Isomerization and ionization of N2O4 on model ice and silica surfaces

Isomerization and ionization of N2O4 on model ice and silica surfaces, hypothesized as key steps in atmospheric HONO formation, were studied using B3LYP and MP2 methods. The models employed are (H2O)(20) for ice and Si8O16H12 for silica. It is found that dangling surface -OH bonds play a key role in the isomerization to generate the 'active', asymmetric ONONO2. However, the computed barrier is high. Potential catalytic effects due to additional water molecules or local heating caused by photoabsorption are discussed. The results suggest that the isomerization of N2O4 into the active ONONO2 form on a variety of tropospheric surfaces having dangling -OH bonds should be considered in heterogeneous NOx chemistry. (C) 2012 Elsevier B.V. All rights reserved.