J. Phys. Chem. 1996, 100, 3115-3120
3115
Uptake of Chlorine Dioxide by Model Polar Stratospheric Cloud Surfaces: Ultrahigh-Vacuum Studies James D. Graham and Jeffrey T. Roberts*,† Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455
Laura A. Brown and Veronica Vaida*,‡ Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: June 14, 1995; In Final Form: August 21, 1995X
An investigation of the interaction of chlorine dioxide (OClO) with the surface of ice is reported. Experiments were carried out under ultrahigh vacuum (UHV) on films between 10 and 100 water monolayers thick. The initial OClO adsorption probability on ice at 100 K is high, approaching unity on both the crystalline and amorphous surfaces. At low coverages, OClO is quantitatively incorporated into the ice bulk, where it resides until sublimation of the ice film near 185 K. An adsorbed state of OClO is formed at moderate exposures. Chlorine dioxide desorbs from ice at approximately 130 K; desorption is first order, with an activation energy equal to 23 kJ mol-1 and a prefactor of 2 × 109 s-1. At high exposures, OClO condenses to form a multilayer film, which sublimes near 135 K; the sublimation energy is 38 kJ mol-1. If adsorbed OClO is covered with an ice film 13 monolayers thick, OClO is trapped within the film, and desorption is completely suppressed until the onset of ice sublimation.
Introduction The surface and/or near surface regions of icy particles catalyze or promote chemical transformations that participate in formation of the ozone “hole” over the Antarctic.1 An important characteristic of the Antarctic stratosphere is the presence of polar stratospheric clouds, or PSC’s. At least two types of PSC particle are important in the Antarctic stratosphere: type I, composed of a solid H2O/HNO3 phase that forms at temperatures below 195 K, and type II, consisting of ice that condenses below 187 K.2 In this context, many laboratory studies have focused on reactions of chlorine-containing compounds, especially HCl, ClONO2, and HOCl, at model type I and II PSC particle surfaces.3-9 However, there are indications that heterogeneous chemistry in the stratosphere needs to be better understood. For example, results from the 1994 ER-2 mission to the Southern Hemisphere suggest that unidentified pathways for heterogeneous chlorine processing may exist.10 The work reported here concerns the subject of chlorine dioxide (OClO) in the stratosphere. Although the OClO mixing ratio in the presence of sunlight is exceedingly low, the mixing ratio at night is comparable to that of ClONO2.11-16 The photochemistry of OClO in the gas phase and in matrices of ice and sulfuric acid has been studied, and interesting differences between the gas and condensed phases have been observed.17-24 The role of these photochemical reactions in ozone depletion has been discussed previously.19,25 The possible role of heterogeneous reactions involving OClO remains largely unexamined. However, chlorine in OClO, like chlorine in HOCl and ClONO2, is highly oxidized, and reactions that involve conversion of OClO to Cl2 are thermodynamically favorable. Here we present a study of the adsorption probability and desorption kinetics of OClO on ice under ultrahigh-vacuum * Authors to whom correspondence should be addressed. † Telephone (612) 625-2363, fax (612) 626-7541, e-mail roberts@ chemsun.chem.umn.edu. ‡ Telephone (303) 492-8605, fax (303) 492-5894, e-mail vaida@ spot.colorado.edu. X Abstract published in AdVance ACS Abstracts, January 15, 1996.
0022-3654/96/20100-3115$12.00/0
conditions. Chlorine dioxide was adsorbed on extremely thin (10-100 monolayer) crystalline and amorphous ice films deposited on the surface of a Pt(111) substrate. The adsorption and desorption rates were measured between 100 and 140 K, well below the temperature range encountered by stratospheric type II PSC ice particles. Nevertheless, the ultrahigh vacuum measurements can be used to estimate the steady state coverage and average lifetime of adsorbed OClO under stratospheric conditions. A companion paper shows how these values can be used to predict total OClO loading in and on ice under stratospheric conditions,26 a first step in the assessment of the importance of heterogeneous OClO processing. Experimental Section The growth and characterization of the ultrathin ice films used in this work are described elsewhere.8,9,27 Briefly, the films were deposited on a 111-oriented Pt surface. Typical film thicknesses, inferred from the water desorption yields, were 6-30 water monolayers (ML). Temperatures were measured using a chromel/alumel thermocouple junction spot-welded to the edge of the crystal. Both amorphous and crystalline ice could be deposited, depending on the deposition conditions. The two types of ice have been characterized previously using infrared spectroscopy.9,28-31 Chlorine dioxide was prepared daily by flowing chlorine gas over sodium chlorite. The compound OClO is potentially explosive and should be stored at pressures less than 400 Torr.32 The product gas was collected in a glass sample bottle and purified by several freeze/pump/thaw cycles using liquid nitrogen as the cryogen. Chlorine dioxide was stored as a solid in a liquid nitrogen-cooled flask and thawed immediately before use. Carbon dioxide remained as an impurity after degassing but had no discernible effect on the OClO measurements reported herein. Deliberate exposure of ice to CO2 resulted in uptake into the bulk, with little or no influence on the OClO temperature programmed desorption (TPD) spectra. Chlorine dioxide was adsorbed onto ice using a directed dosing scheme. During exposure, the sample was positioned © 1996 American Chemical Society
3116 J. Phys. Chem., Vol. 100, No. 8, 1996
Figure 1. Temperature programmed desorption of approximately one OClO monolayer from a 20 ML amorphous ice film deposited on Pt(111). Water and OClO were detected as m/e 20 (H218O+) and m/e 67 (35Cl16O2+), respectively. The heating rate was 6 K s-1.
3-10 mm in front of the doser, a stainless steel tube connected to a leak valve that regulated flow into the vacuum chamber. The inside diameter of the doser was larger than that of the metal substrate, resulting in a uniform flux across the ice surface. The CO2 impurity levels in OClO were highly variable, and exposures were difficult to reproduce. Therefore, the coveragedependent TPD series presented below were extracted from a very large set of desorption spectra. The OClO exposures cited in this paper, which were corrected for the enhancement factor of the directed doser (70 ( 10), are reported in units of langmuirs. Because of the CO2 impurity, the exposures represent upper limits of the actual exposure. Results Thermal Desorption of OClO from Crystalline and Amorphous Ice. The interaction of OClO with both amorphous and crystalline ice was studied. Figure 1 shows a representative set of temperature programmed desorption (TPD) spectra, resulting in this case from the adsorption of approximately one OClO monolayer on a 20 ML amorphous ice film. The ice film was maintained at 100 K during OClO adsorption, and the heating rate during TPD was 6 K s-1. Chlorine dioxide and water are the sole desorption products, with OClO evolving from two states near 130 and 190 K and H2O subliming at 190 K. Chlorine dioxide was identified as a desorption product through a comparison of its mass spectral fragmentation pattern to that of OClO leaked into the vacuum chamber. Specifically, the OClO+:OCl+ ratio is 1.0:0.5 in the desorption product and in the authentic OClO sample. In Figure 1, as in all TPD spectra reported herein, water was detected as m/e 20 (H218O+), a very weak component of the total H2O-derived signal. This scheme preserves the dynamic range of the detection electronics for the less abundant OClO signal. The TPD spectra show no evidence for formation of OCl, Cl2, HCl, and the OClO-H2O dimer. A weak signal at m/e 52 (HOCl+) is observed coincident with OClO evolution at 130 and 190 K but is not associated with a reaction of OClO, at least at low temperatures. Temperature-programmed desorption of OClO from ice-d2 results in the same weak HOCl+ signal at 130 K, with no significant quantity of DOCl+. The HOCl+: OClO+ ratio at 190 K is much higher than at 130 K, and
Graham et al.
Figure 2. Temperature programmed desorption of OClO from 20 ML films of amorphous and crystalline ice as a function of increasing OClO exposure. The OClO exposures were, on amorphous ice, (a) 0.007, (b) 0.014, (c) 0.014, (d) 0.14, (e) 0.13, and (f) 0.11 langmuir and on crystalline ice (g) 0.018, (h) 0.07, (i) 0.14, (j) 0.18, (k) 0.21, and (l) 0.21 langmuir. The OClO source was always contaminated with some CO2, the concentration of which was highly variable. The cited exposures therefore represent upper limits of the true values.
moreover, DOCl+ is observed at 190 K during TPD of OClO from ice-d2. However, for reasons presented in the Discussion, it is not believed that HOCl+ and DOCl+ originate from reactions between OClO and the ice films. The low- and high-temperature OClO states in Figure 1 are associated with OClO at the ice surface and in the ice bulk, respectively. The OClO desorption yield between 170 and 200 K is roughly proportional to the water film thickness between 20 and 80 ML, whereas the low temperature regions of the TPD spectra are independent of thickness. The former behavior is consistent with an assignment of OClO near 190 K to absorbed OClO, i.e. to a bulk state, an assignment that is supported by the near coincidence of ice sublimation and OClO evolution in this temperature region. Conversely, the independence of the desorption spectra on film thickness below 150 K implies an adsorbed or surface phase of OClO. The ice films must be quite uniform and free of gross structural defects, since the OClOaccessible surface areas do not increase with film thickness. The desorption of OClO from ice was investigated over a wide coverage range, from well below a monolayer to a condensed multilayer. Two series of exposure-dependent TPD spectra, corresponding to desorption from amorphous and crystalline ice, are shown in Figure 2. At low exposures, chlorine dioxide is preferentially incorporated into the ice bulk. The bulk state rapidly saturates with increasing exposure, and subsequent changes are confined to the low temperature regions of the desorption spectra. Within the low temperature regions of the spectra, the OClO desorption temperature is initially independent of coverage on both types of ice, but with slightly different desorption temperatures: 127 K from crystalline ice and 133 K from amorphous ice. An analogous although much larger temperature difference was reported for acetone on crystalline and amorphous ice and suggested to be the result of a hydrogen bonded adsorbed state on amorphous ice.33 However, in contrast to acetone on H2O(s) and D2O(s), OClO desorption spectra from amorphous D2O and H2O are essentially identical, with no measurable kinetic isotope effect for desorption, suggesting that hydrogen bonding between OClO and ice
PSC’s Uptake of Chlorine Dioxide by Model PSC’s
J. Phys. Chem., Vol. 100, No. 8, 1996 3117
Figure 3. Temperature programmed desorption of (a) H2O and (b) OClO from a “sandwich” layer. The sandwich was prepared by depositing a 12 ML amorphous ice film on an OClO monolayer which had been adsorbed on a 20 ML amorphous ice film. Shown for comparison in (c) is the TPD spectrum of an OClO monolayer from a 20 ML amorphous ice film.
Figure 4. Auger electron spectra of the Pt(111) surface. Spectra were recorded (a) of the clean surface, (b) after exposing a 20 ML ice film at 100 K to OClO and heating to 700 K, and (c) after exposing the clean Pt(111) surface at 100 K to OClO and heating to 700 K. Spectrum d is the difference spectrum c - a.
is unimportant. In any case, the coverage-independent desorption temperatures are evidence for an adsorbed phase in which the desorption rate expression is first order in OClO coverage. The determination of the rate parameters is discussed below. It is clear from the data in Figure 2 that there is an OClO coverage, roughly corresponding to a 0.2 langmuir exposure, beyond which desorption is no longer first order. The desorption temperature begins to increase with coverage, and the leading edges of the TPD spectra are superimposable. This new state does not saturate with increasing exposure and is also observed when large amounts of OClO are adsorbed on the clean Pt(111) substrate. Such behavior is characteristic of a zeroth order process and is generally observed for the sublimation of a three-dimensional condensed phase. The high exposure spectra are therefore assigned to the sublimation of an OClO multilayer. The coverage at which desorption shifts from the first-order into the zeroth order regime is greater on crystalline than on amorphous ice. One possible origin of the difference in coverage thresholds is discussed below. Although the OClO desorption temperature from ice is quite low (approximately 130 K), desorption is suppressed if the adlayer is buried by a thin ice film. Figure 3a,b shows the evolution of OClO and H2O during TPD of an ice/OClO/ice “sandwich,” consisting of a 20 ML bottom ice layer, a central OClO monolayer, and a 13 ML top ice layer. The two ice layers were amorphous. Also shown for comparison is the desorption of an OClO monolayer from a 20 ML amorphous ice film (Figure 3c). The presence of the top ice layer results in the complete disappearance of the desorption feature attributed to adsorbed OClO. Instead, OClO evolution from the sandwich does not occur until the onset of ice sublimation at 165 K. Chorine dioxide evolution, once it starts to occur, is exceptionally rapid, as indicated by the sharp TPD peak. The simplest possible explanation for OClO evolution from a sandwich is that ice sublimation results in the opening of channels that allow release of OClO into the gas phase. An alternative possibility is that desorption is induced by the crystallization and densification of the amorphous ice film. Regardless of the precise mechanism for OClO release from a sandwich, it is clear that
a top ice layer imposes a significant barrier for desorption. Note that the state associated with OClO in the ice bulk is more intense in the sandwich (Figure 3b,c). This is partly because the intensity of the bulk state is proportional to ice thickness, so that more OClO is incorporated into the 35 ML sandwich than the 20 ML film. However, the bulk state is more intense than would be predicted from thickness arguments alone: sandwich formation leads to enhanced incorporation into the ice bulk. Previous work has demonstrated that ice films as thin as 6 ML deposited on W(100) are free of channels extending from the ice-vacuum to the ice-metal interface, which, if present, would expose the underlying metal substrate to the gas phase.27 Here we show that 20 ML thick films deposited on Pt(111) are also free of channels, at least of diameter on the order of an OClO molecule or larger. In Figure 4 are shown three Auger electron spectra of the Pt(111) substrate: (a) immediately after cleaning, (b) after TPD of a thick OClO multilayer from a 20 ML ice film, and (c) after TPD of OClO from the clean Pt(111) substrate. Unfortunately, the Auger electron spectra of Pt and Cl overlap. Nevertheless, the identical line shapes of the spectra in Figure 4a,b show that no significant amount of Cl (
