Article pubs.acs.org/JPCC
Reaction of Nitrogen Dioxide with Ice Surface at Low Temperature (≤170 K) Jaehyeock Bang, Du Hyeong Lee, Sun-Kyung Kim,† and Heon Kang* Department of Chemistry, Seoul National University, 1 Gwanak-ro, Seoul 151-747, Republic of Korea S Supporting Information *
ABSTRACT: We studied the adsorption and reaction of nitrogen dioxide gas on the surface of an ice film at temperatures of 100−170 K under ultrahigh vacuum (UHV) conditions. Cs+ reactive ion scattering (RIS) and low-energy sputtering (LES) techniques were used to identify and quantify the reactants and products on the surface of the ice film, in conjunction with the use of temperature-programmed desorption (TPD) to monitor the species desorbed. Temperature-ramping experiments were performed to examine the changes in the populations of these species as a function of temperature. Adsorption of NO2 gas on the ice film at 160 K reconverts the dissolved ionic species into NO2 and HONO gas. Interestingly, the products and intermediates detected in the present experiment include all the products of NO2 hydrolysis in aqueous solution (reaction 1): 2NO2 (g) + H 2O(l) → HONO(g) + H+(aq) + NO3−(aq)
(1)
Owing to the similarity of products, it may be claimed that NO2 is as completely hydrolyzed on the ice surface as in aqueous solution. As a counter argument, it may be possible that the observed reactions on the ice surface only correspond to certain intermediate stages of NO2 hydrolysis given that the mobility of water molecules is low and the solvation of adsorbed species must be incomplete. Another notable feature is the formation of diverse hydrolysis products, even at the lowest temperature investigated (≤100 K), although their formation yield is relatively small. The diversity of reaction channels may be a unique characteristic of the ice surface, which may be related to the fact that the ice surface provides a wide variety of binding sites and water-coordinating environments for adsorbing molecules owing to different dangling bond arrangements of the surface and different dipole directions of neighboring water molecules.22,26 4-B. Comparison of NO2 Reactions on Ice and in Liquid Water Films. It is well-known27 that the heterogeneous reactions of NO2 on wet surfaces produce HONO gas, which may be important as a precursor for hydroxyl radicals in polluted urban environments. Kinetic studies of this reaction in the laboratory model experiments show that the rate of formation of nitrous acid is first-order with respect to the concentrations of NO2 and water vapor.28−33 These observations refute the possibility that reaction 1 occurs in a single-step as a termolecular process or involves NO2−NO2 encounter as the rate-determining step. This mechanistic interpretation of the reaction on water films is consistent with the observations on ice films, whereby the efficiency of the reaction of NO2 decreases with increasing NO2 coverage and is inversely correlated with the surface population of N2O4.15 The simplest mechanism that can be deduced from these observations is that NO2 hydrolysis on ice occurs through the reaction between an isolated NO2 molecule and water. Quantum chemical calculations indicate that the reaction of NO2 with small water clusters requires a substantially high activation energy (∼120 kJ mol−1), and the energy barrier is not significantly reduced by the effect of water solvation.34 There is an apparent discrepancy between this high energy barrier for the NO2 reaction with water cluster and the facile occurrence of the reaction on water films at room temperature
5. SUMMARY AND CONCLUSION In the present study, the interaction of NO2 gas with ice surface was investigated by preparing a thick ice film that could be maintained under UHV up to ∼170 K, which is close to the stratospheric temperature (>180 K). The use of a thick ice film eliminated the possibility that the reaction on the sample surface is affected by the metal substrate or a roughening transition of the ice film; these effects could interfere with the measurement of the reaction on a thin (∼4 BL) ice film in the previous work.15 Also, TPRIS and TPLES measurements were performed under the conditions of low NO2 coverage to investigate the reaction mechanism more closely. The following deductions about the reaction of NO2 on the ice surface could thus be drawn. (i) Adsorption of NO2 gas on the ice surface produces various surface species, including NO2δ‑, HONO, NO3−, H3O+, and N2O4. Among these, NO2δ‑ is the major species present on 22022
DOI: 10.1021/acs.jpcc.5b05497 J. Phys. Chem. C 2015, 119, 22016−22024
Article
The Journal of Physical Chemistry C
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