Heterogeneous reactions of toluene/NO2/NH3 on α-Fe2O3/α-Al2O3 particles: The effects of pollutant content and temperature changes on the formation of N-containing organic compounds (NOCs)

Abstract

Heterogeneous reactions of toluene with mineral oxides play a crucial role in the formation processes of N-containing organic compounds (NOCs). However, less attention was paid to the effects of pollutant content and temperature changes on the formation of NOCs under combined pollution. In this study, the combined effects on the heterogeneous reaction of toluene/NO₂/NH₃ with α-Fe₂O₃/α-Al₂O₃ particles were investigated by using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results show that the characteristic peak of the R-ONO₂ group appears at 1295 cm⁻¹, indicating benzyl nitrate formation in the presence of α-Fe₂O₃ with or without α-Al₂O₃. The addition of α-Al₂O₃ promotes the R-ONO₂ formation on the particle surface compared to pure α-Fe₂O₃ as a result of the dispersion and oxidation of the additional particles. The optimal mass ratio of α-Fe₂O₃ and α-Al₂O₃ particles is 3:1. Furthermore, the optimal concentration ratio for toluene, NO₂ and NH₃ is determined as 2.5:2.5:1. It is obvious that NO₂ and toluene are the primary sources of organic nitrates production. Higher concentration proportions of NO₂ and toluene within a certain range lead to higher formation amounts of NOCs during the heterogeneous reaction. Spectral analysis demonstrates that reaction kinetics, such as reaction rate and uptake coefficient, are sensitive to temperature variations. When temperature decreases from 298 K to 253 K, the reaction rate increases from (1.59 ± 0.04) × 10¹⁸ ions g⁻¹ s⁻¹ to (2.38 ± 0.02) × 10¹⁸ ions g⁻¹ s⁻¹. Low temperature favors the production of R-ONO₂, resulting from an exothermic process during the heterogeneous reaction. These findings are helpful for further exploring the combined effects on particulate NOCs formation and partly contribute to understanding of multicomponent reaction systems in real environment conditions.