INFRARED STUDYOF OH AND NH2 GROUPSON DRYSILICASURFACES
and the halides, for example. Unfortunately, not enough data are available to permit comparison of the actinide hydride bond lengths with those of other actinide compounds, such as the monosulfides, in which the cation can be called divalent. Further comparison is possible for the hexagonal trihydrides of neptunium, plutonium, and americium. The heats of formation of these compounds cannot easily be derived accurately from existing pressure
2937
measurements, but the unit cell dimensions are available. I n the heavy rare earth trihydride series, an unequivocal contraction is evident,' but again, no such effect is seen for the three comparable actinide trihydrides.
Acknowledgment. We gratefully acknowledge the determination of the lattice parameters by Finley H. Ellinger.
Infrared Study of OH and NH, Groups on the Surface of a Dry Silica Aerogel
by J. B. Peri Research and Development Department, American OiZ Company, Whiting, Indiana
(Receined March 1+$* 1966)
Infrared and gravimetric study of transparent plates of pure silica aerogel yielded further evidence on the characteristics of the OH groups retained after strong drying, the replacement of OH groups by NHZ groups, and the formation of NH2 groups by chemisorption of NHa. Reactions of D2, DzO, HC1, Cl2, SiC14, and CCl, with surface OH groups were investigated briefly to obtain additional information on the surface structure and the reactivity of surface groups. Spectra and the retention of OH groups were generally as reported for other silicas, but evidence was also found for some rotation or torsional oscillation. The Si-OH bond angle appears to be about 113". Spectra of hot silica showed that OH groups persist on the surface at 800". Exchange of OH groups with Dz was slow on dry silica even a t 700", but D 2 0 exchanged rapidly at 100" or lower. All OH groups left after h y i n g at 600" were on the surface. Chemisorption of NH3 occurred slowly on samples predried at 800", producing bands (3526 and 3446 cm-l) assigned to NH2 groups. Maximum chemisorption was 1.3 to 1.4 moleoules/100 A2. No HC1 was chemisorbed, and it exchanged H only very slowly with surface OD groups. Both C12 and CC1, replaced surface OH with C1, which could in turn be readily replaced with NHz groups. The NHz groups could again be replaced by C1 by treatment with HC1. Reaction with Sic& and subsequent hydrolysis provided evidence that surface OH groups are relatively immobile on "dry" silica, even at 800", and are often paired. Steric factors are probably often important in reactions involving such groups.
Introduction Because their surface properties are both practically and theoretically important, silica gels and other higharea silicas have been intensively studied in many Silica surfaces are usually covered
with a layer of OH groups and adsorbed HzO. Heating a t elevated temperatures ~ m - " e S most of this layer, (1) R. K. Iler, "The Colloid Chemistry of Silica and Silicates," cornell University press,Ithaca, N. y., 1955. (2) J. A. Hockey, Chem. Ind. (London), 57 (1965).
Volume 70,Number 9 September 1966
J. B. PERI
2938
but some OH groups are retained very tenaciously.3 Substitution of other groups for the OH groups is often sought to modify adsorptive and other properties of silicas. Much of our present knowledge of their surface structure has come from infrared studies of high-area sili~as.4-'~ Because typical silica gels scatter much radiation, special forms of high-area silica such as Aerosil (Cabosil) and microporous glass have been used in most studies to date. Neither of these materials represents typical silica gel. llicroporous glass is, moreover, very impure (96Yc silica), containing boria, alumina, and other impurities. Only a few studies of more typical silica gels prepared by "met" methods have been r e p ~ r t e d . ~ , " , To ' ~ minimize scattering losses, these silica gels have been studied as selfsupporting pressed disks. The high pressures used in disk format ion can apparently significantly alter the behavior of the silica, ho~vever.5'1~ Considering the variety of materials studied, the major charwteristics of their infrared spectra are remarkably similar. A sharp band at 3750 em-' is universally assigned t o "isolated" (ie., not H-bonded) OH groups. A tail, or close-lying band, in the 36003750-em-' region appears to be caused by weakly Hbonded OH groups. Absorption centered in the 3400. 3500-cm-l region is attributed to strongly H-bonded OH groups and/or adsorbed HzO. Because the bands in this region are generally associated with a band near 1630 cnl-l (presuniably caused by H20 deformation). Attribu,, assignment to H,O is usuallv favored. tion of a band at 3500 em-' to geminal OH groups (ie., two groups attached to the same silicon atom) has been suggested, however.?!
HzO can be strongly held even after the silica has been dried under vacuum a t 350" or higher.lg Prevailing opinion holds that most of the OH groups in silica gel are on the surface. Characteristically, however, a significant fraction of the OH content, even on silicas preciried at 600", is not removed by ,~~~~ reaction with reagents such as d i b ~ r a n eCHJIgI, and CH3Li,4 suggesting that the groups are held internally rather than on the surface. Absence of reaction could, however, reflect steric factors, possibly arising from the presence of geminal OH groups.2 Little is presently known about the mobility, attachment, or arrangement of OH groups on the surface. It has even been suggested5 that the OH groups left after the silica has been dried at very high temperatures may riot actually exist on the hot surface but may reform only at lower temperatures as traces of H20 are again chemisorbed. Replacement of OH groups on microporous glass by S H 2 groups15 and by fluoridela have been studied, but little infrared study of reactions of OH groups on pure silica has been reported to date. Some XH3 is slowly chemisorbed on silica, but chemisorption never exceeds 1.5 niolecules/100 A2 of surface.21 S o infrared study of such adsorption has been reported, but physical adsorption of SH3 has been
nation of O H stretching with Si-OH bending.14 A band near 870 cm-' has been assigned to deformation of surface OR group^.^ Bands at 1635, 1870, and 2000 cm-' (shoulder) in the spectrum of dry silicas appear to arise from combinations and,/or overtones of lattice +-ibrations,g.IO do weaker bands in the region from 2200 to 2950 em-' in the spectrum of microporous glass.10~'3~17 Spectra in the Ha0 deformation region indicate that after the has little Or no adsorbed H20 been dried above 250°43'0 and that most HzO can be eliminated at considerably lower temperatures. Evidence from reaction of silica gels with A1Ci3 and BC13 has, however. been interpreted to mean that molecular
(12) G. J. Young, J . Colloid SC~., 13, 67 (1958). (13) V. A. Nikitin, 4 . N. Sidorov, and A. Y. Karyakin, Zh. F i z . K h i m . , 30, 117 (1956). (14) J. H. Anderson and K. A. Wickersheini, Surface Sci., 2 , 252 (1964). (15) 31. Folman, Trans. Faraday SOC.,57, 2000 (1961). (16) F. H . Hambleton, J. A. Hockey, and J A. G. Taylor, iyature, 208, 138 (1965). (17) N. Sheppard and D. J. C. Yates, Proc. Rog. Soc. (London), A238, 69 (1956). (18) T. H. Elmer, I. D. Chapman, and 11.E. Nordberg, J . P h m . C h e m , 67, 2219 (1963). (19) H. P. Boehm, hl. Schneider, and F Arendt, 2. Anorg. Allgem. Chem., 320.43 (1963). (20) I. Shapiro and H. G. Weiss, J . Phys. Chem., 57, 2i9 (1953); H. G . w.Veiss, J. 4 . Knight, and I. Shapiro, J . Am. Chem. Soc., 81, 1823 (1959). (21) 17. w.Stober, Kozioid-Z., 145, 17 (1956).
u
The Journal of Physical Chemistry
(3) C. Naccache and B. Imelik, Bull. Soc. Chim. France, 553 (1961); C . Naccache, J. F. Itosettl, and B. Imellk, zbzd., 404 (1959). (4) J. J. Fripiat and J. Uytterhoeven, J . Phgs. (?hem., 66,800 (1962); J. J. Fripiat, M.C. Gastuche, and It. Brichard, %bid., 66, 805 (1962).
(5) R. ibid.* 62, 1168 (1958). (6) A. N. Sidorov, Dokl. Akad. S a u k SSSR, 95, 1235 (1954). (7) K. W. Cant and L. H. Little, Can. J . Chem., 42, 803 (1964). (8) h l . Folman and D. J. C. 1-ates, Proc. Roy. Soc. (London), A246, 32 (1958); J . Phys. Chem., 63, 179 (1959). hlcDonaidi
I N F R A R E D STUDY OF
OH AND NH2 GROUPSON DRYSILICA
2939
SURFACES
Prospects were seen for ohtainiug better spectral data (and avoiding possible prcssure effects) through preparation and study of pure silica aerogel iri the form of transparent plates. Such data are also needed to permit close comparison of the silica surface with those of silica-alumina and other catalysts made from it. Gravimetry and isotopir exchange of surfare OH groups with D, or D,O were used to supplemerit infrared study. Adsorption of NH3, ether, and HCI, and reactions of surface OH groups with C12, CCI,, and SiCI, were investigated to obtain additional information on the surface structure and on the reactivity of surface groups.
Experimental Section AIost of the equipment, techniques, and procedures have been described.*z~z3Various combinations of cells and spectrometers permitted infrared study from 1400 to 5000 cm-' on samples a t room temperature and from 2000 to 5ooo cm-I on hot samples at temperatures up to 900". Ethyl iodide and 1,2,4-trichIorcbenzene2' were used for calihratiori ( 1 2 prism) from 3900 to 4700 cm-'. Calcination in 02,evacuation, and other treatments of the samples were normally carried out while the cells were in place in the spectrnmeten. In one type of cell the sample was suspended from a quartz helix so that weight changes could he followed concurrently with spectral changes and surface area could he measured by Nz adsorption while the sample was in the cell. Such a cell (cell D) containing a silica aerogel plate is shown in Figure 1. Because preparation of strong, clear aerogel plates is an art, the method is given in detail. Typically, 50 ml of stock solution containing 40 vol yoethyl orthosilicate (Fisher Purified) in methanol (Baker and Adamson rewent) was mixed with 30 ml of concentrated HCI (Baker Analyzed reagent), filtered, and poured onto mercury io a crystallization dish 7.25 in. in diameter. The dish was then covered. Within 3 hr, a glass-clear sheet of gel was formed. After an additional 16 hr, the gel was broken into smaller pieces and transferred to a 400-ml bath of 54% aqueous methanol. The following day the bath was changed to 25% aqueous methanol, and later in the day to distilled HIO. During the following week, the H20 was changed four times. The gel plates were then aged for 4 hr in HzO at 100" in a closed autoclave. They were then transferred to 50% aqueous methanol, and after a few hours, to absolute methanol. After five successive changes of methanol (400 ml each) during the next 3 weeks, the plates were transferred to a 250-ml glass autoclave liner. Methanol was added to fill the liner completely, and the plates
ib)
(0)
Figure 1. Cell 1): (a) cell positioned in spectrometer (one side of furnace removed); (h) lower cell showing aerogel plate.
were autoclaved at 25&300" while methanol was vented slowly to maintain pressure between 1200 and 1500 psi until most of the methanol had been vented. After the pressure had dropped to 1 atm, the autcclave was evacuated for 2 hr while cooling. The autoclaving took roughly 7 hr. The final plates were relatively flat, transparent, and free from cracks. Their dimensions ranged up to 1 X 3 in. Thicknesses averaged hetweeri '/la and in. Surface areas, after calcination in O2 and evacuation at 600" for 2 hr, were 750-8.50 m2/g as measured by N2 adsorption. The plates did not lose transparency or develop cracks on heating under vacuum at temperatures up to 900" and lost surface area only slowly at 800" (-3%/hr). Silica gel, made by the same prccedure, except dried normally from the hydrogel (after aging a t 100") gave a surface area of 807 m2/g, showing that autoclave drying of the alcogel had little effect on surface area. Density was ahout 0.18 g/cc and calculated average pore diameter was 2.50 A. Analysis showed less than 0.020% impurities, including, J. B. Peri and R. B. Hannan, J . Phys. Chem.. €4,1526 (19Bo). (23) J. B. l'eri, Adea. IeLern. Cow,. Cotdwae. F. Pa&. 1, 1333 (22)
(1961). (24) N. Acquistn and E. (1952).
K. I'lyler, J . Res. Notl. Bur.
Std., 49, 13
J. B. PERI
2940
as oxides, AIg (0.0074%), AI (0.0060%), Cu (0.0043%), and Fe (0.0017%). The Sic14 (Matheson, 99.8% minimum) was purified and freed from HC1 by vacuum distillation immediately before use. The CCL (Mallinckrodt AR) was dried with P205 and degassed by repeated freezing and evacuation. The Clz (Matheson) was dried with P205. Methanol and ethyl ether (Baker Analyzed AR) were degassed by vacuum distillation, freezing, and evacuation. Sources and purification of HC1, NH3, D2, and D2O have been described.22 Plates used in infrared studies typically weighed 0.15 to 0.30 g, giving about 35 mg/cm2 in the infrared beam. I n typical experiments, a "virgin" aerogel plate was mounted in the cell and calcined in O2 (200 torr) at 600" for 1-2 hr or a t 800" for 1 hr. Final pressure after evacuation was always below torr. Some spectra were recorded while the silica was hot, but usually the sample was first cooled to room temperature. I n experiments involving treatment with SiCI,, the usual cells and procedures were used. Evolved HC1 and excess Sic14 were frozen out in a trap cooled with liquid X2,and HC1 was then determined by the pressure change on warming to -78.5".
Results Calcination and Drying. As removed from the autoclave, the silica aerogel was partially covered with methoxy groups in addition to OH groups and H2O. Calcination in 0 2 at 600" followed by brief evacuation removed all spectral evidence of the methoxy groups, leaving the bands characteristically reported for silica gel. When the silica was dried under vacuum at high temperatures, there was a rapid initial loss of OH groups after which OH groups were lost only very slowly. When the temperature was increased, additional rapid initial loss was again observed. Similar behavior has been reported for microporous g l a d 6 and for y-alumina aerogel.26 The rate of loss of OH groups at 800" in the "plateau" region appeared comparable to the rate of loss of surface area. Because of this behavior, prolonged evacuation seemed neither necessary nor desirable in predrying the plates. By reaction with Sic&, typical plates were found to hold 3.1 and 1.4 OH/100 A2 after evacuation for 2 hr a t 600" and 1 hr at 800", respectively. Figure 2 shows spectra (LiF prism, Perkin-Elmer 12C spectrometer) of an aerogel plate, first calcined in 0 2 a t 600", and then dried by evacuation a t 600, 700, and 800". To provide additional information, the silica was partly exchanged with DzO before drying The Journal of Physical Chemistry
00
l
l
,
i
1
I
I
I
I
I
I
0 - H STRETCH
05
3600
3800 00
4000
COMBINATION
-I 05 i-
2660 o)ii-L--ihkO 2740
7300
4500
4700
WAVENUMBER ( C M ' I
Figure 2. Spectral changes on dehydration. Sample dried at (a) 600°, (b) 700', and (c) 800'.
at 600" to produce a, small OD band. I n the regions shown, the bands left after drying at 600" were reduced to under half their integrated intensities by drying at 800". Bands at 1635, 1870, and 2000 cm-1 were not significantly affected by drying between 600 and 800" or by removal of all OH groups through reaction with Sic&. No bands were evident in the 2500-3000-cm-' range. Exchange of D2 or DzO with OH Groups. The OH groups and adsorbed HzO on silica aerogel exchanged H (or OH) with D20 rapidly and apparently completely at 6 100". The OH bands were replaced with corresponding OD bands at lower frequencies (-2760 cm-1 for isolated OD groups). The OH groups exchanged H far less readily with D2 gas than with D2O. On aerogel predried at 700", only very slow exchange occurred at 300". The rate of exchange increased moderateIy with increasing temperature, but complete exchange required several hours even at 700". Spectra recorded after almost complete conversion of OH groups to OD groups through exchange with Dz at 700" on an aerogel plate predried at 800" are illustrated in Figure 3 for the OH and OD stretching (25) M. J. Rand, J . EEectrochem. SOC.,109, 402 (1962). (26) J. B. Peri, J . Phgs. Chem., 69, 211 (1965).
INFRARED STUDYOF OH AND NHz GROUPSON DRYSILICASURFACES
294 1
0 - H STRETCH [ S A M E TEMPERATURES AS FOR 0-0 BANDS1
-
.- 0-0 COME BAND
05
L
IO
2 m u
1
1 , 3600
I
3400
I
I ,
1
1
3600
3
4000
1 . 5 t
m 2700
0'5k IO
cot=
, , , 2740
I 2780
i 2820
WAVENUMBER ( C M - ' )
Figure 4. Spectra of OH and OD bands for silica at various temperatures.
' 2660
1
1
,
,
2740
2820
I
!
,
2900
WAVENUMBER ( C M ' )
Figure 3. Spectra after exchange with DZ (silica predried at 800').
regions (LiF prism, Perkin-Elmer 12C spectrometer). The characteristics of the OD bands correspond closely to those of the OH bands shown in Figure 2. Weak P and R branches27appear to accompany both the isolated OH and OD stretching bands, but (as indicated in Figure 3) they are evident only when the bands are intense. The band at 3363 em-l (Figure 3), appearing only on deuterated silica, clearly has an origin similar to that of the band a t 4520 cm-' (which disappeared on Dzexchange). Changes in the Isolated OH Bands at High Temperature. Typical spectra obtained at 400, 600, and 800" after the silica had been predried at 800" are shown in Figure 4. The "hot" spectra are distinguished principally by shifting of the isolated OH band to lower frequencies, broadening of this band, and appearance of a small "hot band'' near 3580 cm-l at temperatures above 600". Between -30 and 800", the total shift was about 18 em-' for the OH band and 12-13 em-' for the OD band, being comparable to shifts observed for the OH bands on hot alumina.26 The integrated intensity of the OD band did not change significantly as the band broadened at high temperatures. The number of OH groups remained constant, and all the changes were reversible. Adsorption i f Ether and NH3 on Silica Aerogel. Exchange of OH groups with Dz or D20 does not prove
that the groups are surface groups, since Dz or DzO might penetrate the silica lzttice or internal OH groups might migrate to the surface. If, however, OH groups are affected at low temperatures by adsorbed molecules that cannot penetrate the silica lattice, the OH groups must be on the surface.6 Physical adsorption of NH3 or ethyl ether at room temperature and at pressures below 1 atm completely eliminated bands caused by the isolated and weakly H-bonded OH groups left after the silica had been dried at 600". A broad H-bonded OH band was produced at lower frequencies in both cases. Chemisorption of NH3. Chemisorption of NH3 did not occur readily on silica, and after brief contact nearly all adsorbed NH3 was usually removed by very brief evacuation at room temperature. Immersion of dry aerogel in liquid NH3 did not yield a.ny permanent addition or exchange of NH2 for OH groups after subsequent evacuation. Heating silica in gaseous NH3 up to 800" also failed to replace OH groups with NHz groups. On aerogel completely exchanged with Dz and predried at 800", however, NH3 was slowly chemisorbed a t room temperature on a limited number of sites. Typical spectra are shown in Figure 5. Adsorption initially produced a band at 3419 cm-l, apparently caused by the v 8 stretching vibration which appears at 3417 cm-l for NHI in CCl, solution.28 This band (27) G. Herzberg, "Molecular Spectra and Molecular Structure," Vol. 11, D. Van Nostrand Co., New York, N. Y.,1945. (28) C.G.Cannon, Spectrochim. Acta, 10, 341,425 (1958).
Volume 70,Number 9 September 1966
J. B. PERI
2942
0.0
I
I
I
I
I
l
l
/
l
/
l
IO!
-I
-
i i -NH2 15
:
3300
L
l
3400
I
l
3500
I
1
3600
W A V E N U M B E R ICM
'1
1
1
3700
'hi,
3800
3900
Figure 5. Spectra of NHI adsorbed on dry silica: (a) silica predried a t 800' after deuteration; (b) after adsorption of 1 molecule of NHs/lOOO A 2 ; (c) after exposure to 3 torr of XHa for 0.5 hr plus brief evacuation; (d) after exposure to 5 torr of NHJ for 0.5 hr; (e) after redrying a t 800' and exposure to 10 torr of NH3 for 8 days.
changed slowly with time to give two bands at 3526 and 3446 cm-l. The isolated OH band became more intense. The bands a t 3526 and 3446 em-' are evidently caused by XHZ groups.lS (As was found later, the KHz deformation band occurs near 1555 cm-l.) After the aerogel had been exposed to NH3 a t higher pressures and for a longer time, these bands were more intense, but the maximum chemisorption of NH3 after 8 days at 100 torr was only 1.3 to 1.4 molecules,/100 A2. Chemisorbed NH3 was not readily desorbed at 200" but was mostly removed by heating to 600". Traces remained, however, even after evacuation at 800" for 30 min. These experiments resulted in almost complete exchange of the OD groups initially present. Such exchange precluded clearly establishing that one new OH group is formed for each NH3 molecule chemisorbed, although this is probably the case. Reactions with HCl(g), Clz, and CCC. Pure, dry HC1 was not detectably chemisorbed on dry deuterated silica (
