(426e) Impact of Low Sulfur Gasoline upon Nh3 and N2o Emission during Cold-Start of Three-Way Catalytic Converters

Environmental regulations have placed special interest
in the removal of sulfur from gasoline and Diesel fuels because of the harmful
effect of SO₂ upon the performance of three-way catalytic converters
(TWC) [1]. However, an area that has not received much attention concerns the
effect of low-sulfur fuels upon the emission of side products formed in the TWC
during engine operation. In this work we report how low-sulfur gasoline favors
formation of N₂O and NH₃ as byproducts during light-off
of commercial Pd or Pt-Pd-Rh-TWC and discuss some of the environmental
consequences.

Experimental

Two commercial TWC were utilized, the first one had 6.5
g/ft³ of Pd, while the second contained Pt, Pd, and Rh (9.3, 0.2 and
2.3 g/ft³, respectively). Monolith sections with 16 channels (4x4)
were 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 0-20 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.

Results and discussion

During light-off, a large quantity
of N₂O is produced at low temperature, between 200 and 400ºC, and is independent of the air to fuel ratio. N₂O is the main product initially, but
the selectivity to NH₃ increases as the temperature is raised. This
agrees with Gandhi and Shelef [2], but N₂O was not measured in that
study. In the high temperature range (400-600°C) and lean conditions, N₂O emission is favored by the presence of SO₂ and represents
the main source of N₂O emissions for measurements in both tunnel and
remote sensing studies. There are reports [3] showing a significant upward
trend in NH₃ and N₂O concentrations in air.
Interestingly, that trend correlates with the introduction of TWC and with the
decrease of sulfur in gasoline. As our study shows, the
emission of those two secondary pollutants from gasoline vehicles equipped with
TWC running on low sulfur fuels, although not yet regulated, can be an
important factor in lowering the air quality in urban settings, as well as
affecting global warming.

References

1.      Lou, T., Vohs, J.M., Gorte, R.J.,
J. Catal. 210
(2002) 397.

2. Gandhi, H. S., Shelef, M., Appl. Catal. 77
   (1991) 175.
3. Pérez-Ramírez, J., Kapteijn, J., Schoffel, K., Moulijn, J.A., Appl.
   Catal. B: Environ. 44 (2003) 117