The heterogeneous formation of N2O in the presence of acidic solutions: Experiments and modeling

Abstract

We investigated the heterogeneous processes that contribute towards the formation of N 2 O in an environment that comes as closely as possible to exhaust conditions containing NO and SO 2 among other constituents. The simultaneous presence of NO, SO 2 , O 2 , and condensed phase water in the liquid state has been confirmed to be necessary for the production of significant levels of N 2 O. The maximum rate of N 2 O formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its volume. The replacement of NO by either NO 2 or HONO significantly increases the rate constant for N 2 O formation. The measured reaction orders in the rate law change depending upon the choice of the nitrogen reactant used and were fractional in some cases. The rate constants of N 2 O formation for the three different nitrogen reactants reveal the following series of increasing reactivity:indicating the probable se-NO Ͻ NO Ͻ HONO, 2 quential involvement of those species in the elementary reactions. Furthermore, we observed a complex dependence of the rate constant on the acidity of the liquid phase where both the initial rate as well as the yield of N 2 O are largest at of a H 2 SO 4 /H 2 O solution. The pH ϭ 0 results suggest that HONO is the major reacting N(III) species over a wide range of acidities studied. The N 2 O formation in synthetic flue gas may be simulated using a relatively simple mechanism based on the model of Lyon and Cole. The first step of the complex overall reaction corresponds to NO oxidation by O 2 to NO 2 mainly in the gas phase, with the presence of both H 2 O and active surfaces significantly accelerating NO 2 production. Subsequently, NO 2 reacts with excess NO to obtain HONO which reacts with S(IV) to result in N 2 O and H 2 SO 4 through a complex reaction sequence probably involving nitroxyl (HON) and its dimer, hyponitrous acid.

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

“…The physico-chemical characterization of these samples is presented below. For a detailed description of the procedure used in the study of the acidic solutions corresponding to sulfuric acid and aerosol mimics, see Pires, 1995. Most experiments were performed at standard gas phase concentration conditions: [SO4 = 3000 ppm, [NO] = 650 ppm in purified compressed air.…”

“…In the experiments carried out in the presence of water vapor the relative humidity (rh) was 100% at ambient temperature and 80 to 100% at temperatures up to 368 K. A detailed description of the experimental set-up and procedure may be found in the literature (Pires, 1995). For the experiments performed at low temperatures the N2 flow was saturated with water vapor (or nearly so) by passing it first through water before injection into the reactor.…”

“…and HONO using gas-phase UV absorption is less than 15% (Pires et al, 1995). Most series of measurements of this work were performed at least twice in order to ensure reproducibility of the results.…”

“…Based on experimental results recently presented by Pires and Rossi (1996) using sulfuric acid solutions (0.1 M) as a substrate we propose the following simplified mechanism for heterogeneous N20 formation from exhaust gases of combustion.…”

“…Our efforts are directed at understanding the complex chemistry which will enable us to predict or simulate N20 formation using laboratory data to describe heterogeneous and homogeneous chemical processes that take place in realistic environmental situations. A detailed mechanistic study on the heterogeneous formation of N20 over liquid sulfuric acid solutions has been published recently (Pires et al, 1996). The present work investigates the role of additional condensed phase surfaces such as soot, fly-ash and calcite on the rate of heterogeneous formation of N20.…”