IR Study of the Silica Gel Surface
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979
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average potential, indicative of the inadequacy of the rigid-band model. However, the state density and the bonding orbitals are modified by the introduction of the foreign metal. As a result, the strength of the chemisorption bond can change. In the case of Ir/Ni, such a change is manifested as a weakening of the bond between surface iridium and nitrogen adspecies. Acknowledgment. Support of this research by the Air Force Office of Scientific Research (Contract No. F44620-73-C-0069) is gratefully acknowledged.
V
1
1
I
I
I
I
B
0 N*i/< E X P O S U R E ,.IOLES
x 1011
Figure 6. Effect of H, on catalyst deactivation during N2H, exposure at 373 K.
We conclude from our studies that the surface bonding of N H and N adspecies to Ir is weakened by the addition of Ni atoms. Their presence in subsurface atomic layers affects the nature of the chemisorption bond between the adspecies and the Ir surface atoms. Examination of valence-band electron populations by means of photoelectron spectroscopy have demonstrated that for bimetallic alloys the d states undergo little movement in energy position upon alloy f ~ r m a t i o n . ' ~ Thus J ~ the energy states in the alloy are more affected by the local potential than the
References and Notes (1) B. J. Wood and H. Wise, J . Catal., 39, 471 (1975). 121 J. L. Falconer and H. Wise. J. Cafal.. 43. 220 (1976). (3) A. E. Bucher, W. F. Brinkman, J. P. Maitu, and A: J. Cooper, Phys. Rev. B , 1, 274 (1970). (4) V. Ponec, Cafal. Rev. Sci. Eng., 11, 1-40 (1975). ( 5 ) G. M. Stocks, R. W. Williams, and J. S.Faulkner, Phys. Rev. B , 4, 4390 (19711. (6) R. E. Weber and A. L. Johnson, J . Appl. Phys., 40, 314 (1969). (7) P. W. Palmberg, Anal. Chem., 45, 549A (1973). (8) J. M. McDavid and S. C. Fain, Jr., Surf. Sci., 52, 161 (1975). (9) N. Laegried and G. K. Wehner, J. Appl. Phys., 32, 365 (1961). (10) P. Hou, J. McCarty, and H. Wise, to be published. (11) D. W. Bassett and H. W. Habgood, J. Phys. Chem., 64, 769 (1960). (12) D. A. Dowden, "Proceedings of the Fifth International Congress on Catalysis", North Holland Publishing Co., Amsterdam, 1973, Paper No. 41. (13) D. H. Seib and W. E. Splcer, Phys. Rev. B , 2, 1676 (1970). (14) C. Norris and H. P. Myers, J . Phys. F , 1, 62 (1971).
An Infrared Study of the Silica Gel Surface. 2, Hydration and Dehydration A. J. van Roosmalen" and J. C. Mol University of Amsterdam, Institute of Chemical Technology, Amsterdam, The Netherlands (Received September 29, 1978; Revised Manuscript Received February 28, 1979) Publication costs assisted by the University of Amsterdam
The hydration and dehydration of transparent silica aerogel plates was studied by infrared spectroscopy and thermogravimetry. The following mechanism is proposed for the hydration of silica gel degassed at 875 K: (1)physical adsorption of water on residual hydrogen-bonded vicinal silanol pairs; (2) formation of more vicinal pairs by hydrolysis of surface siloxanes reached by the growing water aggregates; (3) additional water adsorption on these new hydroxyls. Isolated surface silanols appeared to be hardly involved in water vapor adsorption.
Introduction In part 1 of this series we presented a study on the surface structure of dry silica gel plates prepared according to a method described by Peri.1,2 Silica gel was found to have four infrared absorptions in the O-H stretching region after vacuum treatment at 875 K. We assigned these bands to four different types of surface hydroxyls, viz. isolated single (3749 c m l ) and geminal (3742 cm-') silanols, and asymmetric hydrogen-bonded vicinal silanol pairs (3720 and 3500-3700 cm-l). Silica powders, such as Aerosil and Cabosil, show only one absorption (3748 cm-l) in this spectral region after degassing above 700 Ka3From this we concluded that the surface structures of silica gel and silica powder are not identical. Relatively little is known about the hydration mechanism of dry silica geL4l6 In part, this is caused by the bad transparancy of samples obtained by milling and pressing commercial silica gels. The interaction of water vapor with dehydrated silica powders has been extensively studied. Physical adsorption of molecular water was reported to take place on isolated single surface silanols.6%7 However, 0022-3654/79/2083-2485$0 1.OO/O
this result does not necessarily apply to silica gel, because of the observed dissimilarity between silica gel and silica p0wder.l The silica aerogel plates we used in part 1 of this series lack scattering. Therefore, they will be an ideal material for measurements at high degrees of hydroxylation. In the work presented here, infrared spectroscopy and thermogravimetry were combined in order to obtain information about the hydration mechanism and the nature of the hydrogen-bonded hydroxyl groups on silica gel.
Experimental Section The infrared spectra were recorded in the absorbance mode (log Io/l). This allowed direct substraction of spectra obtained with the same sample under different conditions. Difference curves often give more detailed information about (dis)appearing vibrations than the original spectra do. Sample heating by the infrared beam was suppressed by admitting lo3 N m-2 of He to the measuring cell if no other gases were present. In this way, a temperature of 335 f 5 K could be maintained. Details about the @ 1979 American Chemical Society
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The Journal of Physical Chemistry, Vol 83,
No. 19, 1979
A. J. van Roosmalen and J. C. Mol
TABLE I: Weight of a Hydrated Silica Gel Plate as a Function of the Degassing Temperature degas temp, K
wt, mg
a
33.96 33.32 33.02 32.75 32.57
335 500 700 875 a
Measured in 2.7 X
A ~ H ~ o pmol ,
I
I
1
I
m-’
1.71 0.80 0.72 0.48
l o 3N m-2of H,O at 335 K.
measuring cell and the spectrometer are given in our previous paper.l Weight changes during hydration and dehydration of the samples were measured our a Cahn RG electrobalance equipped with a quartz-glass pan and platinum wires. Connected with the Idxnce housing was a conventional stainless-steel high-vacuum system, The weighting accuracy was f0.01. mg. The silica aerogel plates were prepared according t o Peri’s meth0d.l After autoclaving, the plates were cut, and heated in situ for 2 h in 2 X BO4 N m-2 of O2 at 875 M, followed by evacuation for 1 h at the same temperature. Samples treated in this way will be referred to as “dry” silica gel. Hydrated samples were prepared by exposing a dry silica gel for 65 h to 2.7 X IO3N m-’ of H20 at 300 I(. Then, the temperature was raised to 335 K, and kept there for at least 1 h. These samples will be referred to as “wet” silica gel. Immersing the gel plates in liquid water is not possible, as the surface tension will. destroy their macromolecular structure. Results Dehydration. To determine quantitatively the amount of physisorbed water and the number of condensable silanol groups on hydrated silica gel, the weight of a wet silica gel sample was measured as a function of the evacuation temperature, The results of a typical experiment are given in Table I. The evacuation time at each temperature was 45 min, and the find pressure abollt N mP. All weights were determined at the temperature of evacuation, since cooling under vacuum had no measurable effect on the weight of the degassed sample. The loss in surface water, Anw,n,was calculated from the weight change and the specific-surface area (631 m2 gl, BET N?)* Figure 1 shows the infrared spectra of a wet silica gel plate after degassing at increasing temperatures. The degassing procedure was the bame as in the gravimetric experiments mentioned above. Difference curves obtained by graphical substraction of the spectra in Figure 7 are given in Figure 2. These curves appear to be composed of a sharp band at 3720 cm-l and a broader one at 3500-3700 cm-’. No changes were observed in the 1600-1650-cm-’ region (water defmmationei9). Treating a hydrated gel plate with D,O vapor at room temperature lowered the infrared absorbance at 3650 cm-l to less than 0.01. Hence, it appears that internal hydroxyls, as described by Davydov et and adsorbed molecular water are absent under vacuum. The 875 M spectrum from Figure 1 3s identical with that of dry silica gel. Hydration. It is known that silica gel, normally a hydrophylic material, becomes hydrophobic after calcination a t high temperatures. The affinity of silica toward water vapor can be expressed as the time needed to equilibrate with water vapor. We observed that a wet silica gel sample, degassed for 45 min at 335 K, reached constant weight after 11min in 2.7 X IO3 N m-2 of H 2 0 at 335 K. The same sample, degassed for 45 min at 875 K, required 4.5 h to reach
3400
3800
3600 f r e q u e n c y , cm-’
Figure I. Spectra of a hydrated silica gel plate after degassing under vacuum at the indicated temperatures.
0.4 a (0 c
e%
c
I
I
I
I
0.2
Q m
3300
3500
3700
frequency. cm-’
Figure 2. Differences between the curves from Figure 1: (A) 335 500 K; (9)500 - 700 K; (C)700 - 875 K.
constant weight under these conditions. Within experimental error, the final weights were in both cases equal to the weight of the original, wet silica gel. The difference in adsorption rate between silica samples degassed at 335 and 875 K is reflected in their respective infrared spectra during rehydration. After a 15-min reaction with 2.7 X IO3N m-2 of HzO at 335 K the spectrum of the 335 K sample from Figure 1had become identical with the spectrum of wet silica gel, whereas the absorption of the 875 K sample had only slightly increased after the same treatment. Differences between the spectra of the 335 and 875 K samples before and after exposure to water vapor are shown in Figure 3. Figure 4 shows the changes in the water-deformation region after a 22-min reaction. The water treatment had no measurable effect on shape and intensity of the isolated 0-H stretching vibrations a t 3742 and 3749 cm-’. Because the temperature of the sample in the infrared beam was always near 335 K, we could not record spectra of silica aerogel in contact with saturated water vapor. Yet, it is questionable if infrared spectroscopy in saturated water vapor will yield useful information. We observed that a commerical silica gel sample, degassed at 825 K, had more than doubled in weight after reaction with saturated water vapor at room temperature for several days.” Obviously, capillary condensation had taken place,” obscuring all gas-phase adsorption phenomena. Moreover,
IR Study of the Silica Gel Surface
I
I
I
3500
3300
I
I
I
3700
frequency. crn-'
Figure 3. Increase in absorption in the Q-H stretching region after exposure of a degassed silica gel plate to 2.7 X lo3 N m-' of H,Q at 335 K for 15 min: (drawn line) sample degassed at 335 K; (dotted line) sample degassed at 875 K.
7
frequency, crn-'
Flgure 4. Increase in absorption in the H-Q-H deformation region. For assignment of curves see Figure 3. The reaction lime was 22 min.
an enhanced weight loss during degassing at 600-1100 K indicated that the water treatment had introduced internal hydroxyls.'O
Discussion We observed that heating hydrated silica gel under vacuum removes groups with infrared bands at 3720 and 3500- 3700 cm-l. These bands have much in common with the infrared spectra of isolated phenol and methanol dimers,13 and with certain 1:1water-base c0mp1exes.l~In those species, the absorption in the 3000-4000-cm-' region originates from hydroxyl pairs having one "bonded" and one "free" proton. This makes it reasonable to assign the bands in Figure 2 to the two Q-H stretching vibrations in an asymmetric hydrogen-bonded silanol pair (I) 8
SI
1
SI
Ill Ill ub vf =
3500-3700 em-' 3720 em-'
I
Going to higher degassing temperatures, we notice that the intensity ratio. Vb/Vf, decreases, the Vb absorption maximum shifts to higher wavenumbers, and Vb becomes less symmetrical. Hence, it appears that the hydrogen bond weakens,15 evidently because of an increasing distance between the two hydroxyls in the silanol pair. This i s in accordance with the growing effort needed to condense the silanols. Morrow et ale7found groups similar to I after hydration of strongly dehydroxylated silica powder. However, these groups were present in low concentration, and they could be largely removed by evacuation at 525
M. Most silanol pairs can be removed by vacuum treatment at 875 K, although some pairs remain at temperatures up Under vacuum, the aerogel plates hold neither to PO75 physissrbed water, nor internal hydroxyls. For these reasons, the sum of the lower three values in the third
column of Table I, 2.0 pmol M?, will be broadly the same as the concentration of vicinal silanol pairs on the hydrated silica gel surface. This agrees fairly well with the values reported by Armistead et a1.16 It is frequently assumed that She rehydration of annealed silica ultimately leads to the formation of chains of mutual hydro~en-bon~ed silan0lis.~1~ Degassing hydrated silica gel, then, should result in a decrease of the infrared absorption at 3500-3700 cm-' (bound 0-H groups), and in ari increase at, 3720-3750 cm-l (free 0-H groups). the vicinal silanols exist mainly as pairs, dehydration would lower both the 3500-3700 and 3720cm-l bands. We observed that dehydration leads to the simultaneous removal of free and bound hydroxyls. This means that most ~ y d r o g e n " silanols b ~ ~ ~on ~~ ~ gel silica exist as pairs. T'ne integrated Intensities of the difference curves in Figure 2 are in fair accordance with those expected from the observed weight changes (Table I), with the exception of the 3720-cm-' band in Figure 2 8 . The low intensity of the latter could indicate that some vicinal silanols are involve in more than pairwise hydrogen bonding. These has been discussion whether molecular water adsorbs either on isolated silswols, or on some other surface group on silica We found that water treatment did not affect the 3712- and 3 7 4 9 - ~ m -bands, ~ so that the isolated single and geminal silanok presaimably hardly participate in water vapor adsorption. This could account for the observed d.ifficulties in exchanging the isolated single surface hydroxyl groups on silica gel for 8D.l Volkov et aL6 reported a marked decrease in the intensity of the 3'950-cm-' band (isolated single siianoh) on dehydrated silica powder after equilibration with water vapor. Even relative humidities this effect could be obnee, it appears that the water physisorption sites are not the same on silica gel and silica powder. although it remains possible that this dissimilarity is caused by the lower adsorption temperature fm the work of Volkov et al. During the exposcre of a dry silica gel Lo water vapor, an infrared band appears at 3720 em I (Figure 3, dotted line). This indicates the formation of the asymmetric hydrogen-bonded silanol pairs (1). Water adsorption on a wet silica geI, which is physisorption only, was complete in 11min. After a 22-min reaction of water vapor with dry silica gel, the amount of physisorbed water was about one-third of its ~ ~ value. iThis canmbe judged ~ from ~ the intensity of the deformation band at 1620 cm-I (Figure 4, dotted line). Obviously, the amount of physisorbed water is proportional to the amount of chemisorbed water, Le., to the number of vicinal slland pairs. From the foregoing, it ~ Q S ~ O W Sthat physisorption of molecular water on silica gel takes place mainly via mutual hydrogen-bonded siianol pairs. This appears to be true at all degrees of surface hydration. The importarice of adjacent hydroxyl pairs in the adsorption of water vapor on fully hydroxylated silica has already been deduced by Kiselev et al.'? from the heat of adsorption. The amount of physisorbed iniaier is low, viz., about one molecule per vicinal silanol parr (Table I). In spite of this, the spectrum of the adsorbed water, showing absorptions at ,7410 and 3680 cm-' (Figure 3. drawn line), resembles aggregated water ~ n u c h~ Q P Ethan water monomer or dimer.l8J9 As the adsorbed water molecules are, therefore, bonded mainly tc~each other, only few sites will be involved in the adsorption process. Summarizing, we believe that the hydration of annealed silica gel proceeds as foAllows: (I 1 physical adsorption of
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The Journal of Physical Cbemistty, Yo/. 83, No. 19, 1979
water on residual hydrogen-bonded silanol pairs on the silica gel surface; (2) formation of more hydrogen-bonded silanol pairs through hydrolysis of surface siloxanes reached by the growing water aggregates; (3) further water adsorption on these new hydroxyl groups. van Roosmalen, A. J.; Mol, J. C. J . Pbys. Chem. 1978, 82, 2748. Peri, J. 5.J. Pbys. Chem. 1966, 70, 2937. Ryason, P. R.; Russell, B. 6.J. Pbys. Cbem. 1 Little, L. H."Infrared Spectra of Adsorbed Species", Academic Press: London, 1966; pp 258-266. (5) Hair, M. L. "Infrared Spec?roscopy in Surface Chemistry", Marcel Dekker: New 'fork, 1967; pp 84-85. (6) Volkov, A. V.; Kiselev, A. V.; Lygin, V. I. Wuss. J. Phys. Cbem. 1974, 48, 703. (1) (2) (3) (4)
B. W.
Keelan and L. Andrews
(7) Morrow, 5. A,; Cody, I. A,; Lee, L. S. M. J . Pbys. Chem. 1976, 80, 276 1. (8) Lucchesi, P. J.; Glasson, W. A. J . Am. Chem. Soc. 1956, 78, 1347. (9) Benesi, H. A,; Jones, A. C . J . Phys. Chem. 1959, 63, 179. (10) Davydov, V. 'fa.; Kiselev, A. V.; Zhuravlev, L. T. Trans. Faraday Soc. 1964, 60, 2254. (11) van Roosmalen, A. J., unpublished results. (12) &egg, S.J.; Sing, K. S.W. "Adsorption, Surface Area, and Porosity", Academic Press: New 'fork, 1967; Chapter 111. (13) Elellamy, L. J.; Pace, R. J. J . Specfrocblm. Acta 1966, 22, 525. (14) Mohr, S. C.; Wilk, W. D.; Barrow, 0 . D. J . Am. Chem. SOC. 1965, 87, 3048. (15) Lippincott, E. R.; Schroeder, R. J . Cbem. Phys. 1955, 23, 1099. (16) Armislead, C. G.; Tyler, A. J.; Hambleton, F. H.;Mitchell, S.A.; Hockey, J. A. J. Phys. Cbem. 1969, 73, 3947. (17) Kiselev, A. V.; Lygin, V. I. Russ. Cbem. Rev. 1962, 31, 175. (18) Tursi, A. J.; Nixon, E. R. J . Chem. Phys. 1970, 52, 1521. (19) Magnusson, L. 5. J. Pbys. Chem. 1970, 74, 4221.
~t~a~e Infrared s. Spectra of Parent and onded Parent Anions in Solid Argon at I5 K Brian W. Keeilan and Lester Aradrews" Chemistry Deparintent, University of Virginia, Charloffesvi//e, Virginia 2290 1 (Received March 5, 1979) Publication costs assisted by the Petroleum Research Fund
Dilute samples of dihalofluoromethanes in argon were subjected to argon discharge radiation during condensation at 15 E(,New infrared absorptions were grouped by filtered mercury-arc photolysis and assigned to CFXzradicals, CFX2+cations, (CHFX+)Xdaughter cations, parent cations, and two different types of intramolecular hydrogen-bonded parent anions. The two anions produced upon electron capture by CHFClz showed no carbon-13 shift for the hydrogen stretching mode, which suggests the F-H--(CCl2)-and Cl-H--(CFCl)- arrangements. The effect of halogen substitution is demonstrated by the observation of similar F-H--(CBrz)- and F-H--(CIJ- and different Rr---HCFBr and I---WCFI intramolecular hydrogen-bonded anions.
Introduction Intramolecular hydrogen-bonded anions of halogenated methanes have been recently studied in argon matrices. A type 111 species, in which the hydrogen-bonding halide ion has a sufficiently high proton affinity to break the carbon-hydrogen bond,I was first observed by Jacox and Milligan (JM) in studies of CHFC12 and CHFzCl prec u r s o r ~ Type . ~ ~ ~1species, in which the hydrogen-bonding halide ion does not have a sufficiently high proton affinity t o break the carbon-hydrogen bond, were first identified by Andrews et al. in work on CHGI,, CMBr,, and mixed h a l o f o r r n ~ . ~Wydrogen-bonded ,~ anions exhibit unusual spectroscopic properties; in particular, the effect of different halogens in the halide and carbon-bonded positions on the strength of the hydrogen bond is of interest. The most photosensitive species in recent matrix photoionization studies of chlorofluoro- and brornofluoromethanes were identified as parent cations in infrared and optical spectra.6-8 The photolytically stable daughter cations exhibited unusually high carhon-halogen vibrations which were ascribed to increased K bonding in these planar ~ a t i o n s . ~ , ~ The CI-IFC12,CHFBr,, and CHFB2matrix systems were subjected to argon resonance photoionization, and the infrared spectra were analyzed for both positively and negatively charged products. These studies are described in detail here. 0022-3654/79/2083-2488$0 1.OO/O
Experimental Section The experimental methods and apparatus have been discussed in detai1.5~9J0 Samples of dichlorofluoromethane and its deuterium and carbon-13 analogues, dibromofluoromethane and its deuterium analogue, and diiodofluoromethane in argon (Ar/CHFX2 = 250/1, 400/1, or 600/1) were condensed on a 15 K cesium iodide window a t approximately 2 mmol/h while exposed to an open argon microwave discharge through a 1-or 3-mm id. orifice for approximately 17 h. Due to codeposition of argon from the quartz discharge tube, the final matrixlreactant ratios in the matrix were double that of the original sample. The CHFClz and CHFBr2spectra were recorded on a Beckman IR-12 infrared spectrophotometer; high resolution scans of regions of interest were taken before and after each filtered high-pressure mercury arc photolysis by using expanded wavenumber scale a t 10 cm-l/min in the 2002000-cm-' range, and at 15 cm-l/min in the 2000-4000-~m-~ range. The CHFIz spectra were taken on a Perkin-Elmer 521 spectrophotometer with similar experimental parameters. Dichlorofluoromethane (DuPont, Freon 21) and dibromofluoromethane (Peninsular ChemResearch) were purified before use by glass bead distillation. Deuterated dichlorofluoromethane was synthesized by heating CDC13 (99.8% D) and HgF2 to 90 'C for 30 min and separating the products. Carbon-13 enriched dichlorofluoromethane 0 1979 American
Chemical Society
