Preparation and Photocatalytic Activity of TiO₂/Fine Char for Removal of Rhodamine B

2.1. Preparation of FC Carrier

FC was obtained from the U-GAS gasifier. 20 g of the FC was mixed with 200 mL distilled water under magnetic stirring for 6 hours to remove the soluble compounds. The disposed FC was dried at 105°C for 12 hours and then calcined in muffle furnace at 500°C for 3 hours in air.

2.2. Preparation of TiO₂/FC Photocatalyst

Sol-gel method was utilized to prepare the TiO₂/FC composite photocatalyst with tetrabutyl titanate as the precursor and FC as the carrier. Firstly, the tetrabutyl titanate (10 mL) was added into the solution containing glacial acetic acid (5 mL) and absolute ethyl alcohol (30 mL) drop by drop under the vigorous stirring. After stirring for 30 min, FC (10 g) was introduced with continuous stirring for 30 min. Subsequently, the mixing solution of distilled water (4 mL) and absolute ethyl alcohol (6 mL) was slowly dripped into the aforementioned mixture under rapid stirring. Thus, the TiO₂/FC gel was obtained. The gel was aged at room temperature for 24 h, followed by drying at 70°C for 24 h. After calcination in air at 500°C for 2 h, the TiO₂/FC photocatalyst was obtained, and the amount of TiO₂ in TiO₂/FC catalyst was 19%. In contrast, pure TiO₂ nanoparticles were also prepared by the same procedure without the addition of FC.

2.3. Characterization

XRD was performed on a Rigaku X-ray diffractometer with Cu-Ka radiation (Bruker D8 ADVANCE diffractometer, λ = 1.5406 Å), over the 2θ scanning angle range 10 to 80°. The crystallite size of TiO₂ was calculated by using the Scherrer formula, d = kλ/(βcos⁡⁡θ) [13], where d is the crystallite size, k is the shape factor (0.9), λ is the X-ray wavelength (1.5406 Å), β is the line broadening at half of the maximum intensity, and θ is the diffraction angle. SEM (JSM 6700F, Japan) was used to investigate the surface morphology at an accelerating voltage of 5 kv. The surface area (S_BET) was calculated from the nitrogen adsorption isotherms obtained at 77 K using a NOVA 2000e apparatus.

2.4. Photocatalytic Experiment

3.1. Material Characterization

The composition and crystal structure of TiO₂/FC photocatalyst were identified by XRD, and for a comparison, the XRD patterns of FC and TiO₂ were also analyzed. As seen in Figure 1, the diffraction peaks at 2θ = 25.4°, 37.8°, 48°, 55°, and 62.6° can be perfectly indexed to those XRD patterns of anatase TiO₂ nanoparticles [14]. Compared with diffraction peaks of pure FC, the peak intensity of FC in TiO₂/FC was obviously weakened and only two relatively narrow diffraction peaks can be observed at 2θ = 20.8° and 26.5°. XRD analysis strongly confirmed the existence of anatase TiO₂ on FC. The crystallite size of pure TiO₂ was calculated to be about 31 nm and the crystallite size of TiO₂ on the TiO₂/FC was 12 nm, indicating that TiO₂ particle size was significantly decreased after its loading onto FC, which will be favorable for its improvement of photocatalytic activity and efficiency. The BET measurement indicated that the specific surface area of TiO₂, FC, and TiO₂/FC reached 3.235, 28.544, and 39.113 m²/g, respectively, which further suggested that the TiO₂ loading on FC was good for the enhanced photoactivity.

The morphology of FC, TiO₂, and TiO₂/FC was characterized by SEM. As seen in Figure 2(a), FC contained coarse, angular, and irregular particles. For the operational temperature range of U-GAS gasifier is 850~1000°C and FC cannot enter into the liquid state, giving rise to irregular particle shapes. The poor dispersion and obvious agglomeration were observed in the SEM image of TiO₂ (Figure 2(b)). It could be seen from Figure 2(c) that the porous TiO₂ coating was uniformly modified onto the surface of FC, indicating a successful immobilization of TiO₂. Such results were consistent with the XRD analysis.

3.2. Photocatalytic Activity Test

The photocatalytic activity of TiO₂/FC was evaluated by monitoring the degradation efficiency of RhB in aqueous solution, which was also compared with other photocatalysts, for example, TiO₂ and FC. As shown in Figure 3, the degradation efficiency of RhB was low in the dark and reached a ready value after 15 min, indicating that it was difficult for the degradation of RhB in the absence of light. Under UV irradiation, the degradation efficiency of RhB was significantly enhanced and increased with the increase of time. At a given time, the photodegradation efficiency of RhB on TiO₂/FC was obviously higher than that on pure TiO₂ and FC. After UV irradiation for 90 min, the photodegradation efficiency of RhB on TiO₂/FC, TiO₂, and FC reached 90%, 80%, and 33%, respectively. The high photocatalytic activity of TiO₂/FC may be understood from two aspects. On one hand, the TiO₂ modified onto FC possesses a better dispersibility and the number of photocatalytic activity sites will be increased, causing an enhanced photodegradation of RhB molecules. On the other hand, FC is strong adsorbent and can provide a high-density environment for the photocatalytic reaction, speeding up the efficiency of photocatalytic degradation [15].

Photocatalytic degradation of RhB depended on the amount of the catalyst used. Figure 4 shows the photodegradation efficiency of RhB when the concentration of TiO₂/FC was varied from 0.3 to 0.7 g/L. The photodegradation efficiency of RhB increased with an increase in TiO₂/FC concentration up to 0.5 g/L, and after that a further increase in catalyst concentration led to a slow decrease in the photodegradation efficiency of RhB. Such phenomena can be understood from two aspects. On one hand, the increase of TiO₂/FC concentration will increase the number of photoactive sites and also the number of the RhB molecules absorbed. And a further increase of the catalyst concentration beyond 0.5 g/L may cause light scattering and screening effects, which will hinder the penetration of light and reduce the specific activity of the catalyst. On the other hand, at high catalyst concentration, it is difficult to maintain the homogeneous suspension due to particle agglomeration, which may also reduce the catalytic activity. So the photodegradation efficiency of RhB decreases gradually. In the present study, the optimum concentration of TiO₂/FC is found to be 0.5 g/L for the degradation of RhB.

The reusability of the photocatalyst is of great importance in the photodegradation of organic contaminants. To evaluate the reusability of TiO₂/FC, the photodegradation experiment of RhB was performed repeatedly for six times under the same experimental condition. As seen in Figure 5, the photodegradation efficiency of RhB showed a slight change and remained as high as 97.6% after 6 cycles. This indicates that TiO₂/FC can be reusable with meager loss in activity during the photocatalytic oxidation of RhB molecules. The long-term storage stability was also investigated and the obtained results were shown in Figure 6. After storage for 50 days, the photodegradation efficiency of RhB on TiO₂/FC decreased only 3.3% compared to the initial value. The results indicate the satisfactory reproducibility and stability of TiO₂/FC towards RhB degradation.