Langmuir 1989,5, 114-123
114
Infrared Spectroscopy and Microcalorimetric Investigations of 6-6 and K Aluminas Using Basic Probe Molecules: Acetonitrile, Pyridine, 2,6-Lutidine, and n -Butylamine Marguerite H. Healy,*pt Larry F. Wieserman,* Edward M. Arnett,t and Karl Wefersi Department of Chemistry, Duke University, Durham, North Carolina 27706, and Alcoa Laboratories, Aluminum Company of America, Alcoa Center, Pennsylvania 15069 Received May 16, 1988 An in situ infrared investigation was conducted using four basic adsorbates to identify the acidic sites present on 6-8 and K aluminas. For two adsorbates, pyridine and 2,6-lutidine, adsorption studies were conducted between 298 and 983 K and at adsorbate pressures from 1.33 Pa (Pascal) to 1.33 kPa. The adsorption of acetonitrile and n-butylamine was conducted at 298 K by using adsorbate pressures from 0.66 to 8.65 kPa. Lewis acid and hydrogen-bonded complexes were identified on both aluminas for three adsorbates: acetonitrile,pyridine, and 2,6-lutidine. Using n-butylamine as the probe molecule, complexes for Brransted, Lewis, and hydrogen-bonded sites were identified on 6-8 and K aluminas. Results from a calorimetric study of the aforementioned adsorbates and adsorbents are also discussed.
Introduction
Experimental Section
Transition aluminas, formed by the thermal decomposition of aluminum hydroxides under nonequilibrium conditions, have highly disordered structures. Rapid loss of mass but slow structural rearrangement during thermal dehydration of hydroxides is responsible for the high internal porosity of the transition aluminas. In the 6-8 and K forms, pore sizes are on the order of 10-100 nm; specific surface areas are in the range of tens of square meters per gram. Being rough on a nanometer scale with steps and ridges, and having a nonstoichiometric composition, “the alumina surface is certainly an extremely complicated surface”.l Many types of reactive sites are present on the surface, such as Lewis acid sites (exposed aluminum cations), Lewis base sites (oxygen anions), and hydroxyl groups, which can be Brplnsted acid or base sites, as well as a source of hydrogen bonding. As demonstrated by solid-state MAS NMR studies2 and calorimetric ~ t u d i e s , ~ the strength, density, or reactivity of these sites is extremely varied for transition aluminas. Information about the types and strengths of acid sites present on alumina is obtained by using probe molecules of varying basic strength, such as acetonitrile (PKBH+ = -10.00): pyridine (&Ht = 5.23): 2,6-lutidine ( ~ K B H =+ 6.75): and n-butylamine (pKBH+ = 10.63): where the PKBHt of each compound is referred to the usual standard state in water at 298 K. Due to the varied chemical nature of the adsorbates, different complexes are formed on the alumina surface, such as those due to Brcansted acid coordination, Lewis acid coordination, and hydrogen bonding. By use of basic molecules, the type of acidic binding sites present on internal and external alumina surfaces may be probed. In this study the types of adsorption sites were identified and quantified by using infrared spectroscopy. Infrared spectroscopy is 4 valuable tool for characterizing adsorption sites on solids. Not only can the types of adsorption sites be identified, but their relative quantities can be measured. Although numerous infrared studies using a variety of probe molecules on alumina have been published, this study is the first study, known to us, which compares two transition aluminas, 6-6 and K , both of which are nearly anhydrous aluminas? ‘Duke University. Aluminum Company of America.
*
0743-7463/89/2405-0114$01.50/0
Prepared from a bayerite precursor, 6-19 alumina, a mixture of b and 8 aluminas, was 99.99% pure as shown by inductively coupled plasma. It had a surface area of 81 m2/gand a mean pore diameter of 238 A. K alumina (99.99% purity), prepared from gibbsite, possessed a surface area of 30 m2/g and a mean pore diameter of 142 A. Final activation of both aluminas was at 1223 K. The form of each alumina was determined by X-ray diffraction. Particle sizes of less than 7 pm (sized by using an Accucut A12 instrument, manufactured by Majac Corp.) were used for all experiments. The adsorbates, acetonitrile (HPLC grade), n-butylamine (99+%),2,6-lutidine (2,6-dimethylpyridine)(97%),and pyridine (99+%),were obtained from Aldrich Chemical Co. They were purified by using standard literature procedures.6 In general, bases were stirred over a drying agent, either potassium hydroxide or sodium sulfate, and then fractionally distilled at atmospheric pressure. Adsorbates were degassed every day by two to four freeze-pump-thaw cycles prior to the adsorption studies. Infrared spectra were obtained with an IBM IR-98 Fourier transform infrared spectrophotometer equipped with a narrowband mercury cadmium-mercury telluride (MCT) detector, operation of which has been described previously.’ Spectral resolution was 4 cm-’; 128 scans were collected for each spectrum. Absorbance values, corrected for pellet weight and surface area of alumina [A/g of alumina/(m2/g)],are presented at full scale value. Manipulations of the IR optics bench and data were controlled by an Aspect 2000 computer,manufactured by Bruker. Auxillary equipment associated with the conducted FT-IR experiments included a temperature controller, a pressure controller, and vacuum pumps. The temperature of the adsorbent was controlled by a specially modified Bruker variable-temperature unit (Model ER-4111VT). The pressure of the adsorbate (1) Knozinger, H.; Ratnasamy, P. Catal. Rev.-Sei. Eng. 1978,17, 31. ( 2 ) (a) Majors, P. D.; Ellis, P. D. J. Am. Chem. Soc. 1987, 109, 1648. (b) Ripmeester, J. A. J.Am. Chem. SOC.1983,105, 2925. (3) (a) Stradella, L.; Gatta, G. D.; Venturello, G. 2. Phys. Chem. (Munich) 1979, 115, 25 and references therein. (b) Zimmermann, R.; Schneider, H. A.; Wolf, G. Thermochim. Acta. 1988,92,317. (4) (a) Perrin, D. D. Dissociation Constants of Organic Bases in Aqueous Solution; Butterworths: London, 1965. (b) Perrin, D. D.; Dempey, B.; Sejeant, E. P. p K , Prediction for Organic Acids and Bases; Chapman and Hall: London, 1981. (5) Lippens, B. C. In Physical and Chemical Aspects of Adsorbents and Catalysts;Linsen, B. G., Ed.; Academic: New York, 1970, Chapter 4.
(6) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicak, 2nd ed.; Pergamon: New York, 1980. (7) Hanson, B. E.; Wieserman, L.F.; Wagner, G. W.; Kaufman, R. A. Langmuir 1987, 3, 549.
0 1989 American Chemical Society
Langmuir, Vol. 5, No. 1, 1989 115
IR and Microcalorimetric Study of Aluminas
I b' 13.1 Pa
1 1700
, I
1500
I 1300
Wavenumbers em-'
Figure 1. Infrared spectra of pyridine adsorbed on &e alumina
at 13.1 and 1333Pa: (a) 298; (b) 483 K.
present in the system was controlled by a MKS Type 244A pressure flow controller. Two high-accuracy Baratron gauges were used with an operating range of 5 decades (104-100 Pa and 1-105 Pa). Evacuation of the sample chamber was done by first using a roughing pump and then a turbomolecularpump (Pfeiffer Power supply TCP040). The vacuum system was monitored with a Perkin-Elmer Digital Gauge Control1111. A Quadrex 200 mass spectrometer, manufactured by the Inficon Division of Leybold-Hereus and equipped with an electron capture detector, was used to monitor the composition and pressure of gases in the vacuum system. The pump used in this system was a LeyboldHereus Turbotronik NT 150/360 pump and was capable of evacuating the system to 1.33 X Pa. Typically, 20-25 mg of alumina was pressed into a 13-mmdiameter pellet by using a Beckmann pres, operated at 1ton/cm2. Before placing the pellet into the IR cell, it was weighed to an accuracy of lo-' g. In situ activation of the pellet at 1.33 X Pa was achieved by heating the sample to 483 K for 30-45 min. The pellet was then cooled to room temperature, and adsorption studies commenced at 298 K. Infrared spectra were recorded at several pressures from -0.665-1.33 Pa to 1.33-8.66 kPa In awes where temperature studies were conducted the same pellet was used in all experiments. After evacuation of the pellet at 0.133 Pa, the temperature was increased: once the system has restabilized, usually at 1.33 X 109 Pa, adsorption studies were resumed. Aluminas used in calorimetricstudies were dried at 453 K and 133-266 Pa for 18-19 h. Alumina samples were stored and handled in a nitrogen atmosphere drybox. A Tronac 1250 calorimeter was used to determine heats of immersion as described previously? In these experiments 100-200 mg of alumina was placed in a glass ampule, which, when broken, released the solid into an excess of neat adsorbate in the reaction vessel. Heats are expressed in units of mcal/m2 of solid. The calorimeter was calibrated chemically by the immersion of potassium chloride into water. An experimental value of 4.17 i 0.20 kcal/mol was considered to be acceptable compared to the published value of 4.20 h 0.04 kcal/m01.~ Results Infrared Spectroscopy. 1. Pyridine. Adsorption of pyridine on 6-8 and K aluminas occurs via chemical and physical interactions. Figures 1 and 2 present infrared spectra from the adsorption of pyridine on each alumina. Five bands located at 1450,1491,1578,1595,and 1612 cm-' are of particular importance and are assigned by using the nomenclature of Kline and Turkevich'O to the 19b, 19a, 8b, and two 8a modes of pyridine, respectively. The 19a
1700
1500
1300
Wavenumbers em-'
Figure 2. Infrared spectra of pyridine adsorbed on K alumina
at 13.1 and 1333 Pa: (a) 298; (b) 483; (c) (1333Pa only) 583 K.
(1491 cm-') and 8a (1595 and 1612 cm-l) modes are assigned to the totally symmetrical vibrations of the pyridine molecule, whereas the 19b (1450 cm-') and 8b (1578 cm-') modes are assigned to the antisymmetric vibrations in the plane of the molecule. This investigation focused on the 8b and 8a modes, whose frequencies provide information on the coordination sites on the internal and external surfaces of In particular, the stretching mode at 1578 cm-' (8b) was used to quantify the total amount of pyridine adsorbed on the surface,14including physically adsorbed species.16 When physically adsorbed species are present on the surface, the band at 1578 cm-' represenka both the 8a and 8b mode of pyridine.16 The 1595-cm-' band (8a mode) is caused by a hydrogen-bonded species between surface hydroxyl groups on alumina and pyridine,16and the 1612-cm-' band (sa mode) is indicative of a Lewis acid complex.16 The gas-phase spectrum of pyridine has peaks at 1433, 1450,1576, and 1587 cm-', which are attributed to the 19b, 19a, 8b, and 8a vibrational modes, respectively. To quantify the pressure-temperature relationship for the adsorbed species, absorbance values, corrected for pellet weight and surface area of alumina [(A/g of alumina/(m2/g) X 10-7, were plotted as a function of adsorbate pressure and are herein referred to as isotherms. Absorbance values are related directly to the quantities of each adsorbed spe~ies.'~J"Isotherms representing the hydrogen-bonded species (Figure 3) and the Lewis acid coordination complex (Figure 4) on both 6-8 and K alu(11) (12) (13) (14)
Parry, E. P. J . Catal. 1963,2, 371. Hughes, T. R.; White, H. M. J. Phys. Chem. 1967, 71, 2192. Stolz, H.; Knozinger, H. Kolloid-Z. Z . Polym. 1971,243, 71. Paukshtis, E. A.; Shinkarenko, V. G.; Karakchiev, L. G. Kinet.
Catal. 1976. - ,17. - 893. (15)Scokart, P. 0.; Declerck, F. D.; Sempels, R. E.; Rouxhet, P. G. J. Chem. SOC., Faraday Trans. 1 1977, 73,359. (16) Morterra, C.; Chiorino, A.; Ghiotti, G.; Garrone, E. J. Chem. SOC., Faraday Trans. 1 1979, 75, 271. (17) Zecchina, A.; Coluccia, S.; Morterra, C. Appl. Spectroscop. Reo. 7
-
~~
~
1985, 21, 259.
(8) Arnett, E. M.; Moraldo, S. G.; Schilling, S. L.; Harrelson, J. A. J. Am. Chem. SOC.1984,106, 6759. (9) Gunn, S. J. J.Phys. Chem. 1965, 69, 2902. (10) Kline, C. H., Jr.; Turkevich, J. J. Chem. Phys. 1944, 12, 300.
(18)Connell, G.; Dumesic, J. A. J. Catal. 1986, 102, 216. (19) Defosse, C.; Scokart, P. 0.; Rouxhet, P. G. Verres Refract. 1981, 35,50. (20) Paukshtis, E. A.; Yurchenko, E. N. Russ. Chem. Reu. 1983,52, 242.
Healy et al.
116 Langmuir, Vol. 5, No. 1, 1989
Table I. Stabilities of Species Formed on the Alumina Surface with Pyridine as a Function of TemDerature
0 298K 383K A 483K
0 6o
~~
% of species remaining on surfacea at
E
alumina
14:@
383
temp (K) 483 583
783
1612-cm-' Bandb 6-0 K
20
67 26
17 19
47 39
32 34
32 33
0 0
0 0
1595-cm-' Band' 6-0
0
298
1000 2000 Pressure (Pa)
3000
K
18 0
3.5
2.8
4.6
0
"Absorbance (0.133 Pa)/absorbance (1.33 kPa). bLewis acid coordination complex. Hydrogen-bonded complex.
Delta-Theta Alumina I
0
1000 2000 Pressure (Pa)
3000
C I 13.1 Pa
Kappa Alumina
Figure 3. Adsorption isotherms for pyridine on 6-8 alumina at 298, 383, and 483 K and K alumina at 298 and 383 K. Hydrogen-bonded species.
(.06)
133 Pa N
E b-
f 10
lAd!3 533 Pa
0
133 Pa
a
1700 I
0
I
1000 2000 Pressure (Pa)
I 3000
(.W
1500 Wavenumbers cm-1
1300
Figure 5. Infrared spectra of 2,g-lutidineadsorbed on 6-8 alumina. At 13.1 and 533 Pa: (a) 298; (b) 483 K. At 133 and 533 Pa: (c) 583; (d) 783 K.
Delta-Theta Alumina
E
6
f
g
10
0
0 298K
0 383K A 483K 0 583K 783K
0
0
1000
2000
3000
Pressure (Pa)
Kappa Alumina
Figure 4. Adsorption isotherms for pyridine on 6-0 and K aluminas at 298, 383,483, 583, and 783 K. Coordination at Lewis acid sites. minas are presented. To examine the reproducibility of spectral measurements, the adsorption of pyridine on a second sample of K alumina was repeated at 298,383, and 583 K. Adsorption frequencies were identical. Absorbance
values per unit surface area varied no more than 10% for the Lewis acid species. For the hydrogen-bonded species absorbance values varied by more than 10%. This is attributed to the difference in pellet weights: the original pellet was almost 60% heavier than the second pellet. At each temperature, where possible, the stability of the Lewis acid coordinated and hydrogen-bonded species was investigated by comparing the intensity of a particular band at the maximum vapor pressure of pyridine at 298 K (1.33 P a ) to the intensity of the same band after sample evacuation (0.133 Pa). These are listed in Table I. 2. 2,CLutidine. Figures 5 and 6 present infrared spectra from the adsorption of 2,6-lutidine on each alumina. Five bands found at 1414, 1456, 1470, 1582, and 1601 cm-' are of particular importance and are to the 19b, methyl asymmetric stretch, 19a, 8b, and 8a modes of 2,6-lutidine, respectively. The asymmetric (21) Jacobs, P. A.; Heylen, C. F. J. Catal. 1974, 34, 267. (22) Matulewicz, E. R. A.; Kerkhof, F. P. J. M.; Moulun, J. A.; Reitsma, H. J. J. Colloid Interface Sci. 1980, 77, 110. (23) Miyata, H.; Moffat, J. J. Catal. 1980, 62, 357. (24) Corma, A,; Rodellas, C.; Fornes, V. J . Catal. 1984, 88, 374.
Langmuir, Vol. 5, No. 1, 1989 117
IR and Microcalorimetric Study of Aluminas 120
I
0 298K
0 383K A 483K
0 583K N
1
E 80 3
I
; d
40
Q
0
I
I
--
700
350
Pressure (Pa) 13.1 Pa
Delta-Theta Alumina
11
E 80
0 0
350
,-
.
'0
Pressure (Pa)
1500
1700
1300
Kappa Alumina
Wavenumbers cm-'
Figure 6. Infrared spectra of 2,6-lutidineadsorbed on K alumina. At 13.1 and 533 Pa: (a) 298; (b)483 K. At 133 and 533 Pa: ( c ) 583;
(d) 783 K.
Figure 7. Adsorption isotherms for 2,g-lutidine on 68 alumina
at 298,383,483, and 583 K and K alumina at 298,383,483,583, and 783 K. Coordination at Lewis acid and hydroxyl sites.
Table 11. Stabilities of Species Formed on the Alumina Surface with 2,6-Lutidine at Different Temperatures % of species temp, K band position, cm-' remaining on surfaceo b-0 Alumina 298 298 383 383 483 583 783
1599/1593 1614/1593 1599/1593 1615/1593 1576/1591 1574/1591 1605/ 1591
298 298 383 383 483 583 783
1591/1953 1614/1583 1591/ 1593 1612/1591 1614/1591 1609/1589 1572/1583
K
4.0 3.9 2.3 2.4 27 34 19
Alumina 1.6 4.1 1.5 1.8
5.0 20
na
Absorbance (0.133 Pa)/absorbance (1.33 kPa). methyl stretch (1456 cm-') and the 8a (1601 cui') and 8b (1583 cm-') frequencies are useful in assessing adsorbate/adsorbent interactions. The band at 1601 cm-l is attributed to coordination at both a Lewis acid site and a physical adsorption site.22 The gas-phase spectrum of 2,g-lutidine at 298 K has bands at 1 4 6 2 , 1 5 2 2 , and 1591cm-', which are assigned to b,CH, 19a,23and 8a modes, respectively. At 583 K an additional peak at 1560 cm-' is identified and is attributed to the 8b vibrational mode. Isotherms representing the species adsorbed and characterized by the 8a mode (Figure 7) are shown for 6-8 and K aluminas. The stability of the 8a species was investigated by comparing the intensity of the band at saturation (133 Pa) to the intensity of the band after evacuation (0.133 Pa) and is listed in Table 11.
1733 Pa
1700
I
I
1500
1300
Wavenumbers cm-'
Figure 8. Infrared spectra of n-butylamine adsorbed on alumina at 1733 and 4666 Pa and 298 K (a) 6-8 alumina; (b) K alumina.
3. n -Butylamine. Infrared spectra from adsorption studies of n-butylamine on both aluminas at 298 K are shown in Figure 8. In the gas-phase spectrum of n-butylamine, three peaks in the 1700-13OO-cm-' range are found, occurring at 1624,1464,and 1391cm-'. These peaks are assigned%to the NH2 bending mode (1624 cm-') and an angular (1464 cm-') and torsional (1391 cm-') CH2 mode. At low surface concentrations of n-butylamine on K alumina and upon evacuation at 0.133 Pa, peaks at 1605, -1587, -1506,1448, and 1381cm-' are evident. The peak at 1605 cm-' is assigned to a NH2 bend,%*% the peak
-
(25) Fripiat, J. J.; Servais, A.; Leonard,A. Bull. SOC.Chim. Fr. 1962, 635. (26) Morimoto, T.; Imai, J.; Nagao, M. J. Phys. Chem. 1974, 78,704.
1::w
Healy et al.
118 Langmuir, Vol. 5, No. 1, 1989
E
N
9
m n
601
B
0 H-Bond
0
Lewis
20
0
0
5000 Pressure (Pa)
0
10000
4000 8000 Pressure (Pa)
0
Delta-Theta Alumina
Delta-Theta Alumina
0 H-Bond
0 Lewis
j
4
o
u
2 20 -
0
5000 Pressure (Pa)
0
10000
0
Kappa Alumina
Figure 9. Adsorption isotherms for n-butylamine on 6-0 and K aluminasat 298 K. Coordinationat Lewis acid and hydroxyl sitea.
4000 8000 Pressure (Pa)
Kappa Alumina Figure 11. Adsorption isotherms for acetonitrile on 6-8 and K aluminas. Coordination at Lewis acid and hydroxyl sites. Table 111. Heats of Immersion of Various Adsorbates with 6 8 and K Aluminas Determined with a Tronac 1250 Calorimeter at 298 K -Al!Ii,,,:
adsorbate acetonitrile 2,6-lutidine pyridine n-butylamine O 9 5 % confidence level. Obtained by Q.Liu.
2350
2300
2250
2200
Wavenumbers cm-'
Figure 10. Infrared spectra of acetonitrile adsorbed on alumina at 1733 and 6666 PE and 298 K: (a) 6-8 alumina; (b) K alumina.
at 1587 cm-' to a symmetrical NH3+stretch,=s2' the peak at 1506 cm-l to an asymmetrical NH3+stretch,26~26~28 and the peaks at 1448 and 1381 cm-l to an angular and torsional CH2mode.% Similar frequencies, with the exception of the symmetric NH3+ shift being positioned at 1560 cm-', are found on 6-8 alumina. Isotherms obtained from the adsorption of n-butylamine on 6-8 and K aluminas are shown in Figure 9. 4. Acetonitrile. Figure 10 presents infrared spectra from the adsorption of acetonitrile on 6-8 and K aluminas at a variety of adsorbate pressures at 298 K. Acetonitrile The fundamental vibelongs to the C3, point brations which are of importance to this study are v2,the symmetric CGN stretch; v3, the symmetric CH3 defor-
-
( 2 7 ) Ballanato, J. Spectrochim. Acta 1960, 16, 1344. (28) Ghosh, A. K.; Curthoys, G. J. Chem. SOC., Faraday Trum. 1 1984, EO, 99. (29) Knbinger, H.; Krietenbrink, H. J . Chem. SOC.,Faraday Tram. 1 1975, 71, 2421.
6-9 38.6 f 4.4 44.4 f 1.3 50.0 f 9.4 91.5 f 8.lC
mcal/m2 K
48.0 f 4.0 71.3 f 7.7b 61.7 f 8.3 74.3 f 11.1
bObtained by G . V. Barbiera.
mation; and v4, C-C stretching.29i30 The symmetric C=N stretch can be used to investigate the coordination site of the adsorbateJadsorbent interaction. The combination mode v3 v4 is important because this frequency undergoes Fermi resonance with the C=N stretching mode. In the liquid spectrum of acetonitrile, the C=N stretch is located at 2254 cm-l; upon coordination with a Lewis acid site this stretch is shifted +(20-100) cm-1.29931 The combination band is located at 2292 cm-l.% Isotherms obtained from the C=N stretches located at 2254 and 2314 cm-' are shown in Figure 11. Calorimetry. The heats of immersion of 6-8 and K aluminas in various adsorbates are listed in Table 111.
+
Discussion Pyridine. Pyridine is a widely studied probe molecule32 for studies of acid surfaces because the adsorption of pyridine offers so much information about the chemical nature of the solid. Not only is the type of acid-base complex identified, but information about the coordination (30)Venkatsswarlu, P. J. Chem. Phys. 1951,19,293. (31) Sempels, R. E.; Rouxhet, P. G. J. Colloid Interface Sei. 1976,55, 263. (32) Knozinger, H. Adu. Catal. 1976, 25, 184.
IR and Microcalorimetric Study of Aluminas
Langmuir, Vol. 5, No. 1, 1989 119
Scheme I. Adsorption of Pyridine on Alumina
I
H
1
I
1
0 Kappa
0 Delta Theta
-
R
R
- H for pyridine - CH3 for 2,6 lutidlne -3
-2
-1 0 Log (P)
1
2
Figure 12. Ratio of Lewis acid coordination to hydrogen-bonded complexes as a function of pressure on 6-8 and K aluminas at 298 K. Pyridine was the probe molecule.
environments of Lewis acid species is gleaned. In general, two main types of Lewis acid environments are identified on alumina: an "inner" and an "outer" site, where the former is weaker than the latter.13J6 Zechinna et al.= warn that analyses of the type of Lewis acid site(s) present on the solid should not be carried to an extreme because the ring vibrational modes of pyridine are not sensitive to slight changes in electron densities at nitrogen. One disadvantage of using pyridine as a probe molecule is its size: pyridine may not be able to reach all the Lewis acid sitesM due to surface crowding within micropores. 1. Peak Assignments. The initial investigation of pyridine adsorption on solid systems using infrared spectroscopy was done by Parry in 1963." Band assignments were made by comparing the infrared spectra of homogeneous solutions to those obtained on the heterogeneous solid. A coordinately bound pyridine complex (Py:BH3) has frequencies at2' 1455 (19b), 1490 (lga), 1575 (8b), and 1595 (8a) cm-', whereas a pyridinium ion (Py:H+) has frequencies2' at 1450 (19b), 1490 (19a), 1627 (8b), and 1655 (8a) cm-'. In the gas-phase spectrum of pyridine, frequencies are located at 1433,1450,1576,and 1587 cm-l. On K and 6-8 aluminas (298 K and low surface coverage), frequencies occur at approximately 1448,1491,1578,1595, and 1612 cm-'. The major difference between the gas-phase spectrum and the spectra obtained from adsorption of pyridine on alumina is the splitting of the 8a mode on the solid. This is indicative of pyridine adsorption on two different sites or environments on alumina: a hydrogen-bonding site and a coordinately unsaturated aluminum cation. No Brernsted acidity was identified in the present study. Assignment of the adsorption site reflected in the 1595-cm-' band is not straightforward. A previous studyle suggests that this band is due to two different types of sites; at 298 and 383 K, it is really the 8a vibration mode for a Py, species (hydrogen bonded) and a Py2 species (Lewis acid site). Support of this assignment is provided by temperature studies. A t 578 K or higher the band is clearly resolved as two components on alumina. In this study, the band located at 1595 cm-' is assigned to a hydrogen-bonded species for the following five reasons. (a) The peak in question remains single-no peak splitting is evident for the temperature range 483-983 K. (b) A t (33) Zecchina, A,; Guglielminotti, E.; Cerrutti, L.; Coluccia, S. J. J. Phys. Chem. 1972, 76, 571. (34) piviat, F. E.; Petrakis, L. J.Phys. Chem. 1973, 77, 1232.
temperatures greater than 383 K on K alumina and 483 K on 6-19 alumina, this peak is not detected until a relatively high pressure of 1.33 kPa pyridine is reached. This suggests that the residual hydroxyl groups are the last to interact, which makes sense from an energetic point of view? Lewis acid sites at 1612 cm-l are filled a t low pyridine pressures (0.666 Pa). (c) Results from Table I indicate that the species identified by the 1595-cm-' band is much less stable for a given temperature than the species characterized by the 1612-cm-' band. (d) On K alumina at 298 and 383 K, the intensity of the 1595-cm-l peak increases 18% and 14%, in contrast to increases in the Lewis acid band (1612 cm-') of 2.9% and 3.3%. If the site were a Lewis acid site, one would expect increases in intensity similar to those of the 1612-cm-' band. (e) At a pyridine vapor pressure of 1.33 kPa at 298 K, the band is centered at 1587 cm-'. On Alon C, a mixture of y and 8 aluminas, the band is located at 1600 cm-' and on a alumina a t 1597 cm-1.16 The second 8a band, located at 1612 cm-', was assigned to a pyridine species coordinately bound to alumina. This site has been designated as a Py, speciesle or an "outer complex", PyL1,lSand is believed to represent a dual octahedral-tetrahedral site on alumina; that is, it is a bridging vacancy between an Al" and AP atom. This site is classified as a weak site16as opposed to the stronger Py, or PyL" site. Unlike studies with Q alumina, where twoM or three16 different types of Lewis acid species were identified, or 6 alumina,ss on which four Lewis acid sites were found with pyridine as the probe molecule, only a Py, site was identified for 6-8 and K aluminas. A representation of the adsorption of pyridine on the aluminas is shown in Scheme I. 2. IR Spectrum at 298 K. The IR spectra obtained at 298 K are of particular interest to this study because calorimetric studies of 6-19 and K aluminas were done at this temperature. Increasing the pressure of pyridine caused the amount of pyridine on the surface to increase in a systematic way. In general, as the pressure of pyridine increases, the band intensities increase and their positions shift to lower frequencies. For example, the intensities of the 1578-, 1595-, and 1612-cm-' bands increase 18,22, and 2.9 times, respectively. These increases simply represent a greater amount of pyridine adsorption on the surface. From 0.666 Pa to 2.00 kPa, the 1612-cm-' band shifts to 1608 cm-', the 1595-cm-' band to 1589 cm-', and the 1578-cm-' band to 1576 cm-'. We interpret the shift to lower frequencies as follows: As the pressure of the ad(35) Stolz, H.;KnBzinger, H. Fortschrittsber, Kolloide Polym. 1971, 55,16.
Healy et al.
120 Langmuir, Vol. 5, No. 1, 1989 1.0
r Kappa
0 Della Theta
I
0.4 200
I
400
600
800
Temperature (K)
Figure 13. Ratio of Lewis acid coordination complex to total pyridine coverage at 1.33 kPa as a function of temperature for 6-0 and K aluminas.
sorbate is increased, more molecules adsorb on the surface, and steric crowding increases. A decrease in freedom of rotational and vibrational modes occurs. The decrease in frequency is also indicative of weaker complexes being formed on the solid. The most active or energetic sites are filled first and are characterized by higher frequencies. The ratio of the Py, complex (Lewis acid) to Py, complex (hydrogen-bonded species) is monitored by plotting the ratio of the intensity of the two peaks as a function of adsorbate pressure, as shown in Figure 12. The negative slope of the line indicates that the greatest percentage of Lewis acid sites is occupied first at low coverage. As the concentration of pyridine is increased and the number of vacant Lewis acid sites decreases, the dominant interaction becomes hydrogen bonding. An examination of surface stabilities (Table I) shows that a more stable Lewis acid species is formed on 6-8 alumina than on K alumina at 298 K (67% remaining on the surface versus 26% remaining on the surface, respectively). As expected, Lewis acid species are held more strongly on the surface than the hydrogen-bondedspecies. 3. PressumTemperature Relationships. Indicative of type I or Langmuir isotherms,%isotherms obtained for the Lewis acid coordination complex on 6-8 and K aluminas are similar. The isotherms obtained for the Lewis acid coordination (Figure 4) and hydrogen-bonded (Figure 3) complexes are contrasted by the initial slopes of the isotherms: the former have sharp slopes, suggestive of chemical adsorption, whereas the latter possess gradual slopes, indicative of physical adsorption. At 298 and 383 K, the amount of hydrogen-bonded species per surface area is greater on K alumina than on 6-8 alumina. No hydrogen-bonded species were detected on K alumina at 483 K, although these species existed on 6-8 alumina. 4. Effect of Increasing Temperature. In Figure 13 a plot of the ratio of the intensity of the 1612-cm-' band to the 1578-cm-' band versus adsorption temperature is shown. The behavior of the two aluminas is different. On 6 4 alumina the Lewis acidity increases up to a temperature of 483 K and then reaches a constant level. On K alumina the Lewis acidity is constant until 487 K and then increases with increasing temperature. No dramatic shifts in either the 8a or 8b band modes occur with increasing temperature on either solid. It has been noted by othe r ~ that ~ ~pyridine * ~ is ~ transformed to an a-pyridone species on 7 and 6 aluminas above temperatures of 673 K, where the cy-pyridone species is identified by the appear(36) (a) Adamson, A.W.Physical Chemistry of Surfaces, 4th ed.; John Wiley & Sons: New York, 1982. (b) Lowell, S.;Shields, J. E. Powder Surface Area and Porosity, 2nd ed.; Chapman and Hall: New York, 1984.
ance of a carbonyl stretching frequency. In the present study carbonyl bands were absent, and it was concluded that pyridine was not altered on either 6-8 or K aluminas. 2,B-Lutidine. Adsorption studies conducted with 2,6lutidine are influenced greatly by the topography of the solid surface. On the basis of pKBH+values, 2,6-lutidine is a stronger base than pyridine; however, as Brown et al.37 demonstrated, this order is reversed with increasing steric requirements of the reference acid. Analogously, coordination complexes between 2,6-lutidine and alumina are inhibited not only by the steric demands of the methyl groups adjacent to the nitrogen atom but by the location of the Lewis acid site. 2,6-Lutidine has been suggested as a specific probe for protons3s because of their smaller size and accessibility compared to the aluminum cations, which lie below the plane of hydroxyl groups and oxygen atoms in alumina.39 1. Peak Assignments. The original investigation of the adsorption of 2,g-lutidine on a solid (zeolites) was done in 197421and extended to fluorinated aluminas in 197922 and 6 alumina in 1984.24 As with pyridine, band assignments on the solid were accomplished by comparison to IR spectra of homogeneous solutions of coordinately bound 2,6-lutidine and a 2,6-lutidinium ion. For the former, frequencies are found at 1410 (19b), 1455 (6,CH3), 1477 (19a), 1580 (8b), and 1603 (8a) cm-' and for the latter at 1415 (19b), 1452 (6,CH3), 1473 (19a), 1630 (8b), and 1650 (8a) cm-1.21 In the gas-phase spectrum of 2,6-lutidine, frequencies are found (583 K and 13.1 Pa) at 1458, 1508, 1560, and 1591 cm-l and are assigned to the 6,CH3, 19a, 8b, and 8a modes.21 On K alumina (298 K and 1.33 Pa) three bands at 1456 (6,CH3), 1582 (8b), and 1601 (8a) cm-I are present; on 6-8 alumina these bands are at 1456 (6,CH3), 1582 (lib), and 1599 (8a) cm-'. The 19a mode is not present, on either solid, until an adsorbate pressure of 133 Pa is achieved. The major difference between the gas-phase and adsorbateadsorbent spectra is the shift of the 8a and 8b modes to higher frequency. Unlike pyridine, 2,6-lutidine cannot be used to distinguish between different types of Lewis acid sites in the alumina sample.24 It has also been suggested24that the bands assigned to Lewis acidityz1 also possess some hydrogen-bonded or physically adsorbed species. For these reasons, assignmentof the 1599-cm-l band solely to a Lewis acid site is impossible. However, one can state that no Brernsted acidity of sufficient strength to protonate 2,6lutidine is present on either 6-8 or K aluminas. 2. IR Spectra at 298 K. The most dramatic difference between the aluminas at 298 K can be viewed in the isotherm of the 8a mode. The amount of 2,6-lutidine adsorbed per unit surface area on K alumina is approximately 1.5 times that on 6-8 alumina, indicating that K alumina possesses more accessible adsorption sites than 6-8 alumina. Upon initial adsorption, a complex on K alumina is found at 1601 cm-'; the similar peak is located at 1599 cm-' for 6-8 alumina. This indicates that a slightly stronger adsorbate/adsorbent interaction occurs on K alumina. It can be seen that as the pressure of 2,6-lutidine is increased the intensity of the bands increases and the bands shift to lower frequencies. For example, the intensity of the 6,CH3, 8b, and 8a vibrations increases 14,16, and 17 times on 6-0 alumina and 6.7, 6.4, and 8.4 times on K alumina, respectively. The shifts in bands are similar for both (37) Brown, H.C.;Gintis, D.; Domash, L. J. Am. Chem. SOC.1956, 78, 5387. (38) Benesi, H. A. J . Catal. 1973,28, 176. (39) Peri, J. B. J. Phys. Chem. 1965, 69, 211.
Langmuir, Vol. 5, No. 1, 1989 121
IR and Microcalorimetric Study of Aluminas aluminas: from 0.666 Pa to 2.00 kPa for K alumina the band at 1456 cm-' shifts to 1454 cm-', the 1582-cm-' band shifts to 1582 cm-l, and the 1601-crn-' band shifts to 1593 cm-'. Quite surprisingly, upon evacuation a band appears at 1614 cm-' on both aluminas. This stretch is similar to the Py, stretch, representing a coordination complex. The peak seen upon evacuation is, therefore, assigned to coordination at a Lewis acid site. It was previously observed that 2,6-lutidine reacts selectively with hydroxyl groups; that is, hydroxyl groups are "titrated" first, followed by Lewis acid sites.21 Perhaps a certain "barrier" must be overcome before 2,g-lutidine can react with the Lewis acid sites on either 6-0 or K aluminas. The species identified by either the 1599- or 1614-cm-' band on 68and K aluminas is quite unstable (less than 5% remaining on the surface). This is due to increased steric hindrance caused by the methyl groups, which inhibit 2,6-lutidine's interaction with Lewis acid sites on alumina. 3. Pressure-Temperature Relationships. Isotherms from the 8a band are shown in Figure 7. The shapes of the isotherms are very different for the two solids. The isotherm of 6-0 alumina is very gradual whereas the K alumina isotherm is quite steep. More 2,6-lutidine is adsorbed on K alumina than on 6-0 alumina per unit surface area. 4. Effect of Increasing Temperature. A temperature of 483 K provides a demarcation in the infrared analysis. Below 483 K the infrared spectra of the two solids are similar, whereas at and above 483 K the spectra differ considerably. For instance, at 483 K the 6,CH3 stretch is quite different on the two solids: it is a doublet on 6-0 alumina and similar to a quartet on K alumina. Upon evacuation, sharp peaks (peak widths, measured at halfheight, are reported in parentheses in cm-') at 1459 (15), 1493 (29), 1514 (31), 1580 (62), and 1614 (8) cm-' are found on 6-0 alumina, but on K alumina three very broad peaks are evident: 1450 (58), 1474 (120), and 1576 (71) cm-'. At 583 K, although the 6,CH3 splitting on 6-0 alumina becomes more complex and pronounced, the shift of the 8a band, assigned to a Lewis acid site, is quite different for the two solids. For 6-0 alumina, at 6.66 Pa the band is located at 1601 (19) cm-l, at 133 Pa it is located at 1591 (46) cm-l, and upon evacuation at 0.133 Pa it is at 1609 (27) cm-'; for K alumina, at 6.66 Pa the 8a band is located at 1610 (10) cm-', it shifts to 1585 (75) cm-' at 133 Pa, and upon evacuation at 0.133 Pa it is located at 1575 (75) cm-'. At 783 K the bands on 6-8 alumina are quite broad, and only three peaks are identified. On K alumina much sharper peaks are seen. Upon evacuation at 0.133 Pa three broad peaks on 68 alumina are identified 1340 (1201,1460 (89), and 1572 (81) cm-'. On K alumina, sharper, but much less intense, peaks located at 1458 (87), 1491 (68), 1508 (46), and 1605 (79) cm-' are found. The surface of 6-0 alumina is altered significantly at 783 K and resembles the surface of K alumina at 583 and 783 K, as is shown in Figure 14. Another dramatic difference marked by an activation temperature of 483 K is that the stability of the coordinately bound species increases, suggesting that, because of the decrease of hydrogen-bonding interactions, more Lewis sites are available. However, on K alumina, at 483 K and above, band shifts are unpredictable. A t 483 and 583 K, the 8a band shifts to lower frequencies; it is only at 783 K that the band shifts to the expected higher frequency upon evacuation. Even at higher temperatures, the band shifts on 6-0 alumina are as expected. n -Butylamine. Adsorption studies using n-butylamine reveal whether any of the hydroxyl groups on alumina are
s I
L ,-
0
5
"A
0.15
n 4
0.0 1600 1400 Wavenumbers cm-'
Figure 14. Infrared spectra of 2,g-lutidine at 0.133 Pa: (a) 6 6 alumina at 783 K; (b) K alumina at 583 K. Absorbance units not
corrected for pellet weight and surface area.
acidic enough for proton-transfer reactions to occur (Bransted acid sites). Few infrared studies of this molecule on alumina or even silica-alumina have been reported in the l i t e r a t ~ r e .This ~ ~ ~is ~surprising ~ ~ ~ ~ considering that one of the most popular m e t h o d ~ ~ for~ determining p~~ both the a m o ~ n t ~ and " ~ distribution of acid sites& present on acidic solids involves the adsorption of n-butylamine. Although n-butylamine is not a commonly studied adsorbate for infrared investigations, numerous studies employing ammonia as a probe molecule have been conducted.32 Ammonia (pKBH+= 9.24)&is a weaker base than n-butylamine (pKBH+ = 10.63)4in water, yet proton sites on alumina have been detected by using ammonia as a probe mole~ule.~'Depending on the pretreatment of the sample, n-butylamine has also been shown to act as a proton acceptor on some aluminas.26 For example, employing n-butylamine as the probe molecule, Morimoto et al.%detected proton-transfer interactions when an alumina was exposed to air at 373 K; however, when the same alumina was activated in vacuo at 773 K, no proton sites were detected. In a recent study, conducted with alumina samples activated at 1023 K, no proton donor sites were detected when n-butylamine was used as the probe mole~ule.~" The most useful peak for the identification of protonic sites is the symmetric NH3+bending vibration (1550-1504 cm-')% because the asymmetric NH3+vibration (1600-1575 cm-') overlaps with the symmetric NH2 bending mode. The intensity of the NH3+band is relatively weak on 6-0 and K aluminas. In a study conducted by Morimoto et ala,% an intense peak was detected, and in a study of the adsorption of n-butylamine on zeolites,%intense peaks were also reported. Prior to adsorption studies the aluminas in the present study were treated at 353 K; although physically adsorbed water was eliminated, as monitored by infrared spectroscopy, hydroxyl groups, which would serve as the proton source for n-butylamine, remain on the surface. Further aid in making this assignment is provided by evacuation studies, which reveal a peak located at 1587 cm-l. This peak, together with the peak at 1506 cm-', is assigned to the asymmetric and symmetric NH3+stretching vibrations, respectively. Upon increasing adsorbate pressures the band at 1605 cm-' shifts toward higher frequencies. At an adsorbate (40)Sokoll, R.;Hobert, H.; Schmuck, I. J.Chem.SOC.,Faraday Trans. 1 1986,82, 3391.
(41)Tanabe, K.Solid Acids and Bases; Academic: New York, 1970. (42)Atkinson, D.;Curthoys, G. Chem. SOC.Rev. 1979, 8, 475. (43)Tamele, M.W. Discuss. Faraday SOC.1950, 8, 270. (44)Johnson, 0.J. Phys. Chem. 1955,59, 827. (45)Benesi, H.A. J. Phys. Chem. 1957, 61, 970. (46)Albert, A.; Serjeant, E. P. Ionization Constants of Acids and Bases; Wiley: New York, 1962.
Healy et al.
122 Langmuir, Vol. 5, No. 1, 1989
15. The trend of saturating the most energetic sites first followed by the less energetic sites was established and 8 g 1.20 discussed in detail for pyridine. The adsorption isotherms for the Lewis acid species and 1 >g = 1.00 0 0 the hydrogen-bondedspecies are shown in Figure 11. The 04 :z 0.80 isotherm for the Lewis acid coordination complex is eu 0 0 Langmuir like, whereas the hydrogen-bonded complex is $ ? 0.60 reminiscent of a type IV isotherm. A similar number of m2 e 0 =o 0.40 Lewis acid species are present on both solids; however, .e B there are approximately 1.5 times more hydrogen-bonded g .E a; 0.20 species between acetonitrile and 6-8 alumina than between 0 acetonitrile and K alumina. " -0.00 -2 -1 0 1 2 The shift in the CEN stretch caused by coordination Log (P) is, as stated previously, related to the strength of the bond formed.43 The displacement of the CEN stretch at 6.66 Figure 15. Ratio of Lewis acid coordination complex versus Pa on 6-0 alumina is +75 cm-l, whereas on K alumina it hydrogen-bonded/physicallyadsorbed acetonitrile on 6-8 and K aluminas at 298 K. is +81 cm-', with 2253 cm-' as the reference point for the unperturbed C=N stretchaa This indicates that a stronger pressure of 8.66 kPa on both 6-0 and K aluminas it is locoordination complex is formed between acetonitrile and cated at 1622-1624 cm-', which is similar to the frequency aluminum cations on K alumina than on 6 0 alumina. The of the gas phase for the NH3+stretching mode. This indisplacement on a 6 alumina29was +75 cm-' and for ancrease in frequency corresponds to hydrogen-bonded and other alumina3' +77 cm-'. In homogeneous solutions the physically adsorbed species of n - b ~ t y l a m i n e . ~ ~ shift of the C=N stretch upon coordination with SnCl,, The adsorption isotherms from data for the 1605-cm-l AlCl,, and BF3 was +51, +79, and +lo4 cm-', respectivepeak, which shifts to N 1622-1624 cm-l at higher pressures, ly.% It is interesting that the shifts for the aluminas and for 6-0 and K aluminas are shown in Figure 9. Similarly aluminum chloride are so similar. shaped, these isotherms are indicative of type IV isoCalorimetry. 1. Pyridine and t,&Lutidine. Intertherms: at higher pressures the slopes increase, indicating pretation of calorimetric data is enhanced by information an increased uptake of adsorbate in the pores.S6 Apobtained from infrared spectroscopy studies. The infrared proximately 1.5 times more physically adsorbed and hyspectra of pyridine on 6-0 and K aluminas were similar, drogen-bonded complexes are present on K alumina than although a lesser amount of hydrogen-bonded species was on 6-8 alumina at higher adsorbate pressures. The peaks present on the former than the latter. These species could indicative of proton-transfer interactions are not shown perhaps account for the fact that energetically the interon the isotherms; however, the concentration of this species action of pyridine with K alumina was slightly more exois independent of adsorbate pressure and is less than the thermic than with 6-0 alumina. The heats of immersion species characterized by the 1605-cm-' peak. As characof the two solids in 2,6-lutidine differ dramatically: the terized by absorbance values, 68 alumina possesses 5 times reaction is much more exothermic with K alumina than as many acidic hydroxyl groups per unit surface area than with 6-0 alumina. Factors which may contribute to this K alumina. observation include surface coverage and strength of adAcetonitrile. Of the adsorbates already discussed, sorbate/adsorbent interaction; as quantified with infrared acetonitrile is the smallest probe molecule. Ita adsorption spectroscopy, both are greater on K alumina than on 6-0 should be the least affected by the porosity and location alumina. of the aluminum cation in the solids.29 Few infrared 2. n -Butylamine. The heat of immersion of 6-0 aluspectroscopy studies have used acetonitrile or even deumina in n-butylamine is more exothermic than the analteriated acetonitrile on a l ~ m i n a . 'K ~ ~n ~o ~ i n g e found r ~ ~ ~ ~ ~ ogous heat with K alumina. Proton-transfer interactions that acetonitrile is hydrolyzed on alumina by basic hymust dominate both Lewis acid and hydrogen-bonding droxyl groups below 373 K and warned that it was a interactions. Re~ently,~' it was demonstrated on y alumisleading probe molecule. However, acetonitrile is a mina, with triethylamine as the probe, that the proton commonly used solvent in chromatographic separations, donor sites were significantly stronger than the Lewis acid and knowledge of its interactions with chromatographic sites. supports is desirable. 3. Acetonitrile. In acetonitrile a more exothermic heat The peaks at 2255 and 2334 cm-' are attributed to the of immersion was obtained for K alumina than 6-8 alumina. symmetric C=N stretching modes of two different adAs quantified by infrared spectroscopy, the coordination sorbed complexes. The former is attributed to a species complex formed with K alumina is stronger than the cohydrogen bonded or physically ads or bed;=^^^ the latter is ordination complex formed with 6-0 alumina, and it apa species attached to a Lewis acid site on the alumina.*31 pears that this interaction, which is more exothermic than The change in frequency is related to the strength of the hydrogen bonding, predominates. coordination complex Assignment of the Conclusions coordination complex is assisted by the fact that at low This report presents results of an infrared spectroscopy adsorbate coverage the peak at 2334 cm-' is more intense investigation of four basic adsorbates, acetonitrile, pyrithan that at 2255 cm-l; as the amount of adsorbate indine, 2,6-lutidine, and n-butylamine on 6-0 alumina and creases, the intensity of the peak associated with the Lewis K alumina. acid site decreases with respect to the hydrogen-bondInformation pertaining to the type, concentration, ed/ physically adsorbed species, as is illustrated in Figure strength, and environment of acid sites on the two aluminas is gleaned. With all of the adsorbates a Lewis acid (47) Low, M.J. D.; Bartner, P. L. Can. J. Chem. 1970,48, 7. U
-
-
~
(48)Zecchina, A.; Guglielminotti, E.; Coluccia, S.;Borello, E. J.Chem. SOC.,A 1969,2196 and references therein. (49) Angell, C.L.;Howell, M. V. J. Phys. Chem. 1969, 73, 2551.
S.J . Am. Chem. SOC.1966, 88, 919. (51) Aboul-Gheit, A. K.; Al-Hajjaji, M. A. Anal. Lett. 1987, 20, 553. (50) Purcell, K. F.; Drago, R.
Langmuir 1989,5, 123-128 coordination site, designated as an "outer" or "weaker" site, and hydrogen-bonded complexes are identified. Brernsted acid coordination is detected only when a strongly basic adsorbate, n-butylamine, is employed as the probe molecule. No discrimination between the concentration of Lewis acid sites on 6-6 and K aluminas is made by either pyridine or acetonitrile. The strength of the sites, as quantified with pyridine, is similar; however, with acetonitrile, a stronger coordination complex is formed on K alumina than on 6-8 alumina. The environment surrounding the acid site affects the adsorption of 2,6-lutidine; although it is a stronger base than pyridine and should, therefore, interact more strongly with alumina, only a weakly bound species is formed (see Tables I and 11). Although no intrinsic structural property differentiates the transition a l ~ m i n a sit, ~has ~ been suggested that an important variable is hydroxyl content.52 A major dif-
123
ference between the two aluminas is the extent of hydrogen-bonded species formed with an adsorbate. With larger adsorbates, pyridine, 2,6-lutidine, and n-butylamine, more hydrogen-bonded complexes occur on K alumina than on 6-8 alumina. With the smallest adsorbate studied, acetonitrile, there are more hydrogen-bonded species on 6-6 alumina than on K alumina, suggesting that the pore openings of the former are smaller than those of the latter.
Acknowledgment. The assistance of Dr. John W. Novak, Jr., Dr. Meg Martin Thompson, and Raymond Colbert is gratefully acknowledged. M.H.H. thanks the Exxon Education Foundation for support. Registry No. AlzOs, 1344-28-1; acetonitrile, 75-05-8; 2,6lutidine, 108-48-5;pyridine, 110-86-1;n-butylamine, 109-73-9. (52)Stone, F. S.;Whalley, L. J. Catal. 1967,8, 173.
Structure and Composition of Pt(ll1) and Pt(100) Surfaces as a Function of Electrode Potential in Aqueous Sulfide Solutions Nikola Batina, James W. McCargar, Ghaleb N. Salaita, Frank Lu, Laarni Laguren-Davidson, Chiu-Hsun Lin, and Arthur T. Hubbard* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 -0172 Received May 2, 1988. I n Final Form: August 15, 1988 Studies are reported in which surface layers formed by immersion of well-defined Pt(ll1) and Pt(100) electrode surfaces into aqueous NazS solutions were characterized with regard to structure, composition, and reactivity by means of low-energy electron diffraction (LEED),Auger electron spectroscopy, electron energy-loss spectroscopy (EELS), linear scan voltammetry, and coulometry. Voltammetry reveals that only oxidative desorption of S occurs on the Pt surfaces; no S reductive desorption is observed over the useful potential range. Combined surface analysis data (Auger),vibrational spectra (EELS),and structural data (LEED) permit identification of adsorbed layer composition and structure on the Pt(ll1) and Pt(100) surfaces as a function of potential. At potentials between -0.6 and 0.0 V (vs Ag/AgCI), LEED reveals that stable ordered adsorbed sulfur layers are formed on both surfaces: Pt(lll)(d3Xd3)R3O0-S and Pt(100)(d2Xd2)R45°-S. The best clarity of the LEED patterns is found at pH 9. Potentials more positive than 0.0 V give rise to increasingly diffuse intensity related to oxidative desorption of S. Voltammograms for oxidative desorption of S from both surfaces are markedly different, indicating different mechanisms of S oxidation at the two surfaces: at pH 9, four voltammetric peaks are present for S at the Pt(ll1) surface, compared with only one peak for the Pt(100) surface. Coulometric data reveal that approximately six electrons are transferred in oxidation of adsorbed S at both surfaces at pH less than 10. Voltammetric behavior of the sulfur layer is sharply dependent upon pH.
Introduction In previous papers, we reported that ordered layers, adlattices consisting primarily of uncharged adsorbed species, are formed when well-defined Pt(ll1) and Pt(100) surfaces are immersed into aqueous ionic solution^.^-^ Several aspects of surface behavior in these adlattice systems have been investigated, such as relative retention affinities of cations?' pH and potential dependencies of (1) Sticknev, J. L.; Rosasco, S. D.; Salaita, G. N.: Hubbard, A. T. Langmuir 1985, 1, 66. (2)Salaita, G.N.;Stern, D. A.; Lu, F.; Baltruschat, H.; Schardt, B. S.; Stickney, J. L.; Soriaga, M. P.; Frank, D. G., Hubbard, A. T. Langmuir 1986. 2. 828. (3) Stern, D. A.; Baltruschat, H.; Martinez, M.; Stickney, J. L.; Song, D.; Lewis, S. K.; Frank, D. G.; Hubbard, A. T. J.Electround Chem. 1987, 217. 101. (4) Lu, F.; Salaita, G. N.; Baltruschat, H.; Hubbard, A. T. J. Electroanal. Chem. 1987,222, 305. ~~
(5)Salaita, G. N.;Lu, F.; Laguren-Davidson, L.; Hubbard, A. T. J. Electroanal. Chem. 1987, 229, 1.
0743-7463/89/2405-0123$01.50/0
electrochemical adsorption-desorption p r o c e s s e ~ ,and ~*~ metal electrodeposition at electrodes with a well-characterized surface adlatti~e."'~ Such studies are found to reflect the structure and intermolecular interactions within (6)Rosasco, S. D.; Stickney, J. L.; Salaita, G. N.; Frank, D. G.; Katekaru, J. y.;Schardt, B. C.; Soriaga, M. P.; Stern, D. A.; Hubbard, A. T. J. Electroanal. Chem. 1985,188,95. (7)Frank, D. G.;Katekaru, J. Y.; Rosasco, S. D.; Salaita, G. N., Schardt, B. C.; Soriaga, M. P.; Stern, D. A.; Stickney, J. L.; Hubbard, A. T. Langmuir 1985,1, 587. (8) Stickney, J. L.; Rosssco, S. D.; Song, D.; Soriaga, M. P.; Hubbard, A. T. Surf. Sci. 1983, 130, 326. (9)Stickney, J. L; Rmasco, S. D.; Hubbard, A. T. J. Electrochem. SOC. 1984,131, 260. (10)Stickney, J. L.; Stern, D. A.; Schardt, B. C.; Zapien, D. C.; Wieckowski, A.; Hubbard, A. T. J.Electroanal. Chem. 1986, 213, 293. (11)Stickney, J. L.; Schardt, B. C.; Stern, D. A.; Wieckowski, A.; Hubbard, A. T. J.Electrochem. SOC. 1986, 133, 648. (12)Schardt, B. C.;Stickney, 3. L.; Stern, D. A.; Wieckowski, A.; Zapien, D. C.; Hubbard, A. T. Langmuir 1987, 3, 239. (13)Schardt, B. C.;Stickney, 3. L.; Stern, D. A.; Wieckowski, A.; Zapien, D. C.; Hubbard, A. T. Surf. Sci. 1986, 175, 520.
0 1989 American Chemical Society
