1760
J. PhyS. Chem. 1982, 86, 1760-1763
mechanism. Accordingly, the lowest LF excited state of A6Br should labilize the NH3 trans to Br-, that of cis-Brz should labilize a NH3 trans to Br- or a Br- about equally, and that of tram-Brz should labilize Br-. Qualitatively, these results are consistent with the results here, although the stereochemical origin of the labilized NH3 can only be inferred from the observatiorP that NH3is labilized in the LF photochemistry of aqueous trans-Rh(en),(NH3)Brz+. (14)Vanquickenbome, L.G.;Cedemans, A. J. Am. Chem. Soc. 1977, 99, 2208.
(15)Clark, S. F.;Petersen, J. D. Znorg. Chem. 1979,18,3394. (16)Thomas, T. R.; Crosby, G. A. J. Mol. Spectrosc. 1971,38, 118.
Such trans labilization can be attributed to the fact that the lowest energy LF excited state will be those which have excitation largely localized along the ligand axes having the weakest average a-donor strengths. Certainly the requirement in these cases that the NH3 labilized be trans to a Br- is consistent with the very small $NH, and k N H 3 values noted for trans-Brz. Acknowledgment. This research was supported by granta (to P.C.F. and D.M.) from the US. National Science Foundation, by a NATO Research Grant (to P.C.F. and L.S.), and by the Danish Natural Sciences Research Council.
Fourier Transform Infrared Photoacoustlc Spectroscopy of Pyrldine Adsorbed on Silica-Alumlna and y-Alumina Stephen M. Rlsetnan, Franklin E. Mamoth, Q. Mural1 Dhar, and Edward M. Eyrlng' Deparbnent of chemkrby and Depertment of MhIw and Fuels E n g l ~ n g Universtty , of Utah, &If Lake City, Utah 84 1 12 (Received: November 3, 198 1)
Relative numbers of Bronsted acid to Lewis acid sites on silica-alumina have been determined photoacoustically by an infrared analysis of chemisorbed pyridine compared to similar adsorption of y-alumina that has only Lewis sites. Results are similar to those obtained by earlier IR transmission studies that suggested the use of framework vibrations of silica as an internal reference standard. The 20% coverage of the silica-alumina surface by pyridine adsorbed at Bronsted sites found photoacoustically is in good agreement with a previous value of 17% estimated from transmission spectroscopic data. Reproducibilityof the photoacoustic measurements is excellent.
Introduction Photoacoustic spectroscopy (PAS) has been shown to be an effective tool for obtaining optical absorption spectra of organic semiconducting polymers,' highly opaque and light scattering p o w d e r ~ , ~and J chemically modified chromatographic stationary phases."6 Lochmuller and co-workers4+demonstrated PAS to be a simple and reliable method for obtaining the degree of surface coverage of alkyl, phenyl, and aminoalkyl-derivatized silica gel by the detection of overtone absorptions in the near-infrared. Few studies to date,@ however, have made use of the advantages of photoacoustic detection for examining the infrared spectra of catalytically active surfaces. In catalysis, infrared spectroscopy has been used in two ways: to obtain structural characteristics of adsorbed species, and in surface acidity characterization. It has been (1)S.M. Rbeman, 5. I. Yaniger, E. M. Eyring, D. MacInnes, A. G. MacDiarmid and A. J. Heeger, Appl. Spectrosc., in press. (2)D. W. Vidrine, Appl. Spectrosc., 34,313 (1980). (3)M. J. D.Low and G. A. Parodi, J. Mol. Struct., 61, 119 (1980). (4)C. H.LochmOller, S. F. Marshall, and D. R. Wilder, Anal. Chem., 62,19 (1980). (5) C. H. Lochmiiller and D. R. Wilder, Anal. Chim. Acta, 118,101 (1980). ( 6 ) C. H.Lochmiiller and D. R. Wilder, Anal. Chim. Acta, 116, 19 (1980). (7)D.E.Leyden, M. L. Steele, B. B. Jablonski, and R. B. Somoano, Anal. Chim. Acta, 100,545 (1978). (8) M. J. D. Low and G. A. Parodi, Spectrosc. Lett., 11, 581 (1978). (9)J. B.Kinney, R. H. Staley, C. L. Reichel, and M. S. Wrighton, J. Am. Chem. Soc., 103,4273(1981). 0022-3654/82/2086-1760$01.25/0
shown that the surface acidity of catalytic support materials profoundly affects metal ion impregnationlo as well as industrially important catalytic reactions such as cracking, isomerization, and hydrocracking. The following work evaluates PAS as a method for obtaining midinfrared spectra of pyridine chemisorbed on silica-alumina and y-alumina. The object of such measurements is the determination of the relative concentrations of Lewis and Bronsted acid sites on these surfaces, and the comparison of the results obtained by the PAS technique with those previously obtained by conventional infrared spectroscopy, in order that the former can be confidently applied to opaque catalyst surfaces. The use of pyridine to determine the acidity of catalytic surfaces has been well established" owing to the fact that the lone electron pair of the nitrogen can either bind coordinately to Lewis acid sites (LPY) or interact with acidic surface protons to give rise to the pyridinium ion (BPY). The infrared spectrum of the former is clearly distinguishable from that of the latter. The ring vibration modes 19b and 8a from the assignments made by Kline and Turkevich12are observed to be the most sensitive for determining the type of coordination of the nitrogen lone-pair electrons. (10)P. Ratnasamy and S. Sivasanker, Catal. Rev.-Sci. Eng., 22,401 (1980). (11)E. P. Parry, J. Catal., 2, 371 (1963). (12)C. H.Kline and J. Turkevich, J. Chem. Phys., 12, 300 (1944).
@ 1982 American Chemical Society
The Journal of phvslcal Chemistry, Vol. 86, No. 10, 1982 1761
Pyridine Adsorbed on Silica-Alumina and ?Alumina
TABLE I: Assignments of Pyridine Chemisorbed on Silica-Alumina LPY,b BPY,b LPY,c BPY,C vibrational cm-I cm" cm-l assignmenta cm" 1620 1577 1490 1 9 b ~ c c ( ~ ) 1450
~~UCC(N) 8b U C C ( N ) 19avCC(N) a
4000
3500
3000
2500
2000
Reference 12.
1638
1621 1578 1493 1454
1490 1545
1639 1493 1547
This work.
Reference 25.
1500
WAVENUMBERS
Figure 1. Photoacoustic spectra of calcined silica-alumina before (A) and after (6)exposue to 18 torr of pyridine at 150 O C with subsequent pughg. pvrldkre adsorbed on Bronsted sites (BPY) and Lewis add sites (LPY) gives rise to absorbances between 1650 and 1400 cm-' (see Table I).
Experimental Section The silica-alumina used in this work was obtained from Davison Chemical Co. It contains 25% alumina by dry weight, and the surface area was determined to be 426 m2 g-l by the BET method. The y-alumina was obtained from Akzo Chemie and has a surface area of 209 m2 gel. All sample pretreatment was carried out on a conventional vacuum line. The samples were heated to 500 OC under vacuum over a period of 1h followed by calcination in flowing oxygen for 4 h. The samples were evacuated overnight before the temperature was lowered to 150 "C. The catalysts were exposed to pyridine (Aldrich spectrograde) for 1h at 18 torr after the pyridine was subjected to freeze-pump thawing to remove traces of dissolved gases and then distilled through a P205drying column. After exposure, the catalyst was purged in flowing helium for 6 h to remove as much physically adsorbed pyridine as possible. The sample was cooled to room temperature and stored under a positive pressure of helium before it was transferred to the photoacoustic cell in an inert atmosphere, The photoacoustic cell is one of local design and is fitted with a B&K 4165 microphone and NaCl window which permits optical transmission from 4000 to 600 cm-'. All spectral measurements and peak integrations were performed on a Nicolet 7199A Fourier transform infrared spectrometer adapted for photoacoustic detection. Mirror velocities of 0.122 and 0.181 cm s-' were used to obtain the interferograms. These correspond to modulation frequencies of 976 and 1450 Hz a t 4000 cm-l, respectively. Sampling at two different modulation frequencies was undertaken to assure that the spectra were independent of the instrumental parameters used to obtain the data. The resulting interferograms were transformed into the PA spectra after apodization by the HappGenzel method and then normalized against the output of a DTGS pyroelectric detector to yield double-beam spectra with a resolution of 4 cm-'. Two spectra of duplicate samples were taken to assure reproducibility of the technique.
4000 3770 3540 3310
3080 2850
2620 2390
2160
1930
1700
1470
1240
I010
WAVENUM0ERS
Flgure 2. Photoacoustic spectra of calcined y-alumina before (A) and after (6)exposure to 18 torr of pyridine at 150 O C with subsequent purging. LPY absorbances appear between 1625 and 1450 cm-'.
Results Figure 1displays the photoacoustic spectra of calcined silica-alumina before and after exposure to pyridine. The most notable features present in both spectra are the intense Si-0 and A1-0 framework vibrations which occur at frequencies below 1300 cm-' and the hydroxyl stretch of unbound surface SiOH groups located at 3747 cm-'. The two broad bands centered around 1865 and 1635 cm-' have been attributed to S i 0 combination and overtone absorptions, re~pective1y.l~These bands are masked after chemisorption of pyridine, whose absorptions occur in the same region. The water bands observable around 1650 cm-' are due to unpurged water vapor present inside the spectrometer causing them to appear as transmission peaks. No water is observed to be present inside the photoacoustic cell itself. Upon exposure to pyridine with subsequent purging to remove physically adsorbed pyridine, new bands appear which can be attributed to both BPY and LPY. These are shown in Table I along with the results obtained by transmission spectroscopy and their vibrational assignments. It is seen that the two sets of results are in complete agreement with the resolution limits of the measurement (4 cm-'), demonstrating that both types of acid sites are present on calcined silica-alumina. A broad band centered around 3000 cm-'is not observed in these spectra, indicating the absence of physically adsorbed pyridine which would be held on the surface by a hydrogen-bonding interaction with surface hydroxyl groups, if it were present. Figure 2 displays the PA spectra of calcined y-alumina before and after exposure to pyridine. The PA spectra are observed to be devoid of the five hydroxyl stretch vibrations found by Peri and c o - w ~ r k e r s ~in~ the , ' ~ frequency region 3800-3700 cm-'. Their absence can be attributed to the low degassing temperature (500OC)employed in the sample pretreatment since these hydroxyl bands were ~
~~~~~~~~
(13) M. R. Baaila, J.Phys. Chem., 66, 2223 (1962). (14)J. B. Peri and R. B. Hannan, J. Phys. Chem., 64, 1526 (1960). (15)J. B. Peri, J. Phys. Chem., 69,211 (1965).
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7Y)e Journal of phvsical Chemistry, Vol. 86, No. 10, 7982
Riseman et al.
T A B L E I1 : Vibrational Frequencies of Pyridine Chemisorbed on ?-Alumina
a
photo-
transmission,"
acoustic,
cm-' 1453 1495 1578
transmission,"
cm-'
cm-'
photoacoustic,
1447 1493 1578
1614 1622
1614 1621
cm-'
Reference 1 7 .
observed to remain unresolved until pretreatment temperatures exceeded 500 "C. Upon exposure to pyridine, new bands appear which can be attributed to LPY only. These results are in complete agreement with those previously obtained by transmission spectros~opy,'~'~ thus demonstrating that alumina cannot readily furnish protons for acid catalysis because of the inability of these hydroxyls to hydrogen bond to electron-pair donors. These observations are given in Table 11. In particular, the two types of Lewis acid sites on y-alumina are clearly discernible, giving rise to absorbances at 1624 and 1614 cm-'. This is consistent with the observations of Mon617 who was able to show by variabletemperature desorption of chemisorbed pyridine on yalumina that the two sites possess different acidity. Previously,18 only one type of Lewis acid site was postulated to exist on the surface of y-alumina. Discussion I t has been demonstrated that PAS can detect the presence of different pyridinium species present on catalytic surfaces and, as a result, can yield information concerning the nature of the acidity of the surface. Recently, attempts have been made to quantify the PA signal, which is a complex function of the optical and thermal properties of the sample, to give the relative concentration of chromophores. Burggraf and Leyden20were able to account for the effects of saturation and light scattering on the photoacoustic signal magnitude and phase by using the Kubelka-Munk analysis of diffuse reflectance to obtain the corrected photon density at the sample surface in conjunction with the Rosencwaig-Gersho theory21for the generation of photoacoustic signals in thermally thick samples. However, this does not result in an expression which contains a simple correlation between the sample absorption coefficient and signal level when compared to the Beer-Lambert expression for conventional transmission spectroscopy. It should also be noted that Burggraf and Leyden20use the combined photoacoustic magnitude and phase information in their analysis, which is applicable when dispersive measurements are made. However, the phase information is lost when operating in the Fourier transform mode, thereby making the Burggraf and Leydenmanalysis unsuitable for FT experiments. In addition, saturation and light scattering effeds are diminished when long-wavelength infrared radiation is used as the source, so the need to correct for these phenomena is also lessened. Rockley and co-workers22were able to use FT IR PAS for the quantitative analysis of solid mixtures of (16)F. E. Kiviat and L. Petrakis, J. Phys. Chem., 77, 1232 (1973). (17)R.Monb, "Preparationof Catalysta",B. Delmon, P. A. Jacobs, and G. Poncelet, Eds., Elsevier, Amsterdam, The Netherlands, 1976,p 381. (18)T. R. Hughes and H. M. White, J.Phys. Chem., 71,2192(1967). (19)T. R. Hughes, H. M. White, and R. J. White, J. Catal., 13, 58 (1969). (20) L.W.Burggraf and D. E. Leyden, Anal. Chem., 53,759 (1981). (21)A. Rosencwaig, 'Photoacoustics and Photoacoustic Spectroscopy", Wiley, New York, 1980,p 93. (22)M. G.Rockely, D. M. Davis, H. H. Richardson,Appl. Spectrosc., 35,185 (1981).
a
z g m
-
a a
4000
3660
3320
2980
1430
1150
070
590
WAVENUMBERS
Flguro 3. Photoacoustic spectra of calcined silica-alumina before (A) and after (B) exposure to 18 torr of pyridine at 150 O C w K subsequent purging. Attenuation of the 0-H vibrational absorption at 3747 cm-' shown at left Is compared with the constant framework (Si-0) absorption at 1100 cm-' shown at right.
K15N03/K14N03by using a novel internal standard technique which relied only on the ratio of the intensity of the two absorbance bands and not on their absolute signal magnitude. In this way the effects of particle size and sample and gas volumes on the PA signal magnitude are no longer important considerations. This method of quantitative analysis of photoacoustic signals is particularly well suited to the study of catalyst support materials, wherein the intense framework absorptions can provide the internal standard with which to compare bands resulting from adsorbed species. These framework absorptions occur below 1300 cm-' for silica-alumina and below 1050 cm-' for y-alumina. Figure 3 shows the hydroxyl stretch band and Si-0 framework band before and after exposure to pyridine. Adsorption of pyridine onto Bronsted acid sites results in an attenuation of the signal magnitude of the OH band. When each is ratioed against the integrated signal intensity of the intense Si-0 band centered at 1100 cm-', it is observed that 80% of the OH band intensity is retained after pyridine chemisorption, indicating that 20% of the surface hydroxyls are involved in an interaction with pyridine. This value is in good agreement with that of Basila,13who estimated that 17% of the catalyst hydrogen is involved in the reaction of pyridine with silica-alumina under similar reaction conditions. The surface coverage of BPY can then be estimated from a knowledge of total surface hydroxyl concentration. If this is taken to be between 1.0 X 1014and 2.0 X 1014 cm-2 (ref 23 and 24 ) and the pyridine molecule is thought of as a sphere of diameter 9.5 A,25 then the surface coverage by BPY is estimated to lie between 14% and 28% of a monolayer. This analysis is based on the assumption that the hydroxyl groups are not involved in any hydrogen bonding with pyridine. The absence of a broad band in the hydrogen-bonding region of the spectrum (3500-2500 cm-') supports this assumption. (23)W.K. Hall, Acc. Chem. Res., 8, 257 (1975). (24)M.R. Basila and T. R. Kantner, J. Phys. Chem., 71,467(1967). (25)M.R. Basila, T. R. Kantner, and K. H. Rhee, J.Phys. Chem., 68, 3197 (1964).
PyrMlne Adsorbed on Silica-Alumina and y-Alumina
TABLE 111: Photoacoustic Absorption Parameters for 7-Alumina and Silica-Alumina modulation at 1490 cm-', Hz 360 360 540 540
silica-alumina ?-alumina EL14W/E1450
0.35 * 0.36 * 0.34 f 0.34 *
0.09 0.09 0.09 0.09
E14501EIY5
1.6 f 0.4 1.7 * 0.4
[LPYl/ WYI 3.0f 0.4 3.2 * 0.4 2.8 f 0.4 3.0 k 0.4
In previous transmission studies on pyridine chemisorbed on various catalytic s ~ p p o r t s , ' ~ ~it~was * ~ possible ~ to determine the ratio of Lewis to Bronsted sites by an analysis of the infrared absorption bands attributed to LPY and BPY. In this type of analysis, either water or HC1 was added to convert the LPY to BPY in order to derive the relative absorption coefficients of both species, and then use was made of the peak heights or integrated peak intensity to deduce the ratio of the number of sites. Basila and KantneP were able to use the 1490-cm-' band (attributable to both LPY and BPY) to ascertain the ratio [LPY]/[BPY] for a variety of silica-aluminas by using the relative absorption coefficient eB14@)/eL14W) = 6.0 f 0.9 for this band. This value was later corroborated by other a ~ t h o r s . ' ~ Thus, t ~ ~ *when ~ ~ the 1490-cm-l band of silicaalumina is used, it should be possible to determine [LPY]/[BPY] photoacoustically. This is true because the photoacoustic response is proportionalS1to cp where e is the optical absorption coefficient and p is the thermal diffusion depth, which is dependent on the material and the frequency at which the light is modulated. Since it is only the ratio of [LPY] to [BPY] that is desired, the thermal diffusion length dependence drops out of the ratio and signal magnitude becomes proportional to e times the concentration of the absorber as in conventional absorption spectroscopy. It is next possible to determine the ratio of the number of reacted Lewis sites to the number of reacted Bronsted sites by using the 1450 cm-' band (of LPY only) as an internal standard. Since alumina has only Lewis acid sites, (26)M.Lefrancois and C. Malbois, J. Catal., 20,360 (1971). (27)Y. Watanabe and H. W. Habgood, J. Phys. Chem., 72, 3066 (1968). (28)M.R.Basila and T. R. Kantner, J. Phys. Chem., 70,1681 (1966). (29)F. R. Cannings, J. Phys. Chem., 72, 4691 (1968). (30)J. W. Ward, J. Cat& 12, 271 (1968). (31)K. H. Bourne, F. R. Cannings, and R. C. Pitkethly, J. Phys. Chem., 74,2197 (1970).
The Journal of phvsical Chemistty, Vol. 86, No. 10, 1982 1763
one may use this material to ascertain the relative absorption coefficient tL1490/e14&) = 0.35 f 0.09. This then may be used in the following analysis where it is assumed that this experimentally determined ratio is invariant when applied to pyridine adsorbed on silica-alumina: AT1490 = AL1490 + (1) where AT1490 represents the total integrated intensity at the subscripted frequency. AL1490 = 0.35A14~ (2) AB1490 = AT14w - 0.35A1450
(3)
Thus, it follows that [LPY]/[BPY] = 6.0[0.35A14W/(AT1490 - 0.35A1460)I This sequence of equations is analogous to that found in the transmission study by Basila and Kantner.28 The results of the previous analysis are given in Table I11 for two different samples at two different modulation frequencies. The uncertainties quoted in the table were obtained on a percent basis by taking them to be twice the noise-to-signal ratio of the absorbance band. Since integrated peak intensities were used in this analysis rather than peak heights, the estimate of uncertainty is conservative. The agreement between the four data sets reveals that this technique is reproducible within the uncertainty of the measurement, and the ratio [LPY]/[BPY] = 3.0 f 0.4 is found to be within the range reported by other aut h o r for ~ various ~ ~ ~ grades ~ ~ of silica-alumina and other aluminosilicates. The assumption that the ratio of molar absorptivities eL14@)/t14&) will be the same for y-alumina and silica-alumina would appear to be valid since the ratio tL1450/eB1M5 reported by other authors for silica-alumina, mordenite, and group la-Y zeolites are all similar. Finally, it is worth mentioning that the reproducibility of the photoacoustic technique is dependent on there being a vibrational mode (such as the Si-0 vibration of bulk silica-alumina) whose absorption characteristics remain unperturbed when the surface is modified. Acknowledgment. Financial support of this work by a contract from the Department of Energy (Office of Basic Energy Sciences) is gratefully acknowledged. We also express our thanks to Professor Joel M. Harris and Dr. Lindsay B. Lloyd for many stimulating discussions.
