THE HETEROGENEOUS FORMATION OF N2O IN THE PRESENCE OF ACIDIC SOLUTIONS - EXPERIMENTS AND MODELING

We investigated the heterogeneous processes that contribute towards th e formation 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, O-2, and condensed phase water in the liquid state has been confirmed to be necessary for the product ion of significant levels of N2O. The maximum rate of N2O formation oc curred at the beginning of the reaction and scales with the surface ar ea of the condensed phase and is independent of its volume. The replac ement of NO by either NO2 or HONO significantly increases the rate con stant for N2O formation. The measured reaction orders in the rate law change depending upon the choice of the nitrogen reactant used and wer e fractional in some cases. The rate constants of N2O formation for th e three different nitrogen reactants reveal the following series of in creasing reactivity: NO < NO2 < HONO, indicating the probable sequenti al involvement of those species in the elementary reactions. Furthermo re, we observed a complex dependence of the rate constant on the acidi ty of the liquid phase where both the initial rate as well as the yiel d of N2O are largest at pH = 0 of a H2SO4/H2O solution. The results su ggest 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 Ly on and Cole. The first step of the complex overall reaction correspond s to NO oxidation by O-2 to NO2 mainly in the gas phase, with the pres ence of both H2O and active surfaces significantly accelerating NO2 pr oduction. Subsequently, NO2 reacts with excess NO to obtain HONO which reacts with S(IV) to result in N2O and H2SO4 through a complex reacti on sequence probably involving nitroxyl (HON) and its dimer, hyponitro us acid. (C) 1997 John Wiley & Sons, Inc.