5914
J . Phys. Chem. 1994,98, 5914-5983
Interaction of Hydrogen Chloride with an Ultrathin Ice Film: Observation of Adsorbed and Absorbed States James D. Graham and Jeffrey T. Roberts' Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-0431 Received: January 21, 1994; In Final Form: April 3, 1994'
With the objective of gaining insight into how heterogeneous reactions occur in the Antarctic stratosphere, we have initiated a program to study the adsorption and reaction of simple molecules on model polar stratospheric cloud surfaces. In this work, the temperature-programmed desorption of hydrogen chloride from ultrathin (5-20 monolayers thick) water films is described. Two distinct HC1 desorption states, designated a- and 0-HCl, are observed a t 140 and 180 K, respectively. Water sublimation occurs at 180 K and is concurrent with @-HCl evolution. 8-HCl, which is formed exclusively a t low H C l exposures, is derived from the thin film bulk, while a-HC1 is associated with an adsorbed state. P-HCl is assigned to the sublimation of a stoichiometric phase of HC1 and water, probably H C l h H 2 0 , and a-HCl is assigned to the thermal desorption of HCl from the hexahydrate surface. Desorption spectra of HC1 from ice-dz show that H-D exchange between HCl and D 2 0 is much less than would be expected for a dissociatively adsorbed state of HCl. The a state is therefore assigned to molecularly adsorbed HC1. The activation energy for a-HC1 desorption is 33 f 5 kJ-mol-', a value which is highly suggestive of formation of a hydrogen bond between H C l and the hexahydrate surface. Two possible structures of H C l adsorbed on the hexahydrate surface are considered. Implications of these results for heterogeneous polar stratospheric chemistry are discussed.
Introduction The surface and/or near surface regions of icy particles in polar stratospheric clouds (PSC's) catalyze chemical transformations which participate in formation of the ozone "hole" over the Antarctic.' The precise role thesereactionsplay in the atmosphere is still being determined, but it is clear that ice surfaces assist in (i) conversion of photoinactive chlorine reservoirs into photoactive reservoirs and (ii) denitrification of the gas phase, via, for instance, eq 1:2"
At least two kinds of clouds are active in promoting heterogeneous atmospheric chemistry: type I PSC's, composed primarily of particles of a nitric acid hydrate, the composition of which is the subject of some controversy, and type I1 PSC's, the principal constituents of which are solid water particle^.',^ The extreme importance of the ozone hole problem has stimulated much laboratory workon the chemistry of model polar stratospheric cloud surfaces,*" with an emphasis on experiments conducted under conditions like those actually encountered in the atmosphere (180 K I T I 205 K, relevant partial pressures).' This approach is well suited to studying certain aspects of the chemistry of icy surfaces, especially product distributions, thermodynamics, and overall reaction kinetics. There are, however, some issues regarding the surface chemistry of ice which cannot be adequately addressed. In particular, it has proved difficult to separate and study each of the elementary reaction steps which must be involved in any surface-mediated transformation, Le., (i) adsorption or absorption of the reactants, (ii) the reaction itself, and (iii) product desorption into the gas phase. We have initiated a program designed to study the surface chemistry of extremely thin water and waterlnitric acid films (nominal thicknesses 10-200 A) under ultrahigh vacuum ( P = Pa) .8 The films whose surface chemistry we study are grown in situ via deposition of the thin-film component(s) onto a clean, ~~~~
To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, May 1, 1994.
0022-365419412098-5974%04.50/0
single-crystal metal substrate. Advantages of the experimental approach are numerous. First, the high surface area to volume ratio of a very thin film makes it a relatively simple matter to investigate both surface and bulk phenomena using, for instance, temperature-programmed reaction and infrared spectroscopic methods. Second, surface charging is eliminated if films of sufficient thinness are deposited on a conducting material. This allows the straightforward application of electron-based probes, such as X-ray photoelectron spectroscopy (XPS). Finally, under typical ultrahigh-vacuum conditions, the sublimation rate of a film is very low. Surface lifetimes are on the order of several hours or more,9 and the need to include considerations of the solid-gas equilibrium in data interpretation is eliminated. The principal limitation of this approach is that the vapor pressures of water and nitric acid, coupled with the ultrahigh-vacuum requirement, impose an upper temperature limit of =l75 K, which is somewhat below the temperatures encountered in the Antarctic stratosphere.' However, through these experiments, we hope to gain firmer mechanistic insights into the surface chemistry of icy solids than can otherwise be currently obtained. Here we report the adsorption of hydrogen chloride on ultrathin (5-20 monolayers thick) water films deposited on W(lOO), a single-crystal metal surface. We present evidence for HC1 migration into the thin-film bulk upon adsorption at 120 K to form a distinct HCl/H20 phase, the stoichiometry of which is probably HCb6H20. Upon completion of the hydrate phase, hydrogen chloride adsorbs at the surface of the film. Isotopic exchange experiments suggest that the adsorbed state is molecular, Le., that HCI adsorbs on the hydrate film without dissociation. Formation of the adsorbed state is reversible as evidenced by HCl desorption into the gas phase at 140 K. The activation energy for HCI desorption is approximately 33 kJ-mol-', a value which is highly suggestive of a hydrogen-bonding interaction between HC1 and the HC1.6H20 surface. To our knowledge, these experiments constitute the first observation of HC1 adsorbed a t the surface of an icy solid above 100 K. However, the low activation energy for HC1 desorption implies that the steadystate coverage of adsorbed HC1 under stratospheric conditions is so low that it may not be an important intermediate in heterogeneous atmospheric processes. Rather, the likely inter0 1994 American Chemical Society
HCI-Ice Film Interaction
The Journal of Physical Chemistry, Vol. 98, No. 23, 1994 5915
mediate is anionically dissociated species, perhaps residing within the first few layers of a solid or quasi-liquid surface.
rconde
Experimental Section
Experiments were conducted in a stainless steel ultrahighvacuum chamber of base pressure -1 X 10-8 Pa. The chamber was pumped contiuously by a 300 L d ion getter pump and intermittantly by a titanium sublimation pump (both from Physical Electronics). The vacuum system was equipped with a quadrupole mass spectrometer (Extrel C50), a double-pass cylindrical mirror electron analyzer (Physical Electronics 15255 G), an X-ray source (Physical Electronics 04-548 G), and low-energy electron diffraction optics (Physical Electronics 10120). Gases were admitted into thechamber using directed dosers, essentially tubes terminating in leak values (Vacuum Generators Ltd. MD6) which regulate gas flow into the vacuum chamber. Exposures were given in units of P a q the product of the background pressure rise upon opening the doser and the exposure tube. Exposures are uncorrected for the enhancement factors of the dosers. The W( 100) single-crystalsubstrateon which water films were deposited was mounted on a sample manipulator (Thermionics TPM-202-5-2-2-6) which allows for rotation and translation within thevacuumchamber. Thesubstrate was inthermalcontact with a liquid nitrogen-cooled reservoir and could be cooled to approximately 90 K. A tungsten filament positioned =2 mm behind the substrate was used for radiative heating. The substrate could also be biased to +500 V for electron beam heating as required. The temperatureof the tungstensubstrate was measured with a W-5% Re/W-26% Re thermocouplejunction spot-welded to the edge of the crystal; an electronic ice point (Omega MCJC) substituted for a reference junction. Temperature-programmed reaction data were acquired with the W(100) surface positioned in line of site of the mass spectrometer, approximately 20 mm from the ionizer. The mass spectrometer was encased in a stainless steel shield similar to one described previously.1° The shield allowed for interposition of a collimater 2 mm in diameter between the W(100) substrate and the mass spectrometer. The collimater-surface distance was typically =2 mm during an experiment. This arrangement results in the preferential detection of products from the center of the W( 100) surface, where thedefect density islowest. Contributions to the spectra of products evolving from the crystal edges and backside, the support wires, and other parts of the sample manipulator are also minimized. The mass spectrometer was interfaced to an IBM-AT cione PC via a data acquisition board (Keithly DAS-HRES). Data acquisition software allowed for the collection of up to six ion-temperature profiles during a single experiment. The 100-oriented tungsten single crystal was obtained from Metal Crystals Limited (Cambridge, UK) and cleaned in vacuo according to established methods." A typical cleaning cycle involved exposing the hot surface ( T = 1400 K) to oxygen gas ( P 10" Pa) for a period of approximately 5 min. The crystal was then flashed to 2300 K for 30 s, thereby removing oxygen as a volatile tungsten oxide. The crystal was subsequently cooled to approximately room temperature and then briefly flashed to 1100 K to remove any residual CO. Subsequent Auger analysis typically showed the surface to be free of impurities within the limits of detectability of our apparatus (=0.02, 0.01, and 0.04 ML for C, 0, and C1, respectively). Water was deionized and triply distilled before use. It was degassed via several freezepumpthaw cycles before use each day. Water-d2 (Aldrich) was degassed before use each day and otherwise used as received. Hydrogen chloride (Aldrich, 99+%) and oxygen (Matheson, extra dry, 99.6% minimum purity) were used as received.
-
0
200
300
400
5 0
temperature / K
Figure 1. Temperature-programmedreaction of 1.0 X 10-6 Pa.s water on W(lO0). Water and dihydrogen, the sole gas-phase products below 1000 K, were detected as m / e 18 (HzO+) and m / e 2 (Hz+), respectively. The heating rate was 4K.s-1. The exposurewas determinedas described in the text. The pH20 state is just discernibleas a broad shoulder near 210 K on the water sublimation feature. The sharp peak near 165 K in the Hz spectrum is attributed to cracking of water (derived from the multilayer) at m / e 2.
Results Growth and Characterization of Water Films on W(100). Water films were grown via adsorption of gaseous water onto a single-crystal W( 100) surface. During growth of the films, the substrate was positioned =3 mm in front of a doser which regulates the flow of water into the vacuum chamber. The doser is designed so that the inside diameter of the tube (-15 mm) is significantly larger than the diameter of the W( 100) substrate (-6 mm). This arrangement ensures that the water flux across the substrate surface is nearly constant, thereby allowing the deposition of uniform water films. Water was deposited a t a rate of aO.1 mono1ayers.s-1, with the substrate temperature held between 120 and 130 K, conditions which lead to formation of an amorphous layer.12 Deposited films were exceedingly thin, typically 5-20 water monolayers thick. Because the films were so thin, the thermal gradient between W( 100) and a thin film surface was negligible. The temperature at the surfaceof a film can therefore be taken as equivalent to that of the W(100) substrate. Films were analyzed using temperature-programmed reaction mass spectrometry (TPRS). In these experiments, the sample was subjected to a linear temperature ramp (heating rate typically =6 K-s-l, initial temperature approximately 100 K), and thegasphase products were detected mass spectrometrically as a function of substrate temperature. Water (detected as m / e 18, H20+) and dihydrogen ( m / e 2, Hz+) are the only products which evolve into the gas phase below 1000 K during reaction on an initially clean W(100) surface Figure 1. The water desorption spectrum exhibits twodistinct features, at 170 and 210 K. The state at 170 K is attributed to the sublimation of a condensed water film. The intensity of this feature increases without bound with increasing water exposure, and the temperature at which water evolves is in close correspondence with those reported for sublimation of water films deposited on numerous other single-crystal surfaces under ultrahigh vacuum.13 The 210 K state, which in Figure 1 appears as a broad shoulder on the much more intense sublimation
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The Journal of Physical Chemistry, Vol. 98, No. 23, I994
5.4
6.0
6.4 Ti/
6.2
10-3. ~
6.6
6.8
- 1
Figure 2. Zeroth-order plots for water sublimation. The triangular points represent the sublimation of a pure water film, and the circular points represent the sublimation of a water film which was saturated with HCI.
Lines are derived from linear regression analyses of the data points. peak, is designated p H 2 0 and is associated with desorption of reversibly adsorbed water from the W( 100) substrate. In Figure 1, a shoulder at -1 80 K can also be discerned on the trailing edge of the water sublimation peak. This is tentatively assigned to second layer desorption, Le., to desorption of water which is adsorbed on the first adsorbed water layer. Dihydrogen evolution at 3 15 K is associated with decomposition of part of the first layer of water on W(100). Atomically adsorbed oxygen, also a decomposition product, remains on the surface after reaction and can be removed as a volatile tungsten oxide by heating the sample to 2200 K. The TPRS method probes reaction kinetics: peak height (relative to the background level) is proportional to gas evolution rate.I4 The activation energy for water sublimation (&b) may be readily determined from the data in Figure 1. The kinetics for sublimation of a three-dimensional phase are normally well described by a zeroth-order rate expression
where ro is the sublimation rate and u is the frequency factor for sublimation.15 Wheneq 2 isvalid, thelogarithmofthesublimation rate varies linearly with reciprocal temperature. This turns out to be the case for sublimation of a thin water film from W(100) Figure 2). ,!?sub may be extracted from the slope of such a plot (slope = -&b/R). We determine Esub to be 45.8 0.6 kJ-mol-', where the uncertainty represents the standard deviation from five spectra recorded on five separate days. This value compares favorably with those for water sublimation from many other singlecrystal transition metal surfaces13 and is close to the accepted value for the sublimation energy of bulk ice (50 kEmol-1 at 170 K).'6 Film thicknesses were estimated from the water desorption spectra. Thicknesses are given in units of water monolayer equivalents (ML) and were determined using the following method. The water desorption spectra were integrated with respect to time. Since peak height is proportional to gas evolution rate, peak areas are proportional to product yields. The p-HzO yield, which for a multilayer film is independent of film thickness,
*
Graham and Roberts was assigned a value of one monolayer equivalent. The film thickness was then defined as the ratio of the total H20 desorption yield of the p H 2 0 yield. Because of possible packing density differences between water adsorbed at the W( 100) surface and water in the condensed phase and because of possible roughness in the deposited water films, thicknesses should not be considered absolute measurements. They do, however, accurately reflect the relative amounts of water in different films, and they are a rough measure for average film thickness. Moreover, thicknesses determined from the integrated desorption spectra are in excellent agreement with thicknesses determined using an entirely different method, based upon X-ray photoelectron spectroscopy.* There is no simple epitaxial relationship between the W( 100) surface and the crystalline phases of water, so it is likely that deposited water films are rich in grain boundaries, dislocations, and related defects. However, the films are free of micropores which, if present, would result in exposure of the underlying substrate to gaseous HC1. The absence of such micropores was demonstrated as follows. Carbon tetrachloride (CC14) was adsorbed on a 6 M L water film a t 100 K. The substrate was then heated to 400 K, a temperature sufficient to desorb all CC1, and H20 from the W(100) substrate. Subsequent analysis of the surface by Auger electron spectroscopy and X-ray photoelectron spectroscopy showed that the chlorine coverage was below the detectability limits of our apparatus (~0.04 ML). Since CC14 decomposes to C(a)and Cl(a)upon adsorption at 100 K on clean and water-precovered W( loo), this experiment proves that C C 4 is not delivered to the W(100) substrate from the gas phase in the presence of a water thin film. Layers are free of micropores, at least of size on the order of a CC14 molecule or larger. Adsorption/Absorption of Hydrogen Chloride on a Water Thin Film. The interaction of hydrogen chloride (HC1) with a water thin film was studied using TPRS according to the following experimental protocol. A water layer was deposited as described above, cooled to the desired adsorption/ absorption temperature, generally ~ 1 2 K, 0 and exposed to gaseous HCl via a directed doser similar to that used for the introduction of water. Upon reaching the desired exposure, the valve regulating HC1 flow into the chamber was closed, the sample was cooled to -100 K, and an experiment carried out, generally with a heating rate of between 2 and 10 K-s-1. Three gas-phase products are formed during reaction of HCl with a water thin film: HCl, H20, and H2. No other species, in particular HOC1 (which we attempted to detect as m / e 51 and 52, 03'Cl+ and H 0 3 U + , respectively) and Clz ( m / e 70, 35C12+), evolve into thegas phase. Products were unambiguously identified by comparing their mass spectrometric fragmentation patterns with those of authentic HCl, H2, and H20 samples leaked into thevacuum chamber. For most experiments, however, HCl was detected solely as m / e 36 (H3Tl+), the most abundant HClderived ion, H2 was detected as m / e 2, the molecular ion, and HzO was detected as m / e 16 (O+),the least abundant HzOderived ion. This detection scheme prevented saturation of the mass spectrometer electron multiplier by the very intense molecular ion of water and preserved as much of the dynamic range of the instrument as possible for detection of HCl and H2. When hydrogen chloride is adsorbed/absorbed to saturation on a 5 M L film, HC1 evolves into the gas phase at 140 and 180 K (the two peaks are designated a-HCl and j3-HC1, respectively), water sublimes at 180 K, and Hz desorbs at 325 K (Figure 3). The HC1 and H20 desorption spectra also exhibit pronounced shoulders near 185 K; these features are associated with processes occurring a t the thin film-W(100) interface and will not be discussed further. Similarly, dihydrogen desorption occurs well above the temperature a t which water sublimation is complete and is therefore unrelated to any interaction between HC1 and a water thin film. Rather, H2 is formed via the decomposition
HC1-Ice Film Interaction
The Journal of Physical Chemistry, Vol. 98, No. 23, 1994 5977
-
a-HCI P-HCI
.
a-HCI
; :
...e .......e...P-HCI
I.
n 0
150
200
250
300
temperature / K Figure 3. Temperature-programmedreaction of HC1 with a 5 ML thick water film on W(100). The HC1 exposure, determined as described in the text, was 2.4 X 106 Pps. The inset shows the temperatureprogrammed reaction spectrum of H2, which extends to a much higher temperature than either the HCI or H20 spectra. The heating rate was ==5 K d . HCI was detected as m / e 36 (H35C1+),H20 as m / e 16 (O+), and H2 as m / e 2 (Hz+).
of water and HCl at the W(100) substrate. Auger electron analysis of the W( 100) surface after reaction shows that chlorine deposition does occur during an experiment. Chlorine deposition is not, however, inconsistent with the above assertion that water films are free of micropores. Rather, as shown below, a significant amount of HCl is incorporated into the film bulk, thereby opening a pathway for delivery of chlorine to the substrate surface. The a-and 0-HCl yields were measured as a function of film thickness (Figure 4). These results show very clearly that a-HCI is derived from hydrogen chloride which is adsorbed at the thin film surface, while O-HCI originates from within the thin film bulk. Experiments were conducted over a broad thickness range (1-20 ML), and care was taken to ensure that HCl exposures were sufficient to saturate both the a and 0 states. Yields were determined from the time-integrated TPR spectra. Over the entire thickness range studied, the 0-HCl yield increases linearly with film thickness. The P-HCl yield is therefore proportional to thin film volume, indicating that this state originates from the bulk. The linear dependence also implies that HC1 migration through the film is rapid a t the adsorption temperature (120 K). HCl migration through the films leads to the formation of a H20HCl mixture, the composition of which is independent of thin film thickness. For very thin films (
