The heterogeneous formation of N2O over bulk condensed phases in the presence of SO2 at high humidities

In view of the uncertainty of the origin of the secular increase of N₂O, we studied heterogeneous processes that contribute to formation of N₂O in an environment that comes as close as possible to exhaust conditions containing NO and SO₂, among other constituents. The N₂O formation was followed using electron capture gas chromatography (ECD-GC). The other reactants and intermediates (SO₂, NO, NO₂ and HONO) were monitored using gas phase UV-VIS absorption spectroscopy. Experiments were conducted at 298 and 368 K as well as at dry and high humidity (approaching 100% rh) conditions. There is a significant heterogeneous rate of N ₂ O formation at conditions that mimic an exhaust plume from combustion processes.The simultaneous presence of NO, SO₂, O₂ in the gas phase and condensed phase water, either in the bulk liquid or adsorbed state has been confirmed to be necessary for the production of significant levels of N₂O. The stoichiometry of the overall reaction is: 2 NO+SO₂+H₂O N₂O+H₂SO₄. The maximum rate of N₂O formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its volume. A significant rate of N₂O formation at 368 K at 100% rh was also observed in the absence of a bulk substrate. The diffusion of both gas and liquid phase reactants is not rate limiting as the reaction kenetics is dominated by the rate ofN₂O formation under the experimental conditions used in this work. The simultaneous presence of high humidity (90–100% rh at 368 K) and bulk condensed phase results in the maximum rate and final yield of N₂O approaching 60% and 100% conversion after one hour in the presence of amorphous carbon and fly-ash, respectively.Work performed in partial fulfillment of the requirements of Dr ès Sciences at EPFL.