(298d) On The Enhanced Emission Of NH3 And Fine Particles From Gasoline Vehicles Operating With Low Sulfur Gasoline

NH3 is known to contribute to the production of aerosol particles containing ammonium nitrate or ammonium sulfate. Nitrates are formed in the atmosphere by the reaction of HNO3 and NH3, whereas sulfate salts are produced by the reaction between H2SO4 and NH3. Both reactions depend on temperature, relative humidity and most importantly, on the gas phase concentration of NH3 [1]. There is indication that NH3 emission and fine particles increase with the use of low sulfur gasoline by vehicles equipped with three-way catalysts (TWC) [2-3], favoring the formation of fine particles directly on the TWC. In this work we report on experiments with commercial TWC and simulations showing how low-sulfur gasoline favors formation of NH3 and fine particles as byproducts during operation of commercial Pd TWC and discuss some of the environmental consequences. A commercial TWC was utilized; the sample had 6.5 g/ft3 of Pd, 23.81%w of Al2O3, 2.56%w of CeO2, 0.60%w of BaO and 400 cpsi. Monolith section with 16 channels (4x4) was packed in a tubular reactor mounted in an electric furnace. The feed stream composition was 2100 ppm of NO, 515 ppm of C3H8, 6700 ppm of CO, 2200 ppm of H2, 10 % vol. of H2O, SO2 was varied in the range 0-50 ppm, and the gas balance was N2. The air to fuel ratio used was varied between 14.3 and 14.8 (from rich to lean) with a total flow of 950 cm3/min. The analysis of reactants and products was made by GC (HP 6890 and Shimadzu GC-12A) and on line with an FTIR (Bruker Tensor 27) equipped with 0.75 m path-length infrared gas cell heated at 120ºC to prevent condensation. Spectra were acquired at resolution of 4cm-1. The reactor temperature was raised from 25ºC to 600ºC at 2ºC/min. Our results indicate that in the absence of SO2, the TWC produces an important quantity of NH3 at 500°C under rich conditions. Ammonia emission can be explained by the reduction of NO by H2 generated via steam reforming (SR) and water-gas shift reaction (WGS) on the TWC. The presence of SO2 in the feed stream inhibits both reactions, decreasing NH3 formation. This agrees with Gandhi and Shelef [5], but we found that N2O is also formed. During SO2 oxidation, SO3 is the first species formed on the ceria surface. SO3 can react with water to produce H2SO4 [6]. In this environment, the interaction of water, H2SO4 and NH3 produces fine particles of ammonium bisulfate and sulfate. This emission correlates with reports showing that fine particles and NH3 increase during acceleration events using low sulfur gasoline [2-3]. Besides, it has been reported that the fraction of SO2 which forms aerosols increases upon SO2 removal [6]. The emission of NH3 from the TWC correlates with the emission of fine particles. Both NH3 and particles are still considered to be secondary pollutants, but our study shows that as the sulfur level in gasoline decreases, their emission from vehicles equipped with TWC running on low sulfur fuels increases. This is a source of emissions generally disregarded in atmospheric studies, but that has a potentially important effect in lowering the air quality in urban settings, hence affecting the human health. References 1.Salcedo, D., Molina, L.T., Molina, M.J., et. al, Atmos. Chem. Phys. 6 (2006) 925 2.Mejía-Centeno, I., Martínez-Hernández, A., and Fuentes, G.A., Topics Catal , in press 3.Mariq, M.M., Chase, R.E., Xu, N., and Podsiadlik, D.H., Environ. Sci. Technol. 36 (2002) 276 4.Lou, T., Vohs, J.M., Gorte, R.J., J. Catal. 210 (2002) 397. 5.Gandhi, H. S., Shelef, M., Appl. Catal. 77 (1991) 175 6.Hirota, K., Makela, J., and Tokunaga, A., Ind. Eng. Chem. Res. 35 (1996) 3362.