Photonitration of pyrene adsorbed on silica gel with NO2

Abstract

To examine the heterogeneous photonitration of pyrene with NO₂ (approximately 0.2 ppm) on a heavy-traffic road, we studied the photonitration of pyrene adsorbed (pyrene_ads) on silica gel, which was used as SiO₂ in particulate matter (PM), with NO₂ (10.2, 2.0, and 0.20 ppm) under the atmospheric concentration ratio of pyrene_ads to NO₂ and compared the results with those obtained in the dark. The effects of irradiation, wavelength, and oxygen concentration in a NO₂ diluent on the photonitration were examined using a fluidized-bed column irradiated with simulated or real sunlight. Under the UV-light absorption of pyrene, the concentration of pyrene decreased exponentially in accordance with a pseudo-first-order reaction, while in the dark, it decreased sigmoidally in accordance with a H⁺-autocatalyzed reaction. The distribution and the yields of formed nitration products and their photooxidation products were affected by the light intensity, concentrations of NO₂, and oxygen in the NO₂ diluent. In the photonitration experiments using a high-pressure mercury lamp, formed 1-nitropyrene and minor dinitropyrenes were decreased by the transformation into their photooxidation products. Under 8-h exposure of pyrene to 10.2-ppm NO₂, the yield of 1-nitropyrene was 42% in N₂ and 28% in air. The oxygen inhibitory effect can be explained by the energy transfer from ¹pyrene* to oxygen. Radical cation intermediate (pyrene^•+-NO₂⁻) was proposed for 1-nitropyrene formation. Under 24-h exposure of pyrene to 2.0-ppm NO₂, the yields of 1-nitropyrene and the photooxidation products were 21.6% and 8.0%, respectively, in N₂ and 4.9% and 3.8%, respectively, in air. Under 24-h exposure of pyrene to 0.20-ppm NO₂, which is two times the 1-h NO₂ standard in the USA and China, the yields of 1-nitropyrene and the photooxidation products were 2.3% and 3.4%, respectively, in N₂ and 2.1% and 0.9%, respectively, in air. The significant decrease in the yields of 1-nitropyrene and the photooxidation products under the concentration of 0.20-ppm NO₂ can be explained by their easy photodecomposition with the increase in the photolysis of pyrene. Under the concentration of 0.20-ppm NO₂ in air, which is approximately the concentration on heavy-traffic roads, the decay rate of pyrene by the photonitration was increased by own photolysis, although the photonitration was inhibited by oxygen in air.