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N2O and NH3 Formation Over Three-Way Catalytic Converters - the Effect of Low Sulfur Gasoline

Isidro Mejia-Centeno, Dept. of Process Engineering, Universidad A. Metropolitana - Iztapalapa, A.P. 55-534, Mexico, DF, 09340, Mexico and Gustavo A. Fuentes, Department of Process Engineering, Universidad A. Metropolitana - Iztapalapa, A.P. 55-534, Mexico, DF, 09340, Mexico.

The introduction of low sulfur gasoline in many countries has an important positive impact on regulated emissions from gasoline cars equipped with three-way catalytic converter (TWC) [1]. However, the effect of low sulfur gasoline upon N2O and NH3 formation over the TWC is a problem that has not received much attention in the literature. Only a small number of reports have focused on the role of sulfur in the formation of side products from the TWC operating under practical conditions [2].

Real-time data from vehicles showed that the factors that play a major role in the formation of N2O and NH3 from gasoline cars are the catalyst type, driving cycle, TWC temperature, vehicles speed, vehicle power and air-to-fuel ratio [3]. In order to understand how NH3 and N2O are evolving and what will be their impact in the near future, in this work we report the effect of low sulfur gasoline on the operation of commercial converters. This is a matter of concern because N2O is the third most important greenhouse gas in terms of its global warming potential and NH3 contribute to the production of fine particles in the atmosphere, besides, the number of vehicles equipped with TWC is increasing steadily.

Ours result show that a large quantity of N2O is produced at low temperature, between 200 and 400°C, during cold start. The emission of N2O is independent of the air-to-fuel ratio. Under rich conditions, the presence of SO2 promotes N2O formation via NO reduction by CO. Under lean conditions at high temperature (400-600°C) N2O is produced by reduction of NO by C3H8 over TWC.

In the absence of SO2, the TWC produces a large amount of NH3 at 500°C under rich conditions. Ammonia emission can be explained by the reduction of NO by H2 generated via steam reforming and water-gas shift reactions on the TWC. The presence of SO2 in the feed stream inhibits both reactions, decreasing NH3 formation.


Emission of as yet unregulated pollutants such as NH3 and N2O formed in three-way catalytic converter operating with low-sulfur gasoline is a factor that has also important implications upon the formation of particulates and clearly upon human health. Our results also indicate the need to reformulate TWC to address this problem.


We acknowledge the support of CONACYT (Projects 400200-5-38049U, 400200-5-29272U and CIAM 2005-C02-51844/A-1) and UAMI. IMC wishes to thank the Doctoral fellowship from Mexican Institute of Petroleum.


1.   D.D. Beck in “Catalysts Deactivation 1997”, (C.H. Bartholomew and G.A. Fuentes, Eds.) Stud. Surf. Sci. Catal. Vol. 111 p 21 Elsevier, Amsterdam, 1997.

2.   I. Mejía-Centeno, A. Martínez-Hernández and G.A. Fuentes Top. Catal. 42-43 (2007) 381

3.   T. Huai, T.D. Durbin, J.W. Miller and J.M. Norbeck Atmospheric Environment. 38 (2004) 6621