Enhanced emissions of NH3, N2O and H2 from a Pd-only TWC and supported Pd model catalysts: Light-off and sulfur level studies

In this work we study the formation of NH3, N2O and H2 side products during operation of three-way catalytic converters (TWC) in a simulated engine exhaust. We used a commercial Pd-only TWC and supported Pd model catalysts in light-off and during constant temperature tests under rich conditions. N2O is observed primarily during light-off and its low temperature formation (110–320 °C) proceeds via reduction of NO by CO when O2 is present, or by H2 in its absence. We detected also N2O formation at 500 °C that has not been reported previously, proceeding via reduction with CO.NH3 is formed in the 200–600 °C range via reduction of NO by H2. The H2 required to produce NH3 is generated via steam reforming and water gas-shift reactions over the catalysts. On a molar basis, the H2 emission is about five-fold higher than that of NH3 during cold start of TWC.We determined the effect of SO2 level during constant temperature experiments at 500 °C, extending the work of Gandhi and Shelef [1]. In CeO2-containing catalysts the interaction with SO2 favors the production of N2O, but lowers that of NH3, apparently by suppressing selectively the reaction path leading to H2 needed for the reaction.Our study suggests that the use of low and ultra-low sulfur gasoline by cars equipped with TWC lowers the emission of N2O to the atmosphere during high temperature operation but enhances the production of NH3 and H2. This is a matter of concern because NH3 is involved in the formation of nano, fine and ultra fine inorganic particles in the atmosphere.

The presence of SO2, or sulfur level in gasoline, seems to have a dual effect at high temperature, 500 °C. It suppresses selectively the reaction path leading to NH3, but N2O is promoted.Figure optionsDownload as PowerPoint slideHighlights
► Pd-only converters catalyze the formation of NH3, N2O and H2 from engine exhaust.
► NO reacts with CO and H2 to produce N2O below 320 °C.
► NH3 is formed by reduction of NO with H2 between 200 and 600 °C.
► H2 is generated over the catalyst by SR and WGS.