Spectroscopic Study of Nitric Acid and Water Adsorption on Oxide Particles:  Enhanced Nitric Acid Uptake Kinetics in the Presence of Adsorbed Water

Abstract

In this study, the heterogeneous reactivity of nitric acid on oxide particles of some of the most abundant crustal elements is investigated at 296 K. The oxide particles are used as models for mineral dust aerosol found in the atmosphere. Transmission FT-IR spectroscopy is used to probe changes in the spectrum of the oxide particle surface following adsorption of HNO 3 on SiO 2 , R-Al 2 O 3 , TiO 2 , γ-Fe 2 O 3 , CaO, and MgO. It is found that HNO 3 molecularly and reversibly adsorbs on SiO 2 . For the other oxides investigated, HNO 3 dissociatively and irreversibly adsorbs to form surface nitrate. There is also a small amount of molecularly adsorbed nitric acid (<10% of the adsorbed nitrate) on the oxide particle surface in the presence of gas-phase nitric acid. Because adsorbed water may play a role in the heterogeneous uptake of nitric acid in the atmosphere, transmission FT-IR spectroscopy is used to investigate H 2 O adsorption on SiO 2 , R-Al 2 O 3 , TiO 2 , γ-Fe 2 O 3 , CaO, and MgO particles as well. Uptake of water on the oxide particles can be described by a multilayer adsorption isotherm. Water uptake on nitrate-coated oxides remained similar for R-Al 2 O 3 and TiO 2 compared to the uncoated surface; however, for γ-Fe 2 O 3 , CaO, and MgO, the shape of the adsorption isotherm changed for the nitrate-coated particles with an increased amount of water adsorption at a given relative humidity. The infrared spectrum of the surface nitrate shows that water adsorbed on the particle surface can solvate the nitrate ion. The rate of nitric acid uptake on R-Al 2 O 3 and CaO is found to increase by nearly 50-fold when going from conditions near 0 to 20% relative humidity, indicating that the nitric acid dissociation kinetics on the wetted particle surface is significantly enhanced. In the case of MgO and CaO, the amount of nitric acid uptake is increased in the presence of water and is not limited to the surface of the particles, producing saturated solutions of Mg(NO 3 ) 2 and Ca(NO 3 ) 2 . From the studies presented here, atmospheric implications of heterogeneous reactions of HNO 3 with mineral dust aerosol are discussed.

In situ DRIFTS study of hygroscopic behavior of mineral aerosol

“…As shown in Fig. 2a, several bands increased simultaneously with the increase of relative humidity, which are attribute to the vibration of water according to literature (Goodman et al, 2001;AlAbadleh and Grassian, 2003;Gustafsson et al, 2005). The main absorption features observed in the spectra were 3420 cm −1 with a shoulder at 3240 cm −1 , 2100 cm −1 and 1640 cm −1 .…”

“…After this process, the oxides surfaces were covered with nitrate species. When the RH increased from 2% to 95%, these peaks disappeared while the peak at 1650 cm −1 due to the bending mode of water as well as two peaks at 1415 and 1350 cm −1 which were due to watersolvated nitrate (Miller and Grassian, 1998;Goodman et al, 2001) were observed (Fig. 7b).…”

“…The peak in the region of 2600-3800 cm −1 is a combination of OH vibrational modes: symmetric stretch around 3420 cm −1 and asymmetric stretch around 3240 cm −1 . The absorbance of these modes is suitable to monitor water adsorption on the surfaces of the particles; therefore the integrated intensity of the O-H stretching region provides a measure of the amount of water on the surface (Goodman et al, 2001;Gustafsson et al, 2005).…”

Effect of key parameters on the photocatalytic oxidation of toluene at low concentrations in air under 254+185nm UV irradiation

“…The effect of low levels of water can be interpreted as a contribution of adsorbed water to the regeneration of active surface species at low water coverage (i.e., 10% RH), since H 2 O molecules adsorb dissociatively to TiO 2 surfaces at low RH, generating surface OH groups [39,40]. For RH > 15-20%, the amount of water adsorbed on the surface of TiO 2 amounts to more than a monolayer [41], and the net effect is likely a competition with toluene for adsorption to the active surface sites that leads to a 17 decline in toluene reaction rates with increasing RH. Competitive desorption of toluene from TiO 2 surfaces in the dark upon increased water vapor has been described by Demeestere et al (2003), who observed equilibrium adsorption coefficient reductions of up to one order of magnitude when RH increased from 0 to 58% [34].…”

“…The population of surface OH is regenerated through dissociative chemisorption of water vapor under humidified air, thus becoming a limiting factor under dry air [39,40,43]. On the other side, the multilayer water film formed at 66% RH on the surface of the photocatalyst [41] has the potential capacity to retain larger amounts of polar oxidation byproducts and reduce their gas-phase levels. These arguments may justify in qualitative terms the observation of maximum Y F for intermediate humidity conditions at low residence time.…”