Feb., 1959
INFRARED STUDIESOF PHYSICALLY ADSORBEDMOLECULES
183
INFRARED STUDIES OF PHYSICALLY ADSORBED POLAR MOLECULES ASD OF THE SURFACE OF A SILICA ADSORBENT CONTAINING HYDROXYL GROUPS BY M. FOLWAN AND D. J. C. YATES] Ernest Opperiheiiiier Laboratory, Department of Colloid Science, Universilg of Cambridge, England Received J u n e 20, 1967
Further quantit,ative results of infrared studies of physically adsorbed and hydrogen bonded adsorbate molecules are presented in this paper. The shift in frequwcy ( A v ) of the OH absorption band due to this hydrogen bonding has been measured over a range of temperatures for ammonia and acetone. AV decreases with increasing temperature. Values are of the absorption bands of the perturbed OH groups; given for the half width increases ivith increasing temperature of adsorption. The peak optical density of the perturbed OH hand increases in a linear fashion with coverage. On desorption hysteresis in these optical density values occurs, this being related to changes in the distribution of adsorbed molecules between different sites of adsorption. Optical densities have been calculated for some of the CH and NH vibrations of the adsorbed molecules. As the coverage is changed the peak optical density of these bands does not vary linearly. This is due to variation in the apparent extinction coefficient (E,) with coverage. With ammonia the E , values decrease with increasing coverage, but for methyl chloride and acetone an increase in E, takes place. A possihle interpretation of these varintions is suggested.
Introduction In previous work on the hydrogen bonds formed between the OH groups of porous glass and ammonia, acet,one, methyl chloride and sulfur dioxide it has been shown2 that these hydrogen bonds are related to the anomalous effects found in the study of adsorption expansion.3 Further quantitative spectral data from new as well as from existing results on these systems now have been obtained; in particular optical densities both of the perturbed surfxe OH groups and of the N-H and C-H vibrations of the adsorbed molecules have been calculnted from n wide range of spectra and related to the changes in concentration of the adsorbate. As the concentration of the adsorbed molecules is increased, the peak optical density of the adsorption band due to the perturbed OH groups increases linearly and, thus Lambert-Beer’s law is obeyed. Hysteresis in these optical density values occurs on desorption; this is related to the fact that two sites of adsorption exist on the surface of porous glass.2 Where possible the effects due to varying the temperature of adsorption were studied and it was found that the shift Av = 3730 -- V O H (cm.-I) of the perturbed OH band was temperature dependent ; Av decreasing with increasing temperature. In contrast, the half-width of the perturbed OH band VI/^, increases with the increasing temperature. Within the experimental error, no change in Av or of v1l2 was found on changing the coverage. The N-H stretching frequencies of the adsorbed ammonia and the C-H stretching frequencies of the adsorbed acetone and methyl chloride have been measured and compared with lc110n.n values in the gaseous and liquid states. As the amounts of gas adsorbed were measured simultaneously with the spectra, it has been possible to measure the apparent extinction coefficients of some of these stretching frequencies. It has been found that they are dependent on the coverage. Experimental Materials and Apparatus.-As an adsorbent porous silica “Vycor” glass was used, the surface area of the 0.4 mm. (1) School of Mines. C:oluinhia University, New York. (2) M. Folmnri a n d D. J . C . Yntes. Proe. ROU.S o c . ( L o n d o n ) A246,
32 (1958).
thickness porous glass plate was found to he 238 m.Z/g. by the B.E.T. method using argon as the adsorbate, with a . have not been monolayer value Y, of (30.5 ~ m . ~ / gData obtained with the other gases as the B.E.T. method could not be used easily a t temperatures a t which spectra were taken. Approximate urn values have been calculated from cross-sectional areas of the gases obtained using liquid densities. The values in cm.”/g. are: ammonia G9, acetone 33 and methyl chloride 47. The pretreatment of the sample and purification of the adsorbates have heen given previously,2 together with the description of the adsorption system and cell for the spectroscopic measurements. For recording the spectra a Perltin-Elmer model 21 spectrometer was used; as this model has 110 readily accessible main focus, a mirror system was used to provide one. To compensate for losses in the mirrors and cell the slits were widened slightly from those used in normal operation. With the sodium chloride prism used for the ammonia experiments at 3300 cm.-’ the calculated slit width was 25 cm.-l and for the calcium fluoride prism in the other experiments it was 11 cm.-’ at. 3000 cm.-l.
Results The spectrum of the evacuated glass sample has been given and discussed previously.2 From the spectra of the adsorbed gases, the apparent optical densities both of the perturbed OH groups and of the adsorbed molecules themselves were calculated. The densities (d) were obtained from d = log (T0/T)v where T o and T are apparent intensities of the incident and transmitted radiation a t the frequency v. Values of d have been calculated for every gas, for each dose of gas, a t all the temperatures used. The spectra were measured a t intervals about 50 cm.-’ apart, but near the peaks this spacing was considerddy reduced; for each spectrum a t least 30 points were cnlculated. The points are not plotted on the curves because of the overlapping which would have occurred. The figures show only selected examples, in general a t least 10 doses of gas were adsorbed in a given experiment. Even a t the highest pressures used the path length (approx. 1.3 cm.) of the cell used is such that no corrections were needed for any spectral effects due to the unadsorbed gas phase molecules. Figure 1 shows as examples the apparent optical density plots for ammonia adsorbed a t 20 and 150’. The lowest coverage shown is that which produced (3) PI. Folnian and D. ,J. C , Y:ite,q, Tm?in. Pnrodn!, S o c . , 54: 429 (1958).
31. FOLMAX A N D D. J. C. YATES
184 1.2
L
2600
3000 3400 2800 3200 3600 Frequency, cm.-1 Fig. 1.-Optical density bands of the perturhed OH surface groups, and the N H arid CH stretching vibrations, for ammoiiia adsorbed a t 20 and 150' and for acetone and methyl chloride a t 20". The numbers on the curves give the volumes of gas adsorbed in cm.S/g.
1.0
0.8 0.G
0.4
s, .%
2
0.2
acetone is due to the perturbed OH groups, and the weak bands near 3020 and 2930 cm.-' are due to the CH stretching frequencies of the adsorbed molecules. With methyl chloride the OH perturbed band is much weaker and is centered a t 3620 cm.-l. The absorption due to CH stretching frequencies a t about 2070 and 2865 cm.-l is also shown. From all the data, values of the apparent peak optical densities both of the OH groups and the adsorbed molecules have been collected, both on adsorption and on desorption, and are shown in Figs. 2 and 3, as a function of volume of gas adsorbed. In the case of the N-H bands, the corrected nlynrent peak optical density is plotted, obtained by subtracting from the observed peak value the contribution due to the OH band. This contribut ion \vas found by assuming that the OH band was symmetrical about its peak frequency. For the OH groups for each of the molecules adsorbed the points lie on straight lines which do not, in general, intersect the abscissa a t zero coverage. The slopes of the lines on desorption are different from those 011 adsorption; these phenomena will be discussed later. The apparent peak optical density plots for the 3370 cm.-l N H band, in contrast, (Fig. 3) are not linear but do Pass through the origin: as with the OH band the desorptio; curve sh&smarked hysteresis. For methyl chloride (Fig. 2 ) , however, the plots are very nearly linear and do pass through the origin; no hysteresis \\'as observed. Table I gives the absorption frequencies of the adsorbed molecules themselves and comparison frequencies in the gaseous and liquid states. TABLE I COMPARISON OF FREQUENCIES (IN cni.-l) IN THE ADSORBED, GASEOUS A N D LIQUIDSTATES
O.G
G
-G- 0.4
0.2
9-
?
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R,Iolecule
0
NHa
1 .o
08 (CHs)iCO
u.0
0.4
0.2
CHiCl
0
Frequencies of ad 8 orb e d molecules Tepp., (accuracy C. f 10 c m - 1 ) 20 3385 3283 3305 327.5 7.5 3365 3276 100 3300 3275 150 3340 3270 200 20 3020 2935 75 135 20
3015 3015 2470
2930 2930 2865
Frequencies of the cormsponding vibration in t h R liquid or gas phase' 3413(gC) 3336.7 (sH) ,
.
,.
,. ,
.
.. ..
.. ..
3005(1) 2423(1) (accuracy + 5 ctn.-1)
,.
,.
..
2906.2(gH) 2878.8 (gH)
The liquid values were measured using the CaF2 prism, the gas values were taken from Hereberg4 (gH) and Cumining5 (gC). a
2
4
6 V . cm.3/e.
8
10
. V
Fig. 2.-Peak optical densities of the band due t o the perturbed OH groups as a function of the volume of ammonia and methyl cliloritle adsorbed. The dashed lines and vertical bars indicate regioiis of lower accuracy due t o high optical densities. Points obtained by adsorption 0, by desorption S.
the 1oi.r-estmeasurable optical density curves, and the highest is the one for which the peak optical density did not exceed about 1.2. The wide bands are due to the perturbed OH groups and the narrow bands at about 8370 and 3280 cm.-' are due to the N-H stretching frequencies of theadsorbed ammonia molecules. Figure 1 also shows the optical density plots obtained 11ith acetone and methyl chloride adsorbed n,t 20'. The wide band in the c a ~ eof
The apparent extinction coefficients (A'&) of the N-H and the C-H bands have been calculated from E a = l / n log (To/T)vlnaxwhere To and T have been defined previously and n is the number of adsorbed molecules per cm.2 When 1 cme3/g. of gas was adsorbed = 1.3 x 10l8 molecules/ As it is difficult to get good optical density data, particularly a t high cor'erages, the Ea values have been calculated from the smoothed c u n w shown in Figs. 2 and 3, and are given in Fig. 4. Ko E a values are given for acetone as the highest appar(4) G. Hersberg, "Infrared and Raman Spectra," D. Van Nostrand Co. Inc., New York, N. Y.,1945. ( 5 ) C. Ouriming, Can. J. P h y s . , 33, ti85 (1955),
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1SFRARED STUDIES OF PHYSICALLY ADSORBED MOLECULES
elit' optical density value (at 20') obtained is about (100 and 150') 0.21. At this amount adsorbed (12.0 ~ m . ~ / g . ) I f o r the 3020 cm.-' band Ea = 1.4 X lo-*'' cm.z/ 0.6 inc~lcculeand for the 2935 cni.-' baiid 15, = 0.8 X 10 --?O. Both these Eavalues decrease with decreasing 0.4 coverage and are about 35y0 less a t 4.4 cm3/g. :tclsorbed. Liquid acetone, measured a t 20' under 0.2 the same spectroscopic conditions as those used $ 0 foi- the experimen ts with adsorbed acetone, gave '3 e 0.4 an Ea value of 2.6 X a t 3005 c:m.-'. The 3 curves are thus similar in nature to thost: found with d 0.2 methyl chloride. 0
185
3
4
6
8
10
I
I
I
I
I
NH
42 .d
.d
Discussion The Properties of the Evacuated Adsorbent.-
$ 0 0.8
A f u l l discussion of the surface conditions of the
0.4
adsorbent as obtained from the infrared spectrum is given elsewhere.2 The absorption band due to the surface OH group is very strong even after 6 hours evacuation a t 450". This indicates that these O H grolups are cheniically bound to the surface; under such conditions of evacuation all water molecules v.ould have been removed. This OH band has a much larger half-width than the usual absorption bands, and is also asymmetric. This indicates that it probably consists of two overlapping bands. One is probably a narrow band a t about 3740 cm.-l due to isolated OH groups. The other band is much broader and probably is caused by hydrogen bonding between adjacent OH groups. This idea agrees with the work of Sidoromvsand McDonald' who used adsorbents which had a much weaker and narrower OH band centered a t 3749 cm.-'. The Absorption Bands of the Perturbed OH Groups.--It has been shown that some of the adsorbed molecules are attached to the mrface OH groups giving hydrogen bonding.2 The optical densitJycurves given in Fig. 1 show the effects obtained b y varying both the coverage and temperature of adsorption; it may be seen that the bands due to the perturbed OH groups are much more intense than normal absorption bands in this spectral i*egion. This property is a general one for hydrogen bonded systems, both in the gaseous, liquid and solid phases. 'Ilihen the temperature is increased, a t nearly the same amount of adsorbed gas, it will be seen in the case of ammonia and acetone that the intensity of the OH band is decreased. The phenomena is sho\vn more clearly in Fig. 2 where the peak opticnl dtnsity of the OH bands :we plotted as a fuiiction of coverage. The slope of the straight lines decreases v.ith increasing temperature. The existence of these straight lines shows that over this region Lambert-Beer's law is obeyed. Howelver, as these lines do not pass through the origin it follon.s that the first amounts of gas are not adsorbed on the OH groups (see earlier n.ork on the length changes of porous glass due to adsorption3). It has been s h o \ ~ previously ~ n ~ ~ ~ that a t least two different sites of adsorption exist on this glnss, one being the OH groups and the other the si!ica or oxygen at0m.s of the surface. Thus for a gi1.w temperatmureof adsorption the simplest interpret;%tion of the linear optical density plots is that the
0.2 0
(0) A. N. Sidorov, J . P h ! ~ sChem. . ( M o s c o z u ) , SO, 995 (195G). (7) R. S. McDonald, J . Am. Chenz. Soc., 79, 850 (1057).
0
1
2
3
0
1
2
3
4
5
0
7
V , cm.a/g. Fig. 3.--Peak optical densities of the 33iO cm.-' NH stretching vibr:ttioiis of adsorbed ammonia as a fuiict ion of amount, adsorbed. Dashed lines and vertical b:m indicate regions of loner accuracy. Adsoi,ptioii points 0, desorptioii X., CH,CI
5 4
u /
L1 2 6 10 14 hlolecules/cm.* x 10-18. Fig, 4.-Apparent e~tinction coefficients Enin the 3370 cm. -1 band of adsorl3ed rtmnioiiia arid the 2970 cm. - 1 Gnnd of adsorbed methvl chloride as a tiuictioii of niiml)er of adsorlied molecules per cm of optical path Tlie upper curve8 for ammonia a t 100, 150 aiid 200' :we ol~t:iinedf y o m desorption curves.
ratio of the number of adsorbed molecules on the two sites is constant. There may be two causes for the decrease in gradient of these lines with increasing temperature. From desorption data it is evident that the energy of adsorption on the OH sites is less than that on the other. It folloc~~s that a t higher temperatures there will be relatively fewer molecules adsorbed on to the OH sites.
M. FOLMAN AND D. J. C. YATES
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probably is due to experimental difficulties in working with liquid systems over a wide range of tem.peratures. No other data are available for temperature variations in adsorbed systems. As the hydrogen bond is weak it is to be expected that Av should be temperature dependent. Calculated valuesgbof d / d t (Av)lie within the limits 0.2 to 0.G4. Experimental valuesgbfor pure liquids are between 0.3 and 0.5, and for isopropyl alcohol in ether 0.75. We found values of 1.35 for aninionia and 0.54 for acetone. Little seems to be known about the variation in half width of the perturbed OH band in hydrogen bonded systems. However, it has been reported that on increasing the temperature of pure liquids a slight increase in the half width occurs, together with the previously mentioned decrease in intensity. As even greater intensity changes have been found for other hydrogen bonded syst e m it ~~ is to be expected that larger changes will occur in the half width. This is the case with adsorbed ammonia (Fig. 5 ) but the results for acetone on the same figure are rather inconclusive. The Spectra of the Adsorbed Molecules.In Fig. 3 it is shown that the peak optical densities of the N-H stretching bands a t 3370 cm.-' do not vary in a linear fashion with coverage in contrast with the OH intensities. As the coverage increases the gradient of the curve decreases, and in some cases becomes nearly zero. This effect is most marked a t 20°, but is present a t all temperaturep other than 200". On desorption the points lie on a curve which is above the adsorption curve. It must be remembered that the intensity of the N-H band is obtained by subtraction of the OH band on the assumption that the latter band is symmetrical. As the errors in this assumption are larger when the OH band is more intense, it is possible t h t the flattening is due t o this effect. However, the possible errors involved decrease very markedly on desorption (due to the preferential desorption from the OH sites) and if the errors a t high coverages are large it is to be expected that the desorption curve would join the adsorption curve a t the point where the subtraction errors are not significant. Since the desorption curves do not show this behavior it is considered that both the flattening on adsorption and the hysteresis are real and not artifacts. It follows that the apparent extinction coefficient (Ea)changes with the amount adsorbed (Fig. 4). The ammonia molecules adsorbed on a sparsely populated surface have high coefficients; the average value of this becomes lower as more gas is adsorbed. It needs to be emphasized here that our coefficients are 'lapparent" ones, that is, no attempt has been made t o correct for the finite slit width of the prism spectrometer.11 In any case it is justifiable to use apparent Ea values for comparison purposes. Data do not seem to be available for the extinction coefficient of liquid ammonia, to enable any relevant comparisons to be made. As well as the errors involved in the use of finite slit widths,ll the further difficulty exists in adsorbed systems of determining the exact number of molecules in the beam; it is obvious that this factor can be estimated with high (11)
D. A. Rfimsay, J . A n . Chem. Soc., 74, 72 (1952).
Feb., 1959
I N F R . 4 R E D STUDIES O F P H Y S I C A L L Y
accuracy in liquid systems. The values used are averages over the whole sample obtained from pressure measurements, the portion of the sample in the beam may be slightly hotter as previously meiitioned. If this is the case, there mn,y be a somewhat smaller concentration of niolocules in the beam than that estimated. If significant, this effect lowers the E , values relative to the true value; the spectrometer errors12also art in the same direction. Nevertheless, there is no doubt that, I;heextinetioil coefficients decrease with increasing coverage of ammonia. I n the niost general sense it is likely that this effect is due to the heterogeneity of the surface; a t low coverages sites of higher energy of adsorption are active. Cooperative effects between the adsorbed niolecules themselves ase not likely to be present as the largest coverage used was about, 0.25. C a l c ~ l a t i o i i sand ~ ~ experimental results" show that very strong asymmetric dectric fields exist a t ionic surfaces. Consequently the surface field polarizes the adsorbed molecules to quite n large extent, inducing dipoles in the niolecule. For any patticulnr vibration, the intensity of nbsorptioii of rndiation is proportional t o (dg/dq) where dp/dq is the change in dipole moment with respect to the coordinate describing the vibration. Thus dependiiig on the orientations of the vibration with respect to the dipole induced by the surface forces, it c m be seen t'hat (dg/dy)2 can either increase or decrease relative to the free state of the molecule considered. On desorption, moreover, it will be seen that E , for ammonia increnses. As t'he results on the OH bands show, the niolecules at very low coverages are adsorbed only on the sites other than the OH sites. For higher coverages, adsorptioii occurs also 011 the OH sites, and from this point on, the relative numbers of molecules adsorbed on the two different types of sites seem to remain constant (at constant temperature). The desorption experiments show from the peak optical densit#yof the OH bands that t'lie energy of adsorption is less 011 the OH sites, and thus these sites are preferentially depopulated. Consequently the distribiition ratio of adsorbed (12) R. N. Joues and C. SandorEy i n "Cliemirai .$l)i>Iications of Spectroscopy," I n t e t x i e n c e Publihlrers Inc., h-etv York, N. Y..1056, 1). 247. (13) J. H. D e Boer ".4d\,ances in Colloid Science,' 3 , 1 (1050). (14) N. Slieppnrd m d D. .J. C. Y a k s , P m . R o ! i . SOC. ( L o n d o n ) ,
A238, fi0 (195O).
ADSORBEDM O L E C U L E S
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molecules changes very rapidly indeed, and as the desorption proceeds a larger percentage of the molecules are adsorbed 011 the most active sitesthe oxygen or silicon atoms of the surface. The adsorption data a t very low covemges show that the E, value of ammonia is high in this region, thus the change in the E , values on desorption agrees with, mid supplements, tlie datJa ohtnined from the behavior of tlie OH band. The only other comparable data for adsorption systems are those reported by De BoerI5 who studied the absorption of visible light by iodine on calcium fluoride. The extinction coefficient of the molecules adsorbed 011 the most acti1.e sites, a t low co1-erages,was much higher (per molecule) than those a t higher coverages. For methyl chloride, in contrnst to ammonia, tlie curve of optical density of the C-H baiids against coverage is sliglitly coni'ex with respect t'o t'lie abscissa, E , thus increases with coIrerage (Fig. 4). Consideration of the atoms by which the adsorbates become hydrogen bonded t o the surface may explain the varied changes in E,. I n the case of aminonia, the molecules are most proba,bly hydrogen bonded via the nitrogen a t 0 n 1 ~ ~ thus ~ ; large changes can easily occur in all the K-H rvibra t'Ions. The other molecules, in contrast, are hydrogen honded b y groups other than tjhe C-H groups, the chlorine in methyl chloride niid the CO groups in acetone. Only small changes in tlie E , values of the C-H groups are tJliusto he expected as the C-H vibrations are not directly affected b y the adsorption process; it is also possible that the change of sign of variation of E , with coverage is related to this fact. Acknowledgments We are indebted to Professor ,J. H. Schulman, O.B.E., for his interest in the early stages of this work, and t o Dr. K.Sheppnrd for valuable discussions and provision of spectroscopic facilities. Helpful commeiits on the manuscript were made b y Dr. D. M. Sinipson (Mrs. J. N. Agar). The design and construction of the mirror syst,em were due t'o Mr. I,. H. Little. The porous glass used in this work was the gift of Dr. 34. E. Nordberg of the Corning Glass Works, New York. We also wish to thank the Council of the British Cernniic Reeearch Associntion for personal grants. (15) J. H. De Boer "Electron Eniission mid Adsorption Phenomena," Cninbridge University Press London. 1935, 1). 189.
