GARYL. HALLER AND RICHARD W.
4386
RICE
A Study of Adsorption on Single Crystals by Internal Reflectance Spectroscopy by Gary L. Haller* and Richard W. Rice Department of Engineering and Applied Science, Yale University, Nmu Haven, Connecticut 06680 (Received July 10, 1070)
The usefulness of internal reflection spectroscopy in studies of adsorption on surfaces of known structure is demonstrated for the case of alcohol adsorption on single-crystal a-aluminum oxide. The molecular orientation of a Langmuir-Blodgett deposited calcium stearate monolayer and n-amyl alcohol at submonolayer coverages on a (4160) surface was determined qualitatively. Adsorption isotherms have been measured a t several temperatures between 30 and 120’ on three faces of a-aluminum oxide. The resulting isosteric heats of adsorption are 13, 19, and 26 kcal/mol a t 0 = 0.4 on the (OOOl), (1183), and (4150) faces, respectively. An adsorption site involving a coordinatively unsaturated pair of Ala+ ions and an Oa- ion is postulated. A good correlation between the calculated number of such sites and the saturation coverage a t 120’ and between the extent of coordinative unsaturation of the pair site and heat of adsorption on the three surfaces is obtained.
Introduction There now exists a very extensive literature on the infrared spectra of molecules adsorbed on surfaces. Most of this work involves high-area amorphous oxides or metals supported on such oxides. The high area is required in order to obtain detectable infrared absorption in the transmission spectroscopy used. By utilizing internal reflection spectroscopy (irs), it is possible to work with very small surface areas and maintain a detectable signal by requiring the infrared beam to be multiply reflected through the surface. In general, metal oxides are transparent in the infrared and many which are of catalytic interest can be obtained as large single crystals. When these crystals are used as internal reflection prisms, it becomes possible to obtain infrared spectra of molecules adsorbed on surfaces of known structure. Moreover, the electric field amplitudes at the internally reflecting interface are different in the three spatial directions and are a function of beam polarization and angle of refle~tion.~ Thus, it is possible to determine the average orientation of the adsorbed molecule by measuring the absorption as a function of polarization and angle of reflection. The simultaneous measurement of adsorptive capacity, heat of adsorption, and molecular orientation as a function of the structure of the adsorbing surface should allow deductions concerning the stereochemistry of the adsorption sites to be made. The utility of infrared irs for the study of adsorption on single crystals will be demonstrated for the case of alcohol adsorption on a-aluminum oxide. Aluminas of various structures are often used as catalyst supports and/or catalysts. High-purity a-aluminum oxide I -
-
-
are readily available. These considerations and the LEED4 investigations of surface structure on mechanThe Journal of Physical Chemistry, Val. 74, No. 86, I070
ically polished and chemically or thermally etched aaluminum oxide dictated the selection of this oxide for the initial work. Adsorption isotherms and isosteric heats of adsorption of n-amyl alcohol have been obtained on the (OOOl), (1123), and (41E0)6faces of a-aluminum oxide. These faces were chosen on the basis of their distinguishing crystallographic features. They are, respectively, the hexagonal close-packed oxide plane (slip plane), the plane normal to the growth direction of the crystal, and a plane parallel to the c axis (normal to slip plane). The molecular orientation of a Langmuir-Blodgett deposited calcium stearate monolayer and n-amyl alcohol at submonolayer coverages on the (4150) crystal has been investigated both before and after chemical etching of the surface. The deposited stearate monolayer is used as a model system for the orientation work. The orientation of deposited monolayers has been measured by other techniques6 and these constitute an in-
* To whom correspondence should be addressed. (1) (a) M. L. Hair, “Infrared Spectroscopy in Surface Chemistry,” Marcel Dekker, New York, N. Y., 1967; (b) L. H. Little, “Infrared Spectra of Adsorbed Species,” Academic Press, New York, N. Y., 1966; (c) R . P. Eischens and W. A. Pliskin, Advan. Catal., 10, 1 (1958). (2) N. J. Harrick, “Internal Reflection Spectroscopy,” Interscience, New York, N. Y., 1967. (3) Reference 2, p 51. (4) (a) J. M. Charig, Appl. Phys. Lett., 10, 139 (1967); (b) C. C. Chang, J . Appl. Phys., 39, 5570 (1968); (c) J. iM. Charig and D. K. Skinner, “Proceedings of the Conference on the Structure and Chemistry of Solid Surfaces,’’ G. A. Somorjai, Ed., Wiley, New York, N. Y., 1969, p 34; (d) C. C. Chang, “Proceedings of the Conference on the Structure and Chemistry of Solid Surfaces,” G. A. Somorjai, Ed., Wiley, New York, N. Y., 1969, p 77; (e) T. M . French and G. A. Somorjai, J . Phys. Chent., 7 4 , 2489 (1970).
*
the structure of the reflecting surface within l / z o . No deterioration in surface polish was observed during the course of experiments except as noted below.
ADSORPTION ON SINGLE CRYSTALS
4387
dependent check on the theoretical electric field amplitude equations used in the irs work. Moreover, the observed alcohol absorbances can be converted to absolute surface coverages with a minimum number of assumptions by utilizing the measured absorbance of the deposited monolayer.
Experimental Section Apparatus. All spectra were recorded on a Beckman IR 12 a t a resolution of 5 cm-l or better. Polarization of the beam was accomplished using a Perkin-Elmer wire grid polarizer. The irs cell,’ manufactured by Wilks Scientific Corp., can be heated in excess of 600” and evacuated to below lo-’ Torr with the vacuum system used in this work. The cell consists of a stainless steel circular chamber with vacuum, gas inlet, and two window ports on the side. The crystal is clamped into a stainless steel block suspended from the lid which is sealed to the main body of the chamber with a gold “0”ring. Two heater wells with a thermocouple well between them are bored through the lid into the block from outside the chamber. When a crystal is clamped into position, it is in good thermal contact along its edges and enclosed except for the ends. This design reduced the temperature gradient along the crystal and between the crystal and thermocouple to less than 5” even at temperatures above 400”. The windows are in the form of flanged plugs machined out of KBr. When they are in position, they protrude into the chamber and nearly contact the crystal so that the contribution from gas-phase absorbance is negligible. The vacuum seal between the windows and chamber is made with a viton “0”ring. Reagents. Aluminum oxide single crystals were flame fusion grown from 99.98% A1203powder and were supplied, cut, and polished by the Adolf Rteller Co. Unless stated to the contrary, the crystals used are 52 X 20 X 0.5-mm prisms with the ends beveled to 45” to form entrance and exit aperatures; they provide about one hundred 45” internal reflections. The major faces6 were polishe{ with diamond dust to an average roughness of 100 A (measured by Meller). Fisher certified reagent alcohol was dried over Davison 4A molecular sieves and used without further purification. Stearic acid labeled 99% pure was used as recieved from Nutritional Biochemicals Corp. Distilled water used in monolayer preparation was double ion-exchanged, passed through an organic removal filter, and then distilled. All other chemicals used were o f laboratory reagent grade. Procedure. An outgassing at 500” under vacuum Torr for 8 hr was adopted as the stanbelow 1 X dard pretreatment. Alcohol vapor was admitted and/ or exhausted in increments and allowed to equilibrate for 15 min before the spectrum was recorded at X10 ordinate scale expansion. In experiments requiring a deposited monolayer, the
Frequency (cm-’)
Figure-1. Absorbance spectra for a calcium stearate monolayer on (4150) u-AlzOa: - - -, parallel polarized beam; -, perpendicularly polarized beam.
Blodgett method8 was used. The prisms were outgassed under the conditions given above, removed from the vacuum chamber, and immediately submerged in a prepared calcium carbonate buffer, pH 9. Stearic acid was spread above the crystal from a hexane solution, compressed with an oleic acid piston, and allowed to equilibrate about 10 min. The crystal was then drawn through the compressed calcium stearate monolayer mechanically at a rate of 1cm/min and immediately returned to the vacuum chamber.
Results and Discussion Deposited Monolayers. A monolayer was considered useful when the water-supported monolayer change in area during deposition was within 10% of the geometric area of the crystal. The spectrum of a typical calcium stearate monolayer deposited on the (4130) crystal is given in Figure 1. The perpendicular to parallel absorbance ratio, A L I A ,1, for the asymmetric methylene stretch (2918 cm-’) is 1.90 and, as discussed below, indicates that the stearate molecules are orientated nearly normal to the surface of the crystal. This is the expected orientation for deposited monolayers.6 An upper limit for the absorbance ratio can be estimated from the calculated electric field amplitudes at the interface but in the rarer medium assuming unit elec-
(3
(6) G. L. Gaines, Jr., “Insoluble Monolayers at Liquid-Gas Interfaces, Interscience, New York, N. Y.,1968; (b) J. W. Menter and D. Tabor, Proc. Roy. SOC. Ser. A , 204, 514 (1950); (0) L. H . Germer and K. H. Storks, J . Chem. Phys., 6, 280 (1938). (7) A. Yelon and G. L. Haller, “Growth, Structure, and Properties of Surface Oxides,” Final Report on Grant RSA-69-8, Connecticut Research Commission. (8) K. B. Blodgett, J . Amer. Chem. SOC.,57, 1007 (1935).
The Journal of Physical Chemistry, Vol. 74, N o . #6,1070
GARYL. HALLER AND RICHARD W. RICE
4388 tric field for the radiation propagating within the crystal.E1g,10Perpendicular polarization will set up an electric field with an amplitude 2 COS e E, = = 1.75 [I - n312]'/z
t
0
,400 '440
across the 20-mm width of the crystal. Parallel polarization will give rise to elliptical polarkation with amplitude components
along the 62-mm length and
E,
=
C COS 8 sin 8)n322 (1 - n812)'/2[(1 n31') sin2e
+
- n312]'/n= 0.96
normal to the crystal face. If the stearate molecule is normal to the surface, the C-H bonds of the methylene groups will be in a plane parallel to the crystal surface. The fact that the adsorbance ratio for the asymmetric stretch (2918 cm-l) and the symmetric stretch (2856 cm-l) are the same implies that the methylene groups are randomly oriented in this plane as has been found for built-up multilayers by electron diffraction.6b*c The absorbance is proportional to the square of the electric field amplitude so the predicted absorbance = ratio for perfect orientation is AL/dll = EU2/Ez2 2.13. The ratio of 1.90 is in good agreement with this prediction considering surface roughness on the molecular scale. The fact that the ratio is as high as is found is probably accounted for by the ability of compressed films to bridge over surface roughness of the crysta1s.l' Another limiting case is that obtained for completely random orientation.
This is in very good agreement for the absorbance ratio of 1.23 obtained for a thin film of stearic acid formed when the crystal is immersed in a dilute hexane solution and dried. Implicit in the above calculations is the assumption that the energy loss per reflection is small; i.e., the electric fields are not much perturbed by absorption at the interface. This assumption is justified and the reproducibility of our monolayers is demonstrated in Figure 2 by the good linear relationship between absorbance12and number of reflections. OH Absorbances. The background spectrum of the (4150) crystal is shown in Figure 3 and is typical of the single crystal aluminum oxide used in this study. All three crystals are opaque in the infrared below 2100 cm-l, are free of surface 0-H and C-H absorbances after outgassing at 500') and exhibit sharp bands due to 0-H in the bulk of the crystal. The numbers and intensity of these internal bands are properties of the individual crystals. The observed frequencies and The Journal of Physical Chemi8tTy,JVOl.74,No. 26, 19YO
Number of 45. Reflections ("Pathlength) Figure 2. Absorbance for calcium stearate monolayer as a function of number of reflections through the monolayer.
4000
3600
3200
2800
2400
2000
c rn-' Figure 3. Transmittance spectra for clean (41gO) cu-Al~Oa: upper curve, perpendicular polarization; lower curve, parallel polarization.
relative intensities for respective crystals are 3166, 3187, 3218, 3233, and 3280 cm-l (weak) for (OOOl), (9) Perpendicular and parallel polarization refer to the orientation of the electric field vector of the radiation propagating in the crystal relative to the plane defined by the incident and internally reflected beam. (10) 0 is the angle of incidence and is equal to 4 5 O . rial = ndni and ns2 = na/nz. The indices of refraction for aluminum oxide, ni, and gas phase, ns, are 1.70 and 1.00, respectively. The value of nz used was 1.5 for both monolayer [A. C. Hall, J . Phys. Chem., 74, 2742 (1970) ] and alcohol. For calculations concerning water or surface OH, nz = 1.3 (bulk water) was assumed. (11) J. J. Bikerman, Proc. Roy. SOC.Ser. A , 170, 130 (1939). (12) Except for spectra given in Figures 1, 3, 5, and 11, absorbances are those obtained with the partially polarized beam of the spectrometer measured a t the band maximum.
ADSORPTION ON SINGLE CRYSTALS
4389
t
.020 Q)
B5
n
a
L
I
I
I
I
I
I J
3800
3600
3400
3200
3000
2800
2400
c rn-'
Figure 4. Absorbance spectra for water on (0001) ~t-AlsOaat 30": (a) -, weakly adsorbed; (b) - - -, more strongly adsorbed.
3166, and 3280 cm-l (medium) for (11%3),and 3166, 3187, 3218, 3233, and 3313 cm-I (strong) for (4130). This internal 0-H impurity is probably associated with metal impurities and/or aluminum ion vacancies.13 None of the internal 0-H bands are affected by adsorption on the surface or the outgassing procedure, but presumably are deuterium exchangeable above 1000".l4 Figure 4 depicts the absorbance measured for surface OH. When the crystals are evacuated for a few minutes at room temperature, a considerable amount of physically adsorbed water is removed, Figure 4a. When the crystal is subsequently heated to 500" under a vacuum, a further decrease in absorbance below 3600 cm-' results, Figure 4b. Most of the desorption of this strongly bound OH occurs below 370". The physically adsorbed water is rapidly reversible but OH removed at high temperatures requires several hours at ambient water vapor pressure to return to its former state. Accordingly, the physically adsorbed water readily exchanges with DzO at room temperature while other adsorbed water (probably held as surface hydroxyls) exchanges only very slowly. The very large polarization effect indicated in Figure 3 is characteristic of all such absorptions in the three crystals and indicates that the 0-H dipole is in the closepacked oxygen plane (perpendicular to the c axis). This is particularly easy t o see in the case of the (4150) crystal. The geometry used will place the c axis normal to the plane of incidence of the reflected beam. Thus, parallel polarization (electric field normal to the c-axis) results in a very large absorbance while the absorbance for perpendicular polarization (electric field parallel t o c axis) is near zero. If the structure of the surface is represented by the structure of bulk below it (evidence that suggests that this is essentially the case is given below), the surface OH would be expected to be oriented parallel to the surface on the (0001) crystal, nearly parallel on the (1123) crystal, and about the normal t o the surface on the (4150)
crystal. Experimentally, all absorbance due to strongly bound surface OH is found to be consistent with a randomly oriented dipole. An explanation for this and the very broad bands observed (compare Figure 4 to Figure 3) can be given in terms of the mobility of the protons on the surface of the crystals. (Hydrogen bonding also causes band broadening, but it is difficult to imagine the surface OH more extensively hydrogen-bonded than the internal OH). The large band widths that result when boehmite (monohydrate of aluminum oxide) is heated have been attributed to proton delocalization. l5 At elevated temperatures discrete vibrational energy levels above a specified level disappear and the probability of psoton tunneling through the potential energy barrier approaches the absorption frequency. We observe a very similar behavior for internal OH in a-aluminum oxide. At temperatures around 400" these absorptions are very broad and slightly shifted to higher frequency (due to the thermal expansion of the crystal unit cell). If a proton is mobile a t 400" within the crystal lattice, it is reasonable to expect surface proton mobility at room temperature and, therefore, broad peaks which appear to be the absorbances of randomly oriented dipoles. I n contrast to the strongly bound surface OH, the physically adsorbed water does appear oriented. This does not imply an absence of mobility in this layer, but suggests molecular movement with a preferred orientation relative to the surface. If we assume that the molecular water moves randomly across the surface but hydrogen bonding always directs the hydrogens (or the oxygen) toward the surface, we can estimate the angle between the plane defined by the water molecule and the plane of the surface. For the (0001) crystal, this angle is about 11". Another indication that this physically adsorbed water is in close association with the alumina surface is the position of the absorbance maximum. The absorbance maximum of the physically adsorbed water is shifted 30-50 cm-l to higher frequency relative to bulk water at the same temperature. This result may be interpreted as a consequence of diminished intermolecular hydrogen bonding due to the unfavorable geometq imposed by interaction with the surface. One other kind of hydroxyl absorption, the isolated OH absorbing in the 3700-cm-' region observed on higharea y-alurnina,'*l6should be mentioned. These were not seen on the single crystals except in two instances. An absorption at 3750 cm-1 was recorded on the (4150) (13) (a) F. K. Volynets, V. G. Vorobev, and E. A. Sidorova, Z h . Prskl. Spektrosk., 10, 981 (1969); (b) R. F. Belt, J . Apppl. Phys., 38, 2688 (1967).
(14) R. Muller and Hs. H. Gllnthard, J. Chem. Phys., 44, 365 (1966). (15) J. J. Fripiat, H. Bosmans, and P . G. Rouxhet, J . Phys. Chem., 71, 1097 (1967). (16) J. B. Peri and R. B. Hannan, ibid., 64, 1526 (1960).
The Journal of Physical Chemisiry, Vol. 74, No. 26, 1070
GARYL. HALLER, AND RICHARD W. RICB
4390 crystal after repolishing and on the (1123) crystal after heating it above 1000" in hydrogen. This peak was broad on the (4190) crystal on which physically adsorbed water remained and was sharp (comparable to the internal OH) on the (1123) crystal which did not exhibit any other surface OH absorbance. The 3750cm-' band was removed from both crystals on heating to 500" under vacuum and did not reappear when the crystals were exposed to water vapor. Alcohol Adsorption. The spectrum in the C H stretching region of n-amyl alcohol adsorbed on the (4130) crystal a t 1.12 Torr pressure, Figure 5, suggests an ordering in the adsorbed layer. The absorbance ratio for asymmetric stretch of the CH2 group (2935 cm-l) is about 1.6, i.e., the average amyl alcohol molecule is probably more or less erect but not as near the normal to the surface as is calcium stearate. The absorbance ratio for adsorbed amyl alcohol was somewhat lower on the (0001) and (1123) crystals and the interpretation is complicated by depolarization within these crystals, but the absorbed alcohols are clearly ordered here too. More quantitative calculations concerning the average dipole orientation can be made if the absorbance ratio is measured at more than one angle of reflection (the optical system used in this work restricted measurements to 45"). Experimentally, it was noted that absorbance ratios for the asymmetric stretch of the terminal CHs group of straight-chain hydrocarbons are rather less sensitive than those for the CH2 group to the orientation of the molecule. Therefore, the ratio of the absorbance of the asymmetric CHa stretch for n-amyl alcohol (2965 cm-l) absorbed from the gas phase to that observed for the deposited stearate monolayer is taken as a measure of the fractional surface coverage 0. It should be pointed out that 8 defined in this manner is a fraction of the total surface covered and not the fractional coverage of surface sites capable of chemisorbing alcohol. e = 1 is equivalent t o one molecule/20 i i z 18 or 5 x 1014 molecules/cm2. Isotherms obtained on all three crystals at 30 and 120" are shown in Figures 6 and 7, respectively. Below e = 0.1 there always remains some alcohol irreversibly adsorbed which is removed by heating to higher temperatures. The calculated isosteric heats of adsorption at 0 = 0.4 are given in Table I. Isosteres at various coverages on the (0001) crystal are shown in Figure 8 and are representative of those obtained on the other crystals. %-Amyl alTable I Crystal orientation
Isosteric heat of adsorption, e = 0.4, koal/mol
(0001) (1123) (4180)
13 19 26
~
The Journal of Physical Chemistry, Vol. 74, N o . 26, 1970
0.000h
0.044
0.048 2910
2950
2990
2810
2830
Frequency (ern-')
Figure 5. AbEorbance spectra for 1.12 Torr of n-amyl alcohol on (4150) or-AlzOs a t 30": - - -, parallel polarization; -, perpendicular polarization.
,056
I(1123)
,048
(4150)
:
,032
(0001)
-A-:"&
n ,016
8
0
.2'
AI
".6
1 .8
Pressure of
I1.0
1.2
I 1.4
I 1.6
1 1.8
1 2.0
-0
n - Amyl Alcohol, (Torr)
Figure 6. Adsorption isotherms of n-amyl alcohol on three crystal faces of a-Al208 a t 30".
coho1 has been chosen for discussion because it gives reasonably intense CHa stretch absorbance (indicating orientation) and the CH3 group is sufficiently removed from the terminal oxygen so that perturbation corrections on the calculated coverages are unnecessary. l9 (17) It is not possible to find an orientation of a birefringent crystal such as aluminum oxide which will provide a unique index of refraction for the geometry used in this work. I n principle, the beam is double refracted and split into an ordinary and extraordinary ray. The refractive indices for the ordinary and extraordinary rays are not too different for aluminum oxide, being 1,7023and 1.6943,respectively, a t 24' and 3.303 p . Thus, beam divergence is much less a problem than is strain introduced in the polishing operation. Each crystal was checked for beam depolarization by placing a beam polarizer before the crystal entrance and a second po1aril;er rotated 90° after the crystal exit. Both the (0001) 2nd (1123) crystal depolarized about 30% of the beam but the (4150)crystal was free of depolarization effects. (18) Reference 8a, p 162. (19) H. V. Drushel, W. L. Senn, Jr., and J. S, Ellerbe, Spectrochim. Acta, 19, 1815 (1963).
ADSORPTION ON SINGLECRYSTALS
4391
.ole
i-
e (0001)
e
- 0.25 /
A
'
e
A
'
,005' L
(4150)
A .
(0001) A
,000
(1123)
.060
I
I
(IIH3) (4150)
"'7
e
.010
.ooo;
I
(2mm data)x4 A (Y2 rnm pre-anneal)
I
0.5
( V i rnm post-anneal)
I I I I.o 1.5 2.0 Pressure of n-Amyl Alcohol, (Torr)
I 2.5
Figure 9. Adsorption isotherms for (1183) cu-Al~Oaat 30" after various treatments.
0.5 0.0 0.5 1.0 1.5 1.0
L
-c 2.0 2.5 -
Y
3.01 3.5
\
\emo.4
4 4.5 1
2-5 2.6 2.7 2.0 2.9 30 3.1 3.2
3.3 3.4
T-lx l O ? ( O K )
Figure 8. Isosteres for (0001) iu-AlnOa.
If one corrects for perturbation of the CHa group and plots e as a function of reduced pressure, all of the straight-chain C1through Cg alcohols result in a common isotherm within experimental error. In addition) the isosteric heat of adsorption of n-butyl alcohol is the same as n-amyl alcohol on the (0001) crystal. Before attempting to discuss the adsorptive capacity and heats of adsorption on the three crystals in terms of their surface structure, we must consider the question of surface disorder. LEED studies of mechanically polished (0001) surfaces have been reported by several authors4and has also investigated the (1123) surface. The polished surfaces always give weak and diffuse diffraction before heating under vacuum. After heating to between 600 and 800" LEED patterns which have the symmetry of the parent planes are obtained; above 800" the (0001) surface rearranges. To assess the reproducibility of the polishing operation, we determined the n-amyl alcohol adsorption isotherm
on a 2-mm (11'23) crystal, repolished the crystal to 0.5 mm, and again measured the isotherm. The amount of alcohol adsorbed at low coverages was not much changed, but the coverage was about 10% lower on the 0.5-mm crystal at pressures above 0.1 Torr as is shown in Figure 9. Subsequently the 0.5-mm (11'23) crystal was annealed above 1000" in flowing hydrogen, exposed to air at room temperature, and outgassed under vacuum in the usual way. The isotherm of n-amyl alcohol before and after this treatment was the same within experimental error. Of course, this could imply that we have reproducibly contaminated surfaces since our vacuum never exceeds 1 X Torr, although the LEED work on the (0001) surface indicates a bulk structure at the surface even at much higher pressures.4o We hope to check this in the near future by converting to an ultrahigh-vacuum system. It should also be pointed out that, with hindsight, one would choose the heat of adsorption instead of an individual isotherm to probe changes in the surface. Another method of eliminating surface disorder on mechanically polished crystals is chemical etching. When the (0001) surface was etched with phosphoric acid at 400°, a good LEED pattern was obtained.4b The same treatment applied to our (4150) crystal resulted in microscopic etch lines. The highly faceted surface exposes surface of unknown orientation and results in considerable energy loss at higher frequencies due to surface scattering. We report it because it results in an interesting change in orientation of adsorbed alcohol. Figure 10 shows the isotherms before and after etching the (4150) surface. Figure 11 is the spectrum in the CH stretching region of n-amyl alcohol adsorbed on the etched (4150) surface at 1.12 Torr and is to be compared with Figure 5. The absorbance ratio, AL/A 1 1 'for the CH2 asymmetric stretch is now 1.1 compared t o 1.6 before etching. The low ratio and the significant difference between the ratios for the asymmetric and symmetric CH2 stretchz0 are The Journal of Physical Chemistry, Val. 74, N o . 26, 1970
GARYL. HALLER AND RICHARD W. RICE
4392
Table I1 .,048 05t
Ala * pair sites,
Crystal orientation
X 1014 am-*
(0001) (1123) (4160)
5.09 2.45 1.22
Saturation coverage,
x
1200,
1014 em-*
Ssturation coverage/ Al8+ pair sites
1.5 0.83 0.31
0.29 0.34 0.25
.oo* 000
0
I
I
I
I
I
I
I
I
I
I
.2
4
.6
.8
1.0
1.2
1.4
1.6
1.8
2.0
Pressure of n-Amyl Alcohol, (Torr)
Figure 10. Adsorption isotherms at 30" for (4180) C U - A ~ ~ O ~ before and after etching in phosphoric acid a t 400".
both consistent with ordered molecules more or less lying down on the surface. The simplest correlation involving surface structure might be one between number of sites on a given surface and molecules adsorbed by that surface. We will assume that the adsorption site requires one or more surface aluminum ions which are coordinatively unsaturated (coordination number less that 6, the bulk coordination number) and use structural arguments similar to those proposed for a-chromium oxide.21 The aoxides of both aluminum and chromium have trigonal structures that can be represented as a hexagonal closepacked oxide ion lattice with two-thirds of the octahedral holes filled by metal ions. If we form the (0001) surface by cleaving the crystal through a plane of closepacked oxide ions, half of the oxide ions of this plane must be moved t o each surface formed in order to maintain electrical neutrality. Each A13+ in this surface would be five-coordinate but could return to full coordination by adsorption of a water molecule. Movement of a proton from the adsorbed water molecule to an adjacent oxide ion results in a surface of close-packed hydroxide ions. If we reverse this step and desorb a water molecule by condensing any hydroxide and adjacent proton, we generate two five-coordinate surface A13+ions and one two-coordinate 02ion22 (02-is four-coordinate in the bulk). We take these coordinative unsaturated A13+ ions and the accompanying 02-ion as our site for dissociative adsorption of alcohol. The number of such potential sites is just the number of pairs of A13+under the close-packed hydroxide surface [i.e., the number of A13+pairs in any (0001) plane of the bulk]. We can imagine a similar arrangement on the (1123) and (4150) surfaces and again calculate the number of potential sites as the number of AI3+ pairs in the respective planes of the bulk. These calculated densities of potential sitesz3 are tabulated along with the measured saturation coverages at 120" (extrapolation of the linear portion of the isotherms in Figure 7 to zero pressure) in Table 11. The Journal of Physical Chemistry, Vol. 74, No. $6, 1970
0.036
0.040 0.044 0,048 2990
2950
2910 2870 Freguency (ctn-')
2830
Figure 11. Aksorbance spectra for 1.12 Torr of n-amyl alcohol on (4150) cu-AlzOa at 30' after etching in phosphoric acid a t 400'.
We chose saturation coverages to minimize the effect of the varying heats of adsorption and the isotherms at the highest temperature measured to avoid contribution from physical adsorption (hydrogen bonding t o hydroxide or oxide not directly involving A13+ ions) . 2 4 The ratio of adsorbed alcohol molecules t o calculated number of sites is suprisingly constant from surface to surface, being about 0.3 at 120'. This constancy may be, in part, fortuitous in light of the differences in the Al3f pair sites on the three surfaces. The A13+-A13+ (20) The dipole changes for the symmetric and asymmetric stretch of CHz lie in the H-C-H plane and are normal to each other. Therefore, if the CHz group can rotate freely about the axis of the carbon backbone of the molecule or if the molecules are randomly oriented, the ratios Al/'A!, (symmetric stretch) and A i / A I I (asymmetric stretch) will be equal. (21) R. L. Burwell, Jr., G. L. Haller, K. C. Taylor, and J. F. Read, Advan. Catal., 20, 1 (1969). (22) Reference 21, p 9. (23) (a) A hexagonal unit cell with a = 4.7589 and c = 12.991 k was used to calculate site densities. -Perfect close-packing of 02with an average 0 2 - diameter of a / d 3 and a spacing between closepacked planes of c/6 was assumed; (b) R . E. Newham and Y . M. DeHaan, 2. Kristallogr., 117, 235 (1962). (24) R. 0. Kagel, J. Phys. Chem., 71, 844 (1967).
ADSORPTION ON SINGLE CRYSTALS distance is smaller on the (4150) surface than on the other two surfaces (2.65 8 compared to 2.79 A ) . 2 3 b The average coordination number for the pair of A13+ ions in the site is approximately 9, 7,05,25and the average area per site is 20, 40, and 80 A2 on the (OOOl), (1123), and (4130), respectively. Presumably these differences will be reflected in the strength of bonding to a much greater degree than they are in the saturation coverages. In fact, the heats of adsorption increase as coordination of the pair site and intermolecular repulsion decrease (area per site increases) as would be expected, and, as previously emphasized,26there will be a contribution from bonding of the proton that will be a function of the unsaturation of the oxide ion to which it bonds, but it is not possible to define the stereochemistry precisely. The assumed dissociative adsorption has been previously proposed for alcohol adsorption of y-aluminum oxidez4and appears justified on a-aluminum oxide by the high heats of adsorption. The formation of a weak band like that recorded for the strongly bound OH, Figure 4b, accompanies alcohol adsorption and is additional evidence for dissociation. Since this band is not affectedby the manner or degree to which the alcohol is
4393 dried and does not appear when ether is adsorbed, it is not likely due to water impurity in the a l ~ o h o l . ~ ’ , ~ ~ Acknowledgments. The authors wish to acknowledge support of this research under Grant No. RSA-69-8 from the Connecticut Research Commission and Grant No. NSF GI
