Kinetic modeling of promotion and inhibition of temperature on photocatalytic degradation of benzene vapor

This study investigated the effects of temperature, humidity, and benzene concentration on the photocatalytic oxidation of benzene vapor over titanium dioxide. An annular packed-bed photocatalytic reactor was employed to determine the intrinsic oxidation rates for the photocatalysis of benzene. Degussa P-25 TiO2 was used as the photocatalyst and a 15 W near-UV lamp (350 nm) was used as the light source. The experiments were conducted at influent benzene concentrations of 250-450 ppmv, water vapor concentrations of 13,500-27,500 ppmv, and reaction temperatures ranging from 100 to 200 °C. Benzene oxidation rates increased with temperature below 160-180 °C, but decreased with temperature above 160-180 °C. Raising the reaction temperature increased the chemical reaction rates but reduced the reactant adsorption rate on TiO2 surfaces. The overall reaction rate increased with temperature, indicating that the reduction of reactant adsorption rate did not affect the overall reaction, and thus the chemical reaction was the rate-limiting step. As the chemical reaction rate gradually exceeds the reactant adsorption rate with temperature, the rate-limiting step was shifted from the chemical reaction to the reactant adsorption. Additionally, the competitive adsorption between benzene and water for the active sites on TiO2 resulted in the promotion and inhibition of reaction rate by humidity. This study developed a modified bimolecular Langmuir-Hinshelwood kinetic model to simulate the temperature and humidity related promotion and inhibition of the photocatalysis of benzene. The correlation developed here was used as a basis for determining the apparent activation energy of 0.76 kcal/mol and adsorption enthalpies of benzene and water of −20.1 and −13.7 kcal/mol.