Comprehensive study about the heterogeneous conversion of NH3 on nanoscale TiO2 particles: insight into the mechanism and atmospheric implications

Abstract

The heterogeneous reactions of NH₃ on TiO₂ were comprehensively investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and a flow tube reactor. The effect of SO₂ and illumination on the formation of surface and gaseous products was explored. The results reveal that surface hydroxyls and H₂O play important roles in the adsorption of NH₃ while adsorbed sulfur species like sulfite and sulfate can significantly promote the conversion of NH₃ to NH₄⁺ under dark conditions. The reaction mechanism under light conditions is quite different from that under dark conditions. NH₃ can be directly converted into gaseous NO and NO₂ and particulate nitrate on the surface of TiO₂ by photooxidation. Surface sulfate formed from the photooxidation of SO₂ can combine with NH₃ to form ammonium sulfate, which inhibits the oxidation of NH₃ and alters the production ratios of NO/NO₂. The uptake coefficients of NH₃ on TiO₂ are in the range of 1.5–2.5 × 10⁻⁵ regardless of illumination while SO₂ shows an inhibition effect on the initial uptake of NH₃ due to competition in adsorption on surface hydroxyls. These results demonstrate that the heterogeneous reaction of NH₃ on TiO₂ can be a significant sink of NH₃ while its transformation pathway greatly depends on the environmental factors in the atmosphere.