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

Abstract

We investigated the heterogeneous processes that contribute towards the for- mation of N2O in an environment that comes as closely as possible to exhaust conditions containing NO and SO2 among other constituents. The simultaneous presence of NO, SO2, O2, and condensed phase water in the liquid state has been confirmed to be necessary for the production of significant levels of N2O. 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. The replacement of NO by either NO2 or HONO significantly in- creases the rate constant for N2O 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 N2O formation for the three different nitrogen reactants reveal the following series of increasing reactivity: indicating the probable se- NO , NO , 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 N2O are largest at of a H2SO4/H2O solution. The pH 5 0 results suggest that HONO is the major reacting N(III) species over a wide range of acidities studied. The N2O 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 O2 to NO2 mainly in the gas phase, with the presence of both H2O and active surfaces significantly accelerating NO2 production. Subsequently, NO2 reacts with excess NO to obtain HONO which reacts with S(IV) to result in N2O and H2SO4 through a complex reaction sequence probably involving nitroxyl (HON) and its dimer, hyponitrous acid. q 1997 John Wiley & Sons, Inc. Int J Chem Kinet: 29: 869- 891, 1997.