Oxidative photocatalytic degradation of benzene vapor over TiO 2

Abstract

The variables included in a parametric study of the photocatalytic oxidative degradation (PCO) of benzene in a closed reactor over films of Degussa P-25 TiO 2 under fluorescent black-light radiation centered at 367 nm were: concentrations of benzene, O 2 and water vapor; photons/sec incident on the catalyst; and temperature. Concentrations of benzene and CO 2 were measured. All measurements of photocatalytic behavior were performed with freshly deposited films of P-25 which had been preconditioned by irradiating for 15 h while flowing 80 ml min −1 of dry air through the reactor. It was found that water vapor promoted the dark-evolution of CO 2 and inhibited the dark-adsorption of benzene by the preconditioned catalyst. Under irradiation, water vapor promoted the formation of CO 2 and the initial removal of benzene. Dark-adsorption of benzene increased linearly with concentration of benzene vapor. The initial rate of photocatalytic removal of benzene was first order in concentration of benzene vapor but the higher the initial concentration of benzene the more the rate of its removal diminished as reaction proceeded. Photocatalytic production of CO 2 lagged far behind removal of benzene. The initial rates of photocatalytic removal of benzene vapor and generation of CO 2 were, respectively, independent of concentration of O 2 from 800 ppmv to 100% and approximately one-third order in O 2. The initial kinetic orders in number of photons/sec entering the reactor of removal of vaporized benzene and generation of CO, over the range (7–42) × 10 −8 Einstein s −1 were, respectively, 1.85 ± .03 and 1.60 ± .07 in air into which 0.44 mmol of water had been injected and 1.59 ± .07 and 1.41 ± .05 in dry air. Over the range 15–70 °C, the rate of removal of benzene did not vary significantly with temperature in the presence or absence of water vapor while the rate of generation of CO 2 varied in a way corresponding, respectively, to E A = 3.9 ± 0.4 and 9.4 ± 0.9 kJ mol −1 in the presence and absence of water vapor. Mechanistic implications of the data are discussed.