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Infrared Spectroscopic Characterization of the Alumina Surface
Journal of Colloid and Interface Science 209, 16 –19 (1999) Article ID jcis.1998.5735, available online at http://www.idealibrary.com on
Infrared Spectroscopic Characterization of the Alumina Surface 1. Infrared Spectra of Cr(CO)6 Adsorbed at Different Surface Sites on g-Alumina E. Baumgarten1 and P. Dick Institut fuer Physikalische Chemie und Elektrochemie, Abt. f. Angew. Physikalische Chemie, Heinrich-Heine Universita¨t, 40225 Du¨sseldorf, Germany Received January 29, 1998; accepted June 29, 1998
sensitive method. An optimal IR probe molecule should possess the following properties:
Infrared spectra of Cr(CO)6 adsorbed at different surface centers on g-alumina were investigated. The spectra of the probe complex revealed the existence of three types of strong Lewis centers and that of physisorbed Cr(CO)6. In the case of the physisorbed complex Oh symmetry was widely conserved, and addition to strong Lewis centers led to lowering of the symmetry. The assumption of C4v symmetry, caused by interaction of one CO group of the complex with a Lewis center, may explain the spectra. Approximate values for the integral extinction coefficients were determined. © 1999 Academic Press Key Words: alumina surface characterization; adsorption of chromium hexacarbonyl; Lewis surface sites (characterization of).
1. It should distinguish between Lewis and Bronsted centers. 2. The IR bands of molecules adsorbed on different centers should be separated as well as possible. 3. The extinction coefficient should be sufficiently high to give a signal even for the probe molecules adsorbed on centers present in low numbers on the surface only. 4. The probe molecule should be thermally stable on the oxide. 5. The adsorption should not be too strong, in order to be able to measure adsorption isotherms.
1. INTRODUCTION
Literature studies and our own preliminary experiments led to the choice of chromium hexacarbonyl as a probe and of CO for comparison. Chromium hexacarbonyl was used by Zecchina et al. (15, 16) too, who found two types of centers—with well-separated carbonyl bands on alumina pretreated at 1073 K—that were attributed to tetrahedrally and octahedrally coordinated surface aluminum ions. Because these authors made qualitative measurements only, they did not observe that the number of centers is much lower than that of the aluminum ions on the surface, even considering that they are partially covered by hydroxyl ions even at elevated temperatures. Our investigations about the adsorption of Cr(CO)6 on alumina is presented in two papers: the first paper describes the separation of the spectra of the probe on the different centers, based on a semiquantitative method, and serves for a qualitative discussion. In the second paper a band analysis of the separated spectra is the basis for model calculations, allowing measurement of the center numbers and a quantitative description of Lewis strength. In this first paper we present the experimental spectra of adsorption experiments with different amounts of Cr(CO)6 at g-alumina pretreated at various temperatures, and we derive approximate quantitative values, which may be used as starting values for extended calculations. The separated spectra for Cr(CO)6 on different types of centers are also the basis for a qualitative discussion of the adsorption systems.
Transition aluminas play an eminent role in catalysis because they may be used as catalysts directly for different reactions as well as for supporting other catalytically active substances as noble metals. Even under the latter conditions the supporting aluminas may contribute to the overall reaction on the catalyst; i.e., alumina is an active support, and the complete catalyst is a bifunctional one. Our knowledge about the alumina surface is rather limited compared to it’s great importance. This is caused by the fact that the transition aluminas consist of very small primary crystallites, which are hydroxylated at the surface. Comparison of reaction rates with pretreatment temperatures reveals (1, 2) that catalytically active centers are formed with proceeding dehydration after heat treatment above 570 K. As the number of centers— estimated by poisoning experiments—is only on the order of a few percents of the surface places, most surface analytical methods fail. Probably the best way to investigate the centers is the application of adsorbed probe molecules. By using different spectroscopic techniques (nuclear magnetic resonance, electron spin resonance, infrared) (3–14), it was possible to show that Lewis centers are responsible for the reactivity of aluminas. Infrared (IR) spectroscopy turned out to be an especially 1
To whom correspondence should be addressed.
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
16
17
IR SPECTRA OF Cr(CO)6
FIG. 2. Cr(CO)6 adsorbed on g-Al2O3 pretreated at 453 K at equilibrium pressures from 0.31 to 3.25 Pa.
Difference spectra may be obtained, using log(I 0 ( n˜ )/I( n˜ )) of the reference band instead of that of the baseline in [1]. 3. EXPERIMENTAL RESULTS AND DISCUSSION
The experimental results with increasing temperatures of pretreatment are shown in Figs. 2– 6. For clarity only some of the measurements at different equilibrium pressures of Cr(CO)6 are shown. In principle the absorbance A at any given wavenumber n˜ is given by FIG. 1. Measuring cell (opened) with holder for waver.
A~ n˜ ! 5 m w
2. EXPERIMENTAL
The experiments were executed in a measuring cell, shown in Fig. 1 in exploded form, which allowed a heat treatment under vacuum and measurements near room temperature (315 K). The vacuum line was equipped with a dosing system for Cr(CO)6 and a manometer (Balzers TPG 300 with Pirani valve TPR 010). The g-alumina wafers (A 2.2 cm, m > 50 mg) were made from g-Al2O3 type C (Degussa), cleaned from chloride by a special hydrolysis process (17). Cr(CO)6 (Fluka, 98%) was added stepwise, with spectroscopic and pressure control of equilibration. The spectra were taken with a Perkin–Elmer PE 882 spectrometer with a PC data station and are shown in the form of absorbances vs wavenumber. The samples were pretreated at 453, 713, 853, 953, and 1053 K. Quantitative evaluations were made using integral absorbances Ai 5
E F S D log
band
I 0~ n˜ ! I~ n˜ !
S D G
2 log spectrum
I 0~ n˜ ! I~ n˜ !
,
A BET z Aw
O ~G z e ~n˜ !!, n
i
i
i51
a form of Lambert Beer’s law for n surface species, where m w 5 mass of waver/g, A w 5 geometric area of waver/m 2 , A BET 5 BET area of oxide/m 2 g 21 , and G i 5 surface concentration/ mol m 21 . The complete calculation of all e i values requires good starting values to be successful. The following stepwise separation of the spectra of the different adsorption species
[1]
baseline
with the baseline calculated from 13 points on each side to minimize noise effect.
FIG. 3. Cr(CO)6 adsorbed on g-Al2O3 pretreated at 653 K at equilibrium pressures from 0.3 to 4.4 Pa.
18
BAUMGARTEN AND DICK
FIG. 4. Cr(CO)6 adsorbed on g-Al2O3 pretreated at 713 K at equilibrium pressures from 0.06 to 2.5 Pa.
FIG. 6. Cr(CO)6 adsorbed on g-Al2O3 pretreated at 1053 K at equilibrium pressures from ,0.04 to 2.0 Pa.
coefficient for the L1 system, assuming that the physisorption is not influenced by the degree of surface hydroxylation: allows us to derive these starting values which are used in Part 2 of this study and are also the basis for the qualitative discussion in this paper. The spectra given in Fig. 2 are caused by adsorption on a completely hydroxylated alumina surface. Only one band system can be seen, evidently caused by physisorption. From measurements of the equilibrium pressure and from the maximal or integral absorbances it is easy to calculate the corresponding extinction coefficients and adsorption isotherms. All extinction coefficients given in this paper are integral values integrated over the complete spectral area (1500 –2300 cm21).
From the A(L1) values and the equilibrium Cr(CO)6 pressures the integral extinction coefficient (integrated over the whole spectral area again) was obtained:
e i~phys! 5 ~1.118 6 0.105! z 10 28 cm mol21.
e i~L1! 5 ~1.350 6 0.101! z 10 28 cm mol21.
The spectra obtained after pretreatment at 713 K (Fig. 3) show one additional band system caused by adsorption on Lewis centers (15, 16, 18 –23) (L1 centers). Using the information from Fig. 2 (extinction coefficient and adsorption isotherm) it is possible to calculate the isotherm and the extinction
After a pretreatment at 853 K (Fig. 5) a further band system appeared, called the L2 system. At lower coverages its spectrum was widely uninfluenced by both physisorption and L1 spectra, indicating a stronger interaction of the probe with L2 centers than with the other centers. So the subtraction of the physisorption and the L1 spectra (executed like that of the physisorption spectrum in order to isolate the L1 spectrum before) was but a correction. Consequently, this spectrum is comparatively trustworthy.
G~L1! 5 G~observed! 2 G~phys! G~phys! 5 f~ p/p 0! ~isotherm! A~L1! 5 A~observed! 2 A~phys! A~phys! 5 e i~phys! z G~phys! z m w z A BET/A w.
e i~L2! 5 ~1.355 6 0.105! z 10 28 cm mol21.
FIG. 5. Cr(CO)6 adsorbed on g-Al2O3 pretreated at 853 K at equilibrium pressures from 0.06 to 2.2 Pa.
Provided that only physisorption and L1 and L2 centers were present on the alumina, any experimental spectrum could be built up from the three spectra given, but it turned out that there was a systematic deviation that increased with the temperature of the pretreatment. This deviation may be attributed to Cr(CO)6 adsorbed on a third type of Lewis center (L3). Because its spectrum is always accompanied by those of the other ones, its separation— executed in the same way as that of the other species—is less precise; especially position and absorbance of the bands in the 1950 –2050 cm21 region (with strong
19
IR SPECTRA OF Cr(CO)6
overlap) must be seen as rough approximations. This leads also to a greater standard deviation of the extinction coefficient:
TABLE 1 Spectral Activity of Vibrations of Molecules in Corresponding Races in Oh and C4v Symmetry
e i~L3! 5 ~1.443 6 0.373! z 10 28 cm mol21. The separated spectra shown in Fig. 7 serve for the following qualitative discussion. In the physisorption spectrum mainly the T 1u band, active in the undistorted complex with O h symmetry, is to be seen at 1982 cm21 (2000 cm21 in the gas phase). It is accompanied by two weak bands at 2029 (E g ) and 2115 cm21 ( A 1g ), which indicate a slight distortion as they are inactive for gas phase Cr(CO)6 with O h symmetry but are observed for Cr(CO)6 dissolved in acetonitrile too. The spectrum caused by the first type of Lewis center (L1) indicates a lowering of the symmetry to C 4v showing bands at 1867 cm21 ( A 1 ) 3 , 2013 cm21 (E), 2061 cm21 (B 1 ), and 2125 cm21 ( A 1 ) 1 , typical for interactions of hexacarbonyls with Lewis acid centers (15, 16, 18 –23). The spectrum of Cr(CO)6 adsorbed on L2 centers (Fig. 7) widely resembles that from the L1 centers, but the band shifts relative to the position of the gas band are greater than those in the case of the L1 spectrum: 1757 cm21 ( A 1 ) 3 , 2034 cm21 (E), 2088 cm21 (B 1 ), and 2137 cm21 ( A 1 ) 1 . The bands of the L3 system are observed at intermediate positions: 1803 cm21 ( A 1 ) 3 , 2026 cm21 (E), 2078 cm21 (B 1 ), and 2133 cm21 ( A 1) 1. In contrast to the spectrum of physisorbed Cr(CO)6 showing wide O h symmetry, all three types of spectra resulting from adsorption of chromium hexacarbonyl on the different Lewis acid centers show great similarities with additional bands, indicating lowered symmetry. The bands observed are in agreement with C 4v symmetry (23). This indicates a structure with one carbonyl group interacting with a Lewis center. The ( A 1 ) 3 band shows the greatest shift to lower wavenumbers (up to 240 cm21 relative to the T 1v vibration of the gaseous
O h symmetry IR
Raman
A 1g Eg
— —
a a
T 1u
a
—
(A 1 ) 1 (A 1 ) 2 B1 (A 1 ) 3 E
IR
Raman
a a — a a
a a a a a
substance) if Cr(CO)6 is adsorbed on a Lewis center. The other bands are slightly shifted to higher wavenumbers under these conditions. Thus the ( A 1 ) 3 band makes Cr(CO)6 an excellent probe, superior to CO, showing much smaller shifts when it interacts with the same Lewis centers. Table 1 shows IR and Raman activities of vibrations in O h and C 4v symmetry. REFERENCES 1. 2. 3. 4. 5.
6. 7.
8. 9. 10. 11. 12. 13.
14. 15. 16. 17. 18. 19. 20. 21. 22. 23. FIG. 7. Spectra of Cr(CO)6 adsorbed on different strong Lewis acid centers (L1–L3) (normalized to nearly equal absorbances of the ( A 1 ) 3 bands).
C 4v symmetry
McIver, D. S., Wilmot, W. H., and Bridge, J. M., J. Catal. 3, 502 (1964). Kno¨zinger, H., and Ratnasamy, P., Catal. Rev. 17, 31 (1978). Kno¨zinger, H., Adv. Catal. 25, 184 (1976). Zecchina, A., Garrone, E., and Guglielminotti, E., “Catalysis,” Vol. 6, p. 90. R. Chem. Soc., London, 1983. Boehm, H-P., and Kno¨zinger, H., in “Catalysis—Science and Technologie” (J. R. Anderson and M. Boudart, Eds.), Vol. 4, p. 39. Springer-Verlag, Berlin/Heidelberg/New York, 1983. Paukshtis, E. A., and Youchenko, E. N., Russ. Chem. Rev. 52, 42 (1983). Peri, J. B., in “Catalysis—Science and Technologie” (J. R. Anderson and M. Boudart, Eds.), Vol. 5, p. 171. Springer-Verlag, Berlin/Heidelberg/ New York, 1984. Majors, P. D., and Ellis, P. D., J. Am. Chem. Soc. 109, 1648 (1987). Morris, H. D., and Ellis, P. D., J. Am. Chem. Soc. 111, 6045 (1989). Chen, F. R., Davis, J. G., and Fripiat, J. J., J. Catal. 133, 263 (1992). Mastikhin, V. M., Mudrakowsky, I. C., and Filimonova, S. V., Chem. Phys. Lett. 149,2, 175 (1988). Mastikhin, V. M., Colloids Surf. A 78, 143 (1993). Yong, H., Coster, D., Chen, F. R., Davis, J. G., and Fripiat, J. J., in “New Frontiers in Catalysis,” (L. Guczl et al., Eds.), p. 1159. Elsevier, Amsterdam/New York, 1993. Sheng, T.-C., and Gay, I. D., J. Catal. 145, 10 (1994). Zecchina, A., Platero, E. E., and Arean, C. O., Inorg. Chem. 27, 102 (1988). Platero, E. E., and Mentruit, M. P., Inorg. Chem. 33, 1506 (1994). Dedek, D., Dissertation, Heinrich-Heine Universita¨t Du¨sseldorf, 1994. Bilhou, J. C., Theolier, A., Smith, A. K., and Besset, J. M., J. Mol. Catal. 3, 245 (1977/1978). Guglielminotti, E., J. Mol. Catal. 13, 207 (1981). Fubini, B., Giamello, E., Guglielminotti, E., and Zecchina, A., J. Mol. Catal. 32, 219 (1985). Zecchina, A., Rao, K. M., Coluccia, S., Platero, E. E., and Arean, C. O., J. Mol. Catal. 53, 397 (1989). Coluccia, S., Marchese, L., Martra, G., Spoto, G., Zecchina, A., and Louis, C., J. Mol. Catal. 60, 71 (1990). Braterman, P. S., in “Metall Carbonyl Spectra” (P. M. Maitlis, F. G. A. Stone, and R. West, Eds.). Academic Press, London/New York/San Francisco, 1975.
Infrared Spectroscopic Characterization of the Alumina Surface 1. Infrared Spectra of Cr(CO)6 Adsorbed at Different Surface Sites on g-Alumina E. Baumgarten1 and P. Dick Institut fuer Physikalische Chemie und Elektrochemie, Abt. f. Angew. Physikalische Chemie, Heinrich-Heine Universita¨t, 40225 Du¨sseldorf, Germany Received January 29, 1998; accepted June 29, 1998
sensitive method. An optimal IR probe molecule should possess the following properties:
Infrared spectra of Cr(CO)6 adsorbed at different surface centers on g-alumina were investigated. The spectra of the probe complex revealed the existence of three types of strong Lewis centers and that of physisorbed Cr(CO)6. In the case of the physisorbed complex Oh symmetry was widely conserved, and addition to strong Lewis centers led to lowering of the symmetry. The assumption of C4v symmetry, caused by interaction of one CO group of the complex with a Lewis center, may explain the spectra. Approximate values for the integral extinction coefficients were determined. © 1999 Academic Press Key Words: alumina surface characterization; adsorption of chromium hexacarbonyl; Lewis surface sites (characterization of).
1. It should distinguish between Lewis and Bronsted centers. 2. The IR bands of molecules adsorbed on different centers should be separated as well as possible. 3. The extinction coefficient should be sufficiently high to give a signal even for the probe molecules adsorbed on centers present in low numbers on the surface only. 4. The probe molecule should be thermally stable on the oxide. 5. The adsorption should not be too strong, in order to be able to measure adsorption isotherms.
1. INTRODUCTION
Literature studies and our own preliminary experiments led to the choice of chromium hexacarbonyl as a probe and of CO for comparison. Chromium hexacarbonyl was used by Zecchina et al. (15, 16) too, who found two types of centers—with well-separated carbonyl bands on alumina pretreated at 1073 K—that were attributed to tetrahedrally and octahedrally coordinated surface aluminum ions. Because these authors made qualitative measurements only, they did not observe that the number of centers is much lower than that of the aluminum ions on the surface, even considering that they are partially covered by hydroxyl ions even at elevated temperatures. Our investigations about the adsorption of Cr(CO)6 on alumina is presented in two papers: the first paper describes the separation of the spectra of the probe on the different centers, based on a semiquantitative method, and serves for a qualitative discussion. In the second paper a band analysis of the separated spectra is the basis for model calculations, allowing measurement of the center numbers and a quantitative description of Lewis strength. In this first paper we present the experimental spectra of adsorption experiments with different amounts of Cr(CO)6 at g-alumina pretreated at various temperatures, and we derive approximate quantitative values, which may be used as starting values for extended calculations. The separated spectra for Cr(CO)6 on different types of centers are also the basis for a qualitative discussion of the adsorption systems.
Transition aluminas play an eminent role in catalysis because they may be used as catalysts directly for different reactions as well as for supporting other catalytically active substances as noble metals. Even under the latter conditions the supporting aluminas may contribute to the overall reaction on the catalyst; i.e., alumina is an active support, and the complete catalyst is a bifunctional one. Our knowledge about the alumina surface is rather limited compared to it’s great importance. This is caused by the fact that the transition aluminas consist of very small primary crystallites, which are hydroxylated at the surface. Comparison of reaction rates with pretreatment temperatures reveals (1, 2) that catalytically active centers are formed with proceeding dehydration after heat treatment above 570 K. As the number of centers— estimated by poisoning experiments—is only on the order of a few percents of the surface places, most surface analytical methods fail. Probably the best way to investigate the centers is the application of adsorbed probe molecules. By using different spectroscopic techniques (nuclear magnetic resonance, electron spin resonance, infrared) (3–14), it was possible to show that Lewis centers are responsible for the reactivity of aluminas. Infrared (IR) spectroscopy turned out to be an especially 1
To whom correspondence should be addressed.
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
16
17
IR SPECTRA OF Cr(CO)6
FIG. 2. Cr(CO)6 adsorbed on g-Al2O3 pretreated at 453 K at equilibrium pressures from 0.31 to 3.25 Pa.
Difference spectra may be obtained, using log(I 0 ( n˜ )/I( n˜ )) of the reference band instead of that of the baseline in [1]. 3. EXPERIMENTAL RESULTS AND DISCUSSION
The experimental results with increasing temperatures of pretreatment are shown in Figs. 2– 6. For clarity only some of the measurements at different equilibrium pressures of Cr(CO)6 are shown. In principle the absorbance A at any given wavenumber n˜ is given by FIG. 1. Measuring cell (opened) with holder for waver.
A~ n˜ ! 5 m w
2. EXPERIMENTAL
The experiments were executed in a measuring cell, shown in Fig. 1 in exploded form, which allowed a heat treatment under vacuum and measurements near room temperature (315 K). The vacuum line was equipped with a dosing system for Cr(CO)6 and a manometer (Balzers TPG 300 with Pirani valve TPR 010). The g-alumina wafers (A 2.2 cm, m > 50 mg) were made from g-Al2O3 type C (Degussa), cleaned from chloride by a special hydrolysis process (17). Cr(CO)6 (Fluka, 98%) was added stepwise, with spectroscopic and pressure control of equilibration. The spectra were taken with a Perkin–Elmer PE 882 spectrometer with a PC data station and are shown in the form of absorbances vs wavenumber. The samples were pretreated at 453, 713, 853, 953, and 1053 K. Quantitative evaluations were made using integral absorbances Ai 5
E F S D log
band
I 0~ n˜ ! I~ n˜ !
S D G
2 log spectrum
I 0~ n˜ ! I~ n˜ !
,
A BET z Aw
O ~G z e ~n˜ !!, n
i
i
i51
a form of Lambert Beer’s law for n surface species, where m w 5 mass of waver/g, A w 5 geometric area of waver/m 2 , A BET 5 BET area of oxide/m 2 g 21 , and G i 5 surface concentration/ mol m 21 . The complete calculation of all e i values requires good starting values to be successful. The following stepwise separation of the spectra of the different adsorption species
[1]
baseline
with the baseline calculated from 13 points on each side to minimize noise effect.
FIG. 3. Cr(CO)6 adsorbed on g-Al2O3 pretreated at 653 K at equilibrium pressures from 0.3 to 4.4 Pa.
18
BAUMGARTEN AND DICK
FIG. 4. Cr(CO)6 adsorbed on g-Al2O3 pretreated at 713 K at equilibrium pressures from 0.06 to 2.5 Pa.
FIG. 6. Cr(CO)6 adsorbed on g-Al2O3 pretreated at 1053 K at equilibrium pressures from ,0.04 to 2.0 Pa.
coefficient for the L1 system, assuming that the physisorption is not influenced by the degree of surface hydroxylation: allows us to derive these starting values which are used in Part 2 of this study and are also the basis for the qualitative discussion in this paper. The spectra given in Fig. 2 are caused by adsorption on a completely hydroxylated alumina surface. Only one band system can be seen, evidently caused by physisorption. From measurements of the equilibrium pressure and from the maximal or integral absorbances it is easy to calculate the corresponding extinction coefficients and adsorption isotherms. All extinction coefficients given in this paper are integral values integrated over the complete spectral area (1500 –2300 cm21).
From the A(L1) values and the equilibrium Cr(CO)6 pressures the integral extinction coefficient (integrated over the whole spectral area again) was obtained:
e i~phys! 5 ~1.118 6 0.105! z 10 28 cm mol21.
e i~L1! 5 ~1.350 6 0.101! z 10 28 cm mol21.
The spectra obtained after pretreatment at 713 K (Fig. 3) show one additional band system caused by adsorption on Lewis centers (15, 16, 18 –23) (L1 centers). Using the information from Fig. 2 (extinction coefficient and adsorption isotherm) it is possible to calculate the isotherm and the extinction
After a pretreatment at 853 K (Fig. 5) a further band system appeared, called the L2 system. At lower coverages its spectrum was widely uninfluenced by both physisorption and L1 spectra, indicating a stronger interaction of the probe with L2 centers than with the other centers. So the subtraction of the physisorption and the L1 spectra (executed like that of the physisorption spectrum in order to isolate the L1 spectrum before) was but a correction. Consequently, this spectrum is comparatively trustworthy.
G~L1! 5 G~observed! 2 G~phys! G~phys! 5 f~ p/p 0! ~isotherm! A~L1! 5 A~observed! 2 A~phys! A~phys! 5 e i~phys! z G~phys! z m w z A BET/A w.
e i~L2! 5 ~1.355 6 0.105! z 10 28 cm mol21.
FIG. 5. Cr(CO)6 adsorbed on g-Al2O3 pretreated at 853 K at equilibrium pressures from 0.06 to 2.2 Pa.
Provided that only physisorption and L1 and L2 centers were present on the alumina, any experimental spectrum could be built up from the three spectra given, but it turned out that there was a systematic deviation that increased with the temperature of the pretreatment. This deviation may be attributed to Cr(CO)6 adsorbed on a third type of Lewis center (L3). Because its spectrum is always accompanied by those of the other ones, its separation— executed in the same way as that of the other species—is less precise; especially position and absorbance of the bands in the 1950 –2050 cm21 region (with strong
19
IR SPECTRA OF Cr(CO)6
overlap) must be seen as rough approximations. This leads also to a greater standard deviation of the extinction coefficient:
TABLE 1 Spectral Activity of Vibrations of Molecules in Corresponding Races in Oh and C4v Symmetry
e i~L3! 5 ~1.443 6 0.373! z 10 28 cm mol21. The separated spectra shown in Fig. 7 serve for the following qualitative discussion. In the physisorption spectrum mainly the T 1u band, active in the undistorted complex with O h symmetry, is to be seen at 1982 cm21 (2000 cm21 in the gas phase). It is accompanied by two weak bands at 2029 (E g ) and 2115 cm21 ( A 1g ), which indicate a slight distortion as they are inactive for gas phase Cr(CO)6 with O h symmetry but are observed for Cr(CO)6 dissolved in acetonitrile too. The spectrum caused by the first type of Lewis center (L1) indicates a lowering of the symmetry to C 4v showing bands at 1867 cm21 ( A 1 ) 3 , 2013 cm21 (E), 2061 cm21 (B 1 ), and 2125 cm21 ( A 1 ) 1 , typical for interactions of hexacarbonyls with Lewis acid centers (15, 16, 18 –23). The spectrum of Cr(CO)6 adsorbed on L2 centers (Fig. 7) widely resembles that from the L1 centers, but the band shifts relative to the position of the gas band are greater than those in the case of the L1 spectrum: 1757 cm21 ( A 1 ) 3 , 2034 cm21 (E), 2088 cm21 (B 1 ), and 2137 cm21 ( A 1 ) 1 . The bands of the L3 system are observed at intermediate positions: 1803 cm21 ( A 1 ) 3 , 2026 cm21 (E), 2078 cm21 (B 1 ), and 2133 cm21 ( A 1) 1. In contrast to the spectrum of physisorbed Cr(CO)6 showing wide O h symmetry, all three types of spectra resulting from adsorption of chromium hexacarbonyl on the different Lewis acid centers show great similarities with additional bands, indicating lowered symmetry. The bands observed are in agreement with C 4v symmetry (23). This indicates a structure with one carbonyl group interacting with a Lewis center. The ( A 1 ) 3 band shows the greatest shift to lower wavenumbers (up to 240 cm21 relative to the T 1v vibration of the gaseous
O h symmetry IR
Raman
A 1g Eg
— —
a a
T 1u
a
—
(A 1 ) 1 (A 1 ) 2 B1 (A 1 ) 3 E
IR
Raman
a a — a a
a a a a a
substance) if Cr(CO)6 is adsorbed on a Lewis center. The other bands are slightly shifted to higher wavenumbers under these conditions. Thus the ( A 1 ) 3 band makes Cr(CO)6 an excellent probe, superior to CO, showing much smaller shifts when it interacts with the same Lewis centers. Table 1 shows IR and Raman activities of vibrations in O h and C 4v symmetry. REFERENCES 1. 2. 3. 4. 5.
6. 7.
8. 9. 10. 11. 12. 13.
14. 15. 16. 17. 18. 19. 20. 21. 22. 23. FIG. 7. Spectra of Cr(CO)6 adsorbed on different strong Lewis acid centers (L1–L3) (normalized to nearly equal absorbances of the ( A 1 ) 3 bands).
C 4v symmetry
McIver, D. S., Wilmot, W. H., and Bridge, J. M., J. Catal. 3, 502 (1964). Kno¨zinger, H., and Ratnasamy, P., Catal. Rev. 17, 31 (1978). Kno¨zinger, H., Adv. Catal. 25, 184 (1976). Zecchina, A., Garrone, E., and Guglielminotti, E., “Catalysis,” Vol. 6, p. 90. R. Chem. Soc., London, 1983. Boehm, H-P., and Kno¨zinger, H., in “Catalysis—Science and Technologie” (J. R. Anderson and M. Boudart, Eds.), Vol. 4, p. 39. Springer-Verlag, Berlin/Heidelberg/New York, 1983. Paukshtis, E. A., and Youchenko, E. N., Russ. Chem. Rev. 52, 42 (1983). Peri, J. B., in “Catalysis—Science and Technologie” (J. R. Anderson and M. Boudart, Eds.), Vol. 5, p. 171. Springer-Verlag, Berlin/Heidelberg/ New York, 1984. Majors, P. D., and Ellis, P. D., J. Am. Chem. Soc. 109, 1648 (1987). Morris, H. D., and Ellis, P. D., J. Am. Chem. Soc. 111, 6045 (1989). Chen, F. R., Davis, J. G., and Fripiat, J. J., J. Catal. 133, 263 (1992). Mastikhin, V. M., Mudrakowsky, I. C., and Filimonova, S. V., Chem. Phys. Lett. 149,2, 175 (1988). Mastikhin, V. M., Colloids Surf. A 78, 143 (1993). Yong, H., Coster, D., Chen, F. R., Davis, J. G., and Fripiat, J. J., in “New Frontiers in Catalysis,” (L. Guczl et al., Eds.), p. 1159. Elsevier, Amsterdam/New York, 1993. Sheng, T.-C., and Gay, I. D., J. Catal. 145, 10 (1994). Zecchina, A., Platero, E. E., and Arean, C. O., Inorg. Chem. 27, 102 (1988). Platero, E. E., and Mentruit, M. P., Inorg. Chem. 33, 1506 (1994). Dedek, D., Dissertation, Heinrich-Heine Universita¨t Du¨sseldorf, 1994. Bilhou, J. C., Theolier, A., Smith, A. K., and Besset, J. M., J. Mol. Catal. 3, 245 (1977/1978). Guglielminotti, E., J. Mol. Catal. 13, 207 (1981). Fubini, B., Giamello, E., Guglielminotti, E., and Zecchina, A., J. Mol. Catal. 32, 219 (1985). Zecchina, A., Rao, K. M., Coluccia, S., Platero, E. E., and Arean, C. O., J. Mol. Catal. 53, 397 (1989). Coluccia, S., Marchese, L., Martra, G., Spoto, G., Zecchina, A., and Louis, C., J. Mol. Catal. 60, 71 (1990). Braterman, P. S., in “Metall Carbonyl Spectra” (P. M. Maitlis, F. G. A. Stone, and R. West, Eds.). Academic Press, London/New York/San Francisco, 1975.