The heterogeneous reactions of toluene/O₃/NH₃ on hematite nanoparticles: the impact of light illumination on organic ammonium salt formation

Organic ammonium salts which are formed from heterogeneous reactions are one of the important components of nitrogen-containing organic compounds (NOCs) in the atmosphere. In order to investigate the formation process of organic ammonium salts, a gas-flow system with the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique was applied to monitor the influence of a simulated illumination on the heterogeneous reactions of toluene/O-3/NH3 on hematite nanoparticles. The results revealed that toluene was transformed into benzoic acid under the action of oxidants (O-3 and OH radicals). The carboxylic acid was neutralized with NH3 to form ammonium benzoate. The effect of light intensities on the reaction kinetics of ammonium benzoate formation from the heterogeneous reactions was also analyzed. With an increase in the light intensity from dark to 36 mW cm(-2), the reaction rates increased from (1.20 +/- 0.02) x 10(18) ions per g s(-1) to (2.30 +/- 0.09) x 10(18) ions per g s(-1). This induced the formation of abundant active radicals, which accelerated the conversion of toluene to ammonium benzoate on hematite nanoparticles. However, the reaction rates decreased to (1.80 +/- 0.03) x 10(18) ions per g s(-1) as the light intensity continued to increase to 100 mW cm(-2). The yield of organic ammonium salts might be reduced owing to the volatilization of ammonium benzoate at a high light intensity. Meanwhile, the initial uptake coefficient showed a similar change trend. The values of the uptake coefficient increased by 81.1% when the light intensity increased from 0 mW cm(-2) to 36 mW cm(-2) but decreased by 21.1% when the light intensity increased from 36 mW cm(-2) to 100 mW cm(-2). Our results not only propose the heterogeneous reaction kinetics of toluene/O-3/NH3 on the nanoscale hematite surface under different light intensity conditions, but also provide a theoretical support for further understanding the conversion process of volatile organic compounds (VOCs) under combined atmospheric pollution.