Langmuir 1991, 7, 3041-3045
3041
IR Spectroscopic Detection of Lewis Acid Sites on A1203 Using Adsorbed CO. Correlation with AI-OH Group Removal Todd H. Ballinger and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsy 1 vania 15260 Received November 16, 1990. In Final Form: February 22, 1991 CO adsorption at low temperatures has been used to probe A13+ Lewis acid sites created upon dehydroxylation of A1203, using transmission infrared spectroscopy. A1203 was dehydroxylated in the temperature range from 475 to 1200 K. There is an approximate linear correlation between the decreasing integrated absorbanceof the A1-OH IR features and the increasing integrated absorbanceof the adsorbed CO features throughout the full coverage range of hydroxyl groups. Two CO adsorption sites have been identified: the first ( V C O = 2195 cm-l) appears immediately upon mild dehydroxylation, while a second site (VCO = 2213 cm-1) appears after an 800 K dehydroxylation treatment. CO adsorption on the Lewis acid sites formed by dehydroxylation of A1203 involves a binding energy of about 21 kJ/mol.
I. Introduction High surface area alumina, A1203, has been widely used as a as well as a support material for metal and metal oxide catalyst^.^ In both functions the surface AlOH groups and the coordinatively unsaturated (cus) AP+ sites play key roles. As a catalyst, the cus AP+-OH- pair sites on A1203 are accepted to be the active sites for a concerted mechanism in acid-base reaction^.'^^^^ As a metallic catalyst support, A1203surfaces exhibit a high population of diverse surface A1-OH groups5 as well as surface defect sites6which cause high metal dispersion718 compared to most other metal oxide support materials. In addition, the various sites on A1203surfaces may serve as models for geological surfaces of importance in environmental surface chemistry. Characterization studiesgJOof A1203 have focused on the production of surface defect sites on A1203which are believed to consist of cus A13+ sites adjacent to anionic vacanciesgproduced by dehydroxylation during high-temperature treatments. IR spectroscopy, using adsorbed probe molecules,has greatly contributed to understanding these sites. CO is a good probe molecule to use since it is far less basic, and therefore, more selectivell than other frequently used probe molecules such as NH3 and pyridine.1° A. CO Adsorption on AI-OH Sites. Thermal dehydroxylation studies of y-A1203by IR spectroscopy were first executed by Peri12J3 who concluded that A1-OH groups close to one another could form water and desorb from the surface. Associated hydroxyl groups, being (1) Tanabe, K. In Catalysis-Science and Technology;Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1981; Vol. 2, p 231. (2) Knozinger, H. Adu. Catal. 1976, 25, 184. Ratnasamy, P. Catal. Reu. Sci. Eng. 1978, 17, 31. (3) Knozinger, H.; (4) Knozinger, H.In Surface Organometallic Chemistry: Molecular Approaches t o Surface Catalysis;Basset, J. M., Gates, B. C., Eds.; Kluwer: Boston, MA, 1988; p 35. (5) Hardcastle, F.D.;Wachs, I. E. J. Mol. Catal. 1988. 46, 173. (6) Sanchez, M.G.; Gazquez, J. L. J. Catal. 1987, 104, 120. (7)Zaki, M. I.; Vielhaber, B.; Knozinger, H. J . Phys. Chem. 1986,90, 3176. (8) Zaki, M. I.; Tesche, B.; Kraus, L.;Knozinger, H. Surf. Interface Anal. 1988, 12, 239. (9) Boehm, H.P.;Knozinger, H. In Catalysis-Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1983; VOl. 4, p 39. (10) Kung, M. C.;Kung, H. H. Catal. Reo. Sci. Bng. 1985, 27, 425. (11) Zaki, M. I.; Knozinger, H. Spectrochim. Acta 1987, 43A, 1455. (12) Peri, J. B.; Hannan, R. B. J. Phys. Chem. 1960,64, 1526. (13) Peri, J. B. J . Phys. Chem. 1965, 69, 211.
0743-7463/91/2407-3041$02.50/0
closest together, were removed first, leaving isolated AlOH groups to be removed at higher temperatures. The spectra displayed five different VOH frequencies at 313 K for the A1-OH groups. Assuming that the (100) face is dominantly exposed at y&03 surfaces, Peri proposed a model14predicting the existence of five types of AI-OH groups. The model attributes the different VOH frequencies to differing numbers of surrounding surface oxide sites. Later, Knozinger and R a t n a ~ a m yassuming ,~ that all of the low index crystallite faces of A1203have an equal chance of projection at the y-Al203 surface, proposed a model that assigned the different VOH frequencies of the five types of isolated AI-OH groups to differing degrees of coordination of the A13+ site. These assignments are shown in Figure 1. Zaki and Knozinger15undertook a low-temperature IR study of the interaction between A1-OH groups and adsorbed CO, in order to examine the acid-base properties of the hydroxyl groups. They showed that of the five hydroxyl groups on A1203 after dehydroxylation a t 773 K, only three types (those at 3725,3715, and 3695 cm-l) were shifted almost 100 cm-l lower upon CO adsorption a t 80 K. According to Knozinger and Ratnasamy? these three classes of OH groups are bridge-bound to tetrahedrally and octahedrally coordinated AP+ sites and accumulate a net positive charge. Hence the low frequency hydroxyl groups have been considered relatively more acidic than the high frequency A1-OH groups. B. CO Adsorption on A13+Lewis Acid Sites. Early CO adsorption studies on A1203 were conducted by Little and Amberg16using IR spectroscopy. On A1203 pretreated at 673 K, a 2200-cm-1 vco band develops upon CO adsorption at 300 K. This band was not definitely assigned to a particular adsorption site. Using four different structural aluminas (7,y, 8 , Alon C), Della Gatta et al.17made a comprehensive IR study of CO adsorption at 309 K following heating to various temperatures. They observed a vco band between 2203 and 2215 cm-' after 673 K dehydroxylation and a second band between 2235 and 2245 cm-' following dehydroxylation from 773 to 1013 K. They attributed the adsorption to be a-dative bonding from the CO tocus AP+ surface cations. (14) Peri, J. B. J.Phys. Chem. 1965, 69, 220. (15) Zaki, M. 1.; Knozinger, H. Mater. Chem. Phys. 1987, 17, 201. (16) Little, L. H.; Amberg, C. H. Can. J . Chem. 1962,40, 1997. (17) Della Gatta, G.; Fubini, B.; Ghiotti, G.; Morterra, C. J. Catal. 1976, 43, 90.
0 1991 American Chemical Society
3042 Langmuir, Vol. 7, No. 12, 1991
Ballinger and Yates
Surface Hydroxyl Groups on AI,O,
adsorption sites: the first develops following mild dehydroxylation, and the second appears only after dehydroxylation at 800 K.
11. Experimental Section The low temperature-high temperature cell used has been io1 1u io1 lo) It) lo) I f L described previously.20A1203was spraydeposited ontoa tungsten 3800 3750 3700 3600 3400 3200 grid which is held rigidly by nickel clamps. The grid temperature was controlled by cooling with liquid Nz and by electrical Wavenumber (cm-') heating with an electronic controller.21 A chromel/alumel c Isolated Hydroxyl Groups \\ Associated Hydroxyl Groups thermocouple (0.077 mm diameter) spot-weldedto the top center Conligurationr 0 1 isolated -OH groups region of the grid was used to measure the A1203 temperature. accordinQ to Ref 13: The grid support was held in the middle of a stainless steel cell containing ports for gas delivery and for transmission of the IR io1 = octahedral coordination beam. Calcium fluoride optical windows sealed in 23/4 in. IU = tetrahedral coordinarion diameter flanges allow transmission IR measurements in the Figure 1. Diagram showing the OH stretching frequency for 4000-1000 cm-l spectral range. The cell was connected to a varioustypes of AMI3surface sites. The frequency of the isolated stainless steel ultrahigh vacuum system equipped with a liquid A1-OH groups is determined from the coordination of the underlying A1atom(& according to Kncizinger and Ratnasam~.~ Nz cooled zeolite pump and a 30 L/s ion pump, which typically achieves a base pressure of 5 1 X Torr. The A1203 used was Degussa aluminum oxide C (104 m2/g). The A 1 2 0 3 was spray deposited by first slurrying it into an The existence of two vco absorptions demonstrated the appropriate volume (110mL/g support) of a mixture of distilled presence of two different AP+ Lewis acid sites. water and acetone (1:9 ratio), ultrasonically agitated for 30 min. The slurry is then sprayed, by a Nz pressurized atomizer, onto Paukshtis and Yurchenko'* reported that several types the grid which is electrically heated to 323-333 K to flash of complexesbetween CO and differently coordinated AP+ evaporate the solvents.20 The net weight of the A1203 sprayed surface sites exhibit vco absorptions between 2190 and onto the grid (5.2 cm2geometric area) was 36.8 mg (7.1 mg/cm2). 2240 cm-'. Furthermore, they proposed an IR method The sample was prepared for IR studies by outgassing under whereby the CO heat of adsorption could be calculated vacuum at 475 K for 48 h. The carbon monoxide used was 99.9% pure and was obtained from the shift of the vco frequency relative to the gasfrom Matheson Gas Products in a break-seal glass storage bulb. phase frequency. IR spectra of the A1203were obtained in a purged double beam CO adsorption a t 77 K on A1203 dehydroxylated at 1073 Perkin-Elmer Model 580 B infrared grating spectrometer (5.3 cm-l resolution) coupled with a Model 3500 data station for data K was later studied by Zecchina et al.19 Four different vco storage and manipulation. Spectra were signal averaged, with absorptions were observed for increasing amounts of ada data acquisition time of 2.7 s/cm-l acquired at 1 point/cm-l. sorbed CO. They assigned a weak band at 2238 cm-' to The spectra showing adsorbed CO were obtained by subtracting a-bonded CO adsorbed on surface defect sites with strong out a background of CO(g) held in the cell under identical Lewis acid character. The most intense CO feature at measurement conditions. 2210-2190 cm-' was assigned to a-bonded CO to tetra111. Results hedral A13+ ions on the surface. The third band a t 2165 cm-' appears after the second feature maximizes, which A. Effect of Heating A1203 under Vacuum on the authors proposed as a a-bonded CO to octahedral A13+ Surface Hydroxyl Groups. Figure 2 shows the effect surface ions. The final band was assigned as multilayer of increasing temperature under vacuum (- 1X lo+ Torr) on the IR spectra of 7-A1203. After the initial outgassing physisorbed CO species which appeared at 2135-2140 cm-'. at 475 K, the A1203 was cooled to 200 K and spectrum a In the first of two studies, Zaki and KnOzingerl5 dewas obtained. Four 0-H stretching bands can be observed. hydroxylated A1203 at 773 K and observed two low-temThe 3732- and 3680-cm-' bands are due to the dominant perature CO features. The CO adsorption feature at 2152 isolated hydroxylgroups,l*while the 3585- and -3500-cm-' cm-' was assigned to CO hydrogen-bonded to A1-OH bands are due to H-bonded (associated) hydroxyl groups. groups, while the 2140-cm-' band was due to a physisorbed The A1203 was then heated under vacuum to the next layer of CO. The second study" focused on the interaction designated temperature for 30 min and cooled to obtained each spectrum. As the temperature is increased, dehyof CO with the surface Lewis acid sites on similarly droxylation occurs to produce H ~ 0 ( g ) ~ , ~as, ~the~ , ~ ~ , ~ ~ prepared A1203. A 2226-cm-' band was assigned to CO lower frequency associated hydroxyl groups are initially adsorbed on a tetrahedral A13+site, while the 2195-cm-' removed. By 800 K (spectrum c) all such groups have band was assigned to CO adsorbed on an octahedrally been removed from the surface. Further heating causes coordinated AP+ site. the removal of the isolated hydroxyl groups, and two new Despite all of these studies, none has attempted to bands can be resolved at 3798 and 3711 cm-I (spectrum directly correlate the extent of dehydroxylation with the d) after heating to 1000 K. Upon heating to 1200 K, production of Lewis acid sites as a function of the treatment spectrum e, no hydroxyl groups can be detected by infrared spectroscopy. This is in agreement with our previous temperature. Using infrared spectroscopy and low temperature CO adsorption, we have found a direct correlation (20) Basu, P.; Ballinger, T. H.;Yates, J. T.,Jr. Reu. Sci. Instrum. 1988, between the elimination of surface hydroxyl groups and 59, 1321. an increase in AP+-CO species IR intensity at maximum (21) Muha, R. J.; Gates, S. M.; Yates, J. T., Jr.; Basu, P. Reu. Sci. Instrum. 1985,56,613. coverage. Furthermore, we have observed two AIS+ (0)(01~0)
I
--
-
(18) Paukshtis, E. A.; Yurchenko, E. N. Russ. Chem. Rev. 1983, 52, 242. (19) Zecchina, A.; Platero, E. E.; A r e h , C. 0.J. Catal. 1987,107,244.
(22) Hair, M. L. In Infrared Spectroscopy in Surface Chemistry; Dekker: New York, 1967; Chapter 5. (23) Kiselev, A. V.; Lygin, V. I. In Infrared Spectra of Surface Compounds, Wiley: New York, 1975. (24) Cornelius, E. B.;Milliken, T. H.; Mills, G. A.; Oblad,A. G. J . Phys. Chem. 1955,59, 809.
Langmuir, Vol. 7,No. 12, 1991 3043
IR Detection of Lewis Acid Sites on A1203
Development of A I 3 + - co on Dehydroxylated y -AI,O,
Hydroxyl Spectra for Various Thermal Treatments of y -A1,03 ,
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Figure 2. Infrared spectra in the VOH region of A1203 following heating under vacuum at the temperaturesindicated. The A1203 was initially outgassed at 475 K for 48 h. The spectra do not involve a background subtraction. and that of Solymosi,26who observed no hydroxyl stretching bands after an 1173 K evacuation of a Rh/ A1203 catalyst. B. CO Adsorption After Gradual Dehydroxylation. After being heated to each dehydroxylation temperature, the A1203 was cooled to 180 K and CO(g) was added until an equilibrium pressure of 5 Torr was obtained. The IR spectra obtained are shown in Figure 3. After the initial outgassing a t 475 K, two weak C-0 stretching bands are observed in spectrum a. The 2154-cm-' band is due to CO hydrogen bonded to the surface hydroxyl groups as seen on both A120315and Si0227and is more prominent at lower temperatures. The 2189-cm-' band has been assigned to CO coordinated to the Al3+ ~ i t e s . ~ J ~ J ~The -'~J~ weak bonding of these species is demonstrated by the removal of both bands upon evacuation at 180 K. It can be seen in spectrum b that further dehydroxylation to 600 K has decreased the hydrogen-bonded CO, while the 2189-cm-I band has substantially increased in absorbance. By 800 K, when all of the associated OH groups have been removed, the AP+-CO band has shifted to 2194 cm-l (spectrum c). After 1000 K heating under vacuum, no hydrogen-bonded CO is observed by IR, and the A13+-CO feature has shifted to 2195 cm-'. Upon spectroscopic removal of all -OH groups at 1200 K, spectrum e, the 2195-cm-l feature remains, while a shoulder, which was first seen after the 800 K dehydroxylation, is evident at -2213 cm-l. The relationship between the increase in CO adsorption a t 180 K and the removal of OH groups at higher dehydroxylation temperatures is better demonstrated in Figure 4. This plot of the integrated vco absorbance versus the integrated VOH absorbance shows that between dehydroxylation temperatures of 475 and 1200 K there is an approximately linear correlation between the loss of OH absorbance and the development of AP+-CO absorbance. Although this correlation had been previously postu(25) Ballinger, T. H.; Yates, J. T., Jr. J. Phys. Chem. 1991,95, 1694. (26) Solymosi, F.; Pasztor, M. J. Phys. Chem. 1985,89, 4789. (27) Beebe, T. P.; Gelin, P.; Yates, J. T., Jr. Surf. Sci. 1984,148,526.
I
I
I
I
I
250
2150 2050 Wavenumber (cm-' 1 Figure 3. Infrared spectra in the vco region of CO adsorbed at
180K on A13+Lewis acid sites on A1203 after heating under vacuum to the temperature indicated. The spectra involve a CO(g) background subtraction. 3+
Empirical Correlation o f AI -CO with Dehydroxylation o f y-Al,03 I ' T = 180 K Pco = 5 Torr v
0
40
20
60
80
100
3900
JAy d v ( c m - ' ) 3100
OH
Figure 4. Empirical correlation between the integrated absorbance features of physisorbed CO on AP+ sites versus the integrated absorbance features of A1-OH groups on A1203 after successive dehydroxylation treatments. For OH spectral integration a subtraction background (spectrum 2e) was employed. For CO spectralintegration, a CO(g) background was subtracted. lated,L"J7 this is the first experimental result which demonstrates the relationship between the integrated absorbance of these species. It should be noted that the OH integrated absorbance measurements involve (1) the use of the completely dehydroxylated surface, spectrum 2e, as a background subtraction spectrum and (2) an arbitrary integration cutoff at 3200 cm-'. Therefore, the relationship in Figure 4 should be considered to be semiempirical. C. Effect of Temperatureon CO Adsorption Equilibrium after Extreme Dehydroxylation. Figure 5 shows the effect of increasing temperature on CO adsorption capacity of A1203dehydroxylated at 1200 K. All spectra were obtained under an equilibrium CO(g) pressure of 5 Torr. Spectrum 5a, taken at 180 K, is the same as
Ballinger and Yates
3044 Langmuir, Vol. 7, No. 12, 1991 Thermal Behavior of A I 3 + - CO Species
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Figure 5. Infrared spectra of the thermal behavior of the A13+CO species under an equilibrium pressure of 5 Torr. The inset shows the van't Hoff plot whereby the heat of adsorption has been found from the slope of this line. For CO spectral integration, a CO(g) background at the temperature of the measurement was subtracted. spectrum 3e with a prominent vco absorption band at 2195 cm-' and a shoulder at 2213 cm-'. Upon heating, the 2195-cm-' band decreases reversibly in intensity and shifts to a slightly higher frequency (2201 cm-l). At 260 K (spectrum d) this band has notably decreased in intensity and the shoulder a t 2213 cm-l can now be clearly resolved. In spectrum 5e taken at 280 K, the 2201-cm-1 band has almost disappeared, and the 2213-cm-l band has decreased in intensity and shifted to a higher frequency. This trend continues to 350 K, spectrum 5 h, where only a weak CO absorption feature remains at 2226 cm-'. Above 350 K no CO adsorption features can be seen under 5 Torr CO pressure. These equilibrium adsorption measurements will be used to estimate the heat of adsorption of CO on the A P sites. 111. Discussion
A. Dehydroxylation of A1203 There are two models which exist to explain the observed frequencies of the hydroxyl groups on A1203. Peril4first proposed the existence of five different types of isolated Al-OH groups based on Monte Carlo calculations of the dehydroxylation of the (100) face of A1203. The difference in VOH frequency between the Al-OH groups depended upon the number of nearest neighbor oxygen anions that surrounded a particular A1-OH group. Knozinger and Ratnasamy3later considered the OH groups which might exist on the (loo), (110),and (111)faces of A1203. Their modelalso proposed the existence of five Al-OH groups, but the VOH frequencies were believed to vary depending upon the degree of coordinative unsaturation of the AP+ atom or atoms upon which the OH group was bonded. These are shown in Figure 1. Indeed, several studies have resolved13J5all five bands in the 0-H stretching region by using IR spec-
troscopy. Our results are able to clearly resolve four different types of OH groups, depending on the dehydroxylation conditions. The dehydroxylation of A1203occurs upon the condensation of two nearby OH groups to form H20, which desorbs. Such a process leaves behind coordinatively unsaturated (cus)AP+ and oxide ionson the surface. Initially, the mutually hydrogen-bonded OH groups would be expected to form water first since such groups are close together. Indeed, Figure 2 shows this to be true since the broad bands near 3500 and 3585 cm-l disappear by 800 K. Also, the intensity of the isolated OH groups has dramatically decreased, indicating that surface species mobility at such temperatures is great enough to remove some of these species. Our results also show at 800 K (spectrum 2c) that two more types of A1-OH groups can be resolved at 3711 and 3798 cm-'. Above 800 K further dehydroxylation most likely occurs as a result of high surface species mobility on the surface. Finally, after heating at 1200 K for 30 min, dehydroxylation has been completed to the point where no IR detectable A1-OH groups remain. B. CO Adsorption on A1203. It can be seen in Figure 3 that the adsorption of CO (equilibrium pressure of 5 Torr) onto A1203 after initial outgassing at 475 K probes two classes of sites on the surface. The 2154-cm-' band has already been attributed to CO, which hydrogen bonds to the AI-OH groups, and similar behavior has also been observed on Si-OH groups.27 However, because the temperature of adsorption was 180 K, only a small amount of CO will adsorb in this manner as seen by the small absorption feature at 2154 cm-l and a lack of observable OH group frequency perturbation upon adsorption. The 2189-cm-' feature has previously been assigned to CO adsorbed on an AP+ site and this frequency is higher than the gas-phase CO frequency of 2143 cm-l. As dehydroxylation continues, more cus AP+ sites will be produced and more CO adsorption results. This can be seen by the intensity increase in Figure 3, and in Figure 4, where a direct empirical correlation between the integrated area of the hydroxyl IR absorption feature and the CO IR absorption feature occurs throughout the entire range of dehydroxylation temperatures. Thus, it is clear that AlOH removal increases CO adsorption on the A13+ sites. CO adsorption on A13+ sites is believed to occur by one of several processes. Since electrons in the 5a molecular orbital are mostly associated with the carbon atomz8and have weak antibonding c h a r t l ~ t e relectron , ~ ~ ~ ~donation ~ from this 5a orbital into an acceptor orbital on A13+ forms a dative a bond with the oxide surface. The depopulation of the antibonding orbital then strengthens the C-0 bond, which causes the increase in the C-0 stretching frequency above that of the free molecule. Hush and Williams30have calculated the effects of electric fields along the C-0 axis. Positive fields cause a decrease in the C-0 bond length, an increase in the force constant, and, hence, an increase in the stretching frequency as compared to a CO molecule in the absence of an electric field. Therefore, they conclude that similar fields must be imposed on adsorbed CO by the metal cations. However, they also emphasize that a change in frequency upon adsorption does not necessarily imply a change in the CO stretching force constant. Diatomic molecules adsorbed perpendicular to the surface will experience a mechanical increase in stretching frequency (28) Huo, W. M. J. Chem. Phys. 1965,43, 624. (29) Griffin, G. L.; Yates, J. T., Jr. J. Chem. Phys. 1982, 77, 3751. (30) Hush, N. S.;Williams, M. L. J. Mol. Spectrosc. 1974, 50, 349.
IR Detection of Lewis Acid Sites on A1203
Langmuir, Vol. 7, No. 12, 1991 3045
without a change in C-0 force constant due to the linking force constant between the surface metal ion and the carbon atom. Such an effect was also demonstrated by 0thers.3~932Larsson et al.33have used a different calculation of the effect of binding CO to a cation but have reached the same conclusion about the sign of the effect as Hush and Williams.30 C. CO Adsorption on a Heavily Dehydroxylated A1203Surface. 1. Two Forms of A13+-C0. Besides the main CO absorption feature at 2195 cm-l, a second lower intensity CO absorption feature at -2213 cm-' develops following more extensive dehydroxylation at higher temperatures. This species was first observed as a shoulder on the 2195-cm-' band following partial dehydroxylation at 800 K, as shown in Figure 3c. This is in agreement with studies of Della Gatta et al.17 and Zecchina et al.19 With higher levels of dehydroxylation, the 2213-cm-' shoulder became more prominent. The two forms of AP+-CO are not resolved well enough in this work to measure their separate intensities. 2. Estimation of the Heat of Adsorption of A13+CO. Even though the -2213-cm-l and the 2195-cm-1 spectral lines are poorly resolved, we can make a crude estimate of the heat of adsorption of these species (considered together) by using the combined spectral intensity as a measure of coverage, under equilibrium conditions a t constant CO pressure and variable temperature. For the process
-
CO(g)(P) + site ~t CO(a,Bco); T = const an equilibrium expression
(1)
may be written. If the fractional coverage,BCO, is assumed to be equal to the absorbance ratio, Aco/AmaXco,then
Keg= (Aco/Ammco)/(f'co(l- ACO/AmaXCO) (3) A van't Hoff plot of -In Kes vs 1/T will have a slope of AHa/R from AHa/R = -d In Ke,/d(l/T) (4) This method has been employed by Beebe et al. to estimate C02' and N234adsorption enthalpies on Si02 surfaces. The heat of adsorption of CO derived from these measurements is AHa = -20.9 kJ/mol, and the van't Hoff plot for this is shown as an inset in Figure 5. This estimate of AHa is to be considered only as an approximate measurement due to the overlapping of the 2195-cm-l and the 2213-cm-l vco bands. A value of AHa = -27.8 kJ/mol was obtained by Zaki et al." for the heat of adsorption of (31)Okawa, T.; Soma, M.; Bandow, H.; Uchida, K. J. Catal.'1978,54, 439. and references within. (32)Hoffmann, F.M.Surf. Sci.Rep. 1983,3,146, (33)Larsson, R.; Lykvist, R.; Rebenstorf, B. 2.Phys. Chem. (Leipzig) 1982,263,1089. (34)Beebe, T.P.;Yates, J. T., Jr. Surf. Sci. 1985,159, 369.
CO on A13+Lewis acid sites. Paukshtis and Yurchenko18 obtained a heat of adsorption of -32.5 kJ/mol by a method using the CO frequency shift, although Zaki and Knozinger" showed that this method gives higher values than the method used here. The initial appearance of the 2213-cm-' band after the 800 K dehydroxylation temperature is interesting in light of two other experimental results. Knozinger and Ratnasamy reported that the catalytic activity of A1203 only commenced after a 673-773 K re treatment,^ despite the fact that dehydroxylation began at lower temperatures. More recently, Datta35 has studied dehydroxylation of A1203 by XPS and found that heating A1203 up to 673 K caused no changes in the A1 (2p) or 0 (1s) binding energies (BE). However, heating at 773 K caused the A1 (2p) BE to shift 1.7 eV higher and the 0 (1s) BE to shift 0.7 eV lower, indicating an increased positive charge on A1 and an increased negative charge on remaining oxygen. Datta points out that although the dehydroxylation process should cause increased charges on A1 and 0 as a result of the transition from A1-OH to A16+-06- species, the fact that the binding energies do not change before 773 K (well after dehydroxylation has commenced) indicates that a new site is created above -773 K. It is evident from our IR results that a new Lewis acid CO adsorption site, associated with the 2213-cm-l band, is produced after dehydroxylation at 800 K.
IV. Conclusions The following conclusions can be made concerning CO adsorption on A1203: (1)Low temperature CO adsorption on A1203 can be used as a selective probe of Lewis acid sites on the surface of A1-203, monitoring adsorbed species near 2200 cm-l by using IR spectroscopy. (2) There is an approximately linear empirical correlation between the loss of integrated absorbance due to dehydroxylation of A1-OH groups and the increase in integrated absorbance of CO near 2200 cm-l due to CO adsorption on Lewis acid sites produced during dehydroxylation. (3) A second adsorbed CO species is created after dehydroxylation temperatures of 800 K and higher, and it is indicative of the presence of a mixture of AP+ Lewis acid sites on highly dehydroxylated A1203surfaces. (4) This method of probing the Lewis acid sites may find use in understanding the role of these sites in surface chemistry on A1203 surfaces. Such studies may serve as models for the interaction of environmentally important molecules with geological oxide surfaces.
Acknowledgment. We acknowledge,with thanks, the support of this work by the Los Alamos National Laboratories. We also thank Professor Mohamed Zaki for helpful discussions. (35)Datta, A. J.Phys. Chem. 1989,13,7053.
