J . Phys. Chem. 1990, 94, 2505-251 1
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Vibrational Spectroscopy of Porous Vycor Glass: Surface Hydroxyl Perturbations upon Adsorption of Hydrogen T. E. Huber* and C. A. Hubert Department of Physics, University of Puerto Rico, Rio Piedras, Puerto Rico 00931 (Received: September 19, 1988; In Final Form: October 3, 1989)
Infrared absorption measurements have been performed on the hydroxyl vibrational overtone and combinations in porous Vycor glass. The frequency shifts upon adsorption of H2 have been studied as a function of coverage. For a surface layer coverage of H2 a shift of the OH stretching fundamental of 15 cm-' is obtained, much larger than that previously observed in silicas upon adsorption of argon. The shift can be accounted for if van der Waals interactions between adsorbed H2and the surface OH and if the surface layer density of the adsorbate are considered. The contribution of polarization interactions to the OH frequency shift is found to be small. The role of quadrupole and hydrogen-bonding interactions is also examined.
1. Introduction Porous Vycor glass has been used for nearly 40 years as a substrate for surface studies because it has a number of unusual properties. The large internal surface area, about 200 m2/cm3, is one such property. For a sample of typical pore size (40-A radius) 20% of the adsorbate molecules are interfacial. Also, the dimension of the pores is large on an atomic scale but small compared to the wavelength of light, and the material is cohesive and devoid of imperfections on a larger length scale. As a result, elastic scattering is small when compared to that of powders and gels of comparable surface area. Porous Vycor glass can be heat-treated to have various degrees of surface hydration. Water is in part chemisorbed at the surface, changing SiOSi bridges to free SiOH (hydroxyl) units. Heating in a vacuum at temperatures higher than 500 OC shows that the surface hydroxyls are involved in the structure of Vycor, e.g., pore distribution' and stresses.2 Also, surface hydroxyls have been known to participate in the adsorption in porous Vycor glass.3 Considerable literature exists on the subject of adsorbates on porous Vycor glass. Work prior to the mid-1960s is covered in the reviews of the infrared spectroscopy of adsorbates by Hair4 and Little.5 It is well-known that infrared absorption spectroscopy (IRS) when applied to dispersed silicas is very informative of the state of hydration of the surface. Hydroxyl groups in silicas give rise to prominent absorption bands in the infrared. These bands shift to lower wavenumbers if adsorption takes place. For adsorbed species such as N H 3 this effect can be understood in terms of a softening of the OH bond from a specific interaction, possibly hydrogen b ~ n d i n g .A~ OH stretch frequency shift is also observed upon adsorption of the rare gases. McDonald6 investigated the spectral shifts and intensities of the OH band at 3750 cm-' in carefully dehydrated silicas upon adsorption of nitrogen, oxygen, and the heavy rare gases. For 0 2 and the rare gases the effect presumably results from the contribution of van der Waals and polarization energies to the O H bond energy. The frequency shifts scale roughly with molecular (atomic) polarizabilities; a typical shift of 8 cm-I is obtained for a monolayer of argon. For N2, on the other hand, the shift observed is comparatively large. Quadrupolar interactions might be involved for N2.' Here we present a study of the OH vibrational shifts in porous Vycor glass as a function of coverage upon adsorption of molecular hydrogen and deuterium at condensing temperatures. For a surface layer of adsorbed H2 a shift of the O H fundamental of approximately 15 cm-l is observed. The O H frequency shifts for H 2 adsorption do not simply scale with polarizability as for the heavy rare gases. Since the polarizability of H2 is about half that of argon, a factor of 4 is to be noticed when comparing frequency shifts upon H2
(15 cm-l) and Ar (8 cm-I) adsorption. Adsorption isotherms are usually described in terms of the BET (Brunauer-Emmett-Teller)8 model. Implicit in this model is the assumption that adsorbate molecules overlay the surface at the pressure corresponding to the first step of the adsorption isotherm. However, estimates based on the BET model for the surface densities of the light adsorbates, helium, hydrogen, and to a lesser extent neon, on porous substrates are anomalously large.' Although this phenomenon has been known for more than 30 years and the state of the adsorbed layer is relevant to a variety of low-temperature experiments,I0 a microscopic understanding of the underlying physics has been lacking. In this context, important information about the problem of adsorption of H2 can be obtained from IRS. Our results stress the relevance of the interfacial density of the light adsorbates to the understanding of O H frequency shifts upon adsorption of H2. We argue that the shifts of H2 can be explained from van der Waals interactions, typical of physisorption, rather than polarization or hydrogen-bonding interactions. The actual boron concentration at the surface varies with heat treatment. We have carried out some measurements of the OH vibrational shifts on a silica gel, which is not expected to have significant amounts of boron. Our results demonstrate conclusively the irrelevance of the boron content of the silica surface for the hydroxyl shifts upon adsorption. The shifts of the O H bands upon adsorption of foreign species represent a major obstacle for the accurate determination of the infrared-active modes of adsorbates on porous Vycor glass." Adequate knowledge of these shifts is a precondition for the study of such modes. This is particularly true for the bands of hydrogen whose fundamental vibrational frequency (near 4200 cm-l for HZ, 3600 cm-I for HD) is close to the O H fundamental. The results presented here may also be relevant to the study of the surfaceinduced dipole of adsorbates (H2 in particular) on oxide surfacesl2
*To whom correspondence should be addressed. Present address: Harvard University, Department of Physics, Cambridge, MA 021 38. 'Present address: Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 021 39.
5122. (11) Huber, T. E.; Huber, C. A. Phys. Reu. Lett. 1987, 59, 1120. ( I 2) For a discussion of the surface-induced dipole in metals, see: Kromhout, R. A,; Linder, B. A. J . Chem. Phys. 1984, 81, 2516.
0022-3654/90/2094-2505$02.50/0
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(1 ) Elmer, T. H. In Silyluted Surfuces; Leyden, D. E., Collins, W. T., Eds.; Gordon and Breach: New York, 1983. (2) Scherer, G. W. J . Am. Cerum. SOC.1986, 69, 473. (3) Knozinger, H. In The Hydrogen Bond; Shuster, P., Zundel, G., Sandorfy, s.,Eds.; North-Holland: Amsterdam, 1976. (4) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. ( 5 ) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: London, 1966. ( 6 ) McDonald, R. S. J . A m . Chem. SOC.1957, 79, 850. (7) Frohmsdorff, G. J. C.; Kington, G . L. Trans. Faraday SOC.1959,55, 1173.
(8) Brunauer, S.; Emmett, P. H.; Teller, E. J . Am. Chem. SOC.1938,60, 309. (9) Steele, W. A,; Halsey, Jr., G. D. J . Phys. Chem. 1955, 59, 57. Steele, W. A. J . Chem. Phys. 1956, 25, 819. (10) Lie-zhao, Cao; Brewer, D. F.; Girit, C.; Smith, E. N.; Reppy, J. D. Phys. Reu. 1986, B33, 106. Tell, J. L.; Maris, H. J. Phys. Reu. 1983, 828,
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and of the inertness of silica substrates (functionality for hydrogen bonding) at low temperatures. 11. Porous Vycor Glass
Porous Vycor glass (Corning Code 7930) is made from sodium borosilicate glasses whose melts undergo demixing. Upon cooling, the material consists of a silica matrix, the pores of which are filled with a boron-alkali-metal-rich phase. The latter is subsequently leached out with acids to leave a set of interconnecting channels less than 100 8, in diameter. The composition, by weight, of the final porous material is 96% Si02, 3% B203,and 0.4% AI203 with traces of N a 2 0 and As203;its porosity is around 0.3.13 Although some boron and aluminum oxides remain within the structure, normally there would be little boron or aluminum at the interface.I4 According to ref 15, the excess boron at the surface would result from boron migrating to the surface when the Vycor is heated above 500 OC. This would indicate that our samples did not undergo the boron enrichment at the surface, since they were not heated above 150 O C . The normal surface concentration of roughly one B-OH per 30 Si-OH seems to be more appropriate in our case. However, Hair and Chapman’s interpretation16 is that the excess boron at the surface results from the leaching process, and presumably, our sample would then exhibit boron enrichment. On the basis of only the information published in the literature, we favor Low and Ramasubramanian’s interpretation.lS Hair and Chapman’s samples were also subjected to heat treatment at various stages of the experiment. Furthermore, the absence of the B-OH stretch in Low and Ramasubramanian’s spectra after heat treatment under vacuum at 500 OC for 80 h followed by 25 h at 600 O C cannot be explained if the excess boron at the surface were the result of the leaching of the glass as in Hair and Chapman’s argument. Recently, small-angle X-ray and neutron scattering data of Vycor have become available,l’ and an interpretation in terms of a mesostructure has been offered. The microstructure of silica (Si02) is believed to be characterized by a random packing of Si04tetrahedral units. As shown by X-ray studies, the random nature of the glass can be accounted for by allowing large variations in the orientation of neighboring tetrahedra.’* Short-range order evidences itself in the dynamics of the glass, and the three independent vibrational modes of an Si20molecular unit (uI = 450, u2 = 800, and u3 = 1050 cm-]) are a surprisingly good starting point in the description of the Raman spectra of ~ i 1 i c a . l ~It is widely accepted that the surface of porous Vycor glass, as that of silica gels, is terminated with one of the bonds of Si attaching a O H group (hydroxyl).20 The picture is one where, basically, the hydrogen atom partially stripped of its electron is held strongly at a fixed distance from the surface and has some degree of freedom around the bond axis. For a sample degassed at 150 OC, the density of hydroxyls is approximately 5 sites per 100 A2 of Si02s ~ r f a c e .OH ~ groups are not necessarily apart from each other and noninteracting (free hydroxyl groups). They can also attach to adjacent Si atoms on the surface (vicinal hydroxyl groups), and two hydroxyl groups could possibly attach to the same Si atom (geminal groups). Direct evidence has been found for an enhancement of the low-frequency density of states of dehydrated Vycor at 980 and 380 cm-’ from that of bulk silica in Raman scattering and infrared reflectivity mea~urements.’~ These features are believed to arise from the hydroxyl groups bonded to surface silicon atoms, the two frequencies corresponding to the (13) Nordberg, M. E. J . A m . Ceram. SOC.1944, 27, 299. (14) Elmer. T H.; Chapman, 1. D.; Nordberg, M. E. J . Phys. Chem. 1963, 67. 2219. ( I 5 ) Low, M. J. D.; Ramasubramanian, N. J . Phys. Chem. 1965,69,2740. (16)Hair, M.L.;Chapman, I. J . A m . Ceram. SOC.1966, 49, 651. ( I 7) Schmidt. P. W. In Characrerizarion of Porous Solids;Unger, K. K., Ed.; Elsevier: Amsterdam, 1988. (18) Mozzi, R. L.; Warren, B. E. J . Appl. Crysfallogr.1969, 2, 164. (19) Laughlin, R B.; Joannopoulos, J. D. Phys. Reu. 1976, 816, 2942. Murray, C. A.; Greytak, T.J. Phys. Reu. 1979, 20, 3368. (20) Elmer. T. H : Chapman, I. D.; Norberg, I. F. J Phys. Chem. 1962, 66, IS17
0 250 rches
Figure 1. Sample chamber configuration employed for the infrared absorption studies. The Vycor sample is contained in the larger hole of the aluminum filler; the smaller empty hole is for measuring the reference spectrum.
stretching and wagging modes of the Si-OH bond. Although, reportedly, truly physisorbed water can be removed by pumping for long times at room temperature, the free O H stays on the surface upon heating of Vycor under vacuum up to about 200 OC.” At this point adjacent O H groups start condensing and H 2 0 vapor is released. This process continues in various steps up to the sintering point (860 “C). Infrared spectroscopy of the consolidated product shows that a fraction of the O H groups have been incorporated into the bulk. The OH band is observed at 3663 cm-l, which corresponds closely to the O H band in fused silica. The point to be made is that O H groups seem to be very difficult to remove and can only be exchanged for other functional groups,I4 for instance, fluorine substituting for hydroxyl groups in the ammonium fluoride treated Vycor. 111. Experimental Section A. Cell and Gas Handling Techniques. The porous Vycor samples employed were in the form of cylinders 3.58 mm in diameter. Opposite ends of the specimens used in the infrared absorption measurements were polished for entry and exit of the light beam with fine (grit 800) wet sandpaper. The samples were then cleaned from organic material adsorbed from the surrounding atmosphere by boiling in solutions of 30%hydrogen peroxide and rinsing in deionized water. Samples cleaned with this procedure can be burned in a flame without evidence for carbonization. Clean samples were stored in water while not in use. Prior to use they were oven-dried at 60 O C in ambient air for a few hours and then vacuum-baked at 150 OC for 1 h. During assembly of the experimental cell the samples were briefly (5 min) exposed to air. After assembly the cell was flushed at room temperature with purified nitrogen. No attempts were made to dry the samples at higher temperatures in order to avoid possible changes in the pore surface composition and pore s t r u ~ t u r e , llosing ~ + ~ ~the possibility of comparing results to those from other experiments on lowtemperature adsorbates. The sample was contained in a closely fitting copper cell with sapphire windows which was connected to a gas handling system through a nickel capillary tube and mounted in a closed-cycle refrigerator capable of reaching 10 K (Figure 1). Cell temperatures were measured with a calibrated germanium resistance thermometer in a flat copper mount that was bolted to the cell. Temperatures were controlled to within 100 mK by means of an electric heater attached to the refrigerator’s cold end. The germanium resistance calibration was checked within 150 mK by measuring the vapor pressure of H2, D2, and neon in a small copper cell that could be attached to the main cell. Standard volumetric techniques were used to measure the amount of gases taken by the Vycor. Gas doses were metered into the cell from a small (70 cm3) dosing container. The fill valve which closed the fill line was at room temperature and was designed so that its dead volume does not change when operated.
Vibrational Spectroscopy of Porous Vycor Glass Gas pressures in the dosing container were measured with a temperature-stabilized, strain gauge pressure transducer. Later in the work a baratron gauge2] was used. The pressure was monitored with a recorder once the fill valve was opened and as the gas dose was admitted into the cell. When the apparent rate of adsorption was deemed negligible, the fill valve was closed and the infrared spectrum taken. The adsorption measurements were checked by performing the same measurements with a modified cell able to hold a larger volume (0.1 cm3) of Vycor. Details of the procedure for obtaining the hydrogen adsorption isotherms and the nitrogen adsorption isotherms for substrate characterization and for measuring the dead space in the cell are discussed elsewhere.z2 The parahydrogen used in some of the experiments was prepared by liquefying a sample of normal Hz and allowing it to reach equilibrium at 18 K in contact with N d z 0 3 powder which acts as a catalyst.23 N o special activation procedure was followed except that of drying the powder in a hot plate before assembling the cell used for preparation. Samples of parahydrogen were boiled from the cell and stored in a Pyrex container for use the next day. A determination of the ratio of parahydrogen to orthohydrogen could be made by infrared spectroscopy of the liquid during the experiments, our parahydrogen samples having approximately 3% orthohydrogen impurities. B. Infrared Absorption Measurements. The experimental arrangement for observing the overtone spectrum of the hydroxyls in Vycor is quite simple since the band lies in the region from 1.25 to 2.5 pm (4000-8000 cm-I) and optical path lengths of only a few millimeters are needed. The absorption was studied with a single-beam dispersive spectrometer based on a 0.25-m Jarrell-Ash monochromator set up with slits for 5-cm-l resolution and a PbS detector operating at room temperature with a low-frequency cutoff at 2.6 pm. In later experiments the spectral range was extended past the O H fundamental to 3.2 pm with the use of a thermoelectrically cooled PbS detector.24 The spectrometer was calibrated with light from a HeNe laser; the spectra of liquid and solid Hz verified the calibration procedure. The source of radiation was a 10-W filament lamp with borosilicate envelope. The light was focused on the Vycor and on the monochromator entrance slit with a system of off-axis parabolic mirrors and flat mirrors. The effective aperture was 2. The low-pass filter used is a coated Ge filter from Corion. The signal was detected by use of conventional phase-sensitive electronics and recorded with a computerized data acquisition station.25 The absorbance is calculated from A = log ( I o / I ) , where Io is the reference intensity and I is the intensity of light passing through the sample. The two data sets do not necessarily have entries at the same wavenumber; the reference data set is interpolated linearly to match the absorption data set. Since the interpolation range is smaller than the instrumental resolution, this procedure does not lead to errors. Intensity measurements are of considerable predictive value for Hz. Experimentally two conditions must be met: (a) the light reaching the detector should have a frequency spread smaller than the full width at half-maximum (fwhm) of the band being measured, and (b) the recorded signal must be proportional to the energy impinging the detector. The first condition was checked by varying the slit size, and therefore the resolution, and looking for changes in the spectrum measured. None was observed. How well we fulfilled the second condition is difficult to attest. An overall test is the observation that at frequencies of strong absorption the calculated absorbance is proportional to the Vycor sample length for the two sizes used (0.7 and 3.5 mm). (21) Baratron pressure gauge Type 127A, MKS Instruments. This instrument together with associated valves was kept at a constant temperature. (22) Huber, T. E.; Huber, C. A. Extended Abstract of the Symposium T “Fractal Aspects of Materials”, 1989 Fall Meeting of the Materials Research Society. (23) Ashmead, D. R.; Elley, D. D.; Rudham, R. J . Cafal. 1964, 3, 280. (24) Thermoelectrically cooled photoconductive PbS detector, Santa Barbara Research. (25) PAR 124A lock-in amplifier from Princeton Applied Research. 1 / 0 board manufactured by Data Translation. Data handling was performed with a scientific package from Jandel Scientific.
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2507
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IV. Results In this section we present the results of measurements for the characterization of our samples, the OH overtone spectrum in Vycor, and its shifts upon adsorption of hydrogen. A . Substrate Characterization. Figure 2 shows a N2 adsorption isotherm measured at 77 K on a large sample (1.38 cm3) of porous Vycor glass from the same batch as the samples used in the optical experiments. The amount of liquid N 2 required to fill Vycor is 9.05 mmol/cm3, indicating that approximately 3 1% of the sample volume is available to nitrogen. The bulk density (ratio of weight to volume) being 1.41 g/cm3, a skeletal density of 2.04 g/cm3 is obtained. A BET analysis of the isotherm data gives a N2surface density of 2.1 mmol/cm3 or, equivalently, a specific area of 205 m2/cm3. Here we have used the “standard” value of 16.2 A2 for the area of the N2 molecule. Assuming cylindrical pores an average pore radius of 30 8, is obtained. The hysteresis observed in the isotherm (the Vycor fills at around 700 mmHg and empties at the desorption pressure of 490 mmHg) is a well-known effect of porous samples.26 The adsorption-desorption loop observed corresponds to the type of behavior associated with the presence of “ink bottle” pores or tubular capillaries of variable cross section. From the characteristic desorption pressure, 490 mmHg in our sample, and by use of the Kelvin equation to describe capillary condensation, the value of the neck radius r, can be found. After correction for the thickness of the adsorbed film on the surface (4 A), we find r, = 25 A. Analogous behavior is observed for H, adsorption-desorption; the H 2 adsorption isotherm at 18 K has been presented elsewhere.” An estimate for the amount of water adsorbed by the samples during a typical experimental run was obtained in the following way. After one of the experimental runs the sample was taken from the cell and weighed; the weight was 37.2 mg. Then the sample was dried at 300 “ C in ambient air and quickly weighed again; the weight was 36.2 mg. Assuming that at 300 OC all physisorbed water has been removed and neglecting the small percentage of the water removed that is driven off the SiOH at this temperature, we obtain a density of physisorbed water of approximately 6.5/100 A2. Using a value of 2.75 8, for the diameter of the water molecule (determined from the virial coefficientsz7 and consistent with the 2.76-8, average distance between oxygens in ice), a coverage of 39% is obtained for the measured 3% weight increase. The distribution of the physisorbed water is not necessarily uniform; it has been suggested that water organizes in patches on the silica surface.2s B. Infrared Spectrum of the OH Stretch Overtone. It is well-known that information about the bonds of the hydrated phase of silicas and about the state of water at the silica surface can (26) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1967. (27) Hirschfelder, J.; Curtis, C. F.; Bird, R. B. Molecular Theory of Gases and Liquids; Wiley: New York, 1954; Table I-A. (28) Anderson, Jr., J . H.; Wickersheim, K. A. Surf. Sci. 1964, 2, 252.
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5000
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7000 Wavenumber (cm-1)
8000
Figure 3. Near-infrared absorption of a 3.2-mm-thicksample of porous Vycor glass at 21 K. The vertical lines point to features discussed in the text. Curve a is the (absolute) absorbance of the bare Vycor where the reference intensity I , is that transmitted through the small hole in the aluminum filler. Curve b shows the shifts upon adsorption of one surface layer of deuterium. The dotted line indicates the base line (zero) for this
curve. be obtained from measurements of the infrared spectrum of porous Vycor glass. Spectra of Vycor for various hydration-dehydration cycles are available for the region between 3000 and 4000 cm-1.29 These spectra are directly comparable to ours for samples under similar conditions and, also, to the published spectra of silica gel^.^^^ All of these spectra show two prominent absorption bands. One band is at 3750 cm-I and is attributed to freely vibrating O H on the surface. The other band is at 3655 cm-l and corresponds to hydrogen-bonded adjacent silanol surface groups, with possibly a small contribution from O H in the bulk. In addition, at high levels of hydration a band is observed at 3450 cm-I from molecular water. High-resolution spectra of Vycor in the overtone region are not available in the literature. Our measurements in the 4000-8000-cm-’ region at low temperatures are shown in Figure 3a. The reference intensity used for calculating the absolute absorbance is that transmitted through the small reference hole in the aluminum filler of the absorption cell. Comparison with the spectra of a hydrated silica gelZBand of O H dissolved in silica30 in the same frequency region allows us to identify the following lines: (a) The O H stretch overtone at 7310 cm-’. In this region there are also overtone features at 7080,6790, and 6610 cm-’ from adsorbed water molecules. (b) The combination of bending and stretching frequencies of individually adsorbed water molecules at 5130 and 5260 cm-I. (c) A band around 4500 cm-I showing components at 4420,4530, and 4600 cm-’. This band has been identified in silica gels as resulting from a combination of the OH stretching vibration and a bending mode (u 800 cm-l) associated with the Si-OH bond, the bandwidth, and relative intensities of the components depending on the degree of hydration. Combination of the O H stretching fundamental and the u2 (800 cm-I) mode of silica may also contribute to the absorption in this frequency region. A small peak is also observed at 4050 cm-l: its assignment is difficult since it is superimposed on the strong absorption from the O H fundamental. (d) A weak absorption band at about 5820 cm-I probably involving a combination of the stretch-bend of adsorbed water and the u2 mode of the silica or, more likely, the Si-OH bend. Overall, the participation of the internal modes of the silica in the overtone spectrum of Vycor is probably small, as the prominent features can be explained from combinations of the SiOH surface group vibrations. This is in contrast with the overtone spectrum of O H dissolved in silica where the internal modes of silica are believed to participate strongly.30 The spectrum of porous Vycor glass is obviously dependent on the degree of hydration of the sample. It is also temperature
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(29) Chapman, I. D.; Hair, M . L. Trans. Faraday SOC.1965, 61, 1507 (30) Stone, J.; Walrafen. G.E. J . Chem. Phys. 1982, 76, 1712.
Huber and Huber dependent, the features broadening with negligible shifts for increasing temperatures. C. Shifts of the OH Fundamental upon Adsorption of H2 and Dz.The relative absorbance of Vycor after adsorption of a surface layer of deuterium at low temperatures is shown in Figure 3b. The coverage is 3.7 mmol/cm3. D2 was chosen for illustration purposes since adsorbed H2 shows various absorption lines in this spectral region31which make presentation less clear. The reference intensity used for calculating the absorbance in Figure 3b is that free); the dispersive shape transmitted through the bare Vycor (D2 of the spectrum results from the shifts of the Vycor absorption bands. The loss of optical density upon D2 adsorption indicated by curve b lying below the base line (dotted line) in the smoothly varying region (e.g., that between 7500 and 8000 cm-I in Figure 3a) is real as it is observed consistently from sample to sample. We believe this results from refractive index matching and reduced elastic scattering as the samples pores are filled. The area under curve b is small. For example, if we integrate over the O H overtone stretch from u, = 7200 to ub = 7350 cm-’ taking 7 X as the base line, we find l a u b A ( udu )
- 3 cm-l
In comparison the area under curve a of Figure 3, that is, the integrated absorption of the O H hydroxyl overtone, is 40 cm-l, 1 3 times larger than (1). This indicates that the spectral shifts are accompanied by small changes in the features absorption strength. (The apparent loss of optical density, or “negative” integrated absorption, in the region between 5000 and 6000 cm-] is due to noise around 5350 cm-I from atmospheric H 2 0 and C02.) As is clearly observed in Figure 3 for D2 all the main features shift upon adsorption. The relative shifts of the features in the overtone spectrum are quite interesting. In general, we observe that features involving participation of the free OH surface group, Le., those at 4500 and 7300 cm-l, show larger shifts. In contrast, the shift of the individually adsorbed water molecule absorption features at 5260 cm-I, which can be determined rather precisely, is found to be less than 5 cm-’ (the frequency resolution). Although this issue is not the subject matter of this paper, it is important to note that the modification upon adsorption can be isolated for many of the different species of hydroxyl coexisting on the Vycor surface. The fundamental hydroxyl is the most important feature. An indirect m e a ~ u r e m e n of t ~ the ~ O H shifts can be obtained from shifts of the O H stretch overtone at 7310 cm-I and from shifts of the absorption at 4500 cm-I from combination of the O H vibration and the bending mode of the Si-OH bond. One way to obtain such shifts is by measuring the frequency of the peaks in the absolute absorbance spectra (which use as reference intensity that transmitted through the small hole). The frequency shifts can also be obtained by matching relative absorbance spectra, such as that shown in Figure 3b, to a computer simulation of the signal obtained by shifting the bare Vycor spectrum shown in Figure 3a by varying amounts. There is agreement between values of frequency shifts determined by either way, and smooth plots as a function of coverage are obtained. A 20% variation from run to run is observed, however. One probable cause for such variation is the presence of physisorbed water in varying amounts. A careful study as a function of water concentration may produce more precise (and probably slightly larger) values for the shifts. In determining O H shifts from the combination band at 4500 cm-I, we neglect the contribution of the frequency shift of the bending mode of the Si-OH bond to the shift of the combination band upon adsorption. This contribution is probably small since the frequency of the bending mode of the (31) See: Huber, T. E.; Huber, C. A. Phys. Reu. B, in press. Also, a discussion of the overtone absorption spectrum of H, and D, on porous Vycor glass is presented in: Huger, T. E.; Huber, C. A. In Proceedings of the LASST-ACSIS Workshop, Campobello Island, 1989; Grunze, M., Ed. (to appear in Appl. Phys.). (32) A direct measurement of the OH fundamental has not been attempted here as the large absorbance at that frequency would require sample thicknesses too small to be compatible with measurements of coverage.
Vibrational Spectroscopy of Porous Vycor Glass 30 I
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2509 I
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Coverage (71m0~/cm3)
Figure 4. Shifts of the OH stretching mode frequency in porous Vycor glass as a function of H, coverage.
Si-OH bond is only about one-sixth of the combination frequency. Also, such a mode is less likely to be perturbed by the adsorbate. This assumption is confirmed "a posteriori'' as OH shifts determined in this manner from the combination band agree with those determined from the overtone within the precision of the measurements. Within the same precision no significant difference was observed in the frequency shifts for the same coverage of H2, parahydrogen, and D2. Results for H2 adsorption are shown in Figure 4. O H frequency shifts were also measured for N2 adsorption at 77 K; values of 3, 11, and 22 cm-' were obtained from the combination band at 4500 cm-' for relative pressures PIPo of 0.01, 0.09, and 0.35, respectively (Po is atmospheric pressure). The shifts measured for N2adsorption are in agreement with those obtained by McDonald6 in silica gels within the experimental error. An interesting observation can be made from Figure 4 concerning the range of the effect under discussion. At low coverages O H frequency shifts follow, roughly, a linear dependence on coverage. At high coverages we observe that the shifts depend only weakly on coverage. The point at which the transition from one regime to the other occurs corresponds approximately to one surface layer coverage of H, (3.5 mmol/cm3 as determined from adsorption isotherms). The OH frequency still shifting as H2 is added inside the pores, that is, on top of the surface layer, suggests that there is a mechanism inducing shifts that does not require actual contact, as would be the case, for instance, of charge transfer in hydrogen bonds. We believe that the excess boron at the surface results from boron migrating to the surface when the Vycor is heated above 500 OC. This would indicate that our samples did not undergo the boron enrichment at the surface, since they were not heated above 150 OC. The normal surface concentration of roughly one B-OH per 30 Si-OH seems to be more appropriate in our case. This point is particularly important since it could be argued that the B-OH stretch, being displaced from the Si-OH stretch by 40 cm-l, voids our interpretation of the spectra as Si-OH frequency shifts upon H2 adsorption. This argument contradicts the fact that we have observed only one sharp peak in the hydroxyl overtone region (65OC-8000 cm-I). This peak is at 7310 cm-I and is identified as the free Si-OH overtone by comparison with silica in part B of the Results section. The variable measured and plotted in Figure 4 is the maximum of this peak upon H2 adsorption. This is further supported by preliminary measurements we have done on H, and D, on silica gel, which is not expected to have significant amounts of boron. We used a pellet 0.1 mm thick of Davisil (Code 633). Anderson and Wickersheim2*also used Davisil for their infrared studies of water and hydroxyl groups on silica surfaces. The transmission spectra can be studied in the fundamental region where the most prominent sharp feature is the absorption from the Si-OH stretch at 3750 cm-I. Due to the small volume of the sample investigated adsorbate coverages could not be measured. The adsorbate pressure P was determined instead. The temperature was 20 K. For relative pressures PIPo, where Po is the equilibrium vapor pressure, ranging from 0.02 to 1 (full pores) the red shifts observed are approximately 25 cm-I. This value agrees, within experimental error, with that deduced from the overtone shifts in porous Vycor glass. This result indicates that the physics of this particular problem is not related to the actual
surface composition of the two materials. V. Discussion In this discussion of our experimental results the aim is to show that dispersion interactions between the surface layer of adsorbate and the surface O H can account for the O H frequency shift observed upon adsorption of H,. The large frequency shift measured for a surface layer of H2 ( 1 5 cm-I), in comparison to that for Ar adsorption (8 cm-I), can be explained from their relative surface potential strengths and interfacial densities. Also, arguments are presented which indicate that polarization and permanent quadrupole interactions do not significantly contribute to the frequency shifts observed. Consideration of the surface density observed for light adsorbates on porous substrates is important in order to understand the interaction of H 2 with Vycor. Two "standard" values for the surface area of a molecule, u, that are routinely used for measurements of surface areas because they lead to reasonably self-consistent results33 are 16.2 A2 for N2 and 13.8 A2 for Ar. The surface area of a given solid can then be obtained from NeETu, where NBBT is the number of molecules that attach to the surface as determined from a BET fit to the adsorption isotherm data and u is the standard value for the adsorbate used. These standard values are close to those calculated from the bulk liquid densities, indicated by n, in the following manner. Assuming a set of spherical units in a close-packed (hexagonal) arrangement, the area per molecule in the plane of tightest triangular packing can be shown to be = i.0qn~,)-213
(2)
where N, is Avogadro's number. As an example, from the density of liquid nitrogen at 77 K and at standard pressure, n = 28.6 mmol/cm3, a value of c = 16.3 A2 for N, is obtained from eq 2. In contrast to the case of N2 and Ar, application of eq 2 to adsorbed H2 leads to inconsistent results if the assumption is maintained that the surface area of a solid is independent of the adsorbate used to measure it. From our measurements of the adsorption isotherm of H 2 on porous Vycor glass at 17 K and a BET fit to the data, an area per molecule of u = 8.2 A2 is obtained.l' With this value for (r for adsorbed H, eq 2 can be reverted to give n = 81 mmol/cm3. This value is roughly 3 times the density of liquid hydrogen at the same temperature and pressure. This discrepancy between results from adsorption isotherms and from simple surface packing arguments is also exhibited by helium.34 Another special property of adsorbed H2;when compared to Ar, is that its surface potential may not be simply proportional to the polarizability on account of its small Lennard-Jones radius, r = 1.43 A and rAr = 1.7 A. The polarizability of Ar is 1.65 roughly twice that of H,, aH2= 0.81 ~ 4 The ~ relevance . ~ ~of the size of the molecule to the surface potential is simple to show by using the phenomenological form of surface potential given by H ~ i n k e s .Since ~ ~ the potential energy minimum of an adsorbed molecule is proportional to its polarizability and inversely proportional to the third power of its Lennard-Jones radius, we can expect the H2 surface potential to be decreased by only 20% in comparison with that for Ar. Available experimental evidence in this respect is inconclusive; for H2 on glass an adsorption energy of 500 K has been measured,j6 and for Ar on silica gels values as low as8 300 K and as high as3' 1020 K have been reported. The model employed here to describe the stretching vibrations of the hydroxyls is that of a diatomic molecule having only one vibrational degree of freedom. Its intramolecular potential, in units of cm-l, may be written as
f?,
(33) McClellan, A. L.; Harnsberger, H. F. J . Colloid InterfuceSci. 1967, 23, 577. (34) Brewer, D.F.; Champeney, D. C. Proc. Phys. SOC.,London 1962, 79, 855. (35) Hoinkes, H . Reu. Mod. Phys. 1980, 52, 933. (36) Keesom, W. H.; Schweers, J. Physicu 1941, 8, 1007. (37) Kiselev, A. V . Z h . Fiz. Khim. 1964, 38, 2753.
2510
The Journal of Physical Chemistry, Vol. 94, No. 6,1990 V ( x ) = (we2/4Be)x2(1+ a+)
(3) where x = (r - re)/reis the relative displacement from equilibrium, re = 0.95 A being the equilibrium intramolecular distance for O H groups.38 a, is the anharmonicity parameter, and we is the classical harmonic frequency. Be = h/8s2m*cr2 = 19.5 cm-’ is the usual rotational constant,39with m* the reduced mass. w e and a, can be estimated40 from the measured frequencies of the O H fundamental and first overtone, 3750 and 7310 cm-l, respectively, giving w e = 3940 cm-l and a, = -1.61. In general, the frequency of an anharmonic oscillator will be shifted by an external potential. To first order, the frequency shift is given by4, 6we/we = 3(Be/we)a,F (4) where F = -(re/we)U’isthe potential strength parameter, with U the potential energy eigenvalue. The prime notation indicates derivatives with respect to r evaluated at re. For a monolayer coverage of H 2 on porous Vycor glass, the shift of the O H vibrational frequency is 15 cm-’ toward the low-energy side. From (4) we obtain F = 0.16 for the O H interacting with the surface layer of H,. The potential on the OH, U, for a surface layer coverage of adsorbate arises mainly from dispersion (van der Waals), exchange (repulsion), induction (polarization), and permanent quadrupole-dipole interactions with the adsorbed molecules. Since there is a sizable number of O H groups on the surface, the potential on the O H groups is an important part of the total adsorption potential of a surface layer of adsorbate. van der Waals forces are believed to be dominant in physisorption,8 and therefore, we can expect them to contribute to the observed OH frequency shifts. Such forces would produce a red shift which can be estimated in the following manner. If we assume that U is proportional to the polarizability of the O H group, a, then U‘= &’/a. In order to obtain a crude estimate for a‘/a, we assume the molecular polarizability to be additively formed by contributions from individual bonds in the molecule.42 The effect of O H bond changes on a is simple to understand in two extreme cases: (a) For very short bond lengths the electronic structure could be considered similar to that of the fluorine atom, which has a polarizability a F= 0.6 A3.43 (b) Instead, if the O H bond is lengthened past the OH dissociation point, a more appropriate structural model would be that of an oxygen ion (0-)and a proton. The polarizability of the 0- ion is a0- = 3.2 ..43.43 The length over which these structural changes take place is, approximately, the radius of the oxygen ion ro-, which we approximate by the radius of the oxygen atom, 1.5 We can then estimate a’/a to be d / a = 2(ao- - aF)/((ao- aF)ro-) = 0.9 A-l. From the above estimate for a’/@and using the value F = 0.16 obtained from the measured frequency shift for a surface layer of H2, we obtain that if van der Waals interactions are to account for the observed shift, their strength would have to be U = -737 cm-I. This value is not unreasonable, considering the order-ofmagnitude approximations involved in the calculation. To gain some insight into the meaning of U, we can compare it to the adsorption potential of one monolayer of H, per unit area, which we conveniently choose to be 100 A2. The contribution to the adsorption energy of one monolayer of H2 from O H groups would be 3685 cm-’ since there are approximately five OH groups in the said area. From the surface area of the hydrogen molecule, u = 8.2 A2, we estimate that there are approximately 12 H2 per I00 A2. Therefore, the contribution from the OH to the adsorption
+
(38) Pimentel, C. C.: McClellan, A. L. The Hydrogen Bond W. H. Freeman: San Francisco. 1960. re has been estimated from Figure 3.6 for zero-frequency shift. (39) Here the rotational constant is used to group a number of constants and is not meant to imply O H rotations. The subject of O H rotations is discussed in ref 4. (40) King, G.W. Specotroscopy and Molecular Structure; Holt, Rinehart and Winston: New York, 1964. (41) Buckingham, A. D. Proc. R . Soc. London 1960, 255, 32. (42) Hill, N. E.; Vaughan, W. E.; Price, A. H.; Davies, M. Dielectric Properties and Molecular Behavior; Van Nostrand: New York, 1969. (43) Dalgarno, .4 Ado. Phyr. 1962, ! I , 281.
Huber and Huber energy per H2 molecule would be 307 c d , which is comparable to the measured value of about 350 cm-’ for the adsorption energy of H, on glass.36 For the case of Ar adsorption, from the O H frequency shift of 8 cm-’ for a surface layer coverage and by use of eq 4, we obtain F = 0.085. U’can then be calculated to be 352 cm-’/A. Assuming that the O H frequency shift is due to van der Waals interactions with the adsorbed Ar, we find U = 391 cm-l per OH. Since there are 1.45 Ar atoms per OH unit, the contribution of the SiOH to the adsorption energy per Ar atom is 270 cm-I (386 K). This value is in fair agreement with that calculated by using Hoinkes’ expression (601 K) and within the range of the experimental values available (300-1020 K). Our rough calculation probably overestimates the effect of the SiOH interaction with adsorbed H2 and Ar as the SiOH groups are not likely to account for the totality of the adsorption energy and as the effect of repulsive (exchange) interactions has not been included. Shifts of the O H excitation frequencies, in analogy with frequency shifts of O H in a molecule immersed in a nonpolar medium, can also be attributed to a “solvent effect” from the dielectric properties of the adsorbate. The effect of the surrounding medium is to change the electrostatic interaction between the constituents of the O H bond. The expression frequently used for the potential strength parameter is referred to as the Kirkwood-Bauer-Magat (KBM) f o r m ~ l a .With ~ ~ ~our ~ ~choice of parameters it can be written as
F = (re/hcwe)(2pp’/a,3)(t- 1)/(2€ + 1) (5) where p is the O H dipole moment, p’ is the O H effective charge, a, is the radius of Onsager’s sphere,& and t is the optical dielectric constant of the adsorbate. Although the meaning of Onsager’s sphere is very clear in his macroscopic approach, obvious difficulties are encountered in determining it from microscopic parameters. In practice, it is usually preferred to fit a, to available experimental data.47 Values for a, much larger than rOH rg, where rg is the Lennard-Jones radius of the adsorbed molecules, or smaller than rOHcannot be justified in the present context. The dielectric constant can be calculated from the polarizability a by using the Clausius-Mossotti equation. If K = 4aan/3, where n is the adsorbate density, the reaction field factor g = (2/ad)(t - 1)/(2t + 1) can be rewritten as (2/U:)K/(1 + K ) . Our choice of a, determines the value of the reaction field factor. Taking a, = roH = 1 A, which gives the largest shifts, we get g = 0.28 A-3 for H2 ( n = 81 mmol/cm3, a = 0.81 A3) and g = 0.27 A-3 for argon ( n = 37 mmol/cm3, a = 1.65 A3). The O H effective charge p’ can be obtained from infrared absorption measurements of the O H fundamental. From McDonald’s results on silica powders,6 and assuming that there are five free O H groups per 100 A2 of surface, an integrated absorption of 3.3 L (cm2 mol) and an effective charge of 3 X e are obtained!/ A better estimate for the effective charge can be obtained from measurements of the absorbance as a function of the amount of water lost by silica powders upon heating under vacuum.49 From the extinction coefficient at the band maximum, 35 L/(cm mol), the integrated absorption can be calculated assuming a fwhm of 12 cm-I, and the resulting effective charge is 0.02 e. With this value for the effective charge, the O H dipole moment p can be roughly estimated to be 0.03 D. From eq 5 we then obtain F = 3.1 X The contribution from induction interactions to O H frequency shifts appears to be small relative to that from van der Waals interactions. Furthermore, induction interactions as given by eq 5 are unable to explain the enhanced O H frequency shift for H2 adsorption in comparison to that for Ar, their reaction field factors being approximately equal. There are, however, two limitations
+
~
~~
(44) Kirkwood, J. G. Cited by West, W.; Edwards, R. 7.J . Chem. Phys. 1937, 5, 14. Bauer, E.; Magat, M. Physica 1938, 5, 718. (45) Benson, Jr., A. M.; Drickamer, H. G. J . Chem. Phys. 1957,27, 1164. (46) Onsager, L. J . Am. Chem. Soc. 1936, 58, 1486. (47) Yarwood, J.; Ackroyd, R.; Arnold, K. E.; Doge, G.; Arndt, R. Chem. Phys. Lett. 1981, 7 7 , 239. (48) Ibach, H.Surf. Sci. 1977, 66, 56. (49) Borello, E.; Zecchina, A,; Morterra, C. J . Chem. Phys. 1967, 71, 2938.
Vibrational Spectroscopy of Porous Vycor Glass
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2511
to the applicability of eq 5 to our problem. One is that the shifts measured for parahydrogen (J = 0) show no appreciable expression employed for the reaction field factor g is only apdifference with those observed for normal H2. propriate for a problem with spherical symmetry. We have On the basis of this discussion, it is clear that two mechanisms, preferred to assume this simple geometry since such a choice namely dispersion interactions and induction interactions, have probably does not lead to serious errors and because little is known the potential to quantitatively explain the OH frequency shift upon about the actual arrangement of the adsorbed molecules on the adsorption of H2.A short-range mechanism such as a hydroVycor. A more serious limitation is that the permanent dipole gen-bonding interaction, on the other hand, could not by itself p of the free OH groups on the surface of Vycor is not known explain the shifts observed for coverages above a monolayer. from experiments. Considering the approximations involved in In this discussion, the role of physisorbed water and other the calculations, the contribution from induction interactions heterogeneities of the substrate that will undoubtedly make the cannot be entirely neglected with certainty. There is yet another surface potential for H2 adsorption site dependent have not been argument relevant to a discussion of the OH frequency shifts upon specifically considered. The OH frequency shift measured for adsorption of H,. Aside from such shifts, an important mania surface layer coverage of H2 can be accounted for by considering festation of the KBM mechanism is that it induces a dipole the interaction between this particular site (the OH) and the moment on the adsorbate, the spatial range of this induced dipole totality of the adsorbate. We have not attempted at this point being presumably the same as that exhibited by the OH frequency to quantitatively discuss the coverage dependence of the OH shifts. In contrast, we have found that the surface-induced infrared frequency shifts. It is to be noted in this regard that heterogeneities absorption of H, on Vycor saturates at about 50% surface layer of the potential might cause localization near the OH groups. coverage, in agreement with a bilayer model for adsorbed H2." Also, it is believed that the surface layer of H2is organized into Optical effects,50that is, changes arising from changes in the two layers." For the first layer one would expect large effects dielectric environment of the OH groups, have to be included. The from dispersion forces but also important opposing effects from optical theorySois rather involved, and careful consideration is exchange ( r e p u l s i ~ n ) . ~For ~ the second layer the effect from outside the scope of this paper. Also, the structure of the adsorbed dispersion forces would be diminished, and no significant effect film of the light adsorbates is a matter of some c o n t r ~ v e r s y . ~ ~ would ~ ~ ~ ~be~ expected ~ from repulsion interactions. For coverages In ref 50 it is assumed that the structure is planar, and it is not above the surface layer, we have observed that the OH frequency clear to us how the results would be changed if a different gestill shifts as more H2 is added. Although a calculation cannot ometry, such as the spherical one used by us, were to be assumed. be offered here, it is apparent that dispersion interactions are, Also, there are some indications that these effects are not relevant again, a candidate to account for this effect. for H,. The predicted frequency effects give contributions proIn summary, we have studied the OH overtone spectrum in portional to the polarizability of the adsorbed species. Since the porous Vycor glass. We have identified free OH surface groups polarizability of Ar is roughly twice as large as that of H2, the and shown that such groups coexist, at the levels of hydration found optical effects for Ar are twice as large as that of H,. Also, in typical experiments, with adsorbed water. Measurements of the perturbations of the surface OH stretch upon H2 adsorption intensity changes are an integral part of theories of frequency shifts that deal with the electrical dipole moment of the hydroxyls. The are presented. Based on these results, arguments are given which theoretical result in ref 50 exhibits large intensity changes as Ar show that the perturbations observed do not result from hydrogen is adsorbed. Since the frequency shifts are larger for H,, one would bonding and which suggest that such perturbations arise from expect large changes as H2 is adsorbed, something that we have dispersion rather than polarization interactions. These findings not observed. are important to advance the problem of physisorbed H2 and of Finally, we have also considered possible effects on the OH relevance to the more general problem of vibrational spectroscopy frequency shifts from the permanent quadrupole of the H2 of surfaces. molecule. Explanations of the large OH frequency shifts upon Acknowledgment. The authors thank C. L. Barnes for helpful adsorption of N2on Vycor as compared to those for O2adsorption discussions. The technical assistance of C. E. Diaz is also achave been based on the quadrupolar electric field of the N 2 knowledged. This work was supported in part by the National m o l e c ~ l e .Since ~ H2 also has a sizable permanent quadrupole Science Foundation (Experimental Program to Stimulate Commoment, the question arises whether it might have some bearing petitive Research). upon the frequency shifts observed. This is not likely to occur, however, since as shown in ref 7 such an effect cancels out for Registry No. H, 1333-74-0. freely rotating molecules. We have shown such to be the case for H2on the surface of Vycor." Furthermore, the OH frequency (50) Roth, J. G.; Dignam, M. J. Can. J . Chem. 1976, 54, 1388.
( 5 1) The Dhvsical basis for this mechanism and exulicit calculations for some simple mdlecuiar cases are discussed by: Matcha: R. L.; Nesbet, R. K. Phys. Rev. 1967, 160, 72.
