Heterogeneous Reaction of NO₂:  Characterization of Gas-Phase and Adsorbed Products from the Reaction, 2NO₂(g) + H₂O(a) → HONO(g) + HNO₃(a) on Hydrated Silica Particles

Abstract

This study provides the first in-situ characterization of the products in the heterogeneous reaction 2NO 2 (g) + H 2 O(a) → HONO(g) + HNO 3 (a). Transmission FT-IR spectroscopy and UV−vis spectroscopy were used to probe the heterogeneous chemistry of gas-phase nitrogen dioxide on hydrated SiO 2 particles. At 298 K, gaseous nitrogen dioxide reacts with adsorbed water on the surface of SiO 2 particles to form adsorbed nitric acid and gas-phase nitrous acid. FT-IR spectroscopy of the hydrated SiO 2 surface following reaction with NO 2 provides definitive identification of the adsorbed nitric acid product. The reaction does not occur when gas-phase nitrogen dioxide is reacted on dehydrated silica particles. The gas-phase product nitrous acid was detected with both FT-IR and UV−vis spectroscopy. Although homogeneous and heterogeneous reactions can contribute to the observed HONO signal, flow cell measurements can be used to distinguish between heterogeneous and homogeneous reaction pathways leading to HONO production.