Heterogeneous Reaction of NO₂ on α-Al₂O₃ in the Dark and Simulated Sunlight

Abstract

Although there have been a number of publications focused on heterogeneous of NO2 on mineral particles, most of these studies were focused on β-Al2O3 and performed in the dark. Less was known about the reaction process of NO2 on α-Al2O3, especially the effect of sunlight factor. The heterogeneous reaction between NO2 and α-Al2O3 was investigated by using diffuse reflectance infrared Fourier transform spectrometry. The effects of NO2 and O2 concentrations as well as simulated sunlight were examined, and the reaction mechanism including the consumption of surface OH groups, oxidation process of nitrites, and the formation of water was also discussed in detail. It was observed that the formation rates of nitrates and nitrites were sensitive to NO2 concentrations and O2 concentrations. Nitrite was identified to be an intermediate production and disappeared very soon as [NO2] was up to 4.035 × 10(15) molecules/cm(3). Light played an important role in the changes of the electronic configuration of mineral dust, such as electronic donating ability, surface OH groups orientation, as well as the conversion efficiency between proton acid and nonproton acid, all of which could significantly enhance the heterogeneous reaction process. The reaction order for NO2 and O2 was determined to be 0.960 ± 0.111 and 0.620 ± 0.028, respectively. The uptake coefficient of NO2, which dominated the first step of the heterogeneous reaction, was calculated by the infrared absorbance with the use of ion chromatography and determined to be 9.9 × 10(-10) in the dark and varied from 2.54 to 3.33 × 10(-9) under simulated sunlight from 0.45 to 1.35 mW/cm(2). It was also found that γNO2 was independent of [NO2] and sunlight increased the uptake coefficient by three times, indicating that the heterogeneous reaction between NO2 and α-Al2O3 was enhanced under sunlight.

Effect of Formaldehyde on the Heterogeneous Reaction of Nitrogen Dioxide on γ-Alumina

“…These results agree well with previous studies. 11,12 In addition, the band at 1586 cm -1 was observed growing as function of the exposed time.…”

“…15,19 The asymmetric stretching feature was overlapped by the broad bands of nitrate in our study. In the previous study, the band at 1245 cm −1 was assigned to bidentate nitrite by Guan et al, 12 bridging. 31 The band at 1540 cm −1 were assigned to the monodentate nitrate species.…”

“…This result is consistent with the previous study. 12 It was observed that saturation phenomenon of nitrate formation gradually appeared after the reaction proceeding for 40 min. The spectra of heterogeneous uptake of HCHO on γ-Al 2 O 3 particles after pretreatment with NO 2 (4.48 × 10 15 molecules cm −3 ) are shown in Figure 3b, and each spectrum was obtained by referencing to the coating γ-Al 2 O 3 spectrum after preadsorption of NO 2 .…”

“…Previous studies have shown that hydroxyl groups are the active sites for the heterogeneous reaction of NO 2 on the surface of γ-Al 2 O 3 particles. 9,12 However, some of the hydroxyl groups were also consumed by the preadsorbed HCHO, and the more HCHO was introduced, the more hydroxyl groups were consumed, which resulting in the decreasing of the active sites for the heterogeneous reaction of NO 2 , and thus, the formation of nitrate was suppressed. 12 In addition, methylene glycol formed through the preadsorption of HCHO and the formate and nitrate generated during the following heterogeneous reaction of NO 2 may also occupy the active sites for the reaction of NO 2 , which may be another reason for the suppression of nitrate formation.…”

“…12 Therefore, there would be more and more NO 2 reacting with HCHO, resulting in the formation rate of HCOOspecies changed as shown in Figure 4.…”