J. Cent. South Univ. (2013) 20: 495–500

DOI: 10.1007/s11771-013-1511-1

Formation of active radicals and mechanism of photocatalytic degradation of phenol process using eosin sensitized TiO₂ under visible light irradiation

SONG Lin(宋琳)^{1, 2}, ZHANG Xin(张鑫)^{1, 2}, MA Yun-kun(马云坤)^{1, 2},

LI Ming-yu(李明玉)^{1, 2}, ZENG Xiao-long(曾小龙)^{1, 2}

1. Department of Environmental Engineering, Jinan University, Guangzhou 510632, China;

2. Key Laboratory of Water/Soil Toxic Pollutants Control and Bioremediation,

Department of Education of Guangdong Province, Guangzhou 510632, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2013

Abstract: The role of oxygen and the generation of active radicals in the photocatalitic degradation of phenol were investigated using the eosin sensitized TiO₂ as photocatalyst under visible light irradiation. Diffuse reflectance spectra show that the absorbancy range of eosin/TiO₂ is expanded from 378 nm (TiO₂) to about 600 nm. The photocatalitic degradation of phenol is almost stopped when the eosin/TiO₂ system is saturated with N₂, which indicates the significance of O₂. The addition of NaN₃ (a quencher of single oxygen) causes about a 62% decrease in the phenol degradation. The phenol degradation ratio is dropped from 92% to 75% when the isopropanol (a quencher of hydroxyl radical) is present in the system. The experimental results show that there are singlet oxygen and hydroxyl radical generated in the eosin/TiO₂ system under visible light irradiation. The changes of absorbancy indicate that the hydrogen peroxide might be produced. Through the analysis and comparison, it is found that the singlet oxygen is the predominant active radical for the degradation of phenol.

Key words: eosin sensitized TiO₂; visible light; photocatalytic degradation; reaction mechanism

1 Introduction

In recent years, it is of great interest that TiO₂ is used as photocatalyst to eliminate environmental pollutants [1–3]. However, pure TiO₂ has a bandgap of 3.0–3.2 eV, which is too wide to absorb visible light and limits its application on sunlight[4–7]. Therefore, many ways to modify TiO₂ for visible light activity have been reported [8–11]. The surface sensitization as an important method has attracted considerable attention. Surface sensitization means the sensitizer which could absorb visible light was absorbed on the surface of photocatalyst through chemical absorption or physical absorption. The photocatalyst sensitized would be of visible light activity. The sensitizers usually are organic dyes, chlorophyll, humic acid, fulvic acid, unsaturated fatty acid, etc. [8]. The eosin is an excellent sensitizer and the photocatalytic reaction can be carried out smoothly under visible light using eosin sensitized TiO₂. However, most researches of eosin/TiO₂ photocatalyst mainly focused on the photocatalytic effects. The formation of active radicals and which radicals play the most significant role in the photocatalytic process of eosin/TiO₂ system have hardly been reported. Because the degradation of pollutants is mainly dependent on strong oxidation of various active radicals, the research on the formation of active radicals will be significant for TiO₂ photocatalytic technology. In addition to that, the details of the mechanisms of photocatalytic reaction processes at the TiO₂ surface remain elusive [12]. The major uncertain issue is the reaction pathway. Some researchers suggested that oxidation processes of pollutants were attributed to positive holes, while others have suggested a hydroxyl radical-mediated pathway [13]. Therefore, it is necessary to discuss the mechanism of photocatalytic reaction processes in eosin/TiO₂ system. In order to find out the formation of active radicals and the mechanism of eosin/TiO₂ for the degradation of pollutants, the photocatalitic degradation of phenol using the eosin modified TiO₂ was studied under visible light irradiation.

2 Experimental

2.1 Materials

TiO₂, with a crystallographic mode of 80% anatase and 20% rutile, a BET surface area of 50 m²/g and an average particle size of 30 nm, was of supplied by Degussa Co. All other chemicals were of analytical grade and used without further purification. Distilled water was used throughout the experiment.

2.2 Eosin/TiO₂ preparation

A mixture composed of eosin (10 mg) and absolute ethanol (10 mL) was stirred for 4 h to form a clear solution in darkness. TiO₂ (1 g) powder was added to 100 mL absolute ethanol and stirred for 4 h in darkness to obtain a uniform suspension. The eosin solution was added dropwise into the stirring TiO₂ suspension and stirred for another 10 min. The solvent was removed under vacuum when yielding red powder in darkness. Finally, the powder was dried at room temperature. The eosin/TiO₂ photocatalyst was obtained and the mass ratio of eosin to TiO₂ was 1:100. In the same way, the eosin/TiO₂ photocatalysts whose mass ratios of eosin to TiO₂ were 1:50, 1:200, 1:400 and 1:800 respectively were obtained by changing the proportion of eosin.

2.3 Photocatalytic reaction

The photocatalytic reduction was carried out in a cylindrical glass reactor. A 3 W light-emitting diode (Shenzhen Lanbaoli Photo-electricity Technology Co. Ltd., China, lights with a radiant wavelength of 525–531 nm) was used as the source of visible light, which was positioned alongside the reactor. The reactor was open to air to ensure enough oxygen in the reaction solution.0.04 g photocatalyst was suspended into 40 mL aqueous solution of phenol (10 mg/L). Prior to irradiation, the suspension was magnetically stirred by using a magnetic stirrer (JB-2 Shanghai Xinjing Co. Ltd.) in darkness for 30 min to ensure the establishment of adsorption-desorption equilibrium of phenol on photocatalyst surface. After equilibration, the concentration of phenol was measured and taken as the initial concentration (C₀). Then, the light-emitting diode was opened, 5 mL solutions were sampled at appropriate time intervals. The samples were centrifuged immediately at 6000 r/min for 10 min by a high speed freeze centrifuge (KDC-160HR, Keda Innovation Co. Ltd., China) and filtered through 0.45 μm PTFE syringe filter. The filtrates were collected and analyzed. The concentration of phenol was determination by Spectrumlab 54 UV-Visible spectrophotometer (Shanghai Lengguang Lighting Technology Co. Ltd., China) at a wavelength of 510 nm through 4-aminoantipyrine spectrophotometric method [14]. It was found that the removing of 5 mL reaction mixture did not significantly affect the concentration of phenol in the remaining solution. The reaction mixture was maintained in suspension throughout the experiment. The reaction was carried out at room temperature. Duplicate were carried out for each test, and the relative standard deviation was generally less than 5%. The diffuse reflectance spectra (DRS) of eosin/TiO₂ complex and TiO₂ powders were examined by a UV-Visible Spectrophotometer (UV-2501PC, SHIMADZU).

3 Results and discussion

3.1 Diffuse reflectance spectra (DRS) of eosin/TiO₂

Figure 1 shows the DRS of TiO₂ and eosin/TiO₂ powders, which are similar to the polymer/TiO₂ [15–16]. As shown in Fig. 1, there is no absorption above 400 nm for pure TiO₂, while eosin/TiO₂ shows light absorption from 378 to about 600 nm. It is a vital prerequisite for photosensitization that the photosensitizer has been adsorbed onto the surface of semiconductor. Eosin/TiO₂ seems capable of responding to visible light. The absorbency of eosin/TiO₂ is maximum at about 530 nm. So, the green light-emitting diode whose radiant wavelength is 525–531 nm is used as photosource.

Fig. 1 Diffuse reflectance spectra of TiO₂ and eosin/TiO₂

3.2 Effect of mass ratio of eosin to TiO₂ on photocatalytic activity of eosin/TiO₂ under visible light irradiation

As shown in Fig. 2, the eosin/TiO₂ powers with different mass ratios of eosin to TiO₂ all are of visible light catalytic activities. The phenol in different eosin/TiO₂ systems can be degraded in varying degrees under visible irradiation. When the mass ratio of eosin to TiO₂ is 1:800, the photodegradation of phenol is only 55.9%. The photocatalytic activities of eosin/TiO₂ powers are enhanced obviously as the proportion of eosin increases. In the eosin/TiO₂ solutions (the mass ratios of eosin to TiO₂ are 1:400 and 1: 200), the photodegradation of phenol rises to 61% and 72.7%, respectively. The photodegradation of phenol is as high as 92% using the eosin/TiO₂ (m(eosin): m(TiO₂)= 1:100) as photocatalyst. However, the photocatalytic activities of eosin/TiO₂ powders decrease when the proportion of eosin is more than 1%. Only 77.3% phenol is degraded when the mass ratio of eosin to TiO₂ is 1: 50 and the reaction solution becomes red. On one hand, the excessive proportion of eosin leads to the eosin which absorbed on the surface of TiO₂ become easy to fall off. On the other hand, the TiO₂ particles will gather and the partical size of TiO₂ will increase when too much eosin is absorbed on the surface of TiO₂, and the photocatalytic activities of eosin/TiO₂ powders decrease. So, the eosin sensitized TiO₂ (m(eosin): m(TiO₂)= 1:100) is used as photocatalyst in subsequent experiments

Fig. 2 Effect of mass ratio of eosin to TiO₂ on photocatalytic activity of eosin/TiO₂

3.3 Effect of oxygen on photodegradation of phenol under visible light irradiation

As shown in Fig. 3, the degradation of phenol is 92% in air-saturated eosin/TiO₂ system under visible light irradiation while in N₂-saturated photocatalytic reaction is almost stopped. It indicated that O₂ plays an important role in the photocatalytic degradation of phenol. It is well known that O₂ is an efficient electron scavenger [17–22], which can prevent the recombination of the electron and radical cation that is produced by eosin in excited state. Cr₂O₇^2–is also able to trap the photogenerated electron [23]. As shown in Fig. 3, when O₂ is replaced by Cr₂O₇^2– as electron scavenger, the photodegradation of phenol in N₂-saturated system is 36.43%, which is about one-third of that in O₂ as electron scavenger system. It should be pointed out that only a small proportion of the 36.43% degradative phenol is oxidized by Cr₂O₇^2– directly, because the degradation of phenol is only 8.58% under the same experimental conditions without photocatalyst. The reason for the difference between O₂ and Cr₂O₇^2– is that the Cr₂O₇^2– can not generate strong oxidizing radicals which could contribute to the degradation of phenol, but O₂ can. In conclusion, the role of O₂ in the photocatalytic reaction is not only as electron scavenger, but more significantly, it can generate a series of active radicals.

Fig. 3 effect of O₂ on photodegradation of phenol under visible light irradiation

3.4 Formation of singlet oxygen (¹O₂) in eosin/TiO₂ system under visible light irradiation

The formation of singlet oxygen (¹O₂) under visible light irradiation and its role in the phenol degradation process is investigated indirectly. In these experiments, a comparison is made between the original degradation curves of phenol and those obtained after addition of sodium azide (NaN₃), which is a quencher of ¹O₂ [24–25]. As shown in Fig. 4, the degradation ratio of phenol is 92% in the absence of NaN₃ after 2 h under visible light irradiation. The ratio is reduced significantly when NaN₃ is added into the system. It is reduced to 53.89% when the concentration of NaN₃ is 2.5 mmol/L in the solution and decreases gradually with the concentration of NaN₃ increasing. The experimental results suggest that the ¹O₂ which is a low-lying single excited state of molecular oxygen [25–26] might be produced in eosin/TiO₂ system under visible light irradiation. The ¹O₂ may be produced by quenching of the excited state of the photosensitizer by oxygen [8]:

eosin + hv → ¹eosin                           (1)

¹eosin→ ³eosin                               (2)

³eosin + O₂ → eosin + ¹O₂                                  (3)

The oxidation potential of ¹O₂ is about 1 V higher than its ground state, therefore, it is significantly more electrophilic [27]. As a result, it can oxidate phenol. It is also seen in Fig. 4 that when the concentration of NaN₃ is more than 200 mmol/L, the degradation of phenol is almost unchanged, about 30%. Since the quantity of ¹O₂ which is produced in the photocatalytic process is finite in certain condition, nearly all ¹O₂ are quenched by NaN₃ at this moment. In the presence of NaN₃, the degradation of phenol drops from 92% to 30%, which indicates that the degradation of about 62% phenol is caused by singlet oxygen (¹O₂) and the rest of phenol is oxidated by other active species.

Fig. 4 effect of NaN₃ on photodegradation of phenol under visible light irradiation

3.5 Formation of hydroxyl radical (·OH) in eosin/TiO₂ system under visible light irradiation

The methylene blue can be oxidated to form colorless methyleneblue-hydroxylation (MB-OH) by hydroxyl radical (·OH) while can not by general strong oxidizer, such as K₂Cr₂O₆, H₂O₂, etc. Although strong reducing agent can fade methyleneblue, the fade is reversible. In contrast, the MB-OH is stable, which could not be colored through oxidation or reduction reaction. The reason is that the sulfur atoms with intermediate valence of methyleneblue molecular have a high affinity for ·OH [28]. Therefore, ·OH is captured by methyleneblue easily and the formation of ·OH could be demonstrated by spectrophotometry indirectly. As shown in Fig. 5, the absorbancy drops from 0.185 to 0.0018 when the system is irradiated under visible light in the presence of methyleneblue of 10 μmol/L. It suggests that ·OH is formed in eosin/TiO₂ system.

Fig. 5 absorbancy in eosin/TiO₂ system under different conditions

In order to further confirmed the generation of ·OH, control experiments are carried out in the presence of isopropanol, which is a quencher of ·OH [29–30] in eosin/TiO₂ system. As shown in Fig. 6, the degradation of phenol reduces from 92% to 89.46% in the presence of 0.01 mmol/L isopropanol. It decreases gradually with the concentration of isopropanol increasing. The experimental results suggest that the ·OH is likely to be produced in eosin/TiO₂ system under visible light irradiation and it is one of free radicals for the degradation of phenol. It is also seen in Fig. 6 that when the concentration of isopropanol is more than 1 mmol/L, the degradation of phenol is almost unchanged, about 72%. Since the quantity of ·OH which is produced in the photocatalytic process is finite in certain condition, nearly all ·OH is quenched by isopropanol at this moment. In the presence of isopropanol, the degradation of phenol drops from 92% to 72%, which indicates that the degradation of about 20% phenol is caused by ·OH and the degradation of rest phenol is contributed to other active species. It is well known that oxidation potential of ·OH is 2.8 V [8] and its oxidizing is stronger than other active species. However, most of phenol was not oxidated by ·OH. It may be ascribed that ·OH that is produced on the surface of eosin/TiO₂ can not be transformed to solution of phenol well.

Fig. 6 effect of isopropanol on photodegradation of phenol under visible light irradiation

3.6 Formation of hydrogen peroxide (H₂O₂) in eosin/TiO₂ system under visible light irradiation

Because potassium iodide (KI) can be oxidated by oxidizing substance in the presence of hydrochloric acid to form iodine (I₂), which is able to react with starch to form blue iodide [31]. Therefore, the determination of oxidizing substance can be carried out indirectly through the change of absorbancy in eosin/TiO₂ system. As shown in Fig. 7, the absorbancy of eosin/TiO₂ system which is added into KI and starch after being stirred 30 min in darkness is only 0.0075. It indicates that the oxidizing substance can not be generated in eosin/TiO₂ system without visible light irradiation. When the system is irradiated by visible light, the absorbancy increases significantly amounted to 1.327. It indicates that the oxidizing substance is generated in eosin/TiO₂ system under visible light irradiation. The absorbancy drops to 0.758 in the presence of catalase, which can eliminate H₂O₂ through hydrolysis. It suggests that the H₂O₂ may be generated in eosin/TiO₂ system under visible light irradiation.

Fig.7 absorbancy in eosin/TiO₂ system under different conditions

3.7 mechanism of photocatalytic degradation of phenol process using eosin sensitized TiO₂ under visible light irradiation

As mentioned above, the ¹O₂, ·OH and H₂O₂ might be generated in eosin/TiO₂ system under visible light irradiation. Because the equilibrium of photocatalytic reaction is destructed when different kinds of quenchers of the active radicals are added into system, the proportion of each active radical can not be concluded through the photocatalytic reactions under different conditions. However, according to contrast and analysis, the quantity of phenol that is degraded by ¹O₂ is the most, follows by other active radicals. Therefore, the ¹O₂ is the predominant active radical.

The mechanism of eosin/TiO₂ can be concluded as: the eosin molecule absorbed on the TiO₂ surface might be excited by visible light to produce excited state of eosin (¹eosin and ³eosin) [8]:

eosin + hv → ¹eosin                           (1)

¹eosin→ ³eosin                               (2)

The oxidation potential of the excited state of eosin is more negative than the potential of the conduction of TiO₂ under neutral condition. Thus, from a thermodynamical point of view, an electron can be injected from the excited state of eosin to the conduction of TiO₂, with ³eosin being converted to its cation radical (eosin·⁺).

³eosin + TiO₂ →eosin·⁺ + TiO₂(e^–)            (4)

In this reaction, TiO₂ as electron carrier plays an important role in the separation of injected electron and eosin cation radical (eosin·⁺). The injected electron can be scavenged by molecule oxygen and generate the superoxide radical (O₂·^-).

TiO₂(e^–)+ O₂→TiO₂ + O₂·^-                     (5)

The O₂·^-can convert to the H₂O₂, hydroperoxy (HO₂·) and ·OH [19].

O₂·^- + H⁺ →HO₂·                             (6)

HO₂· + O₂·^- + H⁺ → H₂O₂+ O₂                  (7)

HO₂· + H⁺ + TiO₂(e^–) →H₂O₂+ TiO₂                   (8)

H₂O₂ + O₂·^-→ ·OH + OH^- + O₂                          (9)

H₂O₂ + TiO₂ (e^–)→ ·OH + OH^- + TiO₂          (10)

In addition to the above oxidative radicals (O₂·^-, HO₂·, ·OH), ¹O₂ might be also formed under the present conditions. At the same time, the photosensitizer could be regenerated.

³eosin + O₂ → eosin + ¹O₂                         (3)

The degradation of phenol is contributed to all the oxidative radicals above.

4 conclusions

1) The eosin sensitized TiO₂ is of photocatalytic activity. It can absorb the visible light of wavelength from 378 nm to about 600 nm.

2) The proportion of eosin has a marked effect on the photocatalytic activity of eosin/TiO₂. The eosin/TiO₂ (m(eosin):m(TiO₂)=1:100) is the best for the photodegradation of phenol.

3) The singlet oxygen, ·OH and H₂O₂ might be generated in eosin/TiO₂ system under visible light irradiation. The singlet oxygen is the predominant active radical for the degradation of phenol.

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(Edited by HE Yun-bin)

Foundation item: Project(8451063201001261) supported by the Guangdong Natural Science Fund Committee, China; project(LYM08022) supported by the Foundation for Distinguished Young Talents in Higher Education of Guangdong, China; project (2007A032400001, 2008A030202010) supported by the Scientific and Technological Planning of Guangdong Province, China; project(216113132) supported by the Scientific Research Cultivation and Innovation Fund, Jinan University, China

Received date: 2011–12–15; Accepted date: 2012–05–28

Corresponding author: LI Ming-yu, PhD, Professor; Tel.: +86–13302283608; E-mail: tlimy@jnu.edu.cn