J. Phys. Chem. 1992,96, 1395-1400
1395
Surface Vibrational Modes of Sllanol Groups on Silica B. A. Morrow* and A. J. McFarlan Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada, KIN 6N5 (Received: July 17, 1991; In Final Form: October 7 , 1991)
Infrared spectroscopy has been used to study the vibrational modes associated with free (isolated) silanol groups on an aerosil silica and a precipitated silica which have been activated in vacuum in the temperature range from 450 to 800 O C . Both silicas exhibit two Si-O-H angle deformation modes at 760 and 840 cm-l, indicative of two types of isolated silanol species, called type I and 11, respectively. After 450 O C vacuum activation and cooling to 22 O C , there is a sharp but asymmetric OH stretching band near 3747 cm-' for aerosil and near 3743 cm-' for the precipitated silica. However, upon cooling to near liquid nitrogen temperatures, -191 OC,this band splits into two components having maxima near 3750 (I) and 3740 (11) cm-'. In the near-IR region between 4600 and 4500 cm-', two bands at 4580 and 4510 cm-l can be detected for samples at -191 "C,which are combinations of the deformation modes with the OH stretching modes as follows: species I, 760 + 3750 = 4510 c d , species 11, 840 + 3740 = 4580 cm-l. For both silicas, the Si-O(H) stretching mode is near 980 cm-I, and a broad combination band near 3870 cm-' is due to the sum of the OH stretching mode and a low-wavenumber torsional mode near 120 cm-'. The IR bands due to species I1 are preferentially eliminated as the temperature of activation is raised from 450 to 800 O C and these bands have been attributed to weakly perturbed vicinal pairs of silanols. Species I is a truly isolated silanol.
Silica surfaces contain a variety of terminal silanol groups, some of which are isolated single [BOH] or geminal whereas others of either of the above type are involved in hydrogen bonding or are perturbed due to interparticle The characteristic OH stretching vibrations in the 3800-3400-cm-l spectral region have been studied many times by transmission infrared spectroscopy of thin self-supporting silica However, the Si-O(H) stretching and Si-0-H angle bending modes of these silanol group are expected to absorb below 1300 an-',in a spectral region where most silica disks are partially opaque to infrared radiation because of strong absorption by the skeletal modes of silica itself, and fewer studies of this spectral region have been carried o ~ t . ~ * Some " ~ information pertaining to low-frequency modes has been obtained by examining the near-infrared spectral region wherein lie the combination modes of the silanol This region has also received little attention because gr0up.6*'~-~~ (1) Kiselev, A. V.; Lygin, V. I. Infrared Specrra of Surface Compounds; Wiley: New York, 1975. (2) Hair, M. L. Infrared Specrroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. (3) Morrow, B. A. Srud. Surf. Sci. Caral. 1990, 57A, A161. (4) Morrow, B. A,; McFarlan, A. J. J. Non-Cryst. Solids 1990, 120, 61. (5) Morrow, B. A.; McFarlan, A. J. Lungmuir 1991, 7, 1695. (6) Burneau, A.; Barrts, 0.;Gallas, J. P.; Lavalley, J. C. Lungmuir 1990, 6, 1364. (7) Gallas, J. P.; Lavalley, J. C.; Burneau, A.; Barr&, 0. Lungmuir 1991, 7, 1235. (8) References 3-7 are examples of recent papers on the silanol subject and contain a partial list of about 100 earlier papers dealing with this topic. (9) Boccuzzi, F.; Coluccia, S.;Ghiotti, G.; Morterra, C.; Zecchina, A. J . Phvs. Chem. 1978.82. 1298. 110) Brinker, C.'J.; Tallant, D. R.; Roth, E. P.; Ashley, C. S.J. Non-Crysr. Solids 1986, 82, 117. (11) Tripp, C. P.; Hair, M. L. Lungmuir 1991, 7 , 923. (12) Murray, C. A.; Greytak, T. J. Phys. Rev. B 1979, 20, 3368. (13) Cherukuri. S.C.: Pve. L. D.:Chakrabortv. I. N.: Condrate. R. A.: FeGaro, J. R.; Cornilsen; B: C.; Martin, K. Specirosc. Lett. 1985, 18, 123: (14) Sato, R. K.; McMillan, P. F. J . Phys. Chem. 1987, 91, 3494. (15) Wood, D. L.; Rabinovich, E. M. Appl. Spectrosc. 1989, 43, 263. (16) Peri, J. B. J. Phys. Chem. 1966, 70, 2937. (17) Bumeau,A.; Barr-, 0.;Gallas, J. P.; Lavalley, J. C. Proceedings of
rhe Internaiional Workshop FTIR Spectroscopy; Vansant, E. F., Ed.; University of Antwerp: Antwerp, 1990; p 108. (18) Anderson, J. H.; Wickersheim, K. A. Surf. Sci. 1964, 2, 252. (19) Klier, K.;Shen, J. H.; Zettlemoyer, A. C. J . Phys. Chem. 1973, 77, 1458. (20) Davydov, V. Y.; Kiselev, A. V.; Lokutsievskii, V. A.; Lygin, V. I. Russ. J . Phys. Chem. (Engf.Transl.) 1974, 48, 1342. (21) Kiselev, A. V.;Lokutsievskii, V. A,; Lygin, V. I. Russ. J . Phys. Chem. (Engf. Trawl.) 1975, 49, 1053. (22) Tsyganenko, A. A. Russ. J . Phys. Chem. (Engl. Transl.) 1982,56, 1428.
0022-3654/92/2096-1395$03.00/0
of experimental difficulties associated with low extinction coefficients, and because older spectrometers rarely had the capability to scan above 4000 cm-I. This paper is concerned with a study of the fundamental and combination vibrational modes associated with surface silanol groups on two silicas having similar surface areas but of differing origin. By means of variable-temperature thermal activation, physical adsorption of CO, and H/D exchange, coupled with the ability to record IR spectra of samples at near liquid nitrogen temperatures, we have been able to unravel some hitherto unexplained anomalies concerning the vibrational modes of surface silanol~.~~*~~
Experimental Section Two nonporous silicas were used in this work, a pyrogenic or aerosil type silica, Cab-0-Si1 HSS [BET Nzsurface area 325 f 5 m2/g] and a precipitated silica from RhanePoulenc France [285 f 5 m2/g]. The aerosil and precipitated silica will be designated by the terms A-x and P-x respectively where x is the temperature of activation in vacuum for 1 h in degrees Celsius. Some properties of these two silicas have been described previou~ly.4*~ For IR transmission studies, self-supporting disks 19 mm diameter were compacted at about lo7 Pa and contained from 2.5 to 80 mg of silica/cm2, depending on the spectral region under investigation (this will be specified in the Results section). The IR cell, which has been described previ0usly,2~consisted of a 22 mm i.d. quartz tube 15 cm long which could be heated to over 1000 "C via an external quartz tube furnace, or cooled by immersing the middle 10 cm in liquid nitrogen. Efficient cooling of the sample further required the addition of helium as a conducting gas. Optimum cooling was achieved using He pressures in the range 0.1-2 TOKwhich gave a temperature near the sample surface of -191 f 1 OC. Some experiments were carried out with samples which were in a low equilibrium pressure of CO, and if the pressure was in the range 0.1-2 Torr, the temperature was also -191 O C . At lower pressures, the temperatures were slightly higher, being about -186 O C for a pressure of 0.02TOKand -188 OC at 0.05 TOK.The same temperatures were also found for these pressures of He, and we have verified that no spectral artifacts appeared in subtracting a spectrum of a sample at -191 OC from one at -186 O C . Room temperature was 22 f 1 O C . Most of the Fourier transform infrared spectra were recorded at a resolution of 2 cm-l using either a Bomem Michelson or a (23) Kustov, L. M.; Borovkov, V. Yu.;Kazanskii, V. B. Russ. J . Phys. Chem. (Engl. Transl.) 1985, 59, 1314. (24) McFarlan, A. J.; Morrow, B. A. J . Phys. Chem. 1991, 95, 5388.
0 1992 American Chemical Society
Morrow and McFarlan
1396 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 A400
Aerosil
.5-I
4800
I
4600
cm-'
4400
1
P-800
I
I
4800
4600 cm-'
4400
Figure 1. Infrared spectra of 40 mg/cm2 disks of A-800 and P-800at 22 "C (A and A', respectively) and at -191 OC (B and B', respectively). The absorbance scale applits to curves A and A', and curves B and B'
Precipitated Silica
A
4600
P-450
4400
un-l
Figure 2. Baseline-corrected spbctra of aerosil and precipitated silica (40
mg/cm2) at -191 OC after activation at 450,800, or lo00 'C.
have been displaced for purposes of presentation.
i \
Aerosil
IT
Bomem DA3-02 instrument. For high-resolution spectra (0.5 cm-I), the DA3-02 was used.
Results (a) spectrrrl R e g h Figure 1, A and A', shows infrared spectra of thick disks (40mg/cm2) of A-800 and P-800 at 22 "C, respectively. In both cases there is a broad flat-topped peak centered near 4550 an-'which is superimposed on a sloping background. The sloping background is the result of decreasing transmission due to increased light scattering at higher wavenumber. This transmission loss is more severe for the precipitated silica, the absorbance at any wavenumber being about 4 times greater than for aerosil. Consequently, the S/Nratio is poorer for P-800 than for A-800 for comparable observation times. Upon cooling from 22 to -191 "C,each broad peak progressively split into two components with increasing separation as the temperature decreased (not shown). At -191 "C,a well-resolved doublet having maxima at 4580 and 4510 cm-I was observed (Figure 1, B and B'). The increased resolution at low temperature appears to be due to a combination of band narrowing and frequency separation of these two peaks. Figure 2 shows spectra of both silicas at -191 "C following vacuum activation at 450,800, or 1000 "C.These spectra have been baseline corrected to eliminate the sloping background so as to provide a better visual measure of the relative intensities of the two peaks. Further,the spectra of the precipitated silica have been smoothed so as to improve the apparent S/N ratio. For both silicas, the intensity of the 458O-cm-1 peak decreases much faster than that of the 4510-cm-' peak as the temperature of activation is increased, and this change is much more pronounced for the precipitated silica. However, after 1000 "C activation, the spectra of A-1000 and P-10oOare similar. Infrared spectra from 4700 to 3200 cm-'of A450 (80 mg/cm2) and P-450 (40 mg/cm2) at -191 OC are shown in Figure 3. The massive absorption near 3750 cm-l is due to the OH stretching vibration of isolated SiOH groups; this spectral region will be discussed in the next section. Following a high degree of H/D
1
4500
4000
m-'
3500
Precipitated Silica
I
4500
4000 cm-'
3500
Figure 3. Top: spectra of A-450(80 mg/cm2) at -191 O C before (A) and after (B)exchange with D20. Bottom: spectra of P-450(40 me/ cm2) at -191 "C before (A') and after (B') exchange with NDp
exchange using either D 2 0 or ND3,the 4580/4510-~m-~ doublet is eliminated [as is most of the 3750-cm-' peak] and is replaced by a single weak feature at 3370 an-'. The 3370-cm-' band was also observed for all other deuterated activated silicas. Finally, the weak band near 3870 cm-' in Figure 3A,A' shifted to near 2850 cm-' in the spectra of the deuterated silicas. (b) 4000-3000-cm-1 Spectral Region. The infrared spectrum of silica in the region of the OH stretching vibrations varies considerably as the activation temperature is raised from 22 to 450 OC.'-*Hydrogen-bonded pairs or chains of silanols have IR bands near 3720 an-'(free OH) and 3520 cm-I (OH*-OH), and
Silanol Groups on Silica
A
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1397
I
Aerasil
I
\< .
A-600
.A400 1
3800
3750
em-'
3700
3650
3?50
3600
d e c i p i t a t s d Silica
IT
em-'
h
x\ I
P-800
3800
3750
an-'
3650
3800
Figure 4. Infrared spectra (SiOH stretching region) of aerosil and precipitated silica (5 mg/cm2) at 22 O C after vacuum activation at 450,600, or 800 OC (spectral resolution 0.5 cm-I).
TABLE I: Frequency and Bandwidth Data precipitated silica peak max, fwhh, cm-l cm-l ~
450
600 800 a
22 -191 22 -191 22 -191
3141.2 3750.6 3141.3 3151.1 3148.0 3151.7
9.4
10.5 8.2 9.1 6.2 6.9
3743.5 3746.5 3146.2 3149.3 3141.1 3151.2
P-450
J
1
3iOO
aerosil silica activation sample peak max, fwhh, cm-' cm-I temp. O C temD. OC
3650
3?00
Precipitated Silica
U
a 11.1 13.3 6.8 8.3
See text.
silanols which are perturbed by interparticle contact have an IR band near 3650 cm-'. Both types are largely eliminated during thermal activation in vacuum up to 450 OC,leaving a relatively sharp but asymmetric peak near 3745 m-'.Examples of spectra showing these changes for these and other silicas have been published In this section we will only discuss the residual 3745-cm-' band which has been attributed to isolated non-interacting silanols. Figure 4 shows 0.5-cm-' resolution infrared spectra of aerosil and precipitated silicas ( 5 mg/cm2) recorded at 22 OC,and Figure 5 shows the corresponding spectra after coaling to -191 OC. The full width at half-height (fwhh) and the wavenumber of the peak maxima are given in Table I. In the room temperature spectra of either silica, there is a smooth asymmetry to lower wavenumber, and this is so pronounced for P-450 that the apparent fwhh has not been listed in Table I. The asymmetry diminishes sisnificantlyupon going from 450 to 800 OC activation, and the peak maximum shifts slightly to higher wavenumber. There is no evidence of a shoulder to the low-wavenumber side of this asymmetric band for any activation temperature. Upon cooling to -191 OC (Figure 5), a distinct low-wavenumber shoulder emerges near 3738 cm-' in the spectra of A-450 and P-450, indicated by the arrows. This disappears in the spectra of the 800 OC activated samples but is still evident in the spectrum of P-600. Upon comparing the data for the fwhh at -191 OC with that at 22 OC (Table I), it can be noted that the apparent width
3i50
P-800 an-'
3700
1
3650
Figure 5. Infrared spectra of the same samples used in Figure 4 but after cooling each disk to -191 O C (resolution 0.5 cm-I).
of this composite peak increases upon cooling. [That the peak frequency is also higher at the lower temperature has been established previo~sly?~J~] This apparent increase in the bandwidth upon cooling mimics the behavior previously discussed for the combination band near 4550 cm-'and further demonstratesthat there are two components in the 3745-cm-' profile, components which individually are probably narrower and have a greater frequency separation at -191 than at 22 OC. Lastly, there is an additional parallel insofar as the 3738-cm-I shoulder (designated the 374O-cm-I band hereafter) decreases relative to that near 3750 cm-' as the temperature of activation increases. Therefore, the intensity of the 3740-cm-' band closely follows that of the highwavenumber 4580-cm-' band in the combination region. Finally, the spectrum of deuterated P-450 at 22 OC (see ref 24) shows the well-known asymmetric peak at 2760 cm-', and in the -191 "Cspectrum this clearly splits into a doublet having a main peak at 2762.7 cm-l and a shoulder at 2756 cm-', paralleling the spectral behavior of the nondeuterated sample. (c) 1000-500-cm-' Spectral Region. The infrared spectrum of a thin (2.5 mg/cm2) self-supporting disk of A-450 at -191 O C is shown in Figure 6A and that of P-450 in Figure 7A. Both spectra are characterized by a moderately intense peak at 800 cm-l due to the silica substrate; on either side of this peak, the background spectrum incream toward regions of total absorption above 1020 cm-' and below 500 cm-'. The peak near 980 cm-' in the spectrum of P-450, which is a shoulder in the spectrum of A-450, is known to be due to the Si-OH stretching mode of isolated silanol groups.3~9J2J7,27,28 When 2.03 Torr of CO was added to A-450, or 0.53 Torr to P-450, the spectra shown as the dashed curves in Figures 6B and 7B, respectively, were observed. The spectral changes can be more clearly seen in difference spectra after subtracting the background spectrumfrom that observed after addition of CO. A series of such difference spectra for decreasing pressures of CO, and using an expanded absorbance scale, are shown in CUNH C, D, and E (25) Morrow, B. A.; Cody, I. A. J . Phys. Chem. 1973, 77, 1469. (26) Ryason, P. R.; Russel, B. G. J . Phys. Chem. 1975, 79, 1276. (27) Heilweil, E. J.; Casassa, M. P.; Cavanaugh, R. R.; Stephenson, J. C. J . Chem. P h p . 1985,82, 5216. (28) Morrow, B. A.; McFarlane, R. A. J . Phys. Chem. 1986, 90,3192.
Morrow and McFarlan
1398 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 A-430
900 800 M-' 760 600 Figure 6. (A) Infrared spectra of A-450 (2.5 mg/cm2) in 0.84 Torr of helium at -191 O C ; (B) after evacuation of A followed by addition of 2.03 Torr of CO at -191 OC (dashed line). (C) Difference spectrum, curve B minus curve A. Curves D and E are difference spectra where the equilibrium pressure of CO was 0.52 and 0.1 Torr, respectively.
ITo.os
A-0w
900
800
cm-'
700
600
900
800
cm-'
700
600
Figure 8. Low-temperature difference spectra of A-800 and P-800 computed as in Figures 6 and 7 for the following equilibrium pressures (Torr) of CO: (A) 0.05; (B) 0.1; (C) 0.5; (D) 1.0; (A') 0.07; (B') 0.5; (C') 1.0; (D') 1.5.
and without the benefit of spectral subtraction. These modes were variously attributed to Si-OH stretching and Si-O-H bending modes. In our spectra we observe the apparent disappearance of vibrational modes at 840 and 760 cm-l, and the growth of a strong new feature at 870 cm-I which is accompanied by a shoulder near 900 cm-'.Also, there is a decrease in the intensity of the Si-OH mode at 980 and its replacement by a new feature near 995 cm-'. Details of the interaction between silica and CO, or other physically adsorbed probe molecules, will be discussed in a subsequent publication. The important point at present is that the interaction of CO with P-450 produces a negative 840-cm-l band which is more intense than the negative 760-cm-' band (Figure 7). The relative intensities of these bands for A-450 are reversed (Figure 6). The same trend was also found for the 4580/4510cm-' bands in the spectra of P-450 and A-450 shown in Figure 2. Figure 8 shows a series of difference spectra after addition of CO to cooled P-800 and A-800. It can be seen from the spectra in Figures 6-8 that the ratio of the intensity at 840 cm-l to that at 760 cm-l does not vary significantly as a function of CO 900 800 an-' 700 600 pressure, for the same sample. However, this ratio is lower for Figure 7. (A) Infrared spectra of P-450 (2.5 mg/cm2) in 0.50 Torr of P-800 vs P-450 and for A-800 vs A-450. The ratio of the intensity helium at -191 OC; (B) after evacuation of A followed by addition of 0.53 of the 4580-cm-' band to that of the 4510-cm-I band also deTorr of CO at -191 OC (dashed line). (C) Difference spectrum, curve creased when the activation temperature was increased from 450 B minus curve A. Curves D and E are difference spectra where the to 800 "C (Figure 2). equilibrium pressure of CO was 0.07 and 0.02 Torr, respectively. Figure 9 shows the difference spectra of A-450 and P-450 recorded at -191 OC following H/D exchange at 22 OC (curves of Figures 6 and 7 (curve C in each figure corresponds to curve A and A', respectively). The Si-OH stretching band at 980 cm-l B minus curve A; see the figure caption for details). Peaks going shifts to about 960 an-'. The negative 840/760-cm1 pair of bands downward represent the loss of a spectral feature upon adsorption are replaced by a broad positive band having a maximum near of CO, and those going upward show the creation of a new spectral 610 cm-l. The replacement of the 840/760-cm-l pair by a single feature. The physical adsorption of CO on silica at low t e m p e r a t ~ r e s ~ * ~ ~band at 610 cm-l following H/D exchange mimica the replacement of the 4580/4510-cm-' pair by a single band at 3370 cm-I (see causes a downward shift of the isolated SiOH stretching frequency Figure 3). from 3750 to 3655 cm-l. Upward shifts of 'modes" in the 900When CO was added to the above cooled and exchanged 75oCm-' spectral region have also previously been noted by Ghiotti samples, the spectra shown in Figure 9,B and B', for A-450 and et but that study was carried out using thicker silica disks, P-450, respectively, were observed. The major feature to note is the negative peak at 610 cm-', and the broad positive feature ( 2 9 ) Ghiotti, G.; Garrone. E.; Morterra, C.; Boccuzzi, F.J . Phys. Chem. near 640 cm-l, due to perturbation of SiOD by adsorbed CO. The 1979,83, 2863. shift of this single band to higher wavenumber in the SiOD (30) Beebe, T. P.;Gelin, P.; Yates, J. T. Surf.Sci. 1984, 148, 526.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1399
Silanol Groups on Silica
1'n
H/D
lio2
TABLE II: Observed Frequencies and Assignments for Isolated Surface Silanol Species at -191 O c a
Exchange
obsd, cm-I 610
I 900
I
800
m-'
700
600
CO Adsorption
In,
760 840 960 980 2756 2763 2850 3370 3740 3750 3870 4510 4580
assignment b(Si-0-D) of I/IIb G(B-0-H) of I G(Si-0-H) of I1 v(Si-OD) of 1/11 u(Si-OH) of 1/11 v(Si0-D) of I1 v(SiO-D) of I r(Si0D) + v(Si0-D) of 1/11 G(Si-0-D) + u(Si0-D) of 1/11 v(Si0-H) of I1 u(Si0-H) of I r(Si0H) + v(SiO-H) of 1/11 b(Si-0-H) + v(Si0-H) of I G(Si-O-H) u(Si0-H) of I1
+
O b = angle deformation mode. T = torsional mode. Y = stretching mode. 61/11 refers to a mode assigned to a silanol species of type 1 and/or of type 11. See text for a description of these silanol types.
tivation at or above lo00 OC, there is virtually no low wavenumber a ~ y m m e t r y . ~ I In - ~ this ~ case the silanol band is entirely due to species I and the peak maximum is near 3748 cm-' in room temperature spectra, or near 3752 cm-' at -191 OC. Being the last silanol type to be eliminated due to thermal activation, it has always been attributed to single isolated s i l a n o l ~ . ' - ~As ~ ~will ~-~~ be discussed in more detail later, the relative proportions of species 900 800 m-' 700 600 I and I1 are probably more correctly reflected in the relative Figure 9. Infrared difference spectra of A-450 and P-450 at -191 O C intensities of the OH stretching bands near 3750 cm-' (Figures after deuterium exchange (curves A and A', respectively); (B) spectrum 4 and 5) whereas the relative intensities of the angle deformation observed after evacuation of He and the addition of 2.1 Torr of CO to modes near 800 cm-'are anomalous because of vibrational mixing A, (B') spectrum observed after addition of 1.0 Torr of CO to A'. with nearby bulk Si02modes. Therefore, species I1 is a minority species which is preferentially eliminated relative to I as the spectrum is analogous to that of the 840/760-cm-' bands in the temperature of activation is increased. It is tempting to assign SiOH spectrum (see Figures 6 and 7). The residual intensity at the 3740-cm-' band of species I1 band to geminal silanols, Si(0840/760 cm-' in Figure 9B,B' is due to unexchanged OH groups. H)2. Theoretical calculation^^^ have shown that the vibrational Discussion modes of this species are expected to be very close to those of isolated single SiOH. However, 29SiNMR spectroscopy has All of the spectra presented pertain to silicas which have been indicated that the ratio of geminal to single isolated silanols does activated at or above 450 OC. Activation at 450 OC removes not vary significantly according to the type of silica used or with virtually all of the H-bonded silanolsl-8 and this discussion will the temperature of activation under In both respects only be concerned with the infrared spectrum of isolated silanols. this is contrary to the apparent behavior of the 3750/374O-cm-l The wavenumbers of the observed bands and the assignments to pair of bands. Therefore, if we accept the NMR evidence, it is be discussed below are summarized in Table 11. not logical to attribute the 3740-cm-' band to geminal silanol The observation of two bands in the 4550-cm-' combination species. region whose separation and peak intensities increase as the temperature is lowered (Figure 2) is not unique to this ~ o r k . ~ ~ , ~In ~a previous investigation of the rehydration of highly or totally dehydroxylated silica, we observed the initial appearance of a band A pair of bands in the 850-75O-cm-' spectral region (Figures 6-8) at 3742 cm-'when micromole quantities of water were added.3'.32 has also been previously rep0rted.99~~33 However, the assignment This band was attributed to a weakly interacting pair of vicinal of the combination bands has been based on the assumption that single silanols that were formed during the first stage of rethere is a single isolated SiOH frequency near 3750 cm-'. Tsyhydration of this silica. Further rehydration of the same sample ganenko22proposed that bands at 4590 and 4515 cm-' could be caused the intensification of the 3748-cm-' band. (All spectra assigned to combinations of the 3750-cm-l mode with the SiOH in that s t ~ d y ~were ' , ~ recorded ~ at room temperature.) In the deformation mode at 840 cm-l and with the S i 0 stretching mode present study, we added micromole doses of water at room temat 765 cm-I, respectively. Kustov et proposed the reverse perature to a silica which had been activated at 1100 O C , so as assignment for the 840- and 765-cm-' bands. to develop the 3748/3742-cm-l bands. When this sample was The above assignments are incorrect because the isolated Si-OH stretching frequency at 980 cm-l is firmly e s t a b l i ~ h e d . ~ ~ ~ J ~ J ~cooled , ~ ~ , ~to~-191 OC,the 3748-cm-I peak shifted upward to 3752 cm-' and that at 3742 cm-' shifted downward to 3738 cm-'. Further, the 840- and 760-m-' bands cannot be assigned to SiOH Therefore, we conclude that the 3738-cm-' band of species I1 in angle deformation modes belonging to a single surface species the present study is due to the same vicinal pair of isolated silanols because their relative intensity changes with the temperature of activation, and similar changes are also observed in the pairs of (31) Morrow, B. A.; Cody, I. A. J . Phys. Chem. 1975, 79, 761. bands at 3740/3750 cm-l (Figures 4 and 5) and at 4580/45120 (32) Morrow, B. A.; Cody, I. A.; Lee, L. S.M. J . Phys. Chem. 1976,80, cm-I (Figure 2) for borh silicas. Considering these changes in 2761. relative intensity of the components of all three doublets as a (33) Hoffmann, P.; KnBzinger, E. Surf. Sei. 1987, 188, 181. (34) Sauer, J.; SchrMer, K. P. 2.Phys. Chem., Leipzig 1985,266, 379. function of the activation temperature, the bands at 4510, 3750, (35) Maciel, G. E.; Sindorf, D. W. J . Am. Chem. Soc. 1980,102,7606. and 760 cm-' can be attributed to a unique surface silanol species (36) Sindorf, D. W.; Maciel, G. E. J . Am. Chem. Soc. 1983,105, 1487. (I), whereas the bands at 4580, 3740, and 840 cm-' are due to (37) Fyfe, C. A,; Gobbi, G. C.; Kennedy, G. J. J. Phys. Chem. 1985.89, a different silanol type (11). This is supported by the perfect 277. (38) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1982.86, 5208. additivity in the combination region: 3740 840 = 4580 and (39) Legrand, A. P.; Hommel, H.; Tuel, A.; Vidal, A.; Balard, H.; Papirer, 3750 760 = 4510. E.; Levitz, P.; Czernichowski, M.; Erre, R.; Van Damme, H.; Gallas, J. P.; The OH stretching band becomes progressively more symmetric Hemidy, J. F.; Lavalley, J. C.; Barres, 0.; Burneau, A.; Grillet, Y. Adu. as the temperature of activation is increased and, following acColloid Interface Sci. 1990, 33, 91.
+
+
1400 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
which were reported earlier. Because of their proximity, it is plausible that these silanols also preferentially condense to liberate water as the temperature of activation is increased. In the spectrum of the deuterated silicas, a single broad band was observed at 610 cm-I (Figure 9) and at 3370 cm-I (Figure 3) but the 2760-cm-* band24 was clearly a doubler having a separation of 8 cm-I. The 337oCm-l band is undoubtedly the sum of the 610- and 2760-cm-' bands and the failure to observe a splitting of the combination band arises due to the lack of measurable splitting of the broad deformation band at 610 cm-'. (Given the breadth of the 3370-cm-l band, a splitting of 8 cm-' due to the splitting of the OD stretching mode would not be detected.) It remains to explain why the deformation modes for species I and I1 are observed in the SiOH spectrum (840/760 cm-I) whereas only a single deformation mode is observed for deuterated silica (610 ad). The M U H deformation mode of a triatomic molecule shifts by a factor of about 1.36 to lower wavenumber upon deuteration.40 However, for more complex molecules, the H/D shift differs considerably, ranging from 1.56 to 1.22 in alcohols,41-44and from 1.40 to 1.28 in silan0ls.4~*~~ These anomalous H/D shifts (and anomalous intensities) arise because of vibrational mixing when there are other nearby mode^.^^^ Finally, the ZnOH deformation modes at 840 and 810 cm-' on zinc oxide both exhibit a 1.32 H/D shift.47 The shifts in the present case are 1.24 and 1.38 for species I and 11, respectively, the mean being 1.31. The stretching modes of the two SiOD species are separated by about 7 By analogy with shifts which arise from classical H bonding in model compound~,4~ the separation of the deformation modes should not be greater than 7 cm-', and the species having the lower stretching frequency will have the higher deformation frequency. As a result, the two SiOD deformations modes would be expected to appear as a single band; the broad band near 610 cm-I is so assigned. For a 1.31-1.32 isotopic shift relative to SiOD, the SiOH deformation modes would be expected to be separated by about (40) Ross, S.D. Inorganic Infrared and Raman Spectra; McGraw-Hill: London, 1972. (41) Shimanouchi, T. Tables of Molecular Vibrational Frequencies,Part 1; National Standard Reference Data Series, NBS 6; US. Department of Commerce: Washington, DC, 1967. (42) Pinchas, S.;Laulicht. I. Infrared Spectra of Labelled Compounds; Academic Press: New York, 1971. (43) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W . H. Freeman & Co.: San Francisco, 1960. (44) BeUamy, L. J. The Infra-redspectra of Complex Molecules, 3rd ed.; Chapman and Hall: London, 1975. (45) Whitnall, R.; Andrews, L. J . Phys. Chem. 1985, 89, 3261. (46) Ismail, Z. K.; Hauge, R. H.; Fredin, L.; Kauffman, J. W.; Margrave, J. L. J. Chem. Phys. 1982, 77, 1617. (47) Tsyganenko, A. A.; Lamotte, J.; Saussey, J.; Lavalley, J. C. J. Chem. Soc.. Faraday Trans. 1 1989,85, 2397.
Morrow and McFarlan 10 cm-I and to lie near 800 cm-l. This predicted frequency is close to the strong silica substrate mode (or modes) at 800 cm-', but the observed separation is about 80 cm-'.Therefore, it is probable that the two SiOH deformation modes couple with the substrate mode($ such that their frequencies are shifted to higher and lower wavenumber (840 and 760 an-')and that their relative intensities for a given temperature of activation bear no direct relationship to the relative intensities of the corresponding stretching modes. Accordingly, there is a separation of the deformation and combination bands in the SiOH spectrum, but not in the SiOD spectrum. The exact nature of this vibrational effect is not central to this paper, and further speculation is unwarranted. However, Burneau et al.I7 recently reported that the room-temperature spectra of silicas which had been activated at two different temperatures showed a similar trend to ours for the relative intensity changes of the 840/760-cm-I bands and the unresolved doublet near 4550 cm-l, and that there was a single band near 3370 and 610 cm-' for the deuterated silica. Their room temperature spectra lacked the resolution afforded by cooling in liquid nitrogen, and they were unaware of the splitting of the SiOH stretching modes. Therefore, their assignments, although plausible, were different from ours. However, they also accounted for the splitting of the deformation modes by invoking the idea of vibrational coupling with the substrate modes. other Modes. There is a broad relatively weak band near 3870 cm-I to high wavenumber of the massive 3750-374O-cm--' isolated SiOH absorption in Figure 3A,A'. This is near 2850 cm-' (not shown) in the spectrum of deuterated silica. Tsyganenkg assigned such a band (3850 cm-I for SiOH) to a combination of the isolated SiOH stretch (3750 cm-')with a low-wavenumber torsional SiOH mode, calculated to be near 100 cm-'. However, Hoffmann and Kn&hgelJ3 have observed very broad bands which were attributed to the isolated SiOH and SiOD torsional modes near 127 and 94 cm-l, respectively, in room-temperature IR spectra. The 3870/285O-cm-' bands are undoubtedly due to a combination of the stretching modes of either species I or I1 with a torsional mode.
Conclusions Two types of isolated silanol species have been identified on an aerosil and a precipitated silica following vacuum activation at or above 450 "C. Type I silanols are truly isolated whereas those of type I1 are a weakly interacting vicinal pair. The type I1 silanols are preferentially eliminated as the activation temperature is increased above 450 OC. The observed wavenumbers of all H/D isotopomers, and all of the assignments, are summarized in Table 11. Acknowledgment. We are grateful to N.S.E.R.C. of Canada for financial support and for a%tgraduate scholarship to A.J.M. Registry No. S O , , 763 1-86-9.
