Abstract :
Diffuse reflectance infrared spectroscopy was applied for investigating the effect of surface water on the reaction of NaCl with NO2. H2O/NaCl was prepared by water adsorption followed by evacuation at room temperature. The bands appearing between 1300 and 1500 cm−1 (asymmetric stretching of NO−3) were used for the kinetic analysis. The growth of the absorption at 1339 and 1452 cm−1 was observed in the initial period, which is followed by the secondary bands at 1358 and 1434 cm−1. The rates of band growths for these two stages were proportional to NO2 pressure. This dependence can be attributed to HNO3 formation on the surface followed by HNO3–NaCl reaction. On the contrary, on NaCl without water pretreatment (dry NaCl), the order of the band growths was quadratic in NO2 pressure, which suggests the reaction via N2O4. The reaction probabilities in the initial period were (1.5±0.2)×10−8 and (4±2)×10−5 for H2O/NaCl and dry NaCl, respectively. This difference is caused by that of the concentrations between NO2 and N2O4. H2O/NaCl showed a comparable or even higher rate than dry NaCl. Finally, the sequential exposure to NO2–water–NO2 was investigated. The spectra and the band growths at each exposure showed that, after a spontaneous stop of the spectral growth, H2O exposure regenerated H2O/NaCl surface which produces NO−3 almost as rapidly as fresh one.
