Kinetics of photocatalytic degradation of gaseous p-xylene on UiO-66-NH2 and LaFeO3 thin films under combined illumination of ultraviolet and visible lights

摘要

The thin film photocatalysts were prepared from solvothermal UiO-66-NH2 and sol-gel perovskite LaFeO3 by a dip-coating technique. The properties of obtained catalysts were investigated by the methods of Brunauer-Emmett-Teller adsorption, XRD, SEM, FT-IR, TGA, and UV-vis spectroscopies. The results proved that the thin film of the thickness of 4.2 and 4.7 mu m was successfully prepared from micro-mesoporous UiO-66-NH2 and LaFeO3 nanocrystals. Possessing small crystals (9-35 nm) and the band gap energy of 2.83 and 1.92 eV, respectively, UiO-66-NH2 and LaFeO3 are shown to be a highly active visible-light photocatalyst for photodegradation of p-xylene-contained gas. The kinetics of photocatalytic degradation of p-xylene under combined illumination of ultraviolet and visible lights over obtained UiO-66-NH2 and LaFeO3 thin films were carried out in a gradientless flow circulating system at room temperature and atmospheric pressure. The results showed that the Langmuir-Hinshelwood kinetic model was successfully applied to correlate the obtained data. The kinetics of the reaction on both catalysts were found to be written by the fractional equation, describing the dependence of the reaction rate on the concentration of p-xylene, oxygen molecules, dissociative adsorbed water vapor, the photon flux, and the inhibition of CO2 product. It implies that the reaction occurred at high surface coverages, molecular p-xylene, and oxygen participated in the reaction in the form of surface molecular; water-in the forms of (OH)-O-center dot and H+. Although there is a general kinetic equation for reaction on various catalysts, the different affinities of the catalysts with the compounds present in the reaction create the kinetic feature of the reaction on each catalyst. Because of the high affinity of catalysts with water vapor, the given reactant increased the rate of p-xylene photocatalytic degradation at low concentration but inhibited it in the high concentration region. The adsorption affinity of LaFeO3 to oxygen is higher compared to UiO-66-NH2; then, there was an optimum concentration of oxygen in the photodegradation of p-xylene on the former, whereas in the latter monotonically increasing relationship was found.