A comparative study on the activity of TiO₂ in pulsed plasma under different discharge conditions

In the present study, a combination of pulsed discharge plasma and TiO₂ (plasma/TiO₂) has been developed in order to study the activity of TiO₂ by varying the discharge conditions of pulsed voltage, discharge mode, air flow rate and solution conductivity. Phenol was used as the chemical probe to characterize the activity of TiO₂ in a pulsed discharge system. The experimental results showed that the phenol removal efficiency could be improved by about 10% by increasing the applied voltage. The phenol removal efficiency for three discharge modes in the plasma-discharge-alone system was found to be highest in the spark mode, followed by the spark–streamer mode and finally the streamer mode. In the plasma/TiO₂ system, the highest catalytic effect of TiO₂ was observed in the spark–streamer discharge mode, which may be attributed to the favorable chemical and physical effects from the spark–streamer discharge mode, such as ultraviolet light, O₃,H₂O₂, pyrolysis, shockwaves and high-energy electrons. Meanwhile, the optimal flow rate and conductivity were 0.05 m³ l^-1 and 10 μS cm^-1, respectively. The main phenolic intermediates were hydroquinone, catechol, and p-benzoquinone during the discharge treatment process. A different phenol degradation pathway was observed in the plasma/TiO₂ system as compared to plasma alone. Analysis of the reaction intermediates demonstrated that p-benzoquinone reduction was selectively catalyzed on the TiO₂ surface. The effective decomposition of phenol constant (D_e) increased from 74.11% to 79.16% when TiO₂ was added, indicating that higher phenol mineralization was achieved in the plasma/TiO₂ system.