Langmuir 1987, 3, 52-58
52
are undoubtedly produced under these conditions. (This latter circumstance, rather than the presence of only a upd silver layer, is presumably also responsible for a previous observation of SERS using 514.5-nm radiation for pyridine on a gold substrate covered by electrodeposited silver.? The presence of substantial silver agglomeration, rather than merely the deposition of a silver monolayer, is therefore apparently required to engender “silver-”,rather than “gold-like”electromagneticSERS properties for these systems. It therefore appears that at least the electromagnetic component of the SERS activity is provided by the underlying gold microstructures, the role of the silver (or copper) upd layers being chiefly to determine the nature and extent of the adsorbate-surface bonding. Also in harmony with this interpretation is the retention of the high degree of reversibility of the SERS signals with respect to potential excursions for the upd surfaces as observed for gold itse1f.l”
These upd systems therefore exhibit desirable SERS properties which suggest their possible application in interfacial electrochemical (or chemical) studies which require surfaces with silver- or copper-like properties but having a greater positive potential range afforded by the upd stabilization of the overlayer. Together with the upd layers on gold formed by using SERS-inactive overlayers,’ these systems therefore provide a promising means of extending the realm of applicability of SERS to a wider range of surfaces of electrochemical significance. Acknowledgment. Technical assistance was provided by D. F. Milner. This work is supported by the National Science Foundation. D.G. is a recipient of a graduate fellowship from the Eastman Kodak Co. as part of the Purdue Chemistry Industrial Associates Program. Registry No. Au, 7440-57-5;Ag, 7440-22-4;Cu, 7440-50-8; C1-, 16887-00-6;Br-, 24959-67-9;thiocyanate, 302-04-5;benzonitrile, 100-47-0;pyridine, 110-86-1.
Microcalorimetric and FT-IR Spectroscopic Study of the Adsorption of Isopropyl Alcohol and Hexafluoroisopropyl Alcohol on Titanium Dioxide Pier Francesco Rossi,t Guido Busca,? Vincenzo Lorenzelli,*t Odette Saur,S and Jean-Claude Lavalley’ Istituto di Chimica, Facoltci di Ingegneria, Universitci, Fiera del Mare, Pad. D, 16129 Genova, Italy, and Laboratoire de Spectrochimie, U.A. 04 414, I.S.M.Ra., Universitd, 14032 CAEN Cedex, France Received June 9, 1986. In Final Form: September 16, 1986
FT-Et experiments of isopropyl alcohol adsorption on TiOzshow that two irreversible species are formed, one identified as an undissociated coordinated species not involved in hydrogen bonding and the other as monodentate isopropoxy groups. Mild heating causes the transformation of the first one into bridged alkoxy species, as better illustrated by the use of isopropyl-d, alcohol. Reversible hydrogen-bonded species appear at higher coverages. Hexafluoroisopropyl alcohol leads instead to only one irreversible species that is dissociated and to very small amounts of reversible hydrogen-bonded species. Volumetric isotherms show that the amounts irreversibly adsorbed are similar in the two cases. Adsorption microcalorimetry shows that the dissociative adsorption of hexafluoroisopropylalcohol is more exothermic (300-160 kJ mol-’) than the irreversible adsorption of isopropyl alcohol (220-145 kJ mol-’). However, the latter is slightly more exothermic than methanol adsorption, in agreement with the different basicity. Introduction Alcohols provide useful and versatile probe molecules in surface chemistry ~tudies.l-~In fact, they are weak Bransted acids but may also act as Lewis bases: may be involved both as proton donors and acceptors in H bonding, and may interact with polyfunctional sites. Moreover, different alcohols and ha loge no alcohol^^^^ constitute a set of molecules whose acid-base properties vary progressively over a wide range. The study of their adsorption may give information on the nature and properties of catalytic surfaces and on the mechanisms of their heterogeneously catalyzed transformations (dehydration and dehydrogenation). Previous studies concerning the adsorption of methanol on Alzo3’ and on TiOZ8have shown, in both cases, the
* To whom all correspondence should be addressed. Istituto di Chimica.
* Laboratorie de Spectrochimie.
utility of joining spectroscopic and calorimetric data to obtain a complete picture of the adsorption phenomena. In the case of TiOz, in p a r t i ~ u l a r , both ~ t ~ dissociation on acid-base surface sites and coordination on Lewis sites are observed at low coverages. To improve our knowledge of the behavior of the TiOz surface and of the alcohol/oxide (1) Greenler, R. G.; J. Chem. Phys. 1962,37, 2094. (2)Jeziorowski, H.;KnBzinger, H.; Meye, W.; Miiller, H. D.; J. Chem. SOC.,Faraday T r a m . 1 1973,69, 1744. (3)Knazinner. H.:Stiibner. B. J. Phvs. Chem. 1978.82. 1526. (4)Derouaht,’J.; Le Calve,’J.; Forel,-M. T. Spectroc’hih. Acta, Part A 1972, B A , 359. (5)Lavalley, J. C.; Travert, J.; Lamotte, J. J. Chim. Phys. 1981, 78, 35.
(6) Travert, J.; Benaissa, M.; Lavalley, J. C. Spectrochim. Acta, Part A 1985, 41A, 573. (7) Busca, G.; Rossi, P. F.; Lorenzelli, V.; Benaissa, M.; Travert, J.; Lavalley, J. C. J. Phys. Chem. 1985,89, 5433. (8)Rossi, P.F.; Busca, G. Colloids Surf. 1985, 16, 95. (9)Busca, G.;Forzatti, P.; Lavalley, J. C.; Tronconi, E. In Catalysis b y Acids and Bases; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1985; p 15.
0743-7463/87/2403-0052$01.50/0 0 1987 American Chemical Society
Langmuir, Vol. 3, No. 1, 1987 53
Microcalorimetric and FT-IR Study of Adsorption
4
306 / - - - - - - -
3
_ _ - - --
/ E
r
250
/ - -
2
-
#
7
0 E
*\
200
0
E
?
150
y
ic ic
D
a
e
2
I p
3
4
Figure 1. Volumetric and calorimetric adsorption isotherms of isopropyl alcohol (full lines) and hexafluoroisopropyl alcohol (broken lines) on TiOz. surface interactions, we have developed our previous work through a study of the adsorption on Ti02 of two alcohols that, in spite of similar steric requirements, have such different acidities as isopropyl alcohol (pK, = 17.1) and hexafluoroisopropyl alcohol (pK, = 9.3). Experimental Section Ti02powder was Degussa P 25 (49 m2g-', 90% anatase from XRD), whose surface properties have been the object of our previous investigation.10 Before adsorption experiments the powder was heated in air at 673 K for 2 h and evacuated at the same temperature for 2 h. Commercial isopropyl alcohol (Carlo Erba) and 1,1,1,3,3,3-hexafluoroisopropylalcohol (Merck) were degassed by multiple freeze-pump-thaw cycles. Deuterated alcohols (CD3)&HOH and (CF3)&HOD were prepared as reported in ref 11. Microcalorimetricexperiments were carried out at room temperature with a Tian-Calvet heabflow calorimeterequipped with a Setaram NV 724 amplifier nanovoltmeterand ServotraceSefram recorder.'V8 FT-IR spectra were recorded with Nicolet MX 1 Fouriertransform spectrometers. Self-supportingpressed disks underwent the same pretreatment into the IR cell as for microcalorimetric measurements. The spectra will be shown in transmittance in the vOH region, while in the other regions they have been plotted in absorbance after subtraction of the TiOz background. Results (a) Adsorption Isotherms. The volumetric adsorption isotherms of i-PrOH and F,-i-PrOH on Ti02are reported in Figure 1. They may be compared with those of methanol on the same sample6 and of different alcohols on anatase samples reported by Carrizosa and Munera.12 The shape is always similar, but the number of molecules involved in the first, "pressure-independent" step (that roughly corresponds to irreversibly adsorbed alcohol) 'and in the second, pressure-dependent step (that roughly corresponds to reversibly adsorbed alcohol) vary in each case. The amount of i-PrOH irreversibly adsorbed on our Ti02sample is about 3 pmol m-2 and corresponds to about ~~~
5
100
to-r
~
(10) Busca, G.; Saussey, H.; Saur, 0.;Lavalley, J. C.; Lorenzelli, V. Appl. Catal. 1986, 14, 245. (11) Gallas, J. P.; Binet, C. Adu. Mol. Relax. Interact. Processes 1982, 22, 39. (12) Carrizosa, I.; Munuera, G . J . Catal. 1977, 49, 174.
50
0
E
2
1
pmol
3
4
m-2
Figure 2. Integral (in the inset) and differential adsorption heats of isopropyl alcohol (full lines) and hexafluoroisopropylalcohol (broken lines) on Ti02 one-half of the number of methanol molecules irreversibly adsorbed on the same surface? This value is slightly higher than that measured by Carrizosa and Munuera12on their samples, probably due to milder activation conditions used by these authors. They had already reported a decrease in the amount of irreversibly adsorbed alcohol by increasing the number of C atoms. Also the number of i-PrOH molecules involved in the pressure-dependent step (1.6 pmol m-2) is much lower than that measured for methanol. In the case of F,-i-PrOH the number of molecules involved in the irreversible step (2.7 pmol m-2) is only slightly smaller than that of the corresponding i-PrOH species while the reversibly adsorbed amount is very small (0.3 pmol m-2). Such values are rather similar to those measured for F,-i-PrOH adsorption on A1203 (2.85 and 0.75 pmol m-2, re~pectively'~). (b) Calorimetric Measurements. The comparison of the calorimetric adsorption isotherms of i-PrOH and of F6-i-PrOH (Figure 1) indicates that, in spite of the lower adsorbed amounts, higher total heats are released when the halogeno alcohol is adsorbed. This is even more clear by comparison of the integral adsorption heat curves vs. the adsorbed amount (Figure 2, inset): the curve of F,i-PrOH is always at higher values than that of i-PrOH and shows a clear change of slope near to 3 pmol m-2, corresponding to the end of the more exothermic irreversible adsorption step. The curves of the differential adsorption heats (Figure 2), which measure the energetic release of each adsorption step, indicate that at low coverages much more heat is kJ mol-') than by i-PrOH released by F6-i-PrOH (~300 adsorption ( ~ 2 2 0kJ mol-'). However, in both cases a plateau is shown at higher coverages near 145 kJ mol-l for isopropyl alcohol and near 160 kJ mol-' for F6-i-PrOH, while when the irreversible step has ended (2.7 pmol m-2 (13) Benaissa, M.; Saur, 0.; Lavalley, J. C. Mater. Chem. 1982, 7,699.
54 Langmuir, Vol. 3, No. 1, 1987
Rossi et al.
8 1
HnnrcllLm
Figure 3. FT-IR spectra of TiOzpreased disk after (a) activation and (b-d) contact with increasingpressurea of hexafluoroisopropyl alcohol vapor. (e) Spectrum of the reversible hexafluoro-isopropanol species obtained by ratioing of the spectra recorded before and after evacuation. adsorbed for F6-i-PrOH,3 pmol m-2 adsorbed for i-PrOH), the adsorption heats decrease toward their condensation heats. This decrease of qdifi takes place suddenly in the case of F6-i-PrOH, according to the negligible amount of reversibly adsorbed species but much more slowly in the case of i-PrOH, according to the further weak adsorption of about 1.6 pmol m-2. In spite of the different number of molecules involved, the qdif;S relative to i-PrOH adsorpion may be compared with those previously reported for methanol? the qdiff values at very low coverages are similar in the two cases, while at the end of the irreversible adsorption range i-PrOH adsorption releases slightly more heat (145 kJ mol-’) than does methanol adsorption ( ~ 1 1 0 kJ mol-’). As already remarked, there are fewer reversibly adsorbed species in the case of adsorbed i-PrOH, and the corresponding step in qdiff values appears to be less well defined.
(c) FT-IR Study. Hexafluoroisopropyl Alcohol. The admission of small amounts of F6-i-PrOH to the activated TiOz sample causes the formation of a strong sharp vOH band at 3635 cm-’ (Figure 3). Simultaneously vCH bands are observed (Figure 3b and 4b) at 2965 (very weak) and 2930 cm-’; aCH at 1365 cm-’ (with a weak shoulder at 1358 cm-l); and vCF3 at 1298, 1250, 1195, and 1100 cm-I; we are reminded that the spectrum of F,-i-PrOH in CC14 dilute solution14is the sperposition of the spectra of two conformers showing vOH’sat 3619 and 3580 cm-l, uCH’S at 2974 and 2938 cm-’, 6CH at 1378 cm-l, and 6 0 H at 1308 cm-’ as well as several vCF3’s. Comparison of Figure 4, parts a and b, shows that no 6oH band is observed for the adsorbed molecule. The detection of a yoH band at higher frequencies with respect to that of the pure molecule (such a shift cannot be due to the perturbation of undissociated alcohol molecules) and the absence of 60, clearly indicate complete dissociation with the formation of a new hydroxy group. This is confirmed by the adsorption of F6-i-PrOD, which forms the same bands in the region of 1450-1000
Figure 4. FT-IR spectra of hexafluoroisopropyl alcohol (a) in CC14solution, (b) irreversiblyadsorbed on TiOz,and (c) reversibly adsorbed on TiO,; arrows indicate the 6(OH) band. cm-’ and a new vOD band at 2680 cm-l. The weak perturbation of vCH’S and &H’s also agrees with the formation of an alkoxide ion, as well as with the breaking of the coupling of ~ C and H boH modes. The splitting of vCH and 6CH indicates that two alkoxide conformers are formed, as expected. By increasing the amount adsorbed into the reversible adsorption range, a broad UOH band near 3350 cm-l progressively increases in intensity (Figure 3d,e), while a weak band is also formed at 1430 cm-l (Figure 4c). Such bands may be assigned to undissociated F,-i-PrOH (UOH and boH, respectively) hydrogen bonded on basic ~ites.’~J’Under the same conditions the 6cHmode shifts to 1378 cm-’ and the stronger UCH to 2936 cm-’, values similar to those of the pure liquid compound or in solution. Such features disappear by evacuation, indicating that undissociated Fc-i-PrOH constitutes the reversibly adsorbed species. The intensity of the bands connected with the reversibly adsorbed species is weak in spite of the high extinction coefficient of vOH of H-bonded hydroxyls and is in agreement with the small amount of such species. Evacuation at room temperature then restores the spectrum of hexafluoroisopropoxide as the only adsorbed species. (d) FT-IR Study. Isopropyl Alcohol. The first admission of i-PrOH vapor into the cell causes the formation of a well-defined, if rather broad, vOH band at 3445 cm-l, while vOH’S of surface titania hydroxyls are not perturbed (Figure 5b). Also vCH bands are formed, very intense, a t 2972,2938, and 2872 cm-l, with shoulders detectable with difficulty at 2920 cm-’ and near 2900 cm-l. Such VCH bands do not differ very much from those of pure isopropyl alcohol (Table I). Moreover, a weakly split aa-CH3 band at ~3 1468, 1458 cm-’ (shoulder), two well-resolved 6 s - ~modes at 1385 and 1367 cm-l, and the YCH mode at 1343 cm-l also appear, together with a relatively broad band near 1290 (14)Murto, J.; Kivinen, A.; Viitala, R.; Hyomaki, J. Spectrochin. Acta, Part A 1973, 29A, 1121.
Langmuir, Vol. 3, No. 1, 1987 55
Microcalorimetric a n d FT-IR Study of Adsorption
Table I. Observed Wavenumbers (om-')and Assignments of Isopropyl Alcohol Adsorbed and Reference Species i-PrOH rev ads i-PrOH chemis isopropoxides ads (i-Pr0)4Ti16J7 assignmentsl8Vm i-PrOH monomer (CC4sol) 3445 m___ Ha 3627, 3612 3295 2968 2974 2959 2973 "a-CH3 2972 2930 2936 2924 2938 l'a-CH3 2930 2938 2857 (2930) 2870 (2935) 2872 YCHa (2938) 2882 -.. 2895 2ba-CH3 1472 1462 1466 1466 1468 ba-CH3 1451 1462 1457 1458 1454 ba-CH3 1390 1400 1390 ~ C H ,6on 1382 1381 1382 1379 1386 k H 3 1369 1364 1370 1370 1365 bCH3 1342 1328 1345 1341 1345 YCH 1310 1252 1292 bOH, k H 1172 1165 1159 1169 wc 1148 1167 wo rCHa
950
1124
1125
aThese bands may be split due to the presence of two conformers.
i I
Y
10.1 It a
"koo'j4oo'~jooo'i!adt6M w"Em
Figure 5. FT-IR spectra of Ti02 pressed disk after activation (a) and after contact with isopropyl alcohol vapor, (b) P < 1torr and (c) P = 5 torr.
cm-' and a very intense complex absorption whose main maximum is at 1125 cm-', having also a sharp component at 1165 cm-' (Figure 6a). The detection of the uOH band at 3445 cm-l as well as the aOHband at 1290 cm-l, observed in monomeric i-PrOH at 1252 cm-', indicates that a particular form of undissociated isopropyl alcohol is adsorbed on Ti02. The slight perturbation of such modes excludes the formation of a hydrogen-bonded species but is a clear indication that this species coordinates to a Lewis acid site as in the case of methanoFg and the complexes formed by interaction of (CD3)&HOH with Lewis acids like A1C13.15 However, the spectrum in the region 1200-1100 cm-l (in particular the shape and intensity of the strong band at 1125 cm-l), appears significantly different from that of the i-PrOH monomer (Figure 8a), having features in common with that of titanium i s o p r o p ~ x i d e . ~ This ~ J ~ suggests that, in spite of the absence of a clear resolution of the UCH and ~ C H modes, two species might be present, one undissociated and one dissociated. Dissociation would also produce a (15) Gallas, J. P.; Binet, C. Adu. Mol. Relax. Interact. Processes 1982, 24, 191. (16) Bell, J. V.; Heisler, J.; Tannenbaum, H.; Goldenson, J. Anal. Chem. 1953,25,1720. (17) Lynch, C . T.; Mazdiyasni, K. S.; Smith, J. S.; Crawford, W. J. Anal. Chem. 1964,36, 2332.
I
ism'
14%.
lim'
I'W'
I-' ik" it" WVElUlOERS
I'ISO'
iim' ibro
Figure 6. FT-IR spectra of isopropyl alcohol adsorbed on TiOz after evauation at (a) raom temperature, (b) 400 K, (c) 430 K, and (d) 470 K.
new OH group which could be hydrogen bonded to the near isopropoxy group formed. This may make its characterization difficult in contrast to the case of the corresponding fluoro compound where hydrogen bonding does not occur. By increasing the isopropyl alcohol pressure, another species is also formed, whose main feature is a very broad and strong uOH band centered near 3280 cm-l (Figure 5c). Under such conditions, the uOH bands due to free surface hydroxy groups of titania have disappeared, while shoulders are detected at 3635, 3600, and 3570 cm-l, possibly due to their perturbation. The relatively sharp vOH band due to chemisorbed isopropyl alcohol instead seems not perturbed, while water is also produced in small amounts as evidenced by the appearance of the deformation band near 1640 em-'. Evacuation at room temperature causes the disappearance of the strong uOH band near 3280 cm-' and of the shoulder near 3570 cm-'. The ratioed spectra indicate that an undissociated form of isopropyl alcohol is indeed desorbed by evacuation at room temperature, whose VCH and 8CH bands are similar to those of liquid isopropyl alcohol, while the 60, mode is observed at 1310 cm-'. The perturbations of the stretching OH (strongly shifted down)
56 Langmuir, Vol. 3, No. 1, 1987
Rossi et al.
I Figure 7. FT-IR spectra of isopropyl alcohol (a) on CCll dilute solution and (b) chemisorbed undissociatively on TiOz and of isopropoxides adsorbed on Ti02. and of the bending OH (strongly shifted up) are a clear indication of the formation of an adsorbed species hydrogen bonded through ita own hydrogen atom to a basic site of the surface.15 After evacuation at room temperature the spectrum is I the same as that obtained by introduction of the first J quantity of isopropyl alcohol and is composed of the superposition of the bands of isopropoxy groups and of chemisorbed isopropyl alcohol with the only exception that Figure 8. FT-IR spectra of isopropyl alcohol species (same conditions as in Figure 7). the surface hydroxy groups of TiOp have disappeared, possibly due to their reaction with i-PrOH to produce isopropoxy groups by a condensation mechanism. EvacTable 11. Observed Wavenumbers (cm-')and Assignments for Some Characteristic Bands of i-Pr-d6-OHAdsorbed and uation at temperatures between 300 and 473 K causes the Reference Species progressive disappearance of the uOH and 8OH bands of chemisorbed isopropyl alcohol (3445 and 1290 cm-l), ini-PrOH-d6 assignmonomer Mcl, i-PrOH-ds i~oproxides-d~ dicating that such a species is either desorbed or dissomenta15~20 (CCL sold comdexls chemis. adsorbed ciated. After such treatment only very weak uOH absorp3612,3628 3518 3445 vOHa tions are observed, while vCH'S and ~ c H ' s still remain strong. 2969 2938 (2925) 2890 VCHa (2921) 2880 The overall spectrum under these conditions may be as1383 1402 1395 ~ C H ,OH signed to surface isopropoxy groups. However, during 1336 1336 1330 1330 YCH evacuation several features become apparent (Figure 6). 1279 1280 OH, ~ C H 1230 In particular, (i) the higher frequency 8cHs mode becomes 'Splitting8 due to the presence of two conformers. split, indicating that the less resistant component (1388 cm-') corresponds to chemisorbed i-PrOH, the more realso the VCH and 6,H modes of the >CH- group are sensitive sistant one (1382 cm-l) being due to isopropoxy groups; to such interactions. So, the adsorption of i-Pr-d6-OHwas (ii) the shoulder at 2890 cm-l is much more evident on the studied to better characterize the nature of the adsorbed spectrum of isopropoxide than when undissociated isospecies and to have further information about the forpropyl alcohol is present also; and (iii) in the complex band mation and the rearrangement of the previously observed due to skeletal vibrations between 1200 and 1100 cm-l the isopropoxide groups. component at 1125 cm-' first decreases slightly and later (e) FT-IR Study. Isopropyl-d6 Alcohol. When increases slightly in intensity, while the sharp component successive small doses of (CD,),CHOH are introduced into at 1168 cm-' always decreases, probably being due to unthe cell, several bands are observed (Table 11) whose indissociated isopropyl alcohol. This behavior of the band tensity increases progressively with the introduced amount. at 1125 cm-', where a V C of~ isopropyl alcohol is probably The presence of a UOH band (3445 cm-l) and of coupled 6 c H superimposedto a v w of isopropoxy groups, suggests some and 8 0 H bands at 1395 and 1275 cm-l confirms the forkind of rearrangement by heating. The spectra of the mation of undissociated coordinated species associated to adsorbed species, "isolated" through the appropriate subthe vco band evident as a shoulder at 1025 cm-', (1042 cm-' tractions of the experimental spectra, are compared in for the monomer in CCl&. Their wavenumbers show that Figures 7 and 8 with those of i-PrOH monomer in CCll the hydroxy groups of such species are not further involved solution. Their observed frequencies are compared in in hydrogen bonding. This species is completely desorbed Table I with those of two reference compounds, the i-PrOH at 455-470 K and is connected to the V C H component at monomer and (i-PrO)*Ti. As expected, the more signifi2940 cm-l. After evacuation at 470 K, strong bands due cant features which determine the nature of the adsorbed to dissociated species are still apparent. Under these , skeletal (VCC and vco) modes, species are the V O H , 8 0 ~and when these are detectable. However, it is ~ ~ o w that ~ ~ ~conditions J ~ Ja UCH ~ band is predominant near 2890 cm-', while the presence of a weaker shoulder at 2925 cm-l evidences the existence of two conformers (Figure 9). The two very (18) Krueger, P. J.; Jan, J.; Wieser, H. J. Mol. Struct. 1970, 5 , 375. strong bands at 1137 and 1054 cm-', already present at
Langmuir, Vol. 3, No. I, 1987 57
Microcalorimetricand FT-IR Study of Adsorption
t
Figure 9. FT-IR spectra of isopropyl-d6 alcohol adsorbed on TiOzafter evacuation at room temperature (full line) and at 450 K (broken line).
room temperature and almost unaffected by evacuation at 473 K (in contrast to the previously cited bands at 3445, 1395, 1275, and 1025 cm-' that all disappear completely), are assigned to isopropo~y-d~ groups. This confirms that dissociation in part occurs already at room temperature. However, two other bands at 1094 and 1013 cm-', weak at room temperature, become rather intense by heat treatment. Further evacuation at 520 K shows that this couple of bands corresponds to a species more resistant to evacuation than that responsible for the bands at 1137 and 1054 cm-'. These resulta may be interpreted assuming that the dissociation of chemisorbed alcohol molecules by heating produces a different type of alkoxy group (bands at 1094 and 1013 cm-'), more strongly bonded than those formed by direct dissociation of room temperature (bands at 1137 and 1054 cm-I). We recall recent results on where methanol forms two types of methoxy groups, having well-resolved uco bands and different thermal behaviors, believed to be monodentate and bridged. Assuming that the two components of the pairs of bands cited above have at least a partial uco character, we may tentatively assign the higher frequency couple of bands to monodentate alkoxy groups (that are accordingly less stable) and the lower frequency pair to bridged species. Such hypothesis may also rationalize the behavior of the 1135-1125-cm-' band of adsorbed undeuterated isopropoxy groups. Heat treatment at 520-570 K under evacuation results in the progressive complete disappearance of isopropoxy groups. This leaves only almost negligible amounts of organic oxidized products on the surface and restores almost completely the absorptions of surface hydroxy groups of anatase'O in the case of i-PrOH, producing OD groups in the case of i-PrOH-d,.
Discussion Microcalorimetric measurements have shown that the adsorption of i-PrOH on TiOz consists of two steps. At coverages lower than 3 pmol m-2, irreversible adsorption occurs, releasing heats that progressively lower from 220 to 145 kJ mol-'. IR spectroscopy shows that under such conditions two different adsorbed species are formed, one dissociated (isopropoxide species) and one undissociated (chemisorbed on Lewis sites). In the absence of other data, the simultaneous presence of such species, as in the case of methanola in spite of the slightly lower acidity of iPrOH, prevents an evaluation of the number of molecules involved and the differential heats released in the two single mechanisms. The comparison with values measured for the irreversible adsorption of methanol (that are very similar at very low coverages, but are significantly smaller at the end of the irreversible step) may reflect the effect of the higher basicity of isopropyl alcohol, whose coordination on Lewis sites would release higher heats. The formation of strongly bonded isopropoxy groups, responsible for a very strong band at 1150-1100 cm-*, is usually observed by isopropyl alcohol adsorption on oxides such as A1203,21Zn0,22 CeOz,23Fe203,24and Cr203.25 However, in our case two types of isopropoxy groups, better distinguishable in the case of the d6 compound, seem to be formed, one of which is predominant at room temperature (species I), while the other one is formed by dissociation of the chemisorbed alcohol at higher temperature and is slightly more stable under heat treatment (species 11). This tentative assignment parallels that of the two types of methoxy groups having different stabilities, (21)Deo,A. V.;Chuang, T. T.; Dalla Lana, I. G.J.Phys. Chem. 1971, 75, 234.
(22)Koga, 0.; Onishi, T.;Tamaru, K. J. Chem. SOC.,Faraday Tram. 1 1980, 76, 19.
(19)Montagne,X.;Lynch, J.;Freund, E.; Lamotte,J.; Lavalley, J. C. J. Chem. SOC.,Faraday T r a m . 1, in press. (20) (a) Tanaka, C. Nippon Kagaku Zasshi 1962,83,661. (b)Green,
J. H. S. Trans. Faraday SOC.1963,59, 1159.
(23)Zaki, M.I.; Sheppard, N. J . Catal. 1983, 80, 114. (24) Busca, G. React. Kinet. Catal. Lett. 1982,20, 373. (25) Osipova, N.A.; Davydov, A. A.; Kurina, L. N.; Loiko, V. E.Zh. Fiz. Khim. 1986,59, 1479.
58 Langmuir, Vol. 3, No. 1, 1987
Rossi et al.
thought to be monodentate and bridged, detected on other oxides such as Th0219and ZrOPE In the case of alumina, we have also observed transformation of coordinated undissociated methanol to bridged methoxide.' H3C CH, \ /
'iH 0
+ M
HC ,
\I
CH3
CH
ci
rc\l
M
M
II
Ill
IV
While heat treatment causes decomposition of isopropoxy groups, leaving oxidized surface compounds, in our case as in the case of TiOz rutile2'such compounds are detected only in negligible amounts. Under the same conditions the OH stretches of the anatase OH groups are restored, suggesting that isopropoxides evolve to propylene, leaving an OH group on the surface. This agrees with the published data concerning the isopropyl alcohol catalytic decomposition28 on and temperature-programmed desorption12from anatase. These data show dehydration to produce propylene to be the largely predominant reaction starting at about 520 K. In agreement with such a mechanism, OD groups are left on the surface after thermal decomposition of deuterated isopropyl alcohol. The lack of detection of relevant amounts of oxidation products would then confirm the predominance of the acid-base behavior of Ti02 responsible for the easy decomposition of isopropoxy groups producing propylene with respect to its oxidizing properties. Oxidation would produce acetone, which is unstable on the anatase surfaces, undergoing an aldol condensation,29as already observed on rutile.3o This behavior is dissimilar from that observed after methanol adsorption, where methoxy groups are produced as the more stable irreversibly adsorbed form.g Nevertheless, methoxy groups are oxidized by heat treatment to produce formates, while in the gas phase CO and HCHO are ob~ e r v e d .Surface ~ OH'S are not restored during methoxy group oxidation. The presence of nondissociatively chemisorbed alcohol molecules was already cited both on anatase12 and r ~ t i l e . ~However, ' a relatively complete (26) Bensitel, M.; Saur, 0.; Lavalley, J. C., unpublished results. (27) Rochester, C. H.; Graham, J.; Rudham, R. J.Chem. Soc., Faraday Trans. 1 1984,80, 2459. (28) Knozinger, H.; Kochloeff, K. Proc. Int. Congr. Catal. 5th 1973, 2, 1171.
(29) Busca, G., unpublished results. (30)Griffitha,D. M.; Rochester, C. H. J. Chem. Soc., Faraday Trans.
characterization of a similar species has only been given in the case of m e t h a n ~ l ~ ~ ~m - aent hd a n e t h i ~ l . ~The ~ TiOz surface seems to allow a particularly good detection of similarly coordinated species characterized by a relatively small perturbation of the VOH and 8oH bands which remain relatively sharp, indicating the formation of nonhydrogen-bonded coordinated species having well-defined geometry (species 111). Similar complexes have been observed on alumina and SbC1,-treated alumina15 by adsorption of (CD3)2CHOH,31but in this case the vibrational frequencies of the OH group indicated that hydrogen bonding was also present (species IV). The spectra of coordinated i-PrOH and i-Pr-&-OH indicate that also the >CH- bond is strongly sensitive to this type of interaction, its stretching vibration being strongly shifted toward higher frequencies. The irreversible adsorption of hexafluoroisopropyl alcohol is instead completely dissociative and very strongly exothermic according to its relative acidity. The very small amount of reversibly adsorbed species in the case of F6i-PrOH which are hydrogen bonded to weakly basic sites seems to indicate that the basic sites, where also methanol and i-PrOH adsorb via hydrogen bonding, are not available, being possibly already involved in the dissociation reaction. In spite of the different acidity, the number of rnolecules involved in the irreversible adsorption steps is similar in the cases of i-PrOH and F,-i-PrOH. These amounts correspond to one molecule per about 55 and 59 812 respectively, indicating that the steric demands of these molecules when chemisorbed is not very different. The number of adsorbed molecules is instead much less than that of the potentially active surface sites (for example, one site per 14.2 A2 for both Lewis acidic Ti cations and basic oxygen anions on the idealized (001) face of anatase). This work has confirmed the existence on the anatase surface of sites allowing several different interactions with alcohol molecules, such as coordination, dissociation, and hydrogen bonding. In agreement with Munuera,12the sites where alcohols chemisorb undissociatively may be constituted by Ti cations having two coordinative unsaturations, placed on (110) facedo or on similar sites on (111) faces. The distance of the exposed cations on the (110) face (4.33 A) allows the thermally activated formation of bridged alkoxides from the undissociated molecule, as we have proposed. Instead, the cations exposed on the (001) faces that have a single coordinative unsaturation would be more effective for dissociation to produce monodentate alkoxy groups. The sites responsible for the dissociation of F6-i-PrOH may be identified as exposed oxide ions where the less acid nonhalogenated alcohols interact via hydrogen bonding.
Acknowledgment. The technical assistance of G. Oliveri in the word-processing preparation of the manuscript is gratefully acknowledged. Registry No. F6-i-PrOH, 920-66-1; i-PrOH, 67-63-0; TiO,, 13463-67-7.
1 1978, 74,403.
(31) Gallas, J. P.; Binet, C. Adu. Mol. Relax. Interact. Processes 1982,
24, 207.
(32) Sauasey, H.; Saur, 0.;Lavalley, J. C. J.Chim. Phys. Phys.-Chim. B i d . 1984, 81, 261.
