(83f) N2O and NH3 Formation Over Three-Way Catalytic Converters - the Effect of Low Sulfur Gasoline

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.

Acknowledgements

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.

References

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