19582
J. Phys. Chem. 1996, 100, 19582-19586
Properties of Crystalline Sulfuric Acid in Ultrahigh Vacuum: Incongruent Melting to the Liquid Azeotrope and Its Slow Recrystallization Liesl R. Schindler and Jeffrey T. Roberts* Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455-0431 ReceiVed: July 24, 1996X
The growth and characterization of ultrathin (≈80 monolayers) films of pure, crystalline sulfuric acid and of the liquid H2SO4 + H2O azeotrope in ultrahigh vacuum (UHV) are described. The films, which were deposited on a Pt(111) substrate, were synthesized in Vacuo via the reaction of co-condensed mixtures of H2O and SO3. Fourier transform infrared reflection-absorption spectroscopy (FTIRAS) shows that the pure, crystalline acid forms when such mixtures are briefly annealed at 260 K. Crystalline H2SO4 is stable in UHV until 285 K, at which temperature FTIRAS measurements suggest the melting occurs. The melting temperature is equal, within experimental error, to the melting point of a macroscopic sample of crystalline H2SO4. Analysis of the gas phase composition during melting reveals that SO3 is expelled from a film as it melts. The liquid is identified as the azeotrope of H2SO4 and H2O, formed in the incongruent melting of H2SO4. The H2SO4 + H2O mixtures can be supercooled to 255 K. Freezing results in the formation of pure, crystalline H2SO4 and is accompanied by the expulsion of H2O into the gas phase. To our knowledge, this work provides the first observation of melting of a molecular solid under UHV.
Introduction The surface chemical properties of sulfuric acid are of relevance to a wide range of environmentally and technologically important phenomena, such as heterogeneous atmospheric chemistry, electrochemistry, and corrosion.1-3 Critical to an understanding of sulfuric acid surface chemistry is a corresponding understanding of adsorbate structure and adsorption/ desorption kinetics. Few methods are available to study these issues, partly because many conveniently implemented analytical probes (infrared spectroscopy, mass spectrometry, etc.) are intrinsically bulk sensitive, preventing their application to macroscopic sulfuric acid samples, which have small surface area-to-volume ratios. By studying sulfuric acid films between 10 and 100 monolayers thick, it should be possible to circumvent surface area-to-volume ratio limitations. Surface analysis could be further simplified by conducting experiments in ultrahigh vacuum (UHV), where numerous surface-sensitive spectroscopic probes are available. It has been demonstrated that this approachsthe study ultrathin films in UHVscan be used to study the surface chemical properties of ice.4-6 The approach has recently been extended to interfacial systems involving sulfuric acid.7 For sulfuric acid, the experimental challenge of a UHV-based approach is to devise a method for film deposition in UHV. The vapor pressure of pure sulfuric acid (≈10-3 Pa at room temperature)8 is too low for a method based on direct condensation to be practicable, particularly since sulfuric acid is exceedingly corrosive. Earlier, we described a method for the synthesis of ultrathin sulfuric acid films in Vacuo.7 The method is very flexible, allowing the synthesis of pure sulfuric acid as an amorphous or crystalline solid. Mixtures of sulfuric acid and water can be deposited as well, and their compositions are variable over a wide range. In this paper we further explore the properties of sulfuric acid in UHV. In particular, we show that pure, crystalline sulfuric acid reversibly melts to form an * To whom correspondence should be addressed. Telephone (612) 6252363, fax (612) 626-7541, e-mail
[email protected]. X Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)02251-4 CCC: $12.00
H2SO4 + H2O azeotrope, and we consider the kinetics and thermodynamics of the solid-liquid and liquid-solid phase transitions. Experimental Section Experiments were conducted in a UHV chamber that is described in detail elsewhere.9 Sulfuric acid films were deposited on a 111-oriented, single crystal platinum substrate that was mounted on a sample manipulator capable of x-, y-, and z-translation and of rotation about the z-axis. The substrate was in thermal contact with a liquid nitrogen-cooled reservoir and could be cooled to approximately 90 K. Radiative heating was provided by a tungsten filament ≈2 mm behind the crystal. Temperatures were measured with a chromel/alumel thermocouple junction that was spot-welded to the Pt substrate; an electronic ice point substituted for a reference junction. Gases were admitted into the chamber using directed dosers. The UHV chamber is equipped with instrumentation for Fourier transform reflection-absorption infrared spectroscopy (FTIRAS) and temperature-programmed desorption (TPD). For FTIRAS, the infrared beam was reflected from the Pt substrate at an angle of 4° from the surface plane. Desorption data were acquired with the substrate positioned in line of sight of a mass spectrometer that was interfaced to a data acquisition board. As many as seven ion-temperature profiles could be collected during a single experiment. The synthesis of ultrathin H2O + H2SO4 films is described elsewhere.7 Briefly, H2O/SO3/H2O “sandwiches” are deposited on the Pt substrate at 90 K. The sandwiches are converted to sulfuric acid by briefly annealing at a temperature between 180 and 260 K, depending on the desired film composition. Film thicknesses cited in this work are given in units of monolayer equivalents (ML) and are based upon the H2SO4 desorption yield during TPD. The water used to synthesize sulfuric acid was deionized and triply distilled and was degassed via several freeze-pump-thaw cycles before use each day. Water-d2 (CIL) and sulfur trioxide (Aldrich) were degassed before use each day and otherwise used as received. © 1996 American Chemical Society
Crystalline Sulfuric Acid in Ultrahigh Vacuum
Figure 1. Single reflection FTIR spectra of (a) crystalline H2SO4 at 100 K, (b) liquid H2SO4 at 285 K, (c) supercooling H2SO4 at 273 ( 2 K, (d) crystallizing H2SO4 at 257 ( 2 K, and (e) recrystallized H2SO4 at 240 ( 2 K. Absorbance scales are not constant between spectra. The two zeroth-order plots are offset in order to better discern the slope change at Tm.
Results Spectroscopic Observation of the Solid-Liquid Phase Transition in UHV. The infrared spectrum of a pure sulfuric acid film, prepared as described above with a 270 K annealing temperature, is shown in Figure 1a. Because the infrared beam is reflected from the underlying Pt(111) substrate, the spectrum contains information about bulk and surface structure. Strategies for the discernment of features associated with the film surface are discussed elsewhere; here we are concerned only with vibrational modes derived from the bulk.9,10 The spectrum in Figure 1a, which was recorded at 100 K, can be unambiguously assigned to the pure, crystalline acid. In particular, the sharp band in the OH stretching region at 2980 cm-1 is consistent only with crystalline H2SO4; spectra of amorphous and liquid sulfuric acid, of H2O + H2SO4 solutions, and of the crystalline sulfuric acid hydrates (H2SO4‚nH2O) exhibit much broader OH stretching bands. None of the vibrational features are attributable to H2S2O7, the anhydride of sulfuric acid.11 Assignments of all the principal vibrational features are summarized in Table 1. The assignments are based upon those given by Gigue´re and Savoie to the spectra of macroscopic H2SO4 samples.11,12 The assignments are supported by the vibrational frequencies of ultrathin, crystalline D2SO4 films, which are also summarized in Table 1. The frequencies and relative intensities of the vibrational modes of crystalline sulfuric acid do not change significantly with temperature until 285 ( 2 K. At that temperature, the spectra undergo changes that indicative of melting (Figure 1b and Table 1). All of the bands, particularly that associated with OH stretching, broaden, suggestive of a decrease of order in the films. In addition, the S-O-H asymmetric bend becomes much more intense, and it shifts in frequency from 1241 to 1291 cm-1. An analogous change has been reported when macroscopic samples of crystalline sulfuric acid are melted.13 It is
J. Phys. Chem., Vol. 100, No. 50, 1996 19583 important to emphasize the spectral changes do not occur gradually with increasing temperature. Rather, they are abrupt, an observation that is strongly suggestive of a phase transition from the solid to the liquid. Notably, the changes occur at a temperature that is, within the uncertainty of the measurements, equal to what Bolsatis reports as the melting point, Tm, of crystalline sulfuric acid (283.45 K).8 A second important spectral change that occurs with increasing temperature is that absorbance decreases above ≈270 K. This is because the sulfuric acid evaporation rate becomes significant (>0.1 ML s-1). The lifetime of sulfuric acid film is obviously dependent on temperature and thickness. From FTIRAS measurements, we estimate that an 80 ML evaporates in approximately 2 min at 280 K. Evaporation below and above the Melting Point. The evaporation of sulfuric acid was studied with TPD, the results of which provide further evidence for melting. The following ions were monitored mass spectrometrically: m/e 18 (H2O+), m/e 48 (SO+), m/e 64 (SO2+), m/e 80 (SO3+), and m/e 98 (H2SO4+). In Figure 2 are shown representative m/e 18, 80, and 98 ion traces from the evaporation of an 80 ML crystalline H2SO4 film at a heating rate of 0.5 K s-1. The spectra can be divided into two temperature regions, below and above the H2SO4 melting point of 283.45 K. Below Tm, the line shapes of the ion traces are essentially identical; the spectra differ only in intensity. The mass spectrum of anhydrous sulfuric acid has not to our knowledge been reported, but measurements of the concentrated acid suggest that the H2SO4 parent ion fragments to H2O+, SO+, SO2+, and SO3+.14 Also, although gaseous H2SO4 coexists with H2O and SO3 at equilibrium, H2SO4 dissociation should be extremely slow in the nearly collisionless UHV environment. The early leading edges of the spectra in Figure 2 are thus attributed to H2SO4 vapor formed in the sublimation of the solid film. Above Tm, two aspects of the evaporation spectra are worthy of note. First, at 285 K, the SO3+ intensity begins to increase more rapidly than do the H2O+ and H2SO4+ intensities. The rapid increase is brief (close to the data acquisition rate), and by 287 K, the SO3+ signal again changes at approximately the same rate as the H2O+ and H2SO4+ signals. Similar changes are observed in the SO+ and SO2+ spectra, but not in the H2O+ and H2SO4+ spectra. The transient bursts of SO+, SO2+, and SO3+ strongly imply that melting is incongruent: H2SO4 melts to form a H2O + H2SO4 mixture, and excess SO3 is expelled into the gas phase. A second noteworthy feature of the desorption spectra is that the ion traces have different line shapes above the melting point. This is a manifestation of the fact that the liquid is a binary system, while the solid is a quasisingle-component system. The vapor pressures of H2O and H2SO4 in a H2O + H2SO4 mixture do not vary with temperature in the same way, and as a result, the composition of the liquid changes with temperature. The rate expression for sulfuric acid evaporation depends upon temperature. Specifically, evaporation is roughly zeroth order both above and below the melting point, but the evaporation energies are different (inset of Figure 2).15 Below Tm, the evaporation energy, which corresponds to the H2SO4 sublimation energy (Esub), is 93 ( 5 kJ mol-1. Above Tm, the vaporization energy (Evap) is 82 ( 3 kJ mol-1. These values are in close agreement with those reported by Bolsatis for the sublimation and vaporization of sulfuric acid.8 Their difference, 11 kJ mol-1, is equal to the heat of fusion of H2SO4 reported by Giauque.16 The trailing edges of the spectra drop of less rapidly than is normally observed in the sublimation spectra of condensed multilayers in UHV.15 This is attributed, at least in
19584 J. Phys. Chem., Vol. 100, No. 50, 1996
Schindler and Roberts
TABLE 1: Infrared Frequencies and Mode Assignments for H2SO4 and D2SO4 frequency/cm-1 mode
crystalline H2SO4 (this work)
crystalline H2SO4 (ref 12)
crystalline D2SO4 (this work)
crystalline D2SO4 (ref 12)
liquid H2SO4 (this work)
liquid H2SO4 (ref 12)
νasym OH(D) νsym OH(D) νasym SdO νsym SdO νasym S-O-H νsym S-O-H
2980 2455 1395 1195 1241 1169
2970 2450 1365 1170 1240 1170
2260 1870 1370 1185 a a
2280 1860 1350 1190 930 nab
3000 2465 1400 1205 1291 1205 (sh)
2970 2450 1368 1195 nab 1137
a
Frequency below cutoff of infrared detector. b Not assigned.
Figure 2. Temperature-programmed desorption of a pure, solid sulfuric acid film deposited on Pt(111). The heating rate was 0.5 K s-1. The melting temperature is indicated by the arrow. The inset shows the zeroth-order plots from which we determine a sublimation energy of 93 ( 5 kJ/mol and a vaporization energy of 82 ( 3 kJ/mol.
part, to pumping speed effects, since the initial layer was quite thick (≈80 ML). It is also possible that the evaporation is not zeroth order at high temperatures, due, for instance, to morphological changes in the films. The transient bursts of the SO+, SO2+, and SO3+ signals are observed during TPD only when the heating rate (β) is e1 K s-1. For β > 1 K s-1, the line shapes of the spectra differ above 285 K, but no pressure bursts are observed near Tm. TPD studies of amorphous ice have shown that the crystallization and sublimation rates are comparable at low β but that sublimation dominates at high β.17,18 In the H2SO4 system, the disappearance of desorption features associated with a phase transition is associated with an instrumental artifact. The data acquisition rate for each mass channel was ≈4 points per second. At heating rates greater than 1 K s-1, adjacent data points are separated by at least 0.25 K, over which temperature range melting occurs. The gaseous products of incongruent melting were therefore not observable at high β. Recrystallization of Liquid H2SO4 + H2O. The liquid sulfuric acid films studied in this work could be recrystallized under appropriate conditions. In Figure 1c-e are shown three infrared spectra of a sulfuric acid film recorded as it cooled; they were obtained at 273 ( 2, 257 ( 2, and 240 ( 2 K,
Figure 3. Temperature-programmed desorption of pure, solid sulfuric acid film during heating to 290 K and subsequent cooling. See text for discussion of important features. The heating rate was 0.5 K s-1 and the cooling rate was ≈2 K s-1.
respectively. (The temperature of a film decreased by ≈4 K during the time required to obtain an infrared spectrum. The temperatures cited above are those at the data acquisition midpoints.) The spectra provide clear evidence for conversion of the liquid H2SO4 + H2O mixture back to crystalline H2SO4 by 257 K. In particular, the broad OH stretching region narrows and acquires a line shape essentially identical to that of the originally crystalline film. The S-O-H asymmetric bond changes as well, decreasing in intensity and shifting in frequency back to 1241 cm-1. Interestingly, the spectra are attributable to the liquid until ≈260 K, well below Tm. Mass spectrometric measurements give additional evidence for recrystallization of the liquid sulfuric acid films upon cooling, and they provide insight into the origin of hysteresis in the liquid-solid phase transition. In Figure 3 are shown three heating/cooling curves for a sulfuric acid film that was originally crystalline and 80 ML thick. During heating (0.5 K s-1) and cooling (2 K s-1), the mass spectrometer was tuned to m/e 98 (H2SO4+), m/e 80 (SO3+), and m/e 18 (H2O+). Important features of the heating/cooling curves are as follows. First, mass spectral intensity is always greater on the cooling part of the curve. Second, there is no SO3+ burst during cooling. Finally,
Crystalline Sulfuric Acid in Ultrahigh Vacuum water is released from a film as it cools through ≈255 K, approximately the temperature at which infrared spectra indicate that crystallization occurs. All of these phenomena are associated with the cooling and/or crystallization of liquid H2SO4 + H2O. The release of water is a consequence of the fact that freezing, like melting, is incongruent, since the composition of crystalline H2SO4 is different from that of liquid H2SO4 + H2O. Water evolution below Tm indicates that the liquids pass through a metastable, supercooled state as they cool. That the m/e 98, 80, and 18 cooling curves are not superimposable on their corresponding heating curves is at least partly attributable to the fact that the cooling films are metastable between 285 and 255 K: the vapor pressure of a metastable substances is always greater than that of a stable substance. Because the vacuum system is subjected to a gas load during heating, the higher mass spectrometer signals upon cooling may also be associated with pumping speed effects. However, it is unlikely that pumping speed effects account for the different heating and cooling curves in Figure 3. In our hands, the heating and cooling curves of an 80 ML crystalline ice film are nearly superimposable. If pumping speed effects were responsible for all hysteresis in the sulfuric acid cooling curves, they should be manifest as well in the cooling curves of ice. Preliminary work suggests that the sulfuric acid freezing kinetics are a function of film thickness and cooling rate. Thick films freeze at lower temperatures than thin films, and the freezing temperature decreases with increasing cooling rate. The origin of these effects is currently under investigation, as is the possibility that the freezing kinetics are influenced by the underlying film substrate. Discussion The results presented herein establish that ultrathin films of pure, crystalline sulfuric acid can be deposited on a Pt(111) substrate in UHV and that the films melt incongruently at 285 K. The resulting liquids can be supercooled to ≈260 K (Tc), at which temperature they crystallize. Also, the close correspondence of the ultrathin film infrared spectra, as well as Tm, Esub, and Evap, with those of macroscopic sulfuric acid samples suggests that the properties of the ultrathin films are essentially equal to those of bulk sulfuric acid. The significance of these findings is twofold. First, they provide what is to our knowledge the first observation of melting of a molecular solid under ultrahigh vacuum. Detailed kinetic studies of the solidliquid and liquid-solid phase transitions in ultrathin sulfuric acid films may now be carried out, as they were for the amorphous-crystalline phase transition in ultrathin ice films.17,18 Second, by establishing a protocol for the preparation and characterization of liquid and solid sulfuric acid in UHV, they open the possibility of using UHV-based methods to probe differences between the surface chemical properties of the solid and liquid acids. That crystalline sulfuric acid melts incongruently to form a mixture of H2SO4 and H2O is demonstrated by the SO3 pressure burst at 275 K during TPD and by the release of water vapor that accompanies conversion of the H2SO4 + H2O mixture back to crystalline H2SO4. The driving force for incongruent melting is minimization of the free energy. For liquid sulfuric acid at its melting point, the minimum free energy system is a highly concentrated H2SO4 + H2O solution (i.e., an azeotrope), the formation of which requires the decomposition of some H2SO4 to H2O, which remains in the liquid and gaseous SO3. The azeotropic composition in UHV was not determined in this work, but the thermodynamic properties of highly concentrated sulfuric acid solutions at pressures between 100 and 1000 Torr
J. Phys. Chem., Vol. 100, No. 50, 1996 19585 were studied some time ago by Kunzler19 and Giauque.16 They found that the mole fraction of H2SO4 in an equilibrium H2SO4 + H2O solution at its boiling point smoothly increases with decreasing externally applied pressure, from 0.920 at 1.33 × 105 Pa to 0.937 at 1.33 × 104 Pa. Extrapolation to UHV, where the pressure is essentially zero, implies an H2SO4 mole fraction of between 0.94 and 0.95 in an ultrathin liquid film at its melting point. Since vapor pressure increases with temperature, the composition of an H2SO4 + H2O mixture in equilibrium with the gas phase changes with temperature. Specifically, the solutions gradually become enriched in water. This accounts, at least in part, for the different line shapes of the m/e 98, 80, and 18 TPD spectra above the sulfuric acid melting point. Mass spectrometric and FTIRAS measurements of the cooling H2SO4 + H2O films show that they do not freeze at Tm, but supercool, eventually crystallizing at approximately 257 K, ≈30 K below Tm. Supercooling is a well-known phenomenon in concentrated sulfuric acid solutions, which are viscous and reach equilibrium slowly.20 However, it is surprising that recrystallization is inhibited to such an extent in a film only several tens of monolayers thick. Slow recrystallization may be due in part to the fact that freezing requires the migration of excess H2O from the film interior to the surface and its subsequent expulsion into the gas phase. In any case, because supercooled films of ultrathin sulfuric acid films are so stable, UHV experiments on the surface of liquid sulfuric acid are now possible. At 262 K, just above the transition temperature from the supercooled liquid to the crystalline solid, the vapor pressure of sulfuric acid is so low that the lifetime of an 80 ML thick film is several minutes. Under such conditions, reactive and nonreactive sticking of numerous gases can be investigated, as can adsorbate and absorbate structure. These subjects are currently under investigation in this laboratory. Note Added in Proof. Since this paper was submitted for publication, a paper appeared concerning the surface composition of liquid H2SO4 + H2O solutions, as studied by Auger electron spectroscopy and X-ray photoelectron spectroscopy.21 This work provides convincing evidence that molecular liquids like sulfuric acid can be studied in ultrahigh vacuum. Acknowledgment. This work was supported by the National Science Foundation through Grant CHE-9527665. References and Notes (1) Hofmann, D. J.; Solomon, S. J. Geophys. Res. 1989, 94, 50295041. (2) Stuve, E. M.; Kizhakevariam, N. J. Vac. Sci. Technol. 1993, 11, 2217-2224. (3) Dai, Q.; Hu, J.; Freedman, A.; Robinson, G. N.; Salmeron, M. J. Phys. Chem. 1996, 100, 9-11. (4) Schaff, J. E.; Roberts, J. T. J. Phys. Chem. 1994, 98, 6900-6902. (5) Materer, N.; Starke, U.; Barbieri, A.; VanHove, M. A.; Somorjai, G. A.; Kroes, G. J.; Minot, C. J. Phys. Chem. 1995, 99, 6267-6269. (6) Brown, D. E.; George, S. M.; Huang, C.; Wong, E. K. L.; Rider, K. B.; Smith, R. S.; Kay, B. D. J. Phys. Chem. 1996, 100, 4988-4995. (7) Guldan, E. D.; Schindler, L. R.; Roberts, J. T. J. Phys. Chem. 1995, 99, 16059-16066. (8) Bolsaitis, P.; Elliot, J. F. J. Chem. Eng. Data 1990, 53, 69-85. (9) Schaff, J. E.; Roberts, J. T. J. Phys. Chem., in press. (10) Roberts, J. T. In Proceedings of the 1995 SPIE Meeting on Laser Techniques for Surface Science II; SPIE: Bellingham, WA, 1995; pp 125134. (11) Gillespie, R. J.; Robinson, E. A. In Non-aqueous SolVent Systems; Waddington, T. C., Ed.; Academic Press: New York, 1965; pp 117-210. (12) Gigue´re, P. A.; Savoie, R. J. Am. Chem. Soc. 1963, 85, 287-289. (13) Goypiron, A.; Villepin, J. d.; Novak, A. J. Chim. Phys. Phys.-Chim. Biol. 1978, 75, 889-894.
19586 J. Phys. Chem., Vol. 100, No. 50, 1996 (14) Snow, K. B.; Thomas, T. F. Int. J. Mass Spectrom. Ion Phys. 1990, 96, 49-68. (15) Yates, J. T. In Methods of Experimental Physics; Academic Press: New York, 1985; Vol. 22; pp 425-464. (16) Giauque, W. F.; Horning, E. V.; Kunzler, J. K.; Rubin, T. R. J. Am. Chem. Soc. 1960, 82, 62-70. (17) Speedy, R. J.; Debenedetti, P. G.; Smith, R. S.; Huang, C.; Kay, B. D. J. Chem. Phys. 1996, 105, 240-244. (18) Smith, R. S.; Huang, C.; Wong, E. K. L.; Kay, B. D. Surf. Sci. Lett., in press.
Schindler and Roberts (19) Kunzler, J. E. Anal. Chem. 1953, 25, 93-103. (20) (a) Zhang, R.; Wooldridge, P. J.; Abbatt, J. P. D.; Molina, M. J. J. Phys. Chem. 1993, 97, 7351-7358. (b) Ohtake, T. Tellus 1993, 45B, 138144. (c) Williams, L. R.; Long, F. S. J. Phys. Chem. 1995, 99, 37483751. (21) Fairbrother, D. H.; Johnston, H.; Somorjai, G. J. Phys. Chem. 1996, 100, 13696-13700.
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