J . Phys. Chem. 1994,98, 6900-6902
6900
Structure Sensitivity in the Surface Chemistry of Ice: Acetone Adsorption on Amorphous and Crystalline Ice Films Jason E. Schaff and Jeffrey T. Roberts' Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-0431 Received: February 15, 1994; In Final Form: April 5, 1994"
The adsorption of acetone on ice under ultrahigh vacuum has been investigated using temperature-programmed desorption mass spectrometry and single-reflection Fourier transform infrared spectroscopy. Two types of film were investigated: amorphous ice, which has a high density of free surface OH groups, and crystalline ice, on which free OH groups are not spectroscopically detected. On the amorphous film, two states of acetone are observed during temperature-programmed desorption, one derived from acetone which forms a hydrogen bond with the ice surface and a second which is attributed to the desorption of physisorbed acetone. Only the physisorbed state is observed during desorption from a crystalline ice film. This work provides clear and convincing evidence that the surface properties of amorphous ice are different from those of crystalline ice.
Introduction The surface chemistry of ice has recently come under intense theoretical1Jand experimentaPl1 scrutiny. Interest in the subject is motivated not only by a desire to gain insight into how ice particles catalyze chemical transformations in polar stratospheric clouds12 but also by a more general interest in the surface chemistry of molecular solids, an area about which relatively little is known. For some time now, we have been studying the interaction of small molecules with ultrathin (10-100 monolayers thick) ice films deposited on single-crystal transition metal surfaces under ultrahigh vacuum. Previous work has concerned the adsorption of HCl and Cc14;l3*l4 here we consider acetone adsorption. We show that the chemisorption behavior of acetone is critically dependenton the structure of the deposited ice films. Specifically, on amorphous films, which have a high coverage of free surface 0-H groups, acetone adsorbs to form a hydrogen-bonded complex. On films which have been crystallized by annealing them to the onset of sublimation, dangling 0-H groups are not spectroscopically observed. No hydrogen bonding to acetone occurs on crystalline ice, and acetone is instead physisorbed. To the best of our knowledge, this work provides the first clear and convincing evidence that the surface chemical properties of amorphous ice are different from crystalline ice.
Experimental Section Experiments were conducted in two ultrahigh-vacuum chambers described elsewhere.14.15 Single-crystal platinum and tungsten samples were mounted on manipulators which allowed for x, y , and z translation and rotation about the z axis. The samples were in thermal contact with liquid nitrogen-cooled reservoirs and could be cooled to approximately 95 K. Radiative heating was provided by tungsten filaments =2 mm behind the crystals. The crystals could also be biased to f 5 0 0 V for electron beam heating and sputtering. The tungsten temperature was measured with a W-5% Re/W-26% Re thermocouplejunction; the platinum temperature was measured with a chromel/alumel junction. Electronic ice points substituted for reference junctions. Gases were admitted into the chambers using directed dosers. Singlereflection Fourier transform infrared spectra were recorded using a Nicolet Magna 550 IR. The infrared beam was reflected from the metal surface at an angle of 4" from the surface plane. The
* To whom correspondence should be addressed. Telephone: (612) 6252363, fax: (612) 626-7541. Abstract published in Aduunce ACS Abstracts, July 1, 1994. 0022-3654/94/2098-6900%04.50/0
W(100) and Pt(ll1) single crystals were obtained from Metal Crystals Limited (Cambridge, UK) and cleaned in vacuo according to established meth0ds.16.'~ Water was deionized and triply distilled and was degassed via several freeze-pumpthaw cycles beforeuse each day. Water-dz (CIL) and acetone (Aldrich) were degassed before use each day and otherwiseused as received.
Results Characterization of the Ice Films. Ice films were grown via adsorption of water ontoa single-crystalmetal surface. Desorption measurements were made from films deposited on a W(100) surface, and singlereflection infrared spectra were recorded from films deposited on Pt(ll1). During growth of a film, the crystal was positioned in front of a directed doser designed to deliver a uniform water flux across the surface. Water was deposited at a rate of -0.5 monolayer s-l, with the crystal temperature held near 95 K. Film thicknesses, inferred from the water desorption yields, were typically -50 monolayers. Previous work has shown that water films >6 monolayers thick are free of pinholes which, if present, would exposethe underlying metal to adsorbinggases.14 Therefore, the ultrathin water films investigated in this work are suitable substrates on which to study the surface chemistry of ice. Acetone adsorptionwas studied on two types of ice: unannealed films and films which had been briefly heated to 160 K, just below the temperature at which the water sublimation rate is significant. Thestructuresof the two films aredifferent, as shown by the single-reflection FTIR spectra of annealed and unannealed DzO (Figure 1). In particular, the 0-D stretching region of the annealed film exhibits distinct transitions at 2484, 2430, and 2353 cm-l, indicativeof crystalline DzO, while theO-D stretching region of the unannealed film is generally broad and featureless, suggestive of an amorphous structure.5 There is also a sharp feature at 2727 cm-1 in the spectrum of the unannealed film which is not present in the spectrum of annealed ice. The mode at 2727 cm-1 was previously assigned to the free 0 - D stretch in D20 at an amorphous ice surface, Le., to surface molecules which do not fully participate in a hydrogen-bonding network.7JO It is important to emphasize that the free. 0-D stretch originates from the surface of a low surface area, nonporous film, because (i) the intensity of the mode is independent of film thickness and (ii) the dangling OD stretch is replaced by dangling OH stretch when H20 is adsorbed on amorphous D20.lO The lack of a similar mode on the crystallinesurface is surprisingsince standard models for the surface structure of crystalline ice invoke dangling OH b ~ n d s . ~ *Its J ~absence may be due to an orientational effect: if the dangling OH bonds in crystalline ice were nearly parallel to 0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 28, 1994 6901 AT-6K
0.08/
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I
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frequency / cm" Figure 1. Single-reflectioninfrared spectra of annealed and unannealed D20 films deposited on Pt( 111). Unannealed films were deposited at 95 K, and annealed films were prepared by briefly heating an unannealed film to 160 K.
a-acetone p -acetone
'
1 I
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,,
'
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9-J
temerature / K Figure 2. Temperature-programmed desorption of acetone and water from annealed and unannealed ice deposited on W(100). Water and acetone were detected as m / e 20 (H2'*0+)and m / e 59 (12C213CH60+), respectively. Multiplication factors are uncorrected for the degree of ionization or cracking in the mass spectrometer. Heating rata were -5 K s-l.
the underlying metal surface, the intensity of the OH stretch would be reduced because of the surface dipole selection rule.20 An alternative explanation,supported by the data reported herein, is that the coverage of free OH groups on the crystalline surface is much lower than that on the amorphous surface. Temperature-F'rogrammedDesorption of Acetone from Ice. In Figure 2 are shown the temperature-programmed desorption spectra of acetone and water from unannealed and annealed ice. Nominal ice thicknesses were 80-1 00 monolayers; acetone exposures were near what is required to form a monolayer on the underlying W(100) surface. Water evolution near 180 K is associated with ice sublimation. Line shapes of the two sublimation spectra in Figure 2 are essentiallyidenticalbecause amorphous converts to crystalline ice before significant sublimation occurs. In marked contrast to the water spectra, there are pronounced
D
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temperature / K Figve 3. Temperatureprogrammed desorption of acetone and water from unannealed ice+ and ice-&. Acetone was detected as m / e 58 (C3HsO+),and the heating rate was -5 K s-l. differences between the two acetone desorption spectra. Specifically, two states at 140 and 157 K,designated a-and 8-acetone, respectively, are observed from the unannealed surface, while only the lower temperature state is observed from annealed ice. Neither state can be assigned to the sublimation of condensed acetone, which occurs near 130 K under ultrahigh vacuum.21 Moreover, the desorption yields of a- and &acetone are independent of film thickness over an extremely wide thickness range (10-100 monolayers), indicating that they are derived from adsorbed rather than bulk phases. The activation energies for a- and &acetone desorption are approximately 35 and 40 kJ mol-', respectively, assuming first-order desorption kinetics and frequency factors of 10'3 s-1.22 To gain insight into the nature of the interaction between acetone and ice, we examined the thermal desorption of acetone from ice42 (Figure 3). No H-D exchangewas observed between acetone and D20. However, whereas there is no measurable isotope effect for a-acetone desorption, there is an inverse isotope effect for 8 desorption: the 8-acetone desorption temperature from iced2 is approximately 6 K below that from ice-h2. This implies an activation energy difference EHP - ED,o, of -2 kJ mol-'. From these experiments, we conclude that (i) acetone is entirely molecularly adsorbed and inert with respect to H-D exchange and (ii) &acetone is derived from a hydrogen-bonded state on the ice surface, while a-acetone is physisorbed. Infrared Spectra of Acetone on Ice. Adsorption of acetone on an unannealed ice42 layer results in the complete disappearance of the free 0-D stretch (Figure 4). Similar behavior is observed when amorphous ice clusters are exposed to ethylene oxide.' No other changes are observed in the vibrational spectrum of D20, although there are new features due to adsorbed acetone. The vibrational frequencies of adsorbed acetone are generally close to those of condensed a ~ e t o n e ,except ~ ~ . ~for ~ the carbonyl stretch (Figure 4b), which is 1708 cm-1 in adsorbed acetone and 1717 cm-1 in condensed acetone. Heating the surface to 170 K,past the a- and @-acetonedesorption features, results in the near disappearance of vibrational modes associated with acetone (Figure 4c); the residual C-O stretch is attributed to the entrainment of some acetone in the thin film bulk. The 0-D stretching region of ice changes with heating as well, to resemble the spectrum of an annealed ice layer. Notably, the free 0-D stretch does not reappear upon acetone desorption.
Letters
6902 The Journal of Physical Chemistry, Vol. 98, No. 28, 1994
yJ \ //
-$ 0.04-1
4
(b)
0*021(x 10)
o . o O r J - - - l 2800 2600 2400 2200 1800 1600 frequency / cm-l
Figure 4. Single-reflection infrared spectra of (a) an unannealed D20 film, (b) an unannealed D2O film exposed to acetone, and (c) an unannealed D2O film exposed to acetone and heated to 170 K.
Discussion
The mosting striking aspect of the acetone desorption spectra is that @-acetoneevolves from unannealed ice only. The activation energy for @-acetonedesorption is =40 kJ mol-'. This value is consistent with a process involving scission of a hydrogen bond between the carbonyl oxygen in acetone and an 0-H bond in water,25 especially since desorption also involves the disruption of dispersive forces between acetone and water. Hydrogen bonding in the @ state of acetoneis further implied by the infrared spectra. Of the two types of ice investigated, free surface 0-H groups are observed only on the amorphous films. Furthermore, the free0-D stretch in unannealed ice-d2disappearsupon acetone adsorption, presumably because it shifts under the broad 0-D stretching band from bulk ice-d2. Finally, the frequency of the carbonyl stretch in adsorbed acetone is 8 cm-I below that in condensed acetone, consistent with hydrogen bond formation.24 Taken together, these facts imply the formation of hydrogenbonded complex 1:
H3C,
,CH3 C
The kinetics for @-acetonedesorption exhibit an inverseisotope effect: desorption is more rapid from ice42 than from ice-h2. This observation initially surprised us. Because of zero-point effects, O-.D-O interactions are slighly stronger than 0.-H-O interactions; for this reason, D20 sublimes at a higher temperature than H ~ 0 . 1The ~ inverse isotope effect can be rationalized, however, if the transition state for desorption is late, Le., if it resembles the final state of the reaction. Desorption of @-acetone results in hydrogen bond scission but also leads to the formation of new hydrogen bonds between water molecules, in the bulk and at the surface (Figure 4). Desorption is therefore more rapid from D20 because the stronger D20-acetonehydrogen bond which is broken during desorption has less influence on the isotope effect
than the new hydrogen bonds which are formed. It is also possible that @-acetonedesorption is induced by the removal of free OH groups from the surface during crystallization. The 6 K shift in the @-acetonedesorption temperature (Figure 3) is approximately what is predicted using a first-order kinetic model for desorption which takes into account new hydrogen bonds in the final state.21 Unlike the @ state, a-acetone exhibits no desorption isotope effect. Moreover, a-acetone formation is not dependent on the presence of the free OH groups at the ice surface, since desorption occurs from annealed and unannealed films. Also, the a-acetone desorption temperature is close to the temperature at which condensed acetone sublimes under ultrahigh vacuum. We therefore attribute a-acetone to a physisorbed state. This has potentially important implications for the surface chemistry of ice, becauseit suggests that thecrystallinesurfacecannothydrogen bond to acetone. In that regard, these results are in good agreement with isosteric heat of adsorption measurements reported by Adamson and co-workersin the late 196OSp3Owhich suggested that the surface of crystalline ice is nonpolar. In summary, we have shown that the surfacechemical properties of an amorphous ice layer are qualitatively different from those of crystalline ice. Amorphous films have a relatively high density of free surfaceOH groups, and the surface is quite polar. Because of the free OH groups, adsorbed molecules are also able to form specific hydrogen bonds with the amorphoussurface. Annealed, crystalline films interact with adsorbed acetoneonly via dispersive forces. Acknowledgment. This work was supported by the National Science Foundation through Grant CHE-9200108. References and Notes Kroes, G.; Clary, D. C. Geophys. Res. Lett. 1992,19, 1355-1358. Kroes, G.; Clary, D. C. J . Phys. Chem. 1992,96,7079-7088. Leu, M. Geophys. Res. Lett. 1988,15, 851-854. Tolbert, M. A.; Rossi, M. J.; Golden, D. M. Science 1988,240,1018Devlin, J. P. Int. Rev. Phys. Chem. 1990, 9, 29-65. Buch, V.; Devlin, J. P. J . Chem. Phys. 1991, 94, 4091-4092. Rowland, B.; Devlin, J. P. J . Chem. Phys. 1991, 94, 812-813. Rowland, B.; Fisher, M.; Devlin, J. P. J . Chem. Phys. 1991,95,1378Abbatt, J. P. D.; Molina, M. J. Geophys. Res. Lett. 1992, 19,461Callen, B. W.; Griffths, K.; Norton, P. R. Surf.Sei. 1992, 261, LWL48. (1 1) Hixson, H. G.; Wojcik, M. J.; Devlin, M. S.; Devlin, J. P.; Buch, V. J. Chem. Phys. 1992,97,753-767. (12) Solomon, S. Rev. Geophys. 1988, 26, 131-148. (1 3) Graham, J. D.; Roberts. J. T. J . Phys. Chem. 1994,98,5974-5983. (14) Blanchard, J. L.; Roberts, J. T. Lungmuir, accepted. (15) Graham, J. D.; Roberts, J. T., to be published. (16) Pearlstine, K. A.; Friend, C. M. J . Phys. Chem. 1986,90,4344-4347. (17) Agrawal, V.; Trenary, M. Surf.Sci. 1991, 259, 116-128. (18) Fletcher, N . H. The Chemical Physics of Ice; Cambridge Univeristy Press: London, 1970. (19) Thiel, P. A.; Madey, T. E. Surf.Sei. Rep. 1987, 7, 21 1-385. (20) Vibrational Spectroscopy of Molecules on Surfaces; Yates, J. T. J., Madey, T. E., Eds.; Plenum Press: New York, 1987. (21) Schaff, J. E.; Roberts, J. T., to be published. (22) Yates, J. T. J. Methods Exp. Phys. 1985, 22, 425-465. (23) Dellepiane, G.; Overend, J. Spectruehim. Acta 1966, 593-614. (24) Zhang, X. K.; Lewars, E. G.; March, R. E.; Parnis, J. M. J . Phys. Chem. 1993, 97,4320-4325. (25) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding Marcel Decker: New York, 1974. (26) Adamson, A. W.; Dormant, L. M. J . Am. Chem. Sue. 1966, 88, 2055-2057. (27) Adamson, A. W.; Bormant, L. M.; Orem, M. J . Colloid Interface Sci. 1967, 25, 206-217. (28) Orem, M. W.; Adamson, A. W. J . Colloid Interface Sei. 1%9,31, 278-286. (29) Nair, N . K.;Adamson, A. W. J . Phys. Chem. 1970,74,2229-2230. (30) Adamson, A. W.; Jones, B. R. J . Colloid Interface Sei. 1971, 37, 831-835.
