VISIBLE LIGHT-ACTIVE PHOTOCATALYSIS AND ITS HYBRID SYSTEMS WITH FENTON-

This study investigated visible light active photocatalysis and its hybrid Fenton-like reaction systems for the degradation of organic pollutants. Several visible light active photocatalysts (Am-peroxo-titania, S-TiO2 and g-C3N4-AQ) were synthesized and the photochemical activities for the oxidation of organic compounds were examined. Second, sulfur-doped TiO2 (denoted as S-TiO2) demonstrated its visible light photocatalytic activity with Fenton-like reagents (Fe(III) and H2O2) for the degradation of organic compounds.

In photochemical activity, the S-TiO2/Fe(III) system illuminated with visible light completely degraded 10 μM 4-chlorophenol (4-CP) in 2 h. Photochemical reaction mechanisms for the decomposition of organic compounds in the S-TiO2/Fe(III) system (a) and the S-TiO2/Fe(III)/H2O2 system (b). Mechanisms of photochemical reactions for the decomposition of organic compounds in the system g-C3N4-AQ/Fe(III) in the saturated (a) and deaerated (b) state.

Degradation products of 4-CP by Am-peroxo-TiO2 illuminated with visible light identified by HPLC and LC/MS analyses.

INTRODUCTION

Research background

In detail, the photocatalytic reaction takes place through the illumination of light energy that can overcome the bandgap energy of the photocatalyst. Of the total light spectrum, visible light occupies most of the total radiant intensity of sunlight. However, most traditional photocatalysts require UV light energy to use the energy for the breakdown of organic compounds.

The photocatalytic activity can be limited by the wide bandgap energy of photocatalyst, therefore methods that can (1) compensate the wide bandgap energy and (2) efficiently utilize the large amount of visible light energy have been intensively studied.

Fig. 1.2. The general mechanism of photocatalytic reaction

Research trend of photocatalyst

To develop a combined system of active photocatalyst in visible light (S-TiO2) and Fenton-like system (Fe(III)/H2O2) for the degradation of organic compounds. Degradation of 4-CP by S-TiO2/Fe(III) and corresponding control systems (S-TiO2 and Fe(III)) were examined under visible light illumination (Figure 3.2.4a). Benzoic acid degradation was also negligible in Fe(III), H2O2 and S-TiO2/Fe(III) under visible light illumination (Figure 3.2.6b).14%.

The photochemical degradation of various organic compounds (phenol, 4‒CP, BA and CBZ) was examined in S-TiO2/Fe(III) under visible light illumination (Figure 3.2.8a). The oxidative formation of formaldehyde (HCHO), 4-hydroxybenzoic acid (4-HBA) and 7-hydroxycoumarin (7-HC) was examined under visible light illumination (Figure 3.2.10). Degradation of benzoic acid was negligible in the AQ, g-C3N4, g-C3N4/AQ and g-C3N4-AQ systems under visible light illumination (Figure 3.3.6a).

In the deaerated state, the degradation of phenol by the g-C3N4-AQ/Fe(III) system was hardly affected by the scavenging effect of tert-butanol under visible light, but methanol affected the degradation of phenol in g-C3N4-AQ /Fe(III) system under visible light illumination (Fig. 3.3.10b).

Objectives of the study

MATERIALS AND METHODS

• Reagents
• Synthesis of photocatalyst
• Amorphous peroxo-titania (Am-peroxo-TiO 2 )
• Sulfur doped TiO 2 (S-TiO 2 )
• Anthraquinone anchored graphitic carbon nitride (g-C 3 N 4 -AQ)
• Characterization
• Photochemical experiments
• Analytical methods

A UV/vis/near IR spectrophotometer (Cary 5000, Agilent Co.) was used to obtain the diffuse reflectance spectra. For the photochemical experiments with Am-peroxo-TiO2 and S-TiO2, light illumination was performed using fluorescent lamps (six lamps of 4 W; Shin-Kwang electronics Co.) with a 400 nm long-pass filter in a dark room. Light emission spectra of the fluorescent lamp (a) and the xenon arc lamp (b) with a 400 nm long-pass filter.

The initial pH of the solution was adjusted to 5.0 for the experiment with Am-peroxo-TiO2 and 3.0 for the experiments with S-TiO2 and g-C3N4-AQ. The incident photon, which flowed into the reaction solution was Einstein/L∙s (fluorescent lamp) and Einstein/L∙s (xenon arc lamp), was measured by chemical actinometry using potassium Reinecke's salt (400~650 nm).[ #] The above values ​​were converted to 1.45 mW/cm3 and 5.61 mW/cm3 respectively, where the calculation was derived from the respective light emission profiles shown in fig. The degradation products of 4-CP were analyzed by rapid separation liquid chromatography (RSLC) (UltiMate 3000, Dionex Co.) system coupled with a quadrupole-Orbitrap mass spectrometer (Q Exactive™, Thermo Fisher Scientific Inc.) (LC/MS). v/v) formic acid solution and acetonitrile as eluent with a ratio of 65:35 at a flow rate of 0.3 ml/min.

An electron paramagnetic resonance (EPR) spectroscopy (JES-X310, Jeol Co.), using 10 mM DMPO as a spin-trapping agent, was used to detect ROS.

Fig. 2.1. Light emission spectra of the fluorescent lamp (a) and the xenon arc lamp (b) with a 400 nm longpass filter.

RESULTS AND DISCUSSION

Am-peroxo-TiO 2

• Results
• Synthesis and characterization of Am-peroxo-TiO 2
• Photochemical degradation of organic compounds
• Photostability of Am-peroxo-TiO 2
• Oxidation products of 4-CP
• Discussion
• Photochemical activity of Am-peroxo-TiO 2
• Roles of ROS
• Photochemical reactions of illuminated Am-peroxo-TiO 2
• Mechanism of organic compound degradation by illuminated Am-peroxo-TiO 2

Sulfur doped TiO 2 (S-TiO 2 )

• Synthesis and characterization of S-TiO 2
• Effect of radical scavengers
• Oxidant production
• Repetition test under visible light illumination
• Roles of ROS
• Photochemical mechanisms for the degradation of organic compounds

Measurement of Fe(II) ion during the degradation of 4-CP indicates that about 90% of Fe(III) was reduced by the photocatalytic reaction of S-TiO2 (Fig. 3.2.4b). Degradations of 4-CP by S-TiO2/Fe(III) were investigated at different pH and varied Fe(III) concentration conditions under visible light illumination (Fig. 3.2.5). The gradual increase in the pseudo-first-order rate constant for the degradation of 4-CP was observed with increasing the initial concentration of Fe(III) to 0.1 mM (Fig. 3.2.5b).

The degradation of benzoic acid by S-TiO2/Fe(III)/H2O2 was examined at different pH and different concentration conditions of Fe(III) and H2O2 (Figure 3.2.7). On the other hand, the S-TiO2/Fe(III)/H2O2 system can efficiently degrade target compounds (Figure 3.2.8b). The decomposition of benzoic acid by the S-TiO2/Fe(III)/H2O2 system was significantly inhibited in the presence of methanol and tert-butanol (Figure 3.2.9b).

The effect of •OH scavengers (methanol and tert-butanol) on benzoic acid degradation supports the idea that •OH plays an important role in the S-TiO2/Fe(III)/H2O2 system. To investigate the stability of S-TiO2, the repeated degradation of 4-CP in S-TiO2/Fe(III) system was carried out. First, the target-selective reaction for the degradation of organic compounds cannot be generated by the oxidation reaction of •OH (Fig. 3.2.8a).

Second, the hole-scavenging effect of 4-CP decomposition supports the idea that the hole on the S-TiO2 surface is the main oxidant in the S-TiO2/Fe(III) system (Figure 3.2.9a). On the contrary, experimental results in the S-TiO2/Fe(III)/H2O2 system show that •OH is responsible for the degradation of organic compounds: non-selective targeted degradation (Fig. 3.2.8b), •OH removal effect (Fig. 3.2.9b) and the formation of hydroxylated oxidation products (Figure 3.2.10b). In the S-TiO2/Fe(III) system, it appears that the hole oxidation reaction is primarily responsible for the decomposition of organic compounds.

In S-TiO2/Fe(III)/H2O2 system, the additional reactive oxidant species are generated by the reaction with photochemically reduced Fe(II) and H2O2, which improves the degradation efficiency of organic compounds. The dissolved iron exhibited dual roles in S-TiO2/Fe(III) system and S-TiO2/Fe(III)/H2O2 system.

Fig. 3.2.3. Diffuse reflectance spectrum (a), valence band XPS (b), and electronic band structure (c) of S-TiO 2

• Synthesis and characterization of g-C 3 N 4 -AQ
• Photochemical reaction of g-C 3 N 4 -AQ
• Effect of AQ loading and Fe(III) injection
• Photochemical degradation of various organic compounds
• Effect of ROS scavengers
• Oxidant production

Degradation of benzoic acid using visible light illuminated g-C3N4-AQ system and the respective control systems (AQ, g-C3N4 and g-C3N4-AQ) were investigated in the absence and presence of Fe(III) (Fig. 3.3. 6) . To confirm the effect of AQ loading, different variants of g-C3N4-AQ were prepared as the varied dose of AQ (to form peptide bond) and then the degradation of benzoic acid with g-C3N4-AQ/Fe(III) ) was examined under illumination with visible light (Fig. 3.3.7). Photochemical degradation of various organic compounds (phenol, 4‒CP, BA and CBZ) was investigated in g-C3N4-AQ/Fe(III) system under visible light (Fig. 3.3.9).

To confirm the effect of dissolved oxygen in the g-C3N4-AQ/Fe(III) system, photochemical decomposition of organic compounds was performed under conditions of air saturation and deaeration. Under air-saturated conditions, all selected target organic compounds were efficiently degraded by the g-C3N4-AQ/Fe(III) system under visible light illumination (Figure 3.3.9a). However, CBZ was resistant to photochemical degradation by g-C3N4-AQ/Fe(III) and benzoic acid was barely degraded (kCBZ = 0.29 h−1).

Under air-saturated conditions, the decomposition of benzoic acid by the g-C3N4-AQ/Fe(III) system was significantly interrupted by the addition of methanol and tert-butanol (Figure 3.3.10a). The effect of •OH scavengers (methanol and tert-butanol) on the degradation of benzoic acid supports the idea that •OH plays an important role in the g-C3N4-AQ/Fe(III) system under air-saturated conditions. In the deaerated state •OH plays an insignificant role in the g-C3N4-AQ/Fe(III) system due to the negligible effect of scavenger •OH (tert-butanol) on phenol degradation.

On the other hand, 58 μM HCHO was measured in the g-C3N4-AQ/Fe(III) system illuminated by visible light in the deaerated state, while 4-HBA and 7-HC were negligibly detected (Fig. 3.2 .11b). The role of ROS in the g-C3N4-AQ/Fe(III) system was investigated separately depending on the existence of dissolved oxygen. First, the non-selective degradation of organic compounds supports •OH as the main ROS in visible light g-C3N4-AQ/Fe(III) (Fig. 3.3.9a).

In contrast, visible light illuminated g-C3N4-AQ/Fe(III) system under deaerated condition showed evidence that the role of •OH can be excluded for the degradation of organic compounds: target. The photochemical mechanism of g-C3N4-AQ/Fe(III) system in accordance with dissolved oxygen is shown in Fig.

CONCLUSIONS

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