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IR Spectroscopic Testing of Surfaces in Water Ice and in Icy Mixtures with Prussic Acid or Ammonia Aida V. Rudakova, Vladimir N. Sekushin,† Ilya L. Marinov, and Alexey A. Tsyganenko* Department of Photonics, V. A. Fock Institute of Physics, St. Petersburg State UniVersity, St. Petersburg, Russia 198504 ReceiVed August 27, 2008. ReVised Manuscript ReceiVed NoVember 26, 2008 An experimental technique is developed for spectral studies of ice films deposited at 77 K onto the ZnSe inner windows of a cell, so that the spectra could be registered in the presence of gaseous adsorbate. Pure H2O or D2O ice as well as HCN/D2O and ND3/D2O mixed icy films with different dopant/water ratios (1:10, 1:5, and 1:1) were investigated. The surface area of ice deposited at 77 K H2O estimated from adsorption measurements was about 160 m2/g. Bands of dangling hydroxyl groups disappear on raising the temperature up to 130-160 K when the changes of bulk absorption provide evidence for a phase transition from amorphous to polycrystalline ice. Surface properties of icy films were characterized by low-temperature adsorption of CO and CHF3. Insertion of small doses of HCN or ammonia does not change the acid/base strength of dangling hydroxyl groups or coordinately unsaturated surface oxygen atoms, but it changes the proportion between the concentrations of these sites as compared with pure water ice. For high dopant concentrations, the dangling hydroxyls were not observed, and the dominant adsorption sites for CO are likely to be the unsaturated oxygen atoms, while serious structural changes occur in the bulk of ices.
1. Introduction 1,2
The early works of Burton and Oliver back in 1935 had shown that the structure of ice produced by deposition of water vapor on a solid support at cryogenic temperatures can be not only crystalline but also semicrystalline, or even amorphous. Recently, studies of amorphous ice attracted considerable attention in atmospheric sciences, environmental sciences, as well as in fundamental research in physics and chemistry. Both atmospheric observations and laboratory experimental data indicate a significant role of heterogeneous chemical and photochemical processes on the water ice surfaces, which control the balance of minor constituents and contaminations of the atmosphere. Furthermore, one may expect water ice to provide a way for purification of air that enters the stratosphere. It was suggested that the cirrus clouds act as a tropospheric “scrubbers” (i.e., surfaces where catalytic reactions take place) to remove a variety of organic and inorganic pollutants. It was also found that amorphous ice exists in interstellar clouds and in the atmospheres of some planets and can be produced by amorphization of crystalline ice by irradiation at temperatures below 70 K with high-energy electrons and ions or with 110-400 nm UV light.3 Studies of cryochemistry and photochemistry of watercontaining surfaces should be useful for astrophysicists in understanding processes that occur in interstellar dust and comets. Three principal ways of preparation and investigation of the water ice have been developed, namely, the spectral observations of clusters in the gas phase at cryogenic temperatures,4,5 the spectral studies of molecular aggregates in cryogenic inert gas * Corresponding author. E-mail:
[email protected]. Fax: (+7) 812-428-7240. Telephone: (+7) 812-428-4571. † Present address: Institut fu¨r Experimentalphysik, Freie Universita¨t Berlin, Arnimallee 14, 14195 Berlin, Germany.
(1) Burton, E. F.; Oliver, W. F. Nature 1935, 135, 505–506. (2) Burton, E. F.; Oliver, W. F. Proc. R. Soc. London, Ser. A 1935, 135, 166–172. (3) Baragiola, R. A. Planet. Space Sci. 2003, 51, 953–961. (4) Ewing, G. E.; Sheng, D. T. J. Phys. Chem. 1988, 92, 4063–4066. (5) Rowland, B.; Devlin, J. P J. Chem. Phys. 1991, 94(1), 812–813.
matrices,6-8 and spectroscopy of thin ice films deposited onto solid supports. Recently, the reflection-absorption IR spectroscopy (RAIRS) and the Fourier transform (FT) absorption infrared spectroscopy (AIRS) techniques have been widely used for studies of such systems. In the RAIRS configuration, metals (Al, Cu, Ni, Pt, etc.) and nonmetals (Ge, Si, SiO2, FeO, Cr2O3, etc.) were used as supports for the ice film deposition (see, e.g., refs 9-12). IR-transparent materials,6 such as CsI, MgF2, sapphire, and crystalline silicon, were used as substrates in AIRS measurements. Ice films are formed by gas-phase condensation at cryotemperatures or by water vapor deposition on a preliminary cooled substrate by a seeded jet of a carrier gas (e.g., He, or H2). Properties of the ice films (phase state, density, porosity, film thickness, etc.) always depend on the experimental conditions employed (e.g., temperature, pulse rate, chosen substrate).10-14 It should be noted that infrared spectroscopic testing of such systems is carried out at ultrahigh vacuum conditions but not in the presence of the adsorbate in the gas phase. The aim of the present work is to demonstrate the capabilities of a new experimental technique for preparation and FTIR spectroscopic investigation of pure and mixed water ice films in the 56-273 K temperature range. We report the results obtained for amorphous H2O-D2O and D2O ices as well as for HCN-D2O and ND3-D2O mixtures with different component ratios. The advantage of heavy water ice over the usual one is the position of dangling OD groups, which provide significant information (6) Manca, C.; Roubin, P.; Martin, C. Chem. Phys. Lett. 2000, 330, 21–26. (7) Manca, C.; Martin, C.; Roubin, P. J. Phys. Chem. B 2003, 107, 8929–8934. (8) Knoezinger, E.; Wittenbeck, R. Infrared Phys. 1984, 24(2-3), 135–142. (9) Zondlo, M. A.; Onasch, T. B.; Warshawsky, M. S.; Tolbert, M. A.; Mallick, G.; Arentz, P.; Robinson, M. S. J. Phys. Chem. B 1997, 101, 10887–10895. (10) Kaya, S.; Weissenrieder, J.; Stacchiola, D.; Shaikhutdinov, S.; Freund, H.-J. J. Phys. Chem. C 2007, 111, 759–764. (11) Smith, R. S.; Zubkov, T.; Kay, B. D. J. Chem. Phys. 2006, 124, 114710– 114717. (12) Zubkov, T.; Smith, R. S.; Engstrom, T. R.; Kay, B. D. J. Chem. Phys. 2007, 127, 184707–184711. (13) Drobyshev, A.; Aldijarov, A.; Abdykalykov, K.; Panchenko, G. Low Temp. Phys. 2003, 29(8), 669–673. (14) Kimmel, G. A.; Petric, N. G.; Dohnalek, Z.; Kay, B. D. Phys. ReV. Lett. 2005, 95, 166102.
10.1021/la802792u CCC: $40.75 2009 American Chemical Society Published on Web 12/31/2008
IR Testing of Water Ice Surfaces
about the icy surface. The signal-to-noise ratio in the OD-stretch region is much better than that in the OH-stretch region. Moreover, by using D2O water ice, it is possible to investigate the 3000-3600 cm-1 spectral region, where CH- and NH-stretching vibrations occur. Our choice of dopants can be explained by several reasons. The studies of hydrogen-bonded acid-base complexes are of fundamental importance. The systems under investigation are typical models of such species.15,16 Low-temperature atmospheric processes on surfaces of aerosol ice particles are of ecological importance.17,18 Also, recent laboratory work has shown that both prussic acid and ammonia, the presence of which in space was experimentally proven,19 are likely to play an important role in prebiotic chemistry.20 The acid-base properties of ice mixtures with different dopant/ water ratios (1:10, 1:5, and 1:1) are studied using carbon monoxide and fluoroform as probe molecules. Carbon monoxide became the most popular test molecule. The CO-stretching mode is sensitive to the strength of interaction with adsorption sites (see, e.g., ref 21). The CO-stretching frequency increases during the interaction with the cationic sites of zeolites or oxides, and the value of this frequency shift can be used as a measure of the Lewis acidity (see, e.g., ref 22). Recently, it was demonstrated the possibility of the formation of CO complexes with basic surface oxygen ions of basic zeolites, silica, or methoxylated silica.23,24 The basicity of the surface sites can be tested by adsorption of CHF3. The CH-stretching vibration of this molecule on interaction with weak bases reveals a blue-shifting H-bond when νCH slightly increases and is not sensitive to the strength of basic sites.25,26 That is why, to estimate the basicity of dO centers, we used the frequency of the bending mode of adsorbed CHF3, which occurs at 1379 cm-1 for a free gas molecule and increases on formation of a H-bond with surface oxygen atoms up to 1412 cm-1 for the most basic sites.
2. Experimental Section The used vacuum cell for IR spectroscopic studies of adsorbed molecules at variable temperatures, described elsewhere,27,28 has an extra pair of windows which separate the inner volume with the sample from the surrounding evacuated thermoinsulating compartment. This enabled us to record spectra in the presence of gaseous adsorbates at relatively high pressures (up to 800 Torr) or even to (15) Malaspina, T.; Fileti, E. E.; Riveros, J. M.; Canuto, S. J. Phys. Chem. A 2006, 110, 10303–10308. (16) Szulczewski, G. J.; White, J. M. Surf. Sci. 1998, 406, 194–205. (17) Horn, A. B.; Chesters, M. A.; McCoustra, M. R. S.; Sodeau, J. R. J. Chem. Soc., Faraday Trans. 1992, 88(7), 1077–1078. (18) Souda, R.; Kawanowa, H.; Kondo, M.; Gotoh, Y J. Chem. Phys. 2004, 120(12), 5723–5727. (19) Jenniskens, P.; Blake, D. F.; Wilson, M. A.; Pohorille, A Astrophys. J. 1995, 455(1), 389–401. (20) Bernstein, M. P. Nature 2002, 416, 401–403. (21) Knozinger, H. In Adsorption on ordered surfaces of ionic solids and thin films; Umbach, E., Freund, H.-J., Eds.; Springer Series in Surface Science; SpringerVerlag: Berlin, Heideiberg, 1993; Vol. 33, pp 257-267. (22) Bordiga, S.; Garrone, E.; Lamberti, C.; Zecchina, A.; Arean, C. O.; Kazansky, V. B.; Kustov, L. M. J. Chem. Soc., Faraday Trans. 1994, 90(21), 3367–3372. (23) Tsyganenko, A. A.; Kondratieva, E. V.; Yanko, V. S.; Storozhev, P. Yu J. Mater. Chem. 2006, 16, 2358–2363. (24) Hadjiivanov, K.; Penkova, A.; Centeno, M. A. Catal. Commun. 2007, 8, 1715–1718. (25) Zakharov, N. V.; Litkevich, A. M.; Tsyganenko, A. A.; Travert, A.; Daudin, A.; Mauge´, F. Proceedings of the 13th International Congress on Catalysis, Paris, France, 2004; p 217, http://www.13icc.jussieu.fr. (26) Storozheva, E. N.; Sekushin, V. N.; Tsyganenko, A. A Catal. Lett. 2006, 107(3-4), 185–188. (27) Babaeva, M. A.; Bystrov, D. S.; Kovalgin, A. Yu.; Tsyganenko, A. A J. Catal. 1990, 123, 396–416. (28) Tsyganenko, A. A.; Storozhev, P. Yu.; Otero Arean, C. Kinet. Catal. 2004, 45(4), 530–540.
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Figure 1. (A) Device for ice film preparation and (B) cell for IR spectroscopic studies of adsorbed molecules: (1) volumes for H2O and D2O water, (2) molybdenum lead-ins, (3) valves, (4) kovar-glass joint, (5) kovar tube, (6) stainless steel flange of IR cell, (7) volume for adsorbates and vapor mixture preparation, (8) heating filament, (9) glass capillaries, (10) outlet holes, (11) vacuum O-rings, (12) indium O-ring, (13) thermoinsulating volume, (14) inner windows, (15) outer windows, and (16) volume for liquid nitrogen.
study adsorption of some compounds from the solution in cryogenic solvents, such as liquid oxygen. For ice film deposition, the cell was equipped with a special device (Figure 1). Water vapor from volume (1) was introduced through capillaries (9) heated by a thin nichrome filament (8) to avoid condensation inside the capillary and deposited directly onto the inner side of the internal ZnSe windows (14), preliminarily cooled by liquid nitrogen. The outlet holes (10) are placed at about 5 mm from the central part of the beam, between the windows fixed about 7 mm apart. This corresponds to an average angle of sputtering about 60° with respect to the normal that is the angle providing the highest porosity of sputtered WO3 films29 and is supposed here to be suitable for water deposition as well. In order to produce mixed icy films, such as films of D2O with prussic acid or heavy ammonia, mixtures of desired composition were prepared before cooling the cell in a special volume (7) by sequentially freezing their doses of vapors measured by using a pressure gauge in a calibrated volume. The measured water vapor pressure was always not more than 10 Torr, far enough from the saturation vapor pressure. Spectra of adsorbed substances were studied in the 56-273 K temperature range. The film temperature was monitored by a using thermocouple installed inside the inner compartment of the cell through the tube for coolant filling. Temperatures below 77 K were obtained by lowering the pressure over liquid nitrogen. In low-temperature adsorption experiments, gases and vapors were deposited onto the icy film surface by different ways. The gases (29) Sekushin, N. A.; Tsyganenko, A. A. Kolloidn. Zh. 1987, 49(2), 326–328.
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volatile at 77 K (CO, CH4, etc.) were introduced from the vacuum system directly to the cell and purified by passing through a trap cooled by liquid nitrogen. Less volatile gases at 77 K, such as CHF3, were condensed on the walls of the cell. In order to redeposit it onto the icy film, a slight temperature increase was needed. Gases, soluble in liquid oxygen, could be adsorbed at 77 K, otherwise from the solution in liquid oxygen.26 This way was used previously to follow the temperature dependence of ethene ozonolysis reaction starting from 55 K when the gas pressure of both reactants is zero.30 The vapor pressure of HCN or NH3 is high enough for adsorption only at temperatures above 150 K when the water ice film already loses its porosity. In this case, the adsorbate, previously locked in volume (7), could be deposited through the heated capillarity (8) onto the ice surface (Figure 1). At 77 K, it forms a layer on the ice film. Due to diffusion, adsorbate molecules penetrate inside the ice pores with temperature increase and perturb the dangling hydroxyl groups. Thus, the adsorbate molecules occupy surface sites at temperatures much lower than those needed for adsorption from the gas phase. To prevent the film from heating completely, the codeposited films of water ice with HCN or ND3 were prepared as described above and studied by IR spectroscopy of volatile test molecules. The heavy water was thoroughly mixed with prussic acid or heavy ammonia in desired molar (1:10, 1:5, and 1:1) dopant/water ratios. The specific surface area of amorphous water ice film was estimated using volumetric measurements.31 We found that CO adsorption on water ice at 77 K is completely reversible. The amount of adsorbed CO was determined from comparison of the equilibrium pressure of gas after introducing the carefully measured portions of it into the cell with water film as compared with the same measurements with the empty cell in the same conditions. The quantity of ice after deposition of a well-measured portion from a calibrated volume was not sufficient for the precise volumetric measurements. That is why, after spectrum registration, a much greater dose of water was introduced through the capillary directly from the water-containing volume (1) (Figure 1), so that the thickness of the resulting film, monitored spectroscopically by the optical density in IR region, became about 1 order of magnitude greater. It should be noted that the intensity of the IR signal from dangling OH groups in dispersed amorphous ice is increased proportionally with increasing film thickness. This dependence is in accordance with observations made by Zondlo et al. for low-density ice obtained at 94 K.9 The obtained isotherm of CO adsorption was approximated by the Langmuir isotherm with the correlation factor of 0.95. Surface saturation by adsorbed CO at monolayer coverage occurred at pressures of about 3-3.5 Torr and corresponded to conditions when in the IR spectra the band of dangling OH groups was almost completely perturbed by adsorbed molecules, while the intensity of the CO band at ∼2153 cm-1 was close to its maximum value. The specific surface area of water ice film was determined from the following relation:
S)
xm N A M A m
(1)
where NA is the Avogadro number (6.02 × 1023 molecules per mole); M is the molecular weight of carbon monoxide (in grams per mole); xm is the obtained CO absorbance capacity at monolayer coverage (4.6 × 10-2 g of adsorbed CO per 1 g of H2O); and Am is the adsorbate cross-sectional area (16.2 × 10-20 m2 for carbon monoxide32). Used for the preparation of icy films were liquid H2O (99.9%) as well as D2O (99.9%) samples, as supplied. Hydrogen cyanide was prepared by dropwise addition of H2SO4 to KCN, purified by (30) Manoilova, O. V.; Lavalley, J.-C.; Tsyganenko, N. M.; Tsyganenko, A. A. Langmuir 1998, 14(20), 5813–5820. (31) Sekushin, N. A.; Sekushin, V. N.; Tsyganenko, A. A. In Proceedings of the Scientific-Practical Conference; Chukreev, Yu. Ya., Ed.; Syktyvkar Forest Institute: Syktyvkar, Russia, 2007; p 301. (32) Greg, S. J.; Sing, K. S. W. Adsorption, surface area and porosity., 2nd ed; Academic Press: London, U.K., 1982.
Figure 2. IR spectra of disperse D2O/H2O ice film on ZnSe support at (1) 77 K and (2) 163 K. (Inset) Degradation of OD-dangling band with temperature increase: (1) 77 K, (2) 122 K, and (3) 155 K.
vacuum distillation, and only the first portion of HCN evolved used in experiments. DCN was prepared in a similar manner by using D2SO4. The spectra were recorded with a Nicolet-710 FTIR spectrometer, at 4 cm-1 spectral resolution by coadding 256 scans. The band positions and intensities were determined using OMNIC software.
3. Results and Discussion Pure Water Ice Films. The amorphous water ice films prepared by deposition of water vapor on the inner windows of the cell at 77 K had the estimated as above specific surface area of about 160 m2/g. It is known that rapid deposition at temperatures below 130 K favors production of a porous amorphous water ice with a relatively well-developed internal surface and the surface area 120-500 m2/g, from Brunauer-Emmett-Teller (BET) measurements with nitrogen adsorption.6,33 Higher deposition temperatures lead to the formation of denser layers with the area 5-12 m2/g, corresponding to nonporous structures.34 The value of surface area of 160 m2/g, obtained in this work from CO adsoption, is large enough and typical of porous water ice. Curve 1 in Figure 2 demonstrates the IR spectrum of amorphous D2O/H2O ice film at 77 K on ZnSe windows. Highly intense absorption features with maxima near 3300 and 2440 cm-1 belong to the stretching modes of the OH and OD groups respectively, of hydrogen-bonded water molecules in the bulk. Weak peaks in the 1200-1620 cm-1 region are due to the corresponding bending modes: 1215 cm-1 to δ(DOD), 1493 cm-1 to δ(HOD), and 1614 cm-1 to δ(HOH). At the high frequency side on the OH- and OD-stretching bands, barely distinguished weak features are seen at 3696 and 2727 cm-1. The intensities of these bands are much weaker compared to those of stretching vibrations of bulk hydroxyl groups with the coordination number of 4. According to numerous reported studies, these bands should be assigned to the stretching vibrations of the OH and OD groups in water molecules not involved in hydrogen-bonding and located on surfaces of micropores (see refs 35 and 36, and references therein). These are commonly dubbed as “free” or “dangling” hydroxyl groups. At expanded view the spectra reveal satellites near these bands at 3717 and 2745 cm-1, more distinct near the OD-dangling band (see inset in Figure 2, spectrum 1). The higher and lower frequency features are due to dangling bonds in 2- and 3-coordinated surface water molecules, respectively. Based on (33) Ghormley, J. A. J. Chem. Phys. 1967, 46(4), 1321–1325. (34) Adamson, A. W.; Dormant, L. M. J. Am. Chem. Soc. 1966, 88, 2055– 2057. (35) Devlin, J. P.; Buch, V. J. Phys. Chem. 1995, 99(45), 16534–16548. (36) Devlin, J. P.; Buch, V. J. Phys. Chem. B 1997, 101(32), 6095–6098.
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Figure 3. Models of the surface sites on amorphous water ice surface: (left) dangling hydroxyl group in 2-coordinated water molecule (dH2 site), (middle) dangling hydroxyl group in 3-coordinated water molecule (dH3 site), and (right) coordinately unsaturated oxygen atom (dO site). Dashed lines show H-bonds in the bulk of ice. Table 1. Influence of Adsorbate on ν(dangling OD) Frequency Shift adsorbate
ν(d-OD), cm-1
∆ν(d-OD), cm-1
T, K
H2 CF4 N2 CH4 CHF3 CF2Cl2 CFCl3 CCl4 CO O3 CBr4 C2D2
2717 2718 2712 2708 2708 2708 2702 2695 2693 2687 2681 2674
-10 -10 -15 -19 -19 -20 -26 -33 -34 -40 -47 -53
56 12 56 77 77 100 100 100 77 77 12 77
ref this 38 this this this 17 17 17 this this 38 this
work work work work
work work work
the experimental and theoretical arguments, Buch and Devlin (ref 37) assumed that 2-coordinated surface water molecules form H-bonds with the bulk of ice via the proton and oxygen atoms, while 3-coordinated molecules can be bound to neighbors in two ways: (1) by one H-bond via a proton and two H-bonds via an oxygen atom, or (2) by forming two H-bonds via protons and a single H-bond via an oxygen atom. These surface sites are sketched out in Figure 3 and denoted as dH2, dH3, and dO. It should be noted that after the film evaporation at temperatures above 230 K the spectrum of the cell windows was always identical to that registered before the film deposition. Some experiments were carried out with BaF2 windows, and spectra of water films were exactly identical to those observed with ZnSe windows. We did not detect any bands assignable to the OH (OD) groups formed as a result of dissociative adsorption of water on ZnSe substrate. Dangling hydroxyl groups are highly sensitive to adsorption, with the stretching frequency shifts strongly depending on the nature of the adsorbate. Table 1 presents data on the band position of dangling OD groups after adsorption of different substances on D2O ice film, which demonstrate these adsorbate-induced shifts, measured by us or taken from literature sources. The observed perturbation of hydroxyls infers that most of coordinately unsaturated water molecules are located on the pore surfaces. Spectra of H2O/D2O ices at 77 and 163 K, presented in Figure 2, show that at 163 K the bands of OH- and OD-stretching modes become more intense, narrow, and shifted to lower frequencies by 36 and 22 cm-1, while those of bending modes become more intense and narrow (spectrum 2). These data can serve as evidence for the transition from amorphous to polycrystalline phase for the water ice. It is known that the phase transitions in the ice films do occur in the 77-170 K temperature range.12,39 Different authors report different values of the red shifts for the OH- and OD-stretching vibrations that are caused by the phase transition and depend on the isotopic composition of ices, film thickness, and deposition temperature of water ice films.10-14 The inset in Figure 2 demonstrates degradation of the ODdangling band with the temperature increase. Annealing of the (37) Buch, V.; Devlin, J. P. J. Chem. Phys. 1991, 94(5), 4091–4092. (38) Holmes, N. S.; Sodeau, J. R. J. Phys. Chem. A 1999, 103(24), 4673–4679. (39) Ghormley, J. A. J. Chem. Phys. 1968, 48(1), 503–508.
Figure 4. FTIR spectra of CO (3 Torr in equilibrium) adsorbed at 77 K on mixed icy films with small (a) and large (b) dopant concentrations: (a) (1) pure D2O; (2) HCN/D2O, 1:10; (3) ND3/D2O, 1:10 and (b) (1) pure D2O; (2) HCN/D2O, 1:5; (3) HCN/D2O, 1:1; (4) ND3/D2O, 1:5; (5) ND3/D2O, 1:1. Initial spectra of icy films are subtracted from those after CO adsorption.
film up to 155 K (spectrum 3) leads to the disappearance of the band, which does not restore again after subsequent cooling the sample down to 77 K. This observation is in agreement with previous studies5,35 and can be explained by the temperatureinitiated interaction of the freely vibrating D atoms with unsaturated surface O atoms, finally leading to crystallization. The spectrum of the H2O/D2O ice at 163 K, indeed, is close to that of the crystalline Ic ice with the OH- and OD-stretching modes at 3268 and 2415 cm-1, respectively.40 Thus, annealing of amorphous water ice films up to about 155 K leads to crystallization, accompanied by a collapse of micropores and loss of the internal surface. Interestingly, the available data on the hydrogen-bonding enthalpy13,35 indicate that the temperature of transition from the amorphous to cubic crystalline state in D2O cryocondensates is higher than that for H2O. To characterize surface properties of disperse amorphous ice, IR spectroscopic investigation of carbon monoxide adsorption was carried out. The coverage dependence of spectra of adsorbed CO on dispersed icy films were obtained at 77 K for equilibrium pressures up to 3-3.5 Torr. The observed CO bands for all the studied icy films appear and grow in their intensity with the pressure increase, and both demonstrate saturation. One spectrum of adsorbed CO from these series for each film at the same equilibrium pressure is presented in Figure 4. The spectrum of CO adsorbed on pure amorphous water ice film at 77 K is presented in Figure 4 (spectrum 1). Two CO bands occur at 2153 and 2137 cm-1. The high-frequency band corresponds to CO molecules adsorbed on the dangling OD groups by a weak hydrogen bond.41 The dangling OD groups are analogous to surface silanol groups of silica, which manifest the properties of weak acid sites,42 characterized by a CO frequency increase up to 2160-2156 cm-1 and ∆νOH of 90 cm-1. For ice, all the shifts are seriously smaller, demonstrating weaker protondonating ability (acidity) of the dangling hydroxyl groups. Low-frequency bands of weakly adsorbed CO close to the frequency of a free molecule in the gas phase (2143 cm-1) are usually referred to physisorbed CO molecules. In a FTIR study of codeposited at 5 K water-CO in Ar matrices, Givan et al.43 assigned the band at 2138 cm-1 to molecules trapped in the “bulk pores”, because in their experiments it was observed on heating up to 155 K. In our case, both IR bands of adsorbed CO could be easily removed by evacuation at 77 K, and hence, both belong to somehow weakly adsorbed species. (40) Devlin, J. P. Int. ReV. Phys. Chem 1990, 9(1), 29–65. (41) Devlin, J. P. J. Phys. Chem. 1992, 96, 6186–6188. (42) Storozhev, P. Yu.; Otero Arean, C.; Garrone, E.; Ugliengo, P.; Ermoshin, V. A.; Tsyganenko, A. A. Chem. Phys. Lett. 2003, 374, 439–445. (43) Givan, A.; Loewenschuss, A.; Nielsen, C. J. Vib. Spectrosc. 1996, 12(1), 1–14.
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Figure 5. FTIR spectra of CHF3 adsorbed at 110-120 K on mixed icy films with small (a) and large (b) dopant concentrations. Spectra were recorded at 77 K: (a) (1) pure D2O; (2) HCN/D2O, 1:10; (3) ND3/D2O, 1:10 and (b) (1) pure D2O; (2) HCN/D2O, 1:5; (3) HCN/D2O, 1:1; (4) ND3 /D2O, 1:5; (5) ND3 /D2O, 1:1. Initial spectra of icy films are subtracted from those after CHF3 adsorption.
For silica, the analogous band of adsorbed CO at about 2135 cm-1 was recently shown to consist of two overlapping peaks at ∼2138 cm-1 and a weaker one at 2131 cm-1, with the latter being due to molecules H-bonded to silanol groups via an O atom.42 The former band, taking into account thermodynamic measurements and the estimates of electrostatic interaction of molecular quadrupoles with surface ions, was attributed to the side-on complexes of the CO molecule with the oxygen atoms of siloxane bridges.23,24 This enables us to suppose that the 2137 cm-1 band of CO adsorbed on amorphous water ice film at 77 K, which demonstrates saturability typical of specific interaction with certain surface sites, can be assigned to CO molecules forming the “side-on” complexes with coordinately unsaturated oxygen atoms of surface water molecules (dO sites in Figure 3). Apparently, some contribution to this band could also give molecules, which form H-bonds with the dangling hydroxyl groups via the O atom, but the fraction of such complexes, which coexist in thermodynamic equilibrium with the C-bonded molecules,44 should be comparatively small. Devlin41 has found that CF4 molecules adsorbed on amorphous D2O ice at 58 K, slightly shifting the band of dangling OD groups to lower frequencies (by 8 cm-1), also block the access of CO to the sites which account for the band at 2136 cm-1. A complex of water with tetrafluoromethane has been recently studied by infrared matrix isolation spectroscopy and ab initio calculations.45 It was found to be non-hydrogen-bonded but linked via the oxygen atom of water to the carbon of CF4. It means that CF4 really occupies the dO sites and supports our assumption that the second CO band at 2137 cm-1 is associated with CO molecules interacting with the oxygen atoms of the superficial water molecules. The basicity of surface oxygen is comparatively weak, as follows from CO adsorption (band at 2137 cm-1) and from the band position of the CH-stretching vibration of adsorbed fluoroform, which was observed at ∼3060 cm-1. As far as the νCH mode is not sensitive enough for testing weak basic sites, to estimate the basicity of dO centers, we used the frequency of the bending mode of adsorbed CHF3, which occurs at 1379 cm-1 for a free gas molecule and increases on formation of a H-bond with surface oxygen atoms25 up to 1412 cm-1 for the most basic sites. Figure 5 (spectrum 1) shows the spectrum of CHF3 adsorbed on a pure D2O film. The high-frequency shoulder at about 1392 cm-1 can be considered as evidence for the presence of a certain amount of basic sites at the surface of water ice, which could be associated with the superficial unsaturated oxygen atoms (dO sites). A strong band at 1378 cm-1 with the frequency almost (44) Otero Arean, C.; Turnes Palomino, G.; Tsyganenko, A. A.; Garrone, E. Int. J. Mol. Sci. 2002, 3, 764–776. (45) Mienzwicki, K.; Mielke, Z.; Saldyka, M.; Coussan, S.; Roubin, P. Phys. Chem. Chem. Phys. 2008, 10, 1292–1297.
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coinciding with that of free molecules may be due to its physisorption or adsorption on the dangling hydroxyl groups. Mixed Ice Films. Mixtures of heavy water with prussic acid or heavy ammonia with different dopant/water molar ratios (1: 10, 1:5, and 1:1) were prepared as described in the Experimental Section and deposited on the inner ZnSe windows of the IR cell at 77 K. Their acid-base surface properties have also been tested by CO and CHF3 adsorption to compare with those for the pure heavy water ice. Figure 4a presents the IR spectra of CO adsorbed on (HCN+D2O) and (ND3+D2O) icy films with a component ratio of 1:10 (spectra 2 and 3). The band positions of adsorbed CO for mixed icy films turn out to be exactly the same as that for the bare ice film (spectrum 1). However, the relative band intensities of the two bands observed at the same gas pressure are modified by the presence of dopants. The intensity of the 2153 cm-1 band related to that of the 2137 cm-1 one slightly decreases for the (HCN+D2O)/1:10 mixture, and grows by about 25% with respect to that of pure ice for the (ND3+D2O)/1:10 mixture. Addition of small amounts of dopants in water ice does not influence the position of the OD-dangling band. In the IR spectra of icy mixtures with large dopant concentrations (1:5 and 1:1), deposited at 77 K, the bands of dangling hydroxyl groups are not observed, the corresponding band of adsorbed CO at 2153 cm-1 is absent (Figure 4b, spectra 2-5), while the intensities of other CO bands are much weaker than those for pure ice film. The only surface sites of such films are likely to be unsaturated oxygen atoms. Spectra of CHF3 adsorbed on icy mixtures at 110-120 K are shown in Figure 5. As seen from the figure, small dopant concentrations do not affect the spectrum of adsorbed fluoroform (Figure 5a). However, for the (HCN+D2O) icy mixture with concentrations of 1:5 and 1:1, the shoulder at 1392 cm-1 is negligibly weak (Figure 5b, spectra 2 and 3), while for (ND3+D2O) mixtures (spectra 4 and 5), on the contrary, it is better pronounced as compared with the spectrum of the pure D2O film. It is particularly strong for the 1:5 (ND3+D2O) mixture (spectrum 4), where its maximum is shifted to 1394 cm-1. Thus, we can conclude that the strength of acidic and basic sites is not noticeably affected by addition of small amounts of HCN or ND3. However, as follows from the intensity ratio of the two bands of adsorbed CO, the relative number of sites is modified by the presence of dopants. The sense of this effect is not evident: addition of prussic acid causes a slight increase of the relative number of basic dO sites, while the addition of a base (ammonia) increases the number of acid sites (dangling OD groups) as compared with the bare ice film. Such an effect seems somehow unexpected. For silica, addition of acidic molecules, such as SO2 or NO2, leads to the “induced Brønsted acidity” when the proton-donating ability of silanol groups substantially increases up to the protonation of strong bases, which never occurs on pure silica without coadsorbed acids.46 On the other hand, ammonia adsorption was shown to enhance basic properties of silanol oxygen atom, clearly seen from the spectrum of adsorbed fluoroform.26 Moreover, weak manifestations of mutual enhancement of adsorption were detected for ethylene and fluoroform coadsorbed on the ice surface. One could anticipate that addition of stronger acidic (hydrogen cyanide) or basic (ammonia) dopant to water ice would strongly enhance the acidity or basicity of the surface, respectively; however, we failed to see it. Instead, the influence of small amounts of dopants on relative site concentration was established. (46) Tsyganenko, A. A.; Storozheva, E. N.; Manoilova, O. V.; Lesage, T.; Daturi, M.; Lavalley, J.-C. Catal. Lett. 2000, 70(3-4), 159–163.
IR Testing of Water Ice Surfaces
This effect might be explained if we assume that dopants in small concentrations are included into the bulk of the ice structure. Then, if we take into account the fact that ammonia donates three protons to the H-bonds and accepts only a single one, substitution of a water molecule by ammonium in the bulk should change the proportion between dangling protons and unsaturated oxygen sites at the surface in favor of protons, while at the surface of ice doped by acids oxygen dO sites should be more abundant. The absence of dangling hydroxyls on icy films with high concentration of dopants correlates with the absence of the CO band at 2153 cm-1 (Figure 4b, spectra 2-5). The presence of another band of adsorbed CO at 2138-2140 cm-1 infers that the film is still porous, although the lower intensity of this band registered at the same conditions, apparently, is due to the decrease of surface area. Thus, the only surface sites of such films are likely to be unsaturated oxygen atoms (dO sites). The higher intensity of the CO band at 2140 cm-1 for the (ND3+D2O)/1:5 mixture (spectrum 4 in Figure 4) correlates with the betterpronounced shoulder at 1394 cm-1 in the spectrum of fluoroform adsorbed on such a film (spectrum 4 in Figure 5). A slight shift of this shoulder to higher wavenumbers provides evidence for a small basicity increase with respect to pure water ice. The intensity decrease of this shoulder for the 1:1 mixture (curve 5 in Figure 5) can be explained by surface area lowering, revealed from the spectra of adsorbed CO. A small upward shift of the 1394 cm-1 band as compared with basic oxides and zeolites (up to 1410 cm-1)25 infers that fluoroform interacts rather with O atoms of surface water molecules than with the lone pairs of ammonia. It should be noted that the band intensity of CHF3 can hardly be a measure of surface area, because at 77 K it is not in equuilibrium with gas but frozen and can form at the surface more than a monolayer coverage. Such strong changes of surface properties of icy deposits with high dopant concentrations are not easy to explain. We can only suppose that some structural changes occur, in which rearrangement of hydrogen bonds plays a significant role. For instance, (in ND3+D2O) mixtures, more acidic OD groups should strongly interact with the basic lone pairs of nitrogen atoms, while less basic dO sites and less acidic dangling ND groups have to predominate on the surface of deposits. Bands of dangling ND groups superimpose on the strong absorption of hydrogen-bonded deuteroxyl groups and could hardly be observed. Strong structural changes should influence the spectra of dopants in the bulk of films. Analysis of band positions for the dopants in the spectra of considered icy mixtures and comparison with those for dopant molecules in the gas phase,47 solid films,47,48 and different matrices49-52 show that dopant molecules in diluted icy mixtures seem as being relatively isolated in a solid water matrix. Interaction between the dopant molecules appears to be much weaker than that in the films of pure dopants. Probably, they exist mostly as monomers but not as polymers or associates, which they usually form in their own solid films. So, the spectrum (47) Friedrich, H. B.; Krause, P. F. J. Chem. Phys. 1973, 59(9), 4942–4948. (48) Holt, J. S.; Sadoskas, D.; Pursell, C. J. J. Chem. Phys. 2004, 120(15), 7153–7157. (49) Walsh, B.; Barnes, A. J.; Suzuki, S.; Orville-Thomas, W. J. J. Mol. Spectrosc. 1978, 72, 44–56. (50) Kingpr, C. M.; Nixon, E. R. J. Chem. Phys. 1968, 48(4), 1685–1695. (51) Ribbegard, G. Chem. Phys. 1975, 8, 185–191. (52) Langel, W.; Kollhoff, H.; Knoezinger, E. J. Chem. Phys. 1989, 90(7), 4430–3442.
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of the (HCN+D2O) icy mixture is similar to that of HCN isolated in an Ar matrix,49 although the bands are considerably broader, while the spectrum of the (ND3+D2O) icy mixture is close to that of a thin film of amorphous ND3.48 Spectra of 1:1 mixtures are significantly different. Apparently, different polymeric forms of HCN and ND3 associates are formed with the increased concentration of dopants. These observations will be the subject of a separate publication.
4. Summary In this paper, we describe a device designed for porous icy film deposition at 77 K on the inner ZnSe windows of a variable temperature IR cell. The construction of the latter enables studies of transmission FTIR spectra to be carried out in the presence of considerable pressures of adsorbed gases without breaching the thermoinsulation. The specific surface area of this produced water ice was estimated volumetrically using the carbon monoxide adsorption data and found to be near 160 m2/g. In agreement with earlier studies, IR spectra of amorphous ice films exhibit the bands of the dangling hydroxyl groups. Absorption intensities of IR bands of these species are weak, while their frequencies appear to be higher than those for the hydroxyl groups in the bulk of ice. Gradual annealing of ice films results in phase transition from amorphous to polycrystalline phases, when the OH- (OD-) stretching band of the bulk species substantially shifts to lower frequencies at temperatures above 130 K, while the intensity of free hydroxyl (deuteroxyl) groups decreases. Surface properties of icy films were characterized by lowtemperature adsorption of CO and CHF3. Two bands observed at 77 K in the spectrum of adsorbed CO at 2153 and 2137 cm-1 are assigned to molecules bound to the dangling hydroxyl groups by weak H-bonds, and to the unsaturated surface oxygen atoms, respectively. Fluoroform adsorption was used to characterize the basicity of the latter sites, which is rather weak, and that is why it could only be estimated from small upward shifts of the δCH bending mode from 1379 to 1392 cm-1 for pure ice film. Spectra of icy films of D2O mixed with prussic acid or ND3 with different dopant/water ratios (1:10, 1:5, and 1:1), as well as those of test molecules adsorbed on such films, reveal certain differences from pure water films, depending on dopant concentration. Insertion of small doses of HCN or ammonia does not change the acid/base strength of dangling hydroxyl groups or coordinately unsaturated surface oxygen atoms, but it changes the proportion between the concentrations of these sites as compared with pure water ice. For high dopant concentrations, the dangling hydroxyls are not observed, the dominant adsorption sites for CO are likely to be the unsaturated oxygen atoms, while the surface area decreases, as follows from the lowered overall intensity of adsorbed CO bands. Data on adsorbed CHF3 are consistent with these observations. The absence of dangling hydroxyls and the changes in the spectra of adsorbed molecules as well as in the spectra of dopants point to serious structural changes in the bulk of ices. Acknowledgment. This work was supported by INTAS (Grant 03-51-5698) and RFBR (Grants 06-03-32836a and 06-05-64646). A.V.R. thanks Dr. Kirill M. Bulanin for his help. LA802792U
