J. Phys. Chem. 1996, 100, 19891-19897
19891
X-ray Photoelectron Spectroscopy Studies of the Effects of Water Vapor on Ultrathin Nitrate Layers on NaCl J. M. Laux, T. F. Fister,† B. J. Finlayson-Pitts, and John C. Hemminger* Department of Chemistry and Institute for Surface and Interface Science, UniVersity of California, IrVine, IrVine, California 92697-2025 ReceiVed: May 23, 1996; In Final Form: August 5, 1996X
Understanding the reactive chlorine cycle in the troposphere is of importance to the ozone balance and oxidation of organics in marine regions. Sea salt particles, containing NaCl as the main constituent, are believed to be the major source of reactive tropospheric chlorine. To develop a fundamental understanding of the processes involved, we have carried out studies which utilize X-ray photoelectron spectroscopy (XPS) to follow the surface composition of NaCl single crystals as a function of sequential exposures to gaseous nitric acid and water vapor at room temperature. The uptake of HNO3(g) on NaCl was found to saturate the substrate surface, forming a metastable nitrate layer with a thickness on the order of 1-2 monolayers. Subsequent exposure of the nitrate layer to water at various pressures, well below the deliquescence points of NaCl and NaNO3, induced surface ionic mobility in a quasi-liquid layer. Phase separation occurred, with microcrystallites of NaNO3 recrystallizing three-dimensionally on the substrate surface. This exposed fresh chlorine from the bulk NaCl, making it available for further reaction. The large deficits of Cl- found in many sea salt particles can be explained by this cycling effect. Roughening of the NaCl surface from nitric acid corrosion generated surface defects which enhanced water adsorption. Some active sites induced H2O dissociation and generation of surface OH- species, which was detected by XPS. Experiments on the more defective NaCl (111) surface confirmed the role of surface defects.
1. Introduction
N2O5(g) + NaCl(s) f NaNO3(s) + ClNO2(g)
(2)
Reactive chlorine in the troposphere is one of the most important, yet least understood, of all the common trace elements.1-5 Chlorine atoms, if generated in large enough amounts, can have significant effects on the chemistry of the marine boundary layer and coastal areas. Cl abstracts hydrogen from alkanes much faster than OH radicals and can compete with OH oxidation at Cl atom concentrations near 104 molecules/ cm3, as well as have an influence on the tropospheric ozone balance.1-5 Sea salt aerosol is the dominant source of particulate chlorine in the troposphere, with 1012-1013 kg/yr originating from the breaking of ocean whitecaps, the amount being dependent on wind speed.1,4,6,7 The majority of gaseous inorganic chlorine in the troposphere is due to the reaction of sea salt aerosols to form volatile chlorine-containing gases such as HCl.1-4 Numerous researchers have found a deficit of Cl- in sea salt particles relative to the bulk seawater Cl/Na weight ratio of 1.8, with up to 100% Cl- loss in some heavily polluted air masses.1,4,7-10 This Cl- deficit appears to be dependent on particle size, with higher losses occurring in smaller particles. The chlorine deficit has been related to the reaction of oxides of nitrogen with sea salt particles,10 but this may not account for all of the “missing chlorine”.11,12 The reactions of HNO3, N2O5, NO2, and ClONO2 with NaCl, the major component of sea salt, are shown in (1)-(4):
2NO2(g) + NaCl(s) f NaNO3(s) + ClNO(g)
(3)
ClONO2(g) + NaCl(s) f NaNO3(s) + Cl2(g)
(4)
HNO3(g) + NaCl(s) f NaNO3(s) + HCl(g)
(1)
* To whom correspondence should be addressed. Phone (714) 824-6020, FAX (714) 824-3168, E-mail
[email protected]. † Charles Evans & Associate, 301 Chesapeake Dr., Redwood City, CA 94063. X Abstract published in AdVance ACS Abstracts, December 1, 1996.
S0022-3654(96)01517-1 CCC: $12.00
The kinetics and possible mechanisms have been studied for reaction 1,13-18 reaction 2,16,17,19,20 reaction 3,14,21-29 and reaction 4,19,30 but significant discrepancies exist and more research needs to be done, especially with regard to the effects of water and surface defects. A major question is whether only the surface Cl- reacts due to saturation of the salt surface with a NaNO3 layer or whether the entire bulk Cl- is somehow available for reaction, accounting for some of the extremely large chlorine losses in the more polluted air masses. Salts are known to undergo an abrupt dissolution into a saturated solution droplet at a particular relative humidity, called the deliquescence point (DP). The DP is determined by the Kelvin (droplet curvature) effect and the competing solute effect, which decreases the vapor pressure above the solution with increasing salt concentration. For sea salt particles in the 1-10 µm size range, the Kelvin effect can be considered insignificant.11,31-35 The deliquescence point at 25 °C for NaCl(s) has been measured to be 75.3% relative humidity,34 for NaNO3(s) to be 74.3%, and for mixtures35 of NaCl and NaNO3 to be 67.6%. A deliquescence range for sea salt particles of 71-75% at 20 °C36 and 72-73% at 23 °C21 has been observed. There have been some observations of sea salt particles inland as much as 900 km where the relative humidities are below the deliquescence point for sea salt.37 In addition, NaCl particles have been observed in very low humidity environments such as oil well fires in Kuwait38-42 and in volcanic emissions into the stratosphere.43 Very little is known about the effects of ambient water vapor at pressures below the deliquescence point on the chemistry of sea salt particles in the troposphere. © 1996 American Chemical Society
19892 J. Phys. Chem., Vol. 100, No. 51, 1996 Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) studies suggested that water exposure following the reaction of NO2 with ground powders of NaCl resulted in the recrystallization of the nitrate film to form small NaNO3 crystallites.14 Recent work by Peters and Ewing on single crystals of NaCl(100) showed an enhancement of the rate of reaction due to water. However, their spectra surprisingly did not show any evidence for water-induced recrystallization of the nitrate film.29 In some of our recent research,44,45 the effects of water exposure below the DP for NaCl on ultrathin nitrate layers grown on NaCl crystals were studied. X-ray photoelectron spectroscopy (XPS) was used to follow HNO3(g) uptake on NaCl(100) single crystals to saturation and to measure the resulting nitrate layer thickness (1-2 monolayers). Transmission electron microscopy (TEM) was utilized to show that after water exposure, followed by drying in vacuum, dissolution and recrystallization of the surface nitrate layer occurred. This caused phase separation from the NaCl and 3-dimensional growth into bulk NaNO3 crystallites, consistent with earlier observations by Chung et al.27 This exposed fresh NaCl at the surface, allowing for the reaction of the bulk Cl- with additional HNO3. In these TEM experiments, however, the samples could not be handled in a well-controlled ultrahigh-vacuum (UHV) system. For example, it was necessary to expose the crystals to laboratory air during sample transfer. This limited the control of the water vapor concentrations to which the crystals were exposed. X-ray photoelectron spectroscopy was used in the work we describe here to look in more detail at the effects of water vapor exposure on ultrathin nitrate films grown on NaCl single crystal surfaces, under a well-controlled and characterized UHV environment. It will be shown that the details of the recrystallization and 3-dimensional growth of the nitrate film into bulk NaNO3 are dependent on the water pressure. We observe the recrystallization of the nitrate film at pressures as low as 2 orders of magnitude below the DP for NaCl. Surface defects also appear to play a role in the nitrate film growth as well as causing preferential adsorption and some dissociation of H2O at the surface, generating OH-. Such surface defects can be caused by corrosion of the NaCl surface after reaction with nitric acid. This is confirmed by studies on the more defective NaCl(111) surface. 2. Experimental Section 2.1. Procedure. X-ray photoelectron spectroscopy (XPS) was used to quantify the surface Cl- and NO3- following exposure of the solid NaCl to gaseous HNO3 and then H2O. An ESCALAB MKII photoelectron spectrometer (VG Scientific) was used in these experiments. The ESCALAB MKII is a multitechnique surface analysis instrument based on an UHV system consisting of three separately pumped, interconnected chambers referred to in this paper as the spectroscopy, sample preparation, and fast entry chambers. The sample is easily moved from one chamber to the other under UHV conditions. The XPS experiments were performed in the spectroscopy chamber using a standard Al/Mg twin anode X-ray source and a 150 mm hemispherical electron energy analyzer. For these experiments, an analyzer pass energy of 20 eV and Mg KR X-rays (1253.6 eV) were used. Base pressures during XPS analysis were in the low-to-mid 10-10 Torr range. Nitric acid exposures were used to create the ultrathin nitrate films on freshly cleaved NaCl(100) and NaCl(111) single crystals. As described in our previous paper,13 an all-glass capillary doser with a known conductance, mounted on the
Laux et al. preparation chamber, was used for dosing the nitric acid. The HNO3(g) flux was determined by controlling the acid vapor pressure behind the capillary tube. A 50:50 v/v solution of HNO3 and H2SO4 was used as the source for the dry, gaseous HNO3. NaCl(111) single crystals were purchased directly from Bicron in 5 × 5 × 1.5 mm sizes. These crystals were heated to approximately 575 K in UHV to reduce any surface impurities. Prior to heating, substantial amounts of surface oxygen are observed. After heating, the surface oxygen is reduced substantially but not to zero. The O(1s) binding energy of this residual oxygen impurity is consistent with OH-. The HNO3 exposures were performed in the preparation chamber using the glass capillary doser in order to avoid contamination of the spectroscopy chamber and the X-ray source. The sample was then transferred to the spectroscopy chamber for XPS analysis. All H2O vapor doses were made in the fast sample entry chamber to avoid residual HNO3 contamination. The chamber was back-filled with water vapor from a deionized water source that was purified through consecutive freeze-pump-thaw cycles using liquid nitrogen. The vapor pressure was monitored with a Baratron capacitance manometer. A liquid nitrogen cryotrap was installed on the fast sample entry chamber in order to rapidly (
