water Res. 35 (2001) 135–142.

[22] A.A. Khodja, T. Sehili, J.F. Pilichowski, P. Boule, Photocatalytic degra-dation of 2-Phenyphenol on TiO2 and ZnO in aqueous suspensions, J. Photochem. Photobiol. A Chem. 141 (2001) 231–239.

1370 H.-H. Ou et al. / Journal of Hazardous Materials B137 (2006) 1362–1370 [23] C.A.K. Gouvea, F. Wypych, S.G. Moraes, N. Duran, N. Nagata, P.

Peralta-Zamora, Semiconductors-assisted photocatalytic degradation of reactive dyes in aqueous solution, Chemosphere 40 (2000) 433–440.

[24] B. Dindar, S. Icli, Unusual photoreactivity of zinc oxide irradiated by con-centrated sunlight, J. Photochem. Photobiol. A Chem. 140 (2001) 263–268. [25] C. Hu, Y. Tang, J.C. Yu, P.K. Wong, Photocatalytic degradation of cation blue X-GRL adsorbed on TiO2/SiO2photocatalyst, Appl. Catal. B Environ. 40 (2003) 131–140.

[26] U. Stafford, K.A. Gray, P.V. Kamat, Photocatalytic degradation of 4-chlorophenol: the effects of varying TiO2 concentration and light wave-length, J. Catal. 167 (1997) 25–32.

[27] H. Alekabi, N. Serpone, Kinetic studies in heterogeneous photocatalysis. 1. photocatalytic degradation of chlorinated phenols in aqueous solutions over TiO2supported on a glass matrix, J. Phys. Chem. 92 (1988) 5726–5731. [28] G. Alsayyed, J.C. D’Oliveira, P. Pichat, Semiconductor-sensitized

pho-todegradation of 4-CP in water, J. Photochem. Photobiol. A Chem. 58 (1991) 99–114.

[29] T. Sehili, P. Boule, J. Lemaire, Photocatalysed transformation of chloroaro-matic derivatives on zinc oxide IH: chlorophenols, J. Photochem. Photobiol. A Chem. 50 (1989) 117–127.

[30] M. Fujii, T. Kawai, S. Kawai, Photocatalytic activity and the energy levels of electrons in a semiconductor particle under irradiation, Chem. Phys. Lett. 106 (1984) 517–522.

[31] M.F. Finlayson, B.L. Wheeler, N. Kakuta, K.H. Park, A.J. Bard, A. Cam-pion, M.A. Fox, S.E. Webber, J.M. White, Determination of flat-band position of CdS crystals, films, and powders by photocurrent and impedance techniques. Photoredox reaction mediated by intragap states, J. Phys. Chem. 89 (1985) 5676–5681.

[32] J.R. White, A.J. Bard, Electrochemical investigation of photocatalysis at CdS suspensions in the presence of Methylviologen, J. Phys. Chem. 93 (1985) 1947–1954.

[33] G. Redmond, A. O’Keefe, C. Burgess, C. MacHale, D. Fitzmaurice, Spec-troscopic determination of the flatband potential of transparent nanocrys-talline ZnO film, J. Phys. Chem. 97 (1993) 11081–11086.