M. J. D. Low AND N. RAMASUBRAMANIAN
2740
Infrared Study of the Nature of the Hydroxyl Groups on the Surface of Porous Glass
by M. J. D. Low and N. Ramasubramanian School of Chemistry, Rutgers, The State University, New Brunsurick, New Jersey
(Received November 1, 1966)
The slow dehydration and dehydroxylation of porous Vycor glass was followed by infrared spectroscopic techniques. Two sharp bands a t 3748 and 3703 cm-l and also two shoulders near 3850 and 3650 cm-l were observed. Dehydroxylation, deuteration, fluoridation, and adsorption experiments showed all absorptions to be due to surface hydroxyl species. The 3748-cm-' absorption is due to free surface silanol groups. The 3650-cm-l shoulder is due to OH vibrations perturbed by hydrogen bonding. The 3703-cm-' band has a half-width of 5 to 14 cm-' and can only be observed at relatively low surface coverage. Impregnation of silica and porous glass with boric acid produced a band at 3703 cm-l with the silica and enhanced that found with the glass, leading to the assignment of the 3703-cm-' band to a B-OH surface structure. The nature of the species responsible for the 3850-cm-' shoulder is uncertain.
Introduction Porous Vycor glass has been studied extensively in recent years, emphasis being placed on obtaining information on the structure and properties of the glass surface, the nature of the surface hydroxyl groups, and on reaction of gases with the surface. The results of various studies involving infrared spectroscopic techniques are in general agreement, and the observation in the 0-H region of a single asymmetric infrared band ascribed to isolated surface OH groups was reported in addition to a broad band due to hydrogenbonded OH groups and adsorbed H2O.l-l' It was noticed, however, during a study of the interaction of activated hydrogen with glasses, that the surface of porous Vycor was unstable, contrary to the impression given by the literature. Small weight losses and changes in infrared spectra occurred over long periods of time on heating porous glass specimens in uacuo, and the asymmetric band ascribed to isolated surface OH groups was resolved. This led us to reexamine porous glass surfaces. The existence of a second sharp band in the hydroxyl region has been mentioned previously. Sidorov,2 working with a UR-10 spectrophotometer a t a spectral slit width of 4 cm-', presented a spectrum of porous glass heated in Vacuo at 650" for an unspecified time that The Journal of Physical Chemistry
shows a small, sharp band marked to be at 3700 cm-l. That band is not mentioned elsewhere in the paper, however. Also, Kozirovski and Folman, working with a Perkin-Elmer Model 21 spectrometer with CaFz optics, mention the formation of a 30-cm-l spaced doublet when porous glass was evacuated for 1hr at 900". They state that the nature of the second, ~
~~
(1) T. H. Elmer, I. D. Chapman, and M. E. Nordberg, J . Phys. Chem., 66, 1517 (1962). (2) A. N. Sidorov, Opt. Spectry. USSR, 9,424 (1960). (3) I. D. Chapman and M. L. Hair, J. Catalysis, 2, 145 (1963). (4) L. H. Little and M. V. Mathieu, Actes Congr. Intern. CataZyse, P,Paris, 1, 771 (1960). (5) N. W.Cant and L. H. Little, Can. J . Chem., 42,802 (1964). (6) M. Folman, Trans. Faraday SOC.,57, 2000 (1961). (7) M. V. Mathieu, N. Sheppard, and D. J. C. Yates, Z . Ilektrochem., 64,734 (1960). (8) M. Folman and D. J. C. Yates, Proc. Roy. SOC.(London), A246, 32 (1968). (9) M. Folman and D. J. C. Yates, J . Phya. Chem., 63, 183 (1969). (10)I. D.Chapman and M. L. Hair, Trans. Faraday SOC.,61, 1607 (1965). (11) A. V. Kieselev and V. I. Lygin, Proc. Second Intern. Congr. Surface Adiuity, 2 , 204 (1957). (12) T. H.Elmer, I. D. Chapman, and M. E. Nordberg, J. Phya. Chem., 67, 2219 (1963). (13) A. N.Sidorov, Ruse. J. Phya. Chem., 30, 995 (1966). (14) Y. Kozirovski and M. Folman. Trans. Faraday SOC.,60, 1632 (1964).
INFRARED STUDYOF HYDROXYL GROUPSON POROUS GLASS SURFACES
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sharp band of the doublet is uncertain, and also that the doublet was not always observed, but make the suggestion that a hydroxyl bonded to surface boron atoms may be responsible. The sharp band is not mentioned elsewhere in this paper. More recently, it was reported by Chapman and Hairlo that the reaction of ammonia with fluoridated glass at 200" produced a small band at 3700 em-'. The spectra were obtained with a Perkin-Elmer 221G spectrometer at unspecified spectral slit width. They noted that the position of this band was more suggestive of a hydroxyl stretching frequency (perhaps attached to a boron atom rather than a silicon atom), but proposed that it was due to a single P;;-H vibration where the nitrogen atom is bonded to two silicon atoms. No further discussion of the band is given. It is interesting to note that in the above three studies, and also in subsequent ones by other workers, due consideration was not given to the existence of the second sharp hydroxyl band. The reason for this appears to be that much of the work was carried out with porous glass specimens that had been degassed at relatively low temperatures and/or for short periods of time, and that instruments not capable of resolving the band were used, as will be shown below. We have consequently studied the slow changes occurring on continuous evacuation leading to highly degassed surfaces systematically. The results of the present study agree with previous work in that the general trends of dehydration were confirmed, but differ significantly in the detail of all aspects.
permit passage of the infrared beam. The cellfurnace assembly was positioned in the sample space of a Perkin-Elmer Model 12C spectrometer, permitting spectra to be obtained at temperatures up to 850". The glass as received was clear and colorless. As we wished to observe the reactions over a wide range of temperatures, most of the work was done with the specimens as received. Some duplicate sets of experiments established that identical infrared results were obtained with untreated specimens and with specimens that had been heated in oxygen at 650". Spectra were recorded at IX ordinate magnification with a Perkin-Elmer Model 521 spectrophotometer at theoretical slit widths of 3.9 and 2.4 cm-l at 3500 and 2500 em-*, respectively, or with a Perkin-Elmer Model 12C spectrometer equipped with CaFz optics. Two identical specimens and cells were used to obtain differential spectra. Hydrogen and deuterium were purified by diffusion through hot palladium. Conventional vacuum systems capable of torr were used. HF was prepared by the thermal decomposition of KHF2 in vacuo. D20 of 99.84% isotopic purity was freed from dissolved gases by alternate freezing and thawing in vucuo. In order to avoid the overlapping of spectra, the ordinates have been displaced for the spectra of several figures. When this was done, the % transmittance for a particular spectrum is indicated by a number next to the spectrum at the left ordinate.
Experimental Section
kept below 900" in order to minimize sintering. Nitrogen BET surface areas,17 computed from data at six pressures, were 166, 173, 160, and 154 m2/g after heat treatment in vacuo for 3 hr at 300", 19 hr at 50O0, and 9 and 51 hr at 700", respectively. The increase in area is considered to be brought about through the loss of water, small pores previously blocked by water becoming free and thus available for the penetration and adsorption of nitrogen. This agrees with the ob~ e r v a t i o nthat ~ , ~degassing ~ at 200" is necessary to remove physically adsorbed water and that some chemisorbed water remains at 500". The decrease in area after the 51-hr, 700" treatment represents a loss of about one-tenth of the area a t 500", based on the initial weight of the specimen. If estimates of water loss per gram of starting weight are made, based on gravimetric measurements to be described elsewhere,
Samples of porous glass,l5 Corning Code 7930, purchased from Corning Glass Co., were cut into rectangles 1 X 2 em. For most infrared studies, a specimen of about l-mm thickness was tied with thin platinum wire to a quartz carriage bearing a quartzenclosed magnet. The carriage and specimen were then placed within the body of a Vycor cell similar to that described by Peri and Hannan.16 The specimen could be moved from the region of the cell windows to a furnace wound on the cell body by manipulating an externally applied magnet. Single and differential spectra obtained with such cells were run with the specimen at room temperature. In order to obtain spectra of the hot specimen, another type of cell was used. A piece of Vycor tube was closed off, and the sides were flattened to act as windows. The specimen was placed in the flattened region of this simple cell, which was then connected to a conventional vacuum system. The cell was surrounded by a furnace pierced by two holes directly opposite the cell windows to
Experiments and Results Surface Areas. The degassing temperatures were
(15) M.E. Nordberg, J. Am. Ceram. Soc., 27, 299 (1944). (16) J. B.Peri and R. B. Hannan, J. Phys. Chem., 64, 1526 (1960). (17) 5. Brunauer, P. H. Emmett, and E. Teller, J. Am. Chem. SOC., 60,309 (1938).
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M. J. D. Low AND N. RAMASUBRAMANIAN
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Figure 2. Dehydroxylation. Consecutive heat treatment following treatment at 750" for 24 hr: a t 800°, A, 4 hr; a t 825O, B, 20 hr; C, 68 hr; D, 92 hr.
L 3600
Figure 1. Dehydroxylation. Successive heat treatments, subsequent to 80 hr a t 500". The ordinates are displaced for clarity: at 600", A, 25 hr; a t 650", B, 4 hr; C, 8 hr; D, 20 hr; a t 700", E, 2 hr.
a loss of area of similar magnitude results. This suggests that sintering was not drastic, so that possible effects of this were disregarded. Dehydroxglation. The loss of water by porous glass specimens was followed from room temperature to 825" for long periods of time. Slow and continuous dehydration was found to occur even at 500" and below, but for present purposes only the later stages of d e gassing observed at high temperatures need be considered. Some results are shown in Figure 1. Spectrum A shows the general features previously reported, Le., an asymmetric band generally ascribed to surface silanol groups. Noticeable also is a distinct shoulder near 3850 cm-l. On more severe degassing, the main band becomes sharper, as shown in the sequence A to E of Figure 1. Also, a second sharp band appears. A slow diminution of all bands and shoulders occurred at the higher temperature} as indicated by the successive spectra of Figure 2. Spectrum D of that figure shows a weak shoulder near 3850 cm-l, the faint remnants of a band in the 3680-3550-cm-' region, as well as the two prominent, sharp bands. Various values have been quoted for the position of the hydroxyl band. The present experiments show that, besides the gross shift occurring during degassing The Journal of Phyeical Chemistry
at low temperatures or short times, there is a small shift of the free hydroxyl band until a very high degree of degassing is reached. For example, the position of the band changes from 3740 cm-l (spectrum A, Figure 1) to 3743 cm-l (spectrum E, Figure 1) at medium stages of degassing to a final value of 3748 cm-l (spectra C and D, Figure 2) at high stages of degassing. The position of the free hydroxyl band is dependent on degassing conditions} i.e., on the surface OH concentration. The value 3748 cm-' will be used below to designate the free hydroxyl band. During the heat treatments, a specimen was located within the furnace section of the cell and, at the end of a treatment, was moved to the window section of the cell. The spectrum was then measured with the specimen at room temperature. Heat treatments were also carried out without moving the specimens, spectra of the hot specimen being measured. Some of these spectra are shown in Figure 3. The resolution of those spectra and the degree of dehydration of that specimen were less than those shown in Figure 2. The sequence of spectra of Figure 3 serves to illustrate the slow process of dehydration. Of more importance is the fact that the spectra of hot and cold specimens showed similar structure, this indicating that the weak shoulders near 3850 and 3650 cm-l were not artifacts caused by the readsorption of desorbed water as a specimen cooled to room temperature after a heat treatment. The shift in the broad band when the sample was cooled from 750 to 26" is similar to temperature-induced shifts of OH bands on alumina surfaces,'* and is attributed to the effects of temperature on hydrogen bonding.s* l8 The present spectra distinctly show the presence of a sharp band near 3703 cm-l, in direct contrast to previous workl-la except for the instances noted (18) J. B.Peri, J . Phys. Chem., 69, 211 (1966).
2743
INFRARED STUDY OF HYDROXYL GROUPSON POROUS GLASSSURFACES
,
v,
I
3400 c ~ ' Figure 3. Spectra of hot specimen. Consecutive heat treatment for the times in hours indicated, a t 750'. (The second l/s-hr spectrum followed an 800' treatment.) The spectrum marked 22 (26') wm measured with the sample a t room temperature. Other spectra were obtained a t the temperature of heat treatment. The ordinates are displaced for clarity. 4CkX 37bo
earlier.2*10i14This discrepancy is brought about mainly by the difference in resolution of the spectrometers used and, to some extent, by the degassing conditions employed. Much of the previous work was done with Perkin-Elmer Model 21 instruments fitted with NaCl optics and occasionally with CaFz optics, whereas a Perkin-Elmer Model 521 grating instrument capable of higher resolution was used in this study, and instruments of suitable resolving power were used in the instances noted lol14 As the 3703-cm-l band has a half band width ranging from about 5 to 14 cm-l depending on the degree of degassing, instrument quality is important. However, the sharp 3703-cm-l band would not be detected unless degassing had proceeded beyond a stage such as that shown by spectrum A of Figure 1. The sharp band is completely obscured by the broad shoulder on the low-frequency side of the 3748-cm-l band, is barely detectable in spectrum B, but becomes prominent as in spectra C to E on further degassing of the sample. Kozirovski and Folman were not able to detect the hydroxyl doublet every time. They pointed out that Elmer,
Chapman, and Nordberg had not reported a doublet, and suggested that a cause for this could be a difference in the composition of the glass. We have, however, consistently detected the sharp band with samples taken from four different batches of glass obtained over a period of 2 years if degassing was sufficient and if an instrument capable of resolving the band was used. Deuteration. The reaction with gaseous deuterium of specimens in various states of dehydration was examined at various temperatures and reaction times. The deuteration of a relatively poorly degassed specimen was not noticeable below 300", but proceeded very slowly at 300", the growth of bands in the OD region accompanying the decline of corresponding bands in the OH region. As the frequencies of the deuteroxyl species fall in the region showing a broad band near 2700 cm-' attributed to boron oxide present in the solid, the growth of OD bands on top of this band is difficult to distinguish. Differential spectra were therefore made using identical cells and two specimens brought to similar stages of dehydration. Figure 4 shows a sequence of differential spectra with specimens at a "medium" stage of dehydration. Negative bands appeared in the OH region as shown in plot A, because the specimens were not exactly matched at the beginning of the deuteration. Other differential spectra were obtained with specimens at high degrees of dehydration. The bands and shoulders produced in the OD region by deuteration occurred near 2840, 2760, 2730, and 2700-2600 cm-l. The relations between these and corresponding bands near 3850, 3740, 3703, and 36803550 cm-l in the OH region were 1.357, 1.357, 1.359, and about 1.38, respectively. For the spectra of highly degassed specimens the OH, OD positions and isomer shifts were, respectively, 3748 cm-l, 2760 cm-l,
2ot
I
I
3900
3500
" 3000
2600
Figure 4. Differential spectra of deuteration. A: after the sequence of evacuations resulting in spectrum E of Figure 1. The sample waa then heated in Dz at 10 cm; B: 0.5 hr, 26'; C: 0.5 hr, 400'; D: 1.5 hr, 400'; E: 1 hr, 500'.
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1.3585, 3703 em-', 2728 cm-l, 1.3589. This closely approximates the theoretical value of the isomer shift expected for identical OH and OD structures and, with the observation that the growth of a band in the OD region was accompanied by the decline of a corresponding band in the OH region, indicates that the four absorptions were brought about by surface OH groups, the deuteration involving the exchange of the hydrogen atom of an OH group by a deuterium atom. The bands of the residual OH groups declined at different rates on heating in vacuo, shown, for example, by the sequence A, B, C, D of Figure 2. The exchange reactions similarly occur at different rates. For example, the ratio of peak heights of the sharp OD bands a t 2760 and 2728 cm-I changed from about 0.6 after 2.5 hr at 500" to about 1.1 after 10 min at 600", and to about 1.7 after 75 min at 600". These differences indicate that a distinct surface OH species is responsible for each of the sharp bands. The deuteration was accompanied by a frequency shift of the unexchanged silanol groups, except a t very high stages of degassing. For the data of Figure 4, for example, the free OH band moved from an initial value of 3743 cm-l on the deuterium-free surface to a final value of 3748 cm-l for the residual OH groups. A similar small shift was observed for small and high OD surface concentrations. Identical results could be produced by treating porous glass with DzO, although deuteration of the surface occurred at lower temperatures than with D2. For example, a sample was exposed to D2O vapor at 2 cm pressure at 100" for 4 hr and was then degassed at 400" for 4 hr. Upon repetition of this cycle for five or six times about 90% of the surface OH species could be converted to the corresponding OD species. H F Treatment. A porous glass specimen was degassed at 750" for 16 hr, was then exposed to HF at 1 cm pressure for 1 hr, and then was degassed at 500" for 4 hr. This successive H F treatment and degassing was repeated twice. The results are shown in Figure 5, the spectra showing that all bands could be removed completely by the H F treatment, thus indicating the sharp 3748- and 3703-cm-' bands to be caused by surface species. A slight shift to higher frequency in the Si-OH vibration can be seen as the removal of hydroxyls proceeds. The two sharp hydroxyl bands could be restored by adsorbing water on a fluoridated sample at room temperature and degassing at temperatures varying from 30 to 300". Such experiments, which will be reported in detail elsewhere, indicate that the removal of surface boron through the formation of volatile BF3 could not have occurred to an appreciable extent. The Journal of Physical Chemistry
111. J. D. Low AND N. RAMASUBRAMANIAN
F
I
3dOO
'
3600
I
G O O 6M-l
HNOI Leaching. A porous glass specimen was leached with nitric acid following the procedure of Little, Klauser, and Amberg.lg They had shown the boron content of the glass to be unaffected by this treab ment, but the aluminum and zirconium content as RaOg changed from 0.89 to 0.36%. The spectra of the leached and unleached specimens, after identical degassing, were similar in the OH region and also in the 2700-cm-l region where bands attributed to the boric oxide in the glass occur. As the leaching did not affect the ratio of intensities of the sharp bands, this excludes the possibility that the 3703-cm-l band was caused by an OH associated with R203. Boric Acid Impregnation. Cab-O-Si120 silica was impregnated with boric acid to result in a Si02-2% B2Oa sample approximating the chemical composition of the porous Vycor. A slurry made from Cab-0-Si1 and the required amount of an aqueous boric acid solution was airdried at 135" for 2 hr. Self-supporting 1-in. diameter wafers of about 0.1 g of the pure and the impregnated Cab-0-Si1 were prepared by pressing at 30 tons/in.*. Spectra were recorded at different stages of degassing, (19) L. H. Little, H. E. Klauser, and C. H. Amberg, Can. J. Chem., 39, 42 (1961). (20) Cabot Co., Boston, Mass.
INFRARED STUDY OF HYDROXYL GROUPSON POROUS GLASSSURFACES
3800
3400CM"
3600
Figure 6. Boric acid treatments. Degassing of Cab-0-Sil: A, room temperature, 1 hr; B, 400°, 1 hr; C, 500", 1 hr. Degassing of Cab-0-Si1 boric acid: D, room temperature, 0.25 hr; E, 300°,0.5 hr; F, 400", 0.25 hr; G, 400°, 1 hr. The ordinates are displaced.
+
examples being shown in Figure 6. The spectra of pure Cab-0-Si1 show structure similar to that observed with other silica ads or bent^,'^^^^^^^ but a sharp band at 3703 cm-l is evident in the spectra of the boric oxideimpregnated sample in addition to the hydroxyl band near 3745 cm-l. Another specimen was prepared by boiling a porous glass sample in 0.1% boric acid solution for 0.5 hr. The specimen was then air-dried and degassed. Comparison of the spectra of untreated and boric acidtreated specimens after identical degassing showed that the 3703-cm-l band was enhanced by about 6%.
Discussion The existence of four infrared absorptions in the 0-H region has been shown. In concurrence with others, the 3748-cm-' band is attributed to what are generally referred to as "free hydroxyls," Le., silanol groups so oriented that they do not react with their surroundings. The small frequency shifts from lower wavenumbers to 3748 cm-l during the progress of deuteration or fluoridation of surfaces that have not been subjected to severe degassing, however, and also the small shifts to 3748 cm-l on continued degassing of medium or highly degassed surfaces, indicate that the hydroxyl vibrations are not unperturbed except at very low degrees of surface coverage. A second band or shoulder
2745
in the 3650-cm-l region, responsible for the asymmetry of the low frequency side of the 3748-cm-1 band as in spectra A to C of Figure 1, and which gradually decreased in intensity to form a shoulder as in spectrum A of Figure 2, is similarly ascribed to the perturbation of the 0-H stretching vibration through hydrogen bonding. Spectrum A of Figure 1 shows a shoulder near 3850 cm-I which declines in intensity but becomes somewhat more distinct in the sequence B to E, but which is still to be found in spectrum D of Figure 2 of a highly degassed specimen. Adams and Douglas23have assigned a band at 3846 cm-' found with fused silicas to a combination of the bending vibration of OH within the fused silica and of a SiOl stretching vibration. The 3850-cm-' shoulder of porous glass, however, changed in intensity on degassing and fluoridation in concert with other bands, showed an isomer shift. Other experiments not described here showed that the 3850-cm-l shoulder decreased in intensity and disappeared completely when various amounts of acetone or methanol were adsorbed. The shoulder was restored on degassing the adsorbent at room temperature. Also, for spectra measured at various stages of degassing of the same sample as well as of different samples, the ratio of the area of the 3850-cm-l shoulder and of the area of the rest of the absorption in the hydroxyl region remained constant, pointing to some relation of the 3850-cm-' shoulder to surface hydroxyl concentration rather than to hydroxyls in the bulk of the glass. The exact nature of the species responsible for the 3850-cm-l shoulder is uncertain, however. The various degassing, fluoridation, and exchange experiments described above here indicated that the sharp 3703-cm-l band is caused by an OH species, that the particular grouping is on the surface of the glass, and that the surface grouping is not the same as the silanol responsible for the 3748-cm-l band. These experiments, as well as the results of the nitric acid leaching experiments which exclude the consideration of OH groups bonded to surface Rz03,do not exclude the possibility of existence of a second silanol structure. However, the boric acid impregnation of silica, leading to the formation of a band at 3703 cm-', and the enhancement of the 3703-cm-' band of porous glass by the boric acid treatment, lend strong support to the suggestion that the 3703-cm-1 band is brought about by surface B-OH groupings. That band is thus as(21) H. A. Benesi and A. C. Jones, J . Phys. Chem., 6 3 , 179 (1959). (22) R. S. McDonald, J . Am. Chem. SOC.,79,850 (1957). (23) R.V. Adams and R. W. Douglas, Trans. SOC.Glass Techn., 43, 147 (1959).
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signed to OH groups bonded to surface boron atoms, and appears to be identical with bands mentioned previ~usly.~, It seems likely that, with Chapman and Hair's work,lo the reaction of ammonia with the dehydroxylated porous glass surface had brought about the formation of some B-OH structures. The B-OH band is sharp and symmetrical and is observed at relatively high stages of degassing, indicating that the groups are not perturbed by hydrogen bonding. This assignment agrees with the observation that, for porous glass and for the synthetic SiO2-2010 B2O3 sample, the ratios of the intensities of the 3748- and 3703-cm-I bands are of similar magnitude. However, the boric oxide content of the glass is about 3%, so that an intensity ratio of Si-OH and B-OH of the order of 30 would be expected, for a homogeneous distribution of boron based on the assumption of identical absorptions of the two OH groups. As the boron of the synthetic sample is on the silica surface, this would imply that the surface boron concentrations of the glass and impregnated silica were similar, Le., that an appreciable fraction of the boron contained by the glass was on the glass surface. Such an enrichment could have occurred through the migration of boron to the surface
The Journal of Phyaical Chemistry
M. J. D. Low AND N. RAMASUBRAMANIAN
of the porous glass when heated above 500°1aas in the present study. It is interesting to note that absorption in the 3850-cm-l region and the boron oxide bands near 2700 em-' found with porous glass were not observed with the SiO%-2% Bz03 sample. This indicates that the environment of the boron on the silica surface differed from that of the boron on or in the glass, and lends some support to the above assignment. The present results indicate that the nature of the porous glass surface is more complex than had been supposed. If the surface hydroxyl groups play an active role in determining the characteristics of the surface, the existence of the B-OH groups in addition to the silanol species is of particular interest because the reactivity of the silica surface could be affected by their presence. The existence of multiple hydroxyl species should thus be taken into account in studies of adsorption and catalysis involving porous glass surfaces. Achowledgments. Support for this work through Contract No. DA36439-AMC-O217O(E) monitored by U.S.A.E.L., Contract Nonr-404 (19), and National Science Foundation Grant GP 1434, is gratefully acknowledged. 8
