Abstract:

Objective In order to promote the practical application of deep treatment of organic pollutants in slightly polluted water bodies using heterogeneous photocatalysis, mesoporous TiO₂ photocatalyst as a novel photocatalyst with a pore size of 2-50 nm has a particle size of larger than 200 nm, so it was very easy to recycle, thus avoiding the potential nano-toxicity of the nano photocatalyst.

Method In order to obtain a visible-light-responsive mesoporous TiO₂ photocatalyst, chiral mesoporous TiO₂ with spirally-stacked structure was prepared by a soft template method constructed with chiral surfactants. By means of various characterization methods such as X-ray spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, surface area and pore size analysis, and circular dichroism (CD), the differences in structure and visible light response of chiral mesoporous TiO₂ and achiral mesoporous TiO₂ were compared and analyzed. The photodegradation experiment for Rhodamine B (RhB) under visible light excitation was adopted to evaluate their catalytic performance, thus exploring the mechanism and pathway for degrading RhB by chiral mesoporous TiO₂.

Results The average pore diameters of the two mesoporous TiO₂ were 6.4 nm and 8.6 nm. The specific surface area, pore volume and pore size of chiral mesoporous TiO₂ prepared by chiral surfactants were slightly smaller than those of achiral mesoporous TiO₂. The particle size of the chiral mesoporous TiO₂ particles was about 200 nm, and it presented an obvious helical packing structure, which also showed a significant chiral correspondence effect. On the other hand, the morphology of achiral mesoporous TiO₂ did not show the structure of helical stacking, but only showed the aggregation structure of some particles. Both chiral mesoporous TiO₂and achiral mesoporous TiO₂had two mixed crystal forms of anatase and rutile (Fig.4). The helical stacking structure of chiral mesoporous TiO₂ introduced more defects into the catalyst, so that the contents of Ti³⁺ and oxygen holes were higher than those of mesoporous TiO₂ (Fig.5). Owing to its large specific surface area and excellent visible light response performance, chiral mesoporous TiO₂ had a high degradation activity for RhB (the removal rate reached 78% within 5 h), and the degradation process conformed to first-order kinetics (Fig.6). The photocatalytic performance of achiral mesoporous TiO₂ (the removal rate was only 16% within 5 h) was much lower than that of chiral mesoporous TiO₂(Fig.6). Although the adsorption performance of the two catalysts for RhB was similar, the removal rate of RhB by chiral mesoporous TiO₂ was more than 4 times that of achiral mesoporous TiO₂(Fig.6). Radical trapping experiments and electron spin resonance (ESR) spectroscopy showed that the active species of chiral mesoporous TiO₂ to degrade organic pollutant molecules under the excitation of visible light are ·O2-, ·OH and photogenerated h⁺ (Fig.7 and 8). When capturing ·O2-, ·OH and h⁺ during the photodegradation, the removal rates for RhB by the chiral mesoporous TiO₂ decreased by 19.2%, 39.7% and 60.2%, respectively, compared with the photodegradation process without adding capture agent (Fig.7). It showed that ·O2-, ·OH and h⁺ all participated in the degradation of RhB as active species in the photodegradation process. And h⁺ was the main active species for degrading organic pollutants, followed by ·OH, and ·O2- was the least involved in the photodegradation (Fig.8). The calculation of the Fukui index (f^-) of each atom in the RhB molecule proved that the atomic sites that were more likely to give electrons were easily attacked by photogenerated holes for degradation (Fig.9). By analyzing the intermediate products generated during the degradation process (Tab.2), the main pathway of the RhB degradation by chiral mesoporous TiO₂ under irradiation of visible light was further obtained (Fig.10).

Conclusion From the results of our work, the degradation pathway of RhB pollutants was obtained. The first step was that h⁺ attacked on the C—N bond of the RhB molecules to remove the ethyl group. Then, multiple demethylation and deethylation reactions, and deamination processes were carried out. Until the vulnerable C—N bond site disappears, the h⁺ would attack the carboxyl group with high electron density and the benzene ring to enable the ring-opening reaction to be continued, and finally RhB was mineralized into CO₂, H₂O and other inorganic substances.

Key words: chiral mesoporous TiO₂, visible light photodegradation, Rhodamine B, dye pollutant, degradation pathway, wastewater treatment