||This study investigated the effects of temperature and humidity on the photocatalytic oxidation of volatile organic compound (VOCs) over titanium dioxide. Benzene, methyl tert-butyl ether (MTBE), perchloroethylene (PCE), and toluene were selected to investigate the influences of temperature and humidity on photocatalytic conversion. Among these four VOCs, benzene and MTBE were selected for the investigation of reaction pathways and kinetics. |
This work employed a self-designed annular packed-bed photocatalytic reactor to determine the conversion and reaction rates during photocatalytic degradation of VOCs. Degussa P-25 TiO2 was used as the photocatalyst and a 15 W near-UV lamp (350 nm) served as the light source. Benzene conversions increased with temperature below 160 ºC, but decreased above 160 ºC. Moreover, the conversions of MTBE increased with temperature from 30 to 120 ºC, and the thermocatalytic reaction began above 120 ºC. The conversions of PCE decreased as the temperature increased from 120 to 200 ºC. Toluene conversions almost remained constant at 100~200 ºC. Based on the gas-solid catalytic reaction theory, raising the reaction temperature could promote the chemical reaction rate and reduce reactant adsorption on TiO2 surfaces. The overall reaction rate increased with temperature, indicating that the reduction of reactant adsorption did not affect the overall reaction, and thus the chemical reaction was the rate-limiting step. As the chemical reaction rate gradually increased and the reactant adsorption decreased with temperature, the rate-limiting step could shift from the chemical reaction to the reactant adsorption, while the overall reaction rate decreased with temperature. Additionally, the competitive adsorption between VOCs and water for the active sites on TiO2 resulted in VOCs influent concentration and humidity promoting or inhibiting the reaction rate.
The mineralization of benzene and the selectivity of CO and CO2 were not obviously affected under various temperatures, humidities, and influent benzene concentrations. The benzene mineralization ratios ranged from 0.85 to 1.0, to which CO and CO2 contributed approximately 5~20% and 80~95%, respectively. Temperature and humidity variation did not influence the photocatalytic reaction pathway of benzene. Acetone (AC) and tert-butyl alcohol (TBA) were two major organic products for the photocatalysis of MTBE. The addition of water transferred the reaction pathway from producing AC to TBA, while the temperature increase transferred the reaction pathway from producing TBA to AC.
A modified bimolecule Langmuir-Hinshelwood kinetic model was developed to simulate the temperature and humidity related promotion and inhibition of the photocatalysis of benzene and MTBE. The competitive adsorption of VOCs and water on the active sites resulted in VOCs influent concentration and humidity promoting or inhibiting the reaction. The reaction rate constant increased with temperature while the adsorption equilibrium constants decreased, confirming that increasing reaction temperature enhanced the chemical reaction, but reduced the adsorption of VOCs and water. Furthermore, the correlation developed here was also used for determining the apparent activation energy of photocatalytic oxidation of VOCs and the adsorption enthalpies of benzene, MTBE, water vapor, and oxygen.