Reaction of NO₂(g) with NaCl(100)

The heterogeneous reaction of 2NO 2 (g) + NaCl(100) → NaNO 3 (s) + ClNO(g) has been investigated using infrared spectroscopy. Single crystals, chosen for their well-defined and almost defect-free surfaces, were exposed to NO 2 at four pressures: 0.15, 0.52, 1.5, and 3.5 mbar at 298 K. The formation of NO 3 - ions on NaCl was quantified from the photometry as the reaction progressed. The number of NO 3 - ions produced indicates that the reaction is not limited to the initial NaCl(100) face, but that nitrate ions also penetrate into the upper layers of the substrate. Despite an excess of NO 2, the NaCl crystal transformation to NaNO 3 ceases in a few hours. Gas phase ClNO is also observed spectroscopically. The strongest solid-state feature from the reaction is a well-defined sharp doublet at 1360 and 1414 cm -1 assigned to the ν 3 asymmetric stretching vibration of NO 3 - . Both bands grow in unison as the reaction progresses. We suggest that the doublet results from the splitting of the ν 3 degenerate mode in the monomers, dimers, or small ordered arrays of NO 3 - ions bound to neighboring ions of the NaCl substrate. There are as many as three regions that characterize the growth of NO 3 - at each NO 2 pressure. Two of these regions are used to measure rates of NO 3 - production. The order of the reaction was determined from these rates and was found to be quadratic in NO 2 pressure. A solid−gas reaction probability was also obtained from these reaction rates, assuming N 2 O 4 is the reactant, and was measured to be (1.3 ± 0.3) × 10 -6 . This reaction probability is smaller by nearly 2 orders of magnitude than those reported by other research groups who have used powdered NaCl substrates. Probability differences are not surprising since the kinetics of this reaction is expected to depend on the concentration of defects at the surface. A mechanism, involving fracture of the NaCl substrate and diffusion of NO 2 into its interior, accounts for the different growth regions and the production of NO 3 - . Finally when a small amount of water vapor is introduced with NO 2, the production rate of NO 3 - is enhanced dramatically. The possible roles of H 2 O in heterogeneous chemistry of NaCl are discussed.