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Langmuir 1994, 10, 1812-182
IR Spectroscopic and Microcalorimetric Characterization of Lewis Acid Sites on (Transition Phase) A1203 Using Adsorbed CO C. Morterra,* V. Bolis, and G. hbig11acca Department of Inorganic, Physical and Materials C h e n i ,try, University of Turin, via P. Giuria 7, 1-10125 Turin, C i . i i s Received January 3, 1994. I n Final Form: M a ~ r h18, 1994" On -pA1203and 6,8-A1203, the adsorption of CO at -300 K ( a ~occasionally i at -77 K) has been investigated by IR spectroscopy and adsorption microcalorimetry. 1, p LO three types of a-coordinated CO adspecies form, two of which are ascribed to sites (most likely tetrahedrally coordinated A1 ions) in crystallographically defective configurations, and one to sites (still m:1st likely tetrahedrally coordinated A1 ions) located on regular crystal planes; the relative amounts of the three CO adspecies depend on the alumina crystal phase, and on the temperature of vacuum activatiort. The overall amounts of CO adsorbed are quite small, as compared to the extent of surface dehydration: s;iriace reconstruction and ion shielding effects are supposed to be involved in the activation process. The hedl of adsorption of the three adspecies, extrapolated to zero coverage, correlates nicely with the correspondhg CO stretching frequencies, but at higher CO coverages the observed heats fall to very low values, iricompatible with the a-coordination mechanism proposed and with the observed frequencies. The occurrence of reversed surface mobility processes, brought about by the adsorption phenomenon and invoiiing an endothermic contribution to the net heat release observed, is postulated. Introduction Since Little and Amberg first observed by IR spectroscopy a weak interaction of CO with Al2O3,Ithe CO/A1203 system has been the object of several spectroscopic investigations carried out either at ambient temperature or at temperatures as low as -77 K.2-11 The various IR bands that form on A1203 at L 2160 cm-1upon CO uptake have been ascribed by all researchers to the a-coordination (polarization) of CO onto coordinatively unsaturated (cus) surface AP+ cations, produced during the vacuum thermal elimination of the surface hydrated layer.g Still, some disagreement remains among researchers on the structural and coordinative configuration of the adsorbing sites (e.g., compare the different assignments proposed for a CO band at -2195 cm-l in refs 7 and 8). Energetic aspects of CO adsorbed on A1203 have been considered in some cases: the heats of adsorption have been determined either by direct microcalorimetric measuremen@ or by using vibrational spectroscopic data obtained at various temperatures and constant CO pres~ u r e . ~ , ~Heat , ' , ~values quite different have been reported by the various authors for the same process, i.e., the u-coordination of CO onto cus Al3+ sites. Calorimetric values range between a minimum of -8.5 kJ mol-' and a @
Abstract published in Advance ACS Abstracts, April 15, 1994.
(1) Little, L. H.; Amberg, C. H. Can. J. Chem. 1962, 40, 1997. (2) Della Gatta, G.;Fubini, B.;Ghiotti, G.; Morterra, C. J. Catal. 1976, 43, 90.
(3) Paukshtis, E. A,; Soltanov, R. I.; Yurchenko, E. N.React. Kinet. Catal. Lett. 1981, 16, 93. (4) Soltanov, R. I.; Paukshtis, E. A.; Yurchenko, E. N.Kinet. Catal. 1982, 23, 164. (5) Paulshtis, E. A.; Yurchenko, E. N.Russ. Chem. Reu. 1983,52,242. (6)Zaki, M. I.; Knozinger, H. Mater. Chem. Phys. 1987, 17, 201. (7) Zaki, M. I.; Knozinger, H. Spectrochim. Acta 1987,43A, 1455. (8) Zecchina, A.; Escalona Platero, E.; Otero Arean, C. J. Catal. 1987, 107, 244. (9) Ballinger, T.H.; Yates, J. T.,Jr. Langmuir 1991, 7, 3041. (10) Morterra, C.; Magnacca, G.; Del Favero, N. Langmuir 1993, 9, 642. (11) Marchese, L.; Bordiga, S.; Coluccia, S.;Martra, G.; Zecchina, A. J. Chem. Soc., Faraday Trans. 1993, 89, 3483.
0743-7463/94/2410-l812$04.50/0
maximum PJ - 6 0 kJ mol-', whereas most calculated isosteric h a t .dues are scattered in the narrow range 20-35 kJ moi-' The lack of agreement among energetic data obtained i n different ways is due to various reasons, some of which dre herewith discussed; among the reasons is the fact t h a ~calculated isosteric heats are intrinsically related to the clverage energy of the sites, the surface being considered as a whole, whereas calorimetric adsorption heats are &biermined as a function of CO coverage and are thus sensit IVC-' to the surface heterogeneity. Reproducble nonlinear correlations have been observed between the '30 frequency of CO species formed on cus non-d metal ( ations and the relevant enthalpies of l 3 On the basis of these frequency/enthalpy correlations. )L was argued by Soltanov et ale4that the low adsorption hedt measured for CO uptake on 7-A1203a t relatively high coverages (-8.5 kJ mol-l 2, is incompatible with the obwrved vco figures (of the order of 2195-2205 cm-l), and thus not compatible with a CO a-coordination. The measured heat release was thus ascribed to another process, 1.e. ' ( 3 a physical adsorption of CO, which is definitely ,inlikely at ambient temperature. ture of the CO/A1203system so turns out plete and somewhat controversial, and some spec t 1 trscopic/calorimetric aspects of this system cot that the adsorptive behavior of A1203 dy he somewhat different from that of most i*s examined so far.12J3 :o papers, dealing with the adsorption of CO onto Degusw aluminum oxide C (Alon C), i.e., on one of i high-temperature transition aluminas14 in which the -.-1203phase is by far predominant, have brought us 12 d better understanding of the origin of the CO adsort, 115 sites,gand of the possible structural nature
__
(12) Morterr ,Gerrone, E ;Boils, V Fugini, B Spectrochim Acta 1987,43A, l b - . (13) Bolis. Y . rilbini, B.; Garrone, E.; Morterra, C. J . Chem. Soc., Faraday Trar - 19R9,85, 1383. (14) Lippet.i. 8. I.!.; Steggerda, J. J. InPhysicaland Chemical Aspects of Adsorbent: Catalysts;Linsen, B. G., Fortuin. J. M. H.. Okkersee. C., Steggerde Eds.; Academic Press. London, 1970; p 190. I
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Figure 1. High-resolution transmission electron microscopy images of A m (a), Alm (b), and Alm (c), and segments of the XRD spectrogramsof An3 (d),Alm (e),and Alms (f). The vertical arrowsin trace f indicate some XRD peakstypical of the high-temperature transition phases (6- and 8-A120s).
of the active centers" formed upon surface dehydration of the high-temperature transition aluminas. The aim of the present paper is 2-fold: (i) to complete in some way the recent works presented by Ballinger et aL9 and by Marchese et al.," extending the IR spectroscopic study to the adsorption of CO onto the lowtemperature transition alumina phases, which are most frequently used in catalysis; (ii) to characterize the CO/ A1203 system also from the energetic point of view, and check if the spectroscopicand/or energetic features of the CO/Al203system are a t all peculiar to transition aluminas, or may be reconciled with the generalized behavior, known for CO adsorbed on several non-d and do oxidic systems. To do so, the combined use of IR spectroscopy and adsorption microcalorimetry will be reproposed for a systematicstudy of CO uptake, carried out mostly a t 300 K and occasionally a t -77 K, on two different microcrystallline transition A1203 phases, correspondingto two consecutive steps of the complex phase transformation of aluminum hydroxide (bohemite) into aluminum oxide (corundum).
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Experimental Section A. Materials. A1203 samples were prepared starting from a pure commercial pseudo-boehmite preparation (Disperal Alumina, Condea Chemie), which was first dispersed in water and dried as described elsewhere.lSThe powdery precursor was fired (in air for 5 h) a t TI = 773 and 1273 K, yielding two alumina specimens that will hereafter be referred to as A773 and Alms, respectively. Figure 1 reports some high-resolution transmission electron microscopy (HRTEM)details of the preparationA773, isolated in two different stages of thermal activation in air (300 and 1023 K), and of preparation Alma. The following are noted. (i) The starting system An3 (Figure la) is made up of tiny platelet-like crystallites (average size -5 nm), characterized by some rather sharp edges frequently interrupted by other more stepped particle contours. The top termination of the crystallitesis most frequently along (15) Morterra, C.; Magnacca, G.; Cerrato, G.; Del Favero, N.; Filippi, F.; Folonari, C. V. J. Chem. Soc., Faraday Trans. 1993,89,135.
Morterra et al.
1814 Langmuir, Vol. 10, No. 6, 1994
small flat patches of regular (low-index)crystal planes, in which the interference fringe patterns of the intersecting crystal planes are detected with great difficulty, due to the high transparency of the material and to the shadows of the grooves due to the pore system. (ii) The average particle size and morphology of the A773 system long fired at -1023 K (Figure lb) are not much different from those of the starting A773 system. The crystallites are somewhat more closely packed together, and the sharp edges delimiting the patches of regular crystal planes cross with sharp angles and are less frequently interrupted by stepped, irregular contours. (iii) The A1273 system (Figure IC)is made up of large agglomerates (-40-80-nm side) of what remains of the starting particles. The patches of regular crystal planes, with which most of the crystallites terminate, present rather regular patterns of interference fringes, but the identification of the crystal plane(s) is still virtually impossible due to lack of definition. The side contours of the crystallites are, unexpectedly, more stepped and irregularly shaped than in the case of the material fired at lower temperatures. Note that the overall morphology of A1273 is similar to that reported for the well-knownAlon C preparation,l' but the particles of A1273 are somewhat more irregular, and their average size is definitely smaller. X-ray diffraction (XRD) spectrograms, segments of which are shown in Figure 1, indicate that (i) A773 (Figure Id) is microcrystalline (the XRD peaks are broad and ill defined),and is virtually pure 7-A1203,and (ii)A773 remains mainly 7-A1203for thermal treatments at 21' I 1173 K (Figure le),aminor fractionofthe6,dphasebeinggradually formed for prolonged thermal treatment in air or in uacuo. The Nz Brunauer-Emmett-Teller (BET) surface area of A773 is 186 m2 gl, and declines slowly for thermal treatment at T > 773 K (e.g., after a 3-h treatment in uucuo at 1023 K, the surface area is still 148 m2 g-1); A773 exhibits a very abundant porosity, as expected of y-A1203,14 with a narrow peak in the pore-size distribution centered at -32 A. The XRD data of A1273 (Figure 10 indicate that the crystal phase has changed: the low-temperaturetransition phase ?-A1203 is now virtually absent, whereas the two high-temperature transition phases &A1203 and &A1203 are present, in proportions that are difficult to establish; A1273 will be thus referred to as 6,0-A1203. The N2 BET surface area of A1273 is 120 m2 g-l, and is fairly stable for vacuum thermal treatments at T I1373 K; the overall porosity of A1273 is less abundant than that of A773, and the pore-size distribution curve exhibits a broad envelope of peaks in the range 35-60 A. Alumina samples are designated in the text and figures by their symbol AT^), followed by a second numeral T2 that indicates the temperature (K) at which the samples were activated-oxidized in uacuo prior to adsorption experiments. B. Methods. HRTEM images were obtained with a Jeol JEM 2000 EX (200kV,LaBCfilament, top-entry stage). BET surface areas and porosimetric data were determined with N2 at 78 K with an automatic apparatus Sorptomatic 1900, C. Erba. Crystal-phase data were determined with a Philips PW 1050 diffractometer (Cu Ka). In situ IR spectra of adsorbed CO were run at 2-cm-l resolution at 300 K, and occasionally at -77 K, on a Bruker 113v FTIR spectrometer, equipped with an MCT detector. The spectra of adsorbed CO were computer subtracted of the rotovibrationalspectralcomponentdue to gaseous CO. Unsmoothed segments of the absorbance spectra of adsorbed CO were band-resolved and integrated with a
Pascal program by Bruker (Simband) and, occasionally, with a Fortran-Minuit program described elsewhere,16only for comparison purposes. Heats of adsorption were measured at 303 K by means of a Tian-Calvet microcalorimeter (Setaram), connected to a vacuum/gas volumetric apparatus that enabled the simultaneous determination of the heat released and of the amounts adsorbed during the process. A stepwise adsorption procedure described previo~sly'~ was followed.
Results and Discussion A. Adsorption of CO on A,,,+ IR Spectra. Parts a and b of Figure 2 show the spectral patterns relative to CO uptake at ambient temperature onto A773673 and A773773, respectively. The following are noted. (i) On A773673, CO adsorption yieldsa weak and virtually single band, hereafter referred to as band (CO)A,centered at wavenumbers that vary, with increasing CO coverage, between =2200 and -2195 cm-'. Figure 2d shows the (surface area and sample weight) normalized optical adsorption isotherm of species (CO)Aon A773673, which exhibits a slow-growingtrend and does not reach saturation at the maximum pco examined. The species (CO)Awas observed previously, and was assigned to the a-coordination of CO onto cus A1 sites, most likely possessing an (incomplete)tetrahedral coordination (A.lNW), and located on low-index crystal planes, i.e., in crystallographically "regular" configurations.8J1 The micrograph of Figure l a showed that crystal terminations along relatively extended patches of regular crystal planes are indeed quite frequent on A773. In Figure 3 the vco frequencies of adsorbed CO species are reported as a function of the overall absorbance of CO bands. The bottom plots of parts a and b of Figure 3 show that the spectral position of (CO)Aon A773673 varies linearly with CO coverage, at least in the small range of CO coveragesthat can be covered upon adsorption at -300 K. The frequency at zero CO coverage (i.e., the singleton frequency ( U O ) A ) of (CO)A on A773673 is -2205 cm-'. The change of ( U C O ) A with CO coverage could be thought to reflect inductive effects produced by the charge-releasing adsorbed (CO)Aspecies onto the charge-releasing (CO)A adsorbing species, as observed in the case of CO uptake at -300 K on other oxidic systems.lS2O If this were the case, inductive effects would transmit at the surface of -y-Al& at fairly long distances, in view of the very low coverage attained with CO at -300 K (see ref 2, and the quantitative data reported below). Moreover, with increasing CO coverage, the band of (CO)A broadens progressively on the low-frequency side without shifting, as shown by the all-positive differential spectra reported in Figure 2a (top dotted curves): in the case of adsorbateadsorbate inductive effects, a CO band of virtually unchanged breadth would move downward with increasing CO uptake, and the differential spectra would sxhibit negative peaks at high u and positive peaks at low u. The downward shift of the band maximum of (CO)Awith 0 ~ 0 is thought to derive more from an intrinsic (structural) site heterogeneity than from an induced heterogeneity (16)Lamberti, C.; Morterra, C.; Bordiga, S.; Cerrato, G.; Scarano, D. Vib. Spectrosc. 1993, 4, 273. (17) Fubini, B. Thermochim. Acta 1989,135, 19. (18) Garrone, E.; Bolis, V.; Fubini, B.; Morterra, C. Langmuir 1989, 5,892. (19) Morterra, C.; Orio, L.; Emanuel. C. J.Chem. Soc.. Faradav Trans. 1990,86, 3003. (20) Bolis, V.; Fubini, B.; Garrone, E.; Morterra, C.; Ugliengo, P. J . Chem. Soc., Faraday Trans. 1992,88, 391.
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Figure 2. Spectral features of CO adsorption at =300 K on A779T2: (a) solid-line traces, the adsorption pattern on A773673 @CO varies, from top to bottom, between 1.3 X 102 and 1.5 X 1 W Torr);dotted-line traces, differential spectra, as indicated by the numbers on the curves; (b) solid-line traces, the adsorption pattern on A773773 @eo varies, from top to bottom, between 1.4 X 102 and 1.5 X le2 Torr); dotted-line traces, differential spectra, as indicated by the numbers on the curves; (c) solid-line traces, the adsorption pattern on A7781023 @eo varies, from top to bottom, between 1.4 X lo2and 1.5 X le2 Torr); dotted-line traces, some of the (CO)Bcomponents, computer resolved as described in the text; (d) optical adsorption isotherm of CO on An3673 (integralCO absorbance vspco); (e) optical adsorption isotherms of CO on A773773 (integral CO absorbance vs pco), upper trace, total CO uptake; traces A-C, uptake of species (CO)A,(CO)B,and (CO)c,respectively; (0optical adsorption isotherm of CO on A77s1023 (integralCO absorbance vs pco), upper trace, total CO uptake; traces A-C, uptake of species (CO)A,(CO)B,and (CO)c, respectively. caused by mutual perturbations among adspecies, which (ii) On A773773, the absorption due to adsorbed CO in the present case are too few and too far apart from one becomes more complex (see Figure 2b). The regular species (CO)A (due to CO on regular crystal planes) intensifies, another.
Morterra et al.
1816 Langmuir, Vol. 10, No. 6, 1994 22501
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The uco vs ( A C O plots ) ~ ~ of Figure 3a show that the spectral positions of species (CO)Band ( C 0 ) c vary very little, if at all, with CO coverage, whereas the spectral position of species (CO)Aexhibits a steep linear trend with the overall CO coverage. Also in the case of A773773, the dependence of ( V C O ) A on 6co is thought to reflect more an actual heterogeneity of the relevant adsorbing sites, located on patches of low-index crystal planes, than a heterogeneity induced by inductive effects. This is in view of the fairly low concentration of adsorbed CO (less than 0.1 molecule/ nm2; see the adsorption isotherms discussed below), and of the continuous broadening of band (CO)Aon the lowfrequency side, better shown by the differential spectra of Figure 2b. The singleton frequency (uo)~of the three CO species formed on A773773 is ~ 2 2 1 0cm-' for (CO)A,~ 2 2 1 7cm-l for (CO)B,and -2238 cm-' for ( C 0 ) c . It is also noted that, on passing from A773673 to A773773, ( U O ) A has increased: the frequency of (CO)Adepends on the sample activation temperature; i.e., it is influenced by the residual concentration of (polar) surface OH groups, as observed also for CO on other 0xides.~3Jg,~~ The surface sites responsible for species ( C 0 ) c are characterized by a very high Lewis acidity with respect to the regular species (CO)A,as shown by the uco frequency 25-30 cm-1 higher. The species ( C 0 ) chas been observed previously, and has been ascribed to a family of highly uncoordinated AIIV,, cations localized in crystallographically defective configurations.**" In fact, crystal defects like steps, corners, etc. can be expected to be quite abundant in a system with small crystal sizes and high porosity, as shown by the HRTEM image reported in Figure la. Also species (CO)B,which is characterized by a Lewis acidity higher than that of (CO)A,though not as high as that of (CO)c,reaches a fast saturation with CO coverage already at ambient temperature. It is most likely ascribable to a (different) family of AIIVcuscenters, localized in (different) structurally defective configurations. Consistent with this assignment, lowering the adsorption temperature to -77 K (see the spectral pattern reported in Figure 4a) extensively increases both the overall intensity and band width of the band due to the regular (CO)A species, whose isotherm at 300 K a t the maximum Pco examined was still far from saturation. Lowering the adsorption temperature does not increase appreciably the intensity of the "defective" species (CO)Band (CO)c,which reached saturation already at 300 K; instead, the species (CO)Band ( C 0 ) c become difficult to observe, due to the overwhelming intensity of other lower energy CO adspecies (like (CO)Aand H-bonded ( C O ) H )and , of the abundant adsorbate-adsorbate perturbations therefrom. Figure 5a reports the UO-H spectra of the system A773 vacuum activated a t medium-high temperatures (bottom section), and a differential spectrum therefrom (top section). The complex W-H spectral pattern of transition aluminas in these activation stages is well-known (e.g., see refs 9 and 22 and references therein), and does not require a detailed description. The OH spectra 1 and 2, and the relevant differential spectrum 12-11, indicate that in the narrow interval of activation temperatures 673-773 K one relatively broad OH species ( ~ 3 7 3 0cm-l) increases somewhat, while mainly three OH species decrease: a broad band at -3590 cm-', a sharp low u fraction of the OH band a t -3675 cm-I, and a discrete OH component at -3770 cm-l. The decline of the OH band at ~ 3 5 9 0cm-1
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(*co)tot Figure 3. Dependence of the individual CO frequencies on the overall CO coverage ((uco)~vs ( A C O ) ~(a) ~ )bottom : line (broken trace),the species (CO)Aon A773673; traces A-C, species (CO)A, (CO)B,and (C0)con A773773;(b)bottom line (broken trace),the species (CO)Aon A773673; traces A-C: species (CO)A,(CO)B,and (C0)c on A7731023.
while two new CO adspecies form, hereafter termed (CO)B and (CO)c, respectively, centered a t -2215 and -2235 cm-l. Figure 2e shows the total optical adsorption isotherm (top curve), and the three individual adsorption isotherms of A773773 (curvesA-C), obtained through band resolution and integration of the complex absorption shown in Figure 2b. Species (CO)Band (CO)c,characterized by fairly small intensities, reach a saturation plateau fast, whereas species (CO)Astill exhibits a slow-growing trend and does not approach a saturation plateau in the whole range of CO pressures explored.
(21) Bolis, V.; Morterra, C.; Volante, M.; Orio, L.; Fubini, B. Langmuir 1990, 6, 695.
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Figure 5. (a) IR spectra in the VOH range of AmTz: (bottom) transmittancespectra of A773673 (curve l),Am773 (curve21, and An31023 (curve3); the spectraare presented in the transmittance modeto emphasizethe weakest spectralfeatures;(top)abrbance differentialspectrum [An3773- An36731;the arrows pointing up indicate the spectral components that increased, the arrows pointing down the components that decreased. (b) IR spectra in the VOH range of AIn3Tz: (bottom) transmittance spectra of Alms673 (curve l), Am3773 (curve2), and Aln31023 (curve3; this spectrumunderwenta 2-fold ordinatescalemagnification);(top) absorbancedifferential spectra [Aln3773- Aln36731, [Alm1023 - Ain37731, and [Aim3773 - Ans7731.
ascribed to surface clusters of two octahedral Al ions and
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Figure 4. Spectral features of CO adsorption at =77 K (a) An3773 (pm varies, from top to bottom, between 4.0 X 10' and 1.0 X 10-9 Torr); the dotted-line trace shows the spectrum
observed,on the same sampleand with the same ordinate scale, at -300 K and pm = 40 Torr; (b) An31023 @CO varies, from top to bottom, between 4 X 10' and 1 X 1W Torr); the dotted-line traceshows the spectrumobserved,on the same sampleand with the sameordinatescale,at -300 K andpm = 40Torr;(c)Alm1023 @CO varies, from top to bottom, between 4 X 101 and 1 X 10-9 Torr);the dotted-linetrace showsthe spectrum observed,on the same sample and with the same ordinate scale, at a300 K and pco = 40 Torr.
and the increase of the band at -3730 cm-1 correspond to the eliminationof the last fractions of H-bonded hydroxyls, and to the simultaneous formation of the highest surface concentration of bibridged free OH groups (the OH species termed by Kn6zinger et u Z . , ~ ~ 11, and IIb, which in this system are not resolved). The decline of the other two OH bands represents an early step of the gradual elimination of free surface hydroxyls; in particular, the sharp negative band at =3665 cm-1 in the differential spectrum [2 - 1J seems to correspond to the selective elimination at 773 K of the low-frequency fraction of tribridged free OH groups (the OH species termed IIIJ, which has been (22) Kn-er,
H.; Ratnasamy,P. Catal. Rev.-Sci. Eng. 1978,17,31.
one tetrahedral Al ion.% It is logical to supposethat the increase of species (CO)A and the appearance of species (CO)B and (C0)c are somehow related with the partial decline of the two OH species at -3665 and -3770 cm-l, even if it is not so easy to find a 1:lcorrelation. Still, as it will appear clear below where the results relative to the system A1273are discussed, it is our belief that the species of highest acidity, (CO)c, is mainly formed on Alw, sites created in highly defective configurations by the vacuum thermal elimination of the (most reactive) isolated OH species absorbing at =3770 cm-l (the OH species termed 13,ascribed by Knbinger et aLn to terminal (monocoordinated) OH groups in the coordination sphere of Alw ions. Part c of Figure 2 reports the spectral pattern of CO uptake at 300 K onto A7131023. It must be preliminarilynoted that,when the An3 system is activated at T > 773 K (i.e., at temperatureshigher than that at which the y-Al203 material was first fired during the preparation; this is a situation quite commonly met with the y-Al2O3 systems used in catalytic applications), some irreversible modifications occur in the material. In fact, they-Al203crystalphaseremains basicallyunchanged (at least up to -1200 K see Figure le), but the particle morphology becomes slightly different (as shown by the micrograph of Figure lb), the surface area declines somewhat, as does the overall porosity, and the pore size distribution is modified, while most of the free surface (23) Nortier,P.;Fourre,P.;Mohammed Saad, A. B.; Saw, 0.;Lavalley, J. C. Appl. Catal. 1990,61,141.
1818 Langmuir, Vol. 10, No. 6, 1994
OH species are eliminated (see curve 3 of Figure 5a, relative to A ~ ~ ~ 1 0 2Note, 3 ) . in particular, that the sharp terminal OH component I, at e3770 cm-l is eliminated irreversibly: rehydration with H2O vapor at ambient temperature, followed by a subsequent second dehydration, does not lead to the appearance/decline of the e3770-cm-1OH band anymore, unless the rehydration process is carried out at T 1 673 K.24 This indicates that, at T 1 673 K, besides the normal dehydration processes also surface reconstruction phenomena occur in the 7-A1203phase. Also the chemical surface features of the A773 system activated at T > 773 K, as revealed by the adsorption of CO a t ~ 3 0 K, 0 are modified and become quite complex. The spectral pattern of CO on A7731023,reported in Figure 2c, shows that the overall intensity of the CO absorption has increased appreciably (see also the total optical adsorption isotherm, top curve in Figure 20. This indicates that the dehydration process yielding surface AlTVC,cations of strong acidity prevailed over the loss of surface area. In more detail, Figure 2c indicates the following. (i)The species of strongest acidity, (CO)c,has increased appreciably (by a factor of =3), more in intensity than in bandwidth. Note that the (surface area normalized) intensity of species (C0)c will decline somewhat for prolonged activation at T L 1023 K: the datum is consistent with the hypothesis of a defective nature for the sites responsible for (CO)c,with the hypothesis of the occurrence of surface reconstruction effects, and with the observed irreversible thermal elimination of the free OH species at ~ 3 7 8 cm-l. 0 Still, the species (CO)c, ascribable to metastable AIIVcusconfigurations, must be thought of as peculiar to the y-A1203phase and to its pore system, as this species tends to disappear completely on the hightemperature transition A1203 phases (dealt with below). This hypothesis is confirmed by the data reported in the literature for Alon C.9J1 (ii) The species (CO)A,formed on regular crystal planes, should be expected to have increased appreciably: curve A of Figure 2f, corresponding to the optical adsorption isotherm of species (CO)Aobtained through a “reasonable” deconvolution of the relevant spectral component in the pattern of Figure 2c, indicates that this is actually the case. (Due to the complexity of the spectra of Figure 2c, it is virtually impossible to obtain a unique and unequivocal band deconvolution. The reasonable deconvolution proposed here was obtained by assumingfor (C0)cand (CO)A the same variability range of bandwidths and of Gaussian character observed in the case of A773773, and ascribing all the remaining intensity of the overall absorption to the complex envelope of species (CO)B. The result is a (CO)B band quite symmetrical (see, for instance, the top curves in Figure 2c), with percent Gaussian character and halfbandwidth varying with CO coverage no more than do the other CO components, and varying in frequency as shown by the vco vs (Aco)totplot of Figure 3b). The spectra in Figure 4b show that at e 7 7 K the band ~ A7731023 is very strong, very broad, of species ( c 0 )on and asymmetric on the low-frequency side. This confirms a highly heterogeneous nature for the sites of (CO)Aand confirms that, at 300 K, the uptake of species (CO)* is far from being complete (see,for comparison, the dotted trace in Figure 4b). (iii) The most spectacular changes produced in the spectrum of CO adsorbed a t e300 K on y-A1203after activation at T > 773 K are in the intensity and breadth of the band of species (CO)B, isolated from the overall
Morterra et al. band envelope as outlined above. The vast heterogeneity of this band iridicates that a broad spectrum of AIIVcue sites in slightiy different unsaturated and/or structurally defective configurations has been produced with the thermal treatment. As observed for (CO)c, also the intensity s.i hand (CO)B tends to decline, though very slowly, if tiit- :hermal treatment at temperatures as high as e1023 ti ; S carried out for much longer times: this confirms tlw crystallographically defective nature of the relevant adqcrbing sites, and the partial thermal instability of the pore avitem and of the surface of 7-A1203. B. Adsorption of CO on A1273. IR Spectra. Figure 5b reports some spectra from the medium-high temperature segmenc of the thermal dehydration pattern of A1273 As A1273 corresponds to the 6,8-A1203phase, these spectra and the reltvmt assignment can be compared with the OH spectr;: reported for Alon C by Ballinger et aL9 The overdl ,pectral trend of the dehydration pattern of A1273 is quite similar to that O f A773 (Figure 5a), and the only signiflcaiit differences are the following. (i) The r e d u t i o n of the band due to bicoordinated OH g r o ~ p si?i~ l,w~o ~components (OH species IIb at -3745 cm-l and Q i - i species 11, at e3726 cm-I). The higher v component 1- was not resolved on the y-Al2O3 phase, possibly diit to the overwhelming intensity of the closeby componen? il,. this seems to indicate a higher occupation of tetrahedrdl 3ites in the low-temperature transition aluminas I C (ii) The riluch lower intensity exhibited by the OH component i r 4780 cm-l, attributed to terminal hydroxyls I, on A P ioI:*22 and supposed above to involve preferentially cry~t~ll~graphically defective configurations. Note that on AI_- 2 weak band at ~ 3 7 8 cm-1 0 is still present, and its elir! ii’di ion in the 773-1023 K range is evidenced by the difft.rtsnt,al spectrum [3 - 21 of Figure 5b, whereas on Alon C t hi‘ c IHcomponent is totally a b ~ e n consistent t,~ rystalline order reported for the 6-A1203 The differentialspectrum [2b- 2al in Figure 5b confirms that, a t mrdrwx dehydration stages, the high-temperature transition A I 4 7 phase(s) tends to exhibit lower amounts onents (indicated with downward arrows) tetrahedral coordination of Al, whereas the other O H components that involve the sole octahedral coordinatiori ul A1 are more abundant. Figure 6a-c weports the spectral adsorption patterns of CO on some ,i 273 samples, whereas parts d-f show the correspondim zormalized optical adsorption isotherms. The vco VP ‘-47,))tot plots of CO uptake onto A1273 are reported in F!giire 7. The following can be noted. (i) As in the Lase of A773673 (Figure 2a), on A1273673 there is the iorrnation of only one weak band, ascribable ’ 9 ) ~The . band is asymmetric on the lowand does not shift with increasing CO pressure, birr hcomes broader and broader on the lowfrequency side, as shown by the all-positive differential spectra (see the top curves of Figure 6a). This indicates also for Alz; 4 B heterogeneous nature of the sites responsible ~ linearly with for ( c o ) ~r!’h~> . frequency of ( c o )decreases CO coverage ‘see Figure 7), much as in the case of A773, and the CO frequency extrapolated to zero CO coverage (&)A 2204 cm-l) is lower than in the case of (CO)Aof A773. It is deduced that, on the regular crystal planes with which the particles of A1273 terminate, the site heterogeneity is sinuiar to that of the regular crystal planes Of A773, but the deg I P:: of coordinative unsaturation obtainable by
(24) Zecchina, A.; Coluccia, S.; Morterra, C. Appl. Spectrosc.Reu. 1985, 21, 259.
(25) Reller. A . I>ocke,D. L. Catal. Lett. 1989, 2, 91.
Lewis Acid Sites on A1203
Langmuir, Vol. 10, No. 6,1994 1819
-A
a
1.5 0
d 1
I
4
0.5
L
1
! 2240
2200
2160
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0
60
90
120
150
PCO
Ib
T 2200
e
:::::I-. P
I
I
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2200
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E
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60
90
120
150
PCO
4 AI
C
2240
2200
wavenumber cm'
2160
0
30
60
90
120
150
PCO
Figure 6. Spectralfeatures of CO adsorption at ~ 3 0 K0 on Ain3T2: (a) solid-line traces,the adsorption pattern on Am3673 @CO varies, from top to bottom, between 1.3 X 102 and 1.5 X 10-2 Torr); dotted-line traces, differential spectra, as indicated by the numbers on the curves; (b) solid-line traces, the adsorption pattern on Aln3773 @CO varies, from top to bottom, between 1.4 X 102 and 1.6 X le2 Torr); (c)solid-line traces, the adsorption pattern on Aln31023 @CO varies, from top to bottom, between 1.4 X 102 and 1.5 X Ton); dotted-line traces, differential spectra, as indicated by the numbers on the curves; (d) optical adsorption isotherm of CO on Alns673 (integralCO absorbance vs pco); (e)optical adsorption isotherms of CO on Am3773 (integralCO absorbance vs pco),upper trace, total CO uptake; traces A-C, uptake of species (CO)*, (CO)B,and (CO)c, respectively; (f) optical adsorption isotherm of CO on Aln31023 (integral CO absorbance vs pco), upper trace, total CO uptake; traces A-C, uptake of species (COh, (CO)B,and (CO)c, respectively.
Morterra et al.
1820 Langmuir, Vol. 10, No. 6, 1994
vco 2250
a 2240
2230
2220
221c
220c
2 19c
0.5
3
1
1.5
1
1.5
(ACO)tot 2250
vc0
b 2240
2230
2220
221c
220c
2 19( I)
0.5
(ACOhot
Figure 7. Dependence of the individual CO frequencies on the overall CO coverage ( ( u c o )vs ~ ( A C O ) ~ ~(a) ) : bottom line (broken trace), the species (C0)A on A1273673; traces A and B, species (CO)Aand (CO)Bon A1273773; (b) bottom line (broken trace),the species (CO)Aon ALZ73673; traces A-C, species (CO)A,(CO)B,and (C0)c on A12731023.
cationic sites on the regular crystal planes of A1273 (Le., the Lewis acidity of sites A) is somewhat lower than on A773.
(ii) On passing to A1273773 (Figure 6b), the overall CO uptake increases by a factor of ==2 and there is the appearance of appreciable amounts of the species termed (CO)B. The latter, ascribed to CO adsorbed onto AlrVcus sites on crystal defects, exhibits a relative intensity with respect to (CO)Ahigher than on A773 The species (CO)Bsaturates at very low CO coverages, as it does in the case of A773773 (see Figure 2e), but unlike A773773 the vco frequency declines linearly with CO
coverage, as shown by Figure 7a. In this case this shift effect cannot be ascribed primarily to an intrinsic heterogeneity of the sites responsible for species (CO)B: in fact the saturating intensity of ( C O h is reached for Pco = 10 Torr, whereas the change of ( V C O ) B with 8co continues also after reaching saturation. The change of (VCO)B is due primarily to an induced heterogeneity, brought about by inductive effects (dipole-dipole interactions) produced by the increasing amounts of (C0)Aspeciesbeing adsorbed on adjoining patches of regular crystal planes. A different surface area and porosity and a different occupation of tetrahedral cationic sites are thus not the sole differences between 7-A1203and 6,8-A1203. A different geometry and different mechanisms of transmission of electronic effects at the surface must also exist, consistent with the different particles and pore sizes in the two transition p h a s e ~ . ' ~ J 5 ? ~ ~ (iii) On A12731023, Le., on a 6,8-A1203 system highly dehydrated and on which no irreversible changes of surface area and pore size have occurred yet, the overall CO uptake is almost doubled (see Figure 6c,O. Both (C0)Aand ( C O h species increased, and also a tiny amount of ( C 0 ) c formed: consistent with the previous assignment, the formation of ( C 0 ) c is thought to coincide with the elimination of the weak residual OH band I, at ~ 3 7 8 0 cm-'. The vo frequency of species ( C 0 ) c ( ~ 2 2 3 0cm-l) is lower than on A773, and still does not change with CO coverage, indicating that this species forms at crystal defects somewhat different from those of A773 and that, as on A773, these sites cannot be reached by the adsorbateadsorbate perturbation effects from the abundant (CO)A species that are being adsorbed on regular crystal planes. Unlike (CO)c,the frequency of species (CO)Bis affected by side adsorbate-adsorbate perturbation effects, and still decreases linearly with CO coverage. Also the frequency of the more abundant species (CO)Adecreases with CO coverage, but in this case most of the effect must still be ascribed to the intrinsic heterogeneity of the relevant sites rather than to adsorbate-adsorbate perturbation effects. In fact the band of (CO)Abecomes broader and broader on the low-frequency side, as shown by the differential spectra of Figure 6c in which no negative excursions are observed. The spectral pattern in Figure 4c, relative to CO uptake at =77 K on A12731023, confirms also for the hightemperature alumina phases that the uptake observed at -300 K corresponds to the total amount of CO adsorbable on defective configurations, whereas the fraction of (CO)A species observable at =300 K is fairly small, in terms of both intensity and half-band width, compared to that adsorbable at low temperature. C. Adsorption of CO on A773 a n d A1273. Volumetric a n d Microcalorimetric D a t a . Figure 8 reports, for A773673 and A773773, the (surface area) normalized volumetric adsorption isotherms (part a) and the heats of adsorption q as a function of CO coverage (part b). Figure 9 reports the same kind of data for A1273773 and A12731023. The following can be noted. (i) As already anticipated above, the overall amounts of CO adsorbable at 303 K on aluminas are fairly small, especially in the case of A773 samples. For instance, on A773673 at pco = 80 Torr CO uptake reaches only ~ 0 . 0 6 CO molecule/nm2;on other do metal oxides like Ti0213 or Zr0zZ1pretreated under similar conditions, the adsorptive capacity toward CO is 1 order of magnitude larger. (ii) On passing from A773673 to A773773, there is an increase of the CO uptake, which is mostly ascribable to ~~
~~
(26) Giachello, A,;et al. Data to be presented at the meeting CAPOC3, Brussels, April 1994.
Lewis Acid Sites on A1203
Langmuir, Vol. 10, No. 6, 1994 1821
CO molecules
loo, a25 Y
a
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1
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a06
0.12
0
020
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5
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m
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20
M35
0 0 2 0 4 0 6 0 8 0 1 0 0
OD5
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pco CTorrl
010 015 na CJJ mol m - 3
Figure 8. Quantitative aspects of CO uptake at 303 K onto A773T2: (a) volumetric adsorption isotherm on A773673 (lower curve) and A773773 (upper curve); the empty symbols on the plots correspond to the primary adsorption isotherms and the full symbols to the secondary adsorption isotherms (Le., after CO evacuation at ambient temperature for 1 h); (b) differential adsorption heats of CO adsorbed on A773673 (lower plots) and A713773 (upper plots); empty symbols correspond to the primary adsorption isotherms and the full symbols to the secondary adsorption isotherms (i.e., after CO evacuation at ambient temperature for 1 h). a25
,015
,
,
CO "3
molecules nm-' a? 0.12
aos
L
O J
0
p,,
CTorrl
0.1;
QD5
OD 035 na Cum01 m-2)
OM
025
Figure 9. Quantitative aspects of CO uptake at 303 K onto Aln3Tz: (a) volumetric adsorption isotherm on A1273773 (middle curve), Aln31023 (upper curve),and A773773 (lower broken curve, inserted for comparison);the empty symbols on the plots correspond to the
primary adsorption isotherms and the full symbols to the secondary adsorption isotherms (i.e., after CO evacuation at ambient temperature for 1h); (b) differential adsorption heats of CO adsorbed on Aln3773 (lower plots), Aln31023 (upper plots), and A113773 (broken-line plot, inserted for comparison). The empty symbols on the plots correspond to the primary adsorption isotherms and the full symbols to the secondary adsorption isotherms (i.e., after CO evacuation at ambient temperature for 1 h). the appearance of the high-frequency adspecies (C0)Band (C0)c. The two curves of Figure 8a indicate that this increase does not mean much in quantitative terms (at pco = 80 Torr the increase is -0.02 CO molecule/nm2,i.e., =25% of the total amount adsorbed). This was somehow expected, on the basis of the low IR intensity change observed in the CO bands (see Figure 2a,b,d,e), and considering that in the 2240-2200-cm-l spectral interval the extinction coefficient of u-coordinated CO species does actually change, but not much,20so that the intensity of the CO bands can be taken as a measure of the relevant adsorbed amounts. The very small increase of the CO uptake is apparently in contradiction with the abundant decline of intensity observed in the OH bands (see curves 1and 2 of Figure 5a). This lends support to the hypothesis of surface reconstruction phenomena that could occur at T L 773 K with the dehydration process, so that only a minor fraction of the surface A1 ions liberated by dehydration would actually attain a coordinative unsaturation sufficient for coordinating CO at =300 K.
(iii) On the high-temperature transition alumina phases, e.g., on A1273773, the CO uptake becomes definitely larger than on A773773, as clearly shown by the adsorption isotherms in Figure 9a: at pco = 80 Torr, the coverage is ~ 0 . 1 2molecule/nm2 on 6,8-A1203,and -0.08 molecule/ nm2 on ~-A1203. This indicates that, a t least in the medium-temperature activation stages, the dehydration process is faster on the high-temperature alumina phases than on the low-temperature one, and agrees with the mainly negative differential spectrum [2b - 2al reported in Figure 5b, and relevant to the OH regions of A1273773 and A773773. (iv) On A1273 dehydrated a t high temperature (1023 K; Figure 9a) the overall CO uptake (=0.15 molecule/nm2at pco = 80 Torr) increases by 20.03 molecule/nm2 with respect to A1273773, in agreement with what is indicated by the IR data of Figure 6c,f. In a previous work: the CO uptake measured a t pco = 12 Torr on a different yAlzO3 preparation activated a t 1013K was =0.09 molecule/nm2, very close to the ~ 0 . 0 molecule/nm2 7 measured at the same pco on the present sample A12731023.
Morterra et al.
1822 Langmuir, Vol. 10, No. 6, 1994
(vi) The fast declining trend of all the q vs n, plots of Also on A1273, the increase of the CO uptake with Figures 8b and 9b indicates that the spectrum of energies dehydration a t high temperatures is quite low compared involved in the adsorption of CO on yA1203 and 6,8-A1203 to the decrease of the intensities of the surface OH bands. is extremely broad, confirming the large surface heteroThis confirms that surface mobility and ion shielding geneity postulated above on the basis of the IR spectra. effects are likely to occur at high temperatures, and produce Moreover, at the high CO coverages corresponding surface reconstructions and a net surface unsaturation mostly to the formation of larger and larger amounts of much lower than expected on the basis of the plain dehydroxylation mechanisms normally p r o p o ~ e d .Also ~ ~ ? ~ ~ species (CO)A,q values fall below -10 kJ mol-'. This is a surprisingly low value for CO a-coordinated onto AP+ in the case of the high CO coverages attained at low cations (tetrahedrally coordinated), which are normally temperatures, reported by Ballinger and Y a t e ~ ,the ~ supposed to possess relatively high Lewis acidity. q values decreasing ratio of the integrated intensities of the OH bands is much larger than the corresponding increasing reported so far for the energy of interaction of CO with ratio exhibited by the CO band(&. dometal cations in oxidic12~20~28 and zeolitic systems29range in the interval 80-30 kJ mol-', and are normally correlated Models for some possible reconstructions of low-index with the C-0 stretching frequen~y.'~J3?~9 The behavior crystal planes of A1203have been recently proposed by of the CO/A1203 system so turns out to be somehow Marchese e t d . l l These models, though speculative, are anomalous; in fact, also in a previous work dealing with useful to show that a plain dehydration (which leaves all the adsorption of CO a t ambient temperature on 7-A1203 surface ions in the regular bulk positions) would lead to and 8 - A l ~ 0experimental 3~ adsorption heats as low as -8.5 a surface unsaturation unacceptably high. The reader is kJ mol-' were observed. referred to these pictorial models to see what kind of Site heterogeneity induced by adsorbate-adsorbate migrations of the surface 02-ions are needed to shield cus interactions was argued above to be most likely a minor surface AP+ centers. one in the case of the abundant regular species (CO)A, (v) In the heat vs coverage plots of Figures 8b and 9b, which is the only species that keeps forming at high CO the extrapolation to zero coverage of the continuous curves coverages. As a consequence, the steeply declining trend (not reported, for the sake of brevity) passing through the of the q plots indicates a vast intrinsic heterogeneity for middle points of the experimental histogramd7gives an the Al'",, sites formed on regular crystal planes of both estimate of the initial heat of adsorption (qo). The latter yAl2O3 and 6,8-Al2O3. The heterogeneity of species (CO)A datum represents the adsorption enthalpy of CO on the is confirmed by the large downward shift and by the large most energetic fraction of adsorbing sites. In fact, a t zero half-bandwidth that the (CO)Aband reaches at the high coverage the most energetic sites are covered first, and a t coverages attained at low temperature (see Figure 4 and vanishing coveragethe inductive effects, possibly occurring refs 8, 9, and 11). with increasing CO coverage, are still absent. As a The steep downward trend of the q vs n, plots correctly consequence, the extrapolated qo values are a useful accounts for site heterogeneity, but the very low value parameter of comparison for the acidic strengths of the reached by q at high CO coverage is quite surprising. In various families of cus cationic sites. fact, in the case of a plain u-coordination to do metal cations On A773673, on whose surface only the species (CO)Ais the stretching frequency of adsorbed CO (UCO) and the formed, qo is estimated to be =45 kJ mol-': this should enthalpy of adsorption (A,H) are usually correlated,12J3 be considered as the adsorption heat of the most energetic and a heat of adsorption of -10 kJ mol-' would correspond fraction of the regular (CO)Aspecies, i.e., the adsorption to a uco very low and close to that of the gas (2143 cm-'). enthalpy of the singleton (CO)Aspecies ( ( U O ) A 2208 cm-'1. This would represent CO uptake onto sites of extremely On A773773 (Figure 8b) and on A12731023 (Figure 9b), low acidity (uptake hardly obtainable at ambient temthe qo value has become as high as 4 3 0 and =85 kJ mol-', perature), whereas in the present case the lowest uco respectively, relatively large values for a-coordinated CO observed for (CO)Ais as high as -2195 cm-1. surface complexes. This figure is a measure of the For some reason the adsorption heat measured for adsorption energy of the most energetic fraction of the species (CO)Ais, or becomes with CO coverage, muchlower highly energetic (C0)c adspecies on A773773, and of the than is acceptable for a (weakly) chemisorbed species. In tiny amount of the same (C0)c adspecies still formed on fact, the q values of Figures 8b and 9b tend to values as A12731023, The energies involved in the adsorption of CO low as those normally expected for a plain physical on defects and on regular crystal planes differ by 40 kJ adsorption. The occurrence of a plain physical adsorption mol-': this large difference was actually expected, in view was actually claimed by Soltanov e t ~ l . discussing , ~ the of the large difference of the uco frequency (Au 40 cm-'1, low heat values reported in ref 2. This is obviously not and is in agreement with the correlation proposed by some the case: at -300 K, no CO physisorption can occur, as of us between the adsorption enthalpy of a-coordinated at this temperature CO is a noncoercible gas, and even the CO and the shift of the vco frequency.12J3 weak interaction by H-bonding with surface OH groups On A1273773 (Figure 9b), on which the strongest sites does not occur. (The latter form of interaction becomes are represented by the singleton species (CO)B ( ( U O ) B = indeed observable at low temperatures; see, for instance, 2217 cm-'), the initial heat QO is -65 kJ mol-', i.e., 15 kJ the CO bands at -2160 cm-l, marked (CO)Hin Figure 4, mol-' lower than the qo on A773773 assigned to the (C0)c or similar bands reported in refs 6-9.) species. This is in agreement with the spectral position Isosteric heats reported by some researcher~,~J*7'9 calof (COIBthat is intermediate between (CO)Aand (C0)c. culated from spectroscopic data a t a particular pco, fall Consistently, in the case of A12731023 (Figure 9b), the in the range 25-40 kJ mol-'. Besides any consideration differentialheat plot starts fairly high, due to the presence of the difficulty of knowing the exact temperature of the of small amounts of (CO)c,and reaches -60 kJ mol-' when the CO uptake is =0.03 molecule/nm2and the CO pressure (28) Bolis, V.; Morterra, C.; Fubini, B.; Ugliengo, P.; Garrone, E. is -10 Torr, corresponding to the saturation plateau of Langmuir 1993,9,1521. (29)(a) Egerton, T.A.; Stone, F. S. J. Chem. Soc., Faraday Trans. 1 the relatively abundant (CO)Bspecies.
-
-
(27)Peri, J. B. J . Phys. Chem. 1965,69,220.
1973,69,22.(b) Bolis, V.;Fubini,B.; Garrone, E.; Giamello, E.; Morterra, C. In Structure and Reactioity of Surfaces; Morterra, C., Zecchina, A,, Costa, G., Eds.; Elsevier: Amsterdam, 1989;p 159.
Langmuir, Vol. 10, No. 6, 1994 1823
Lewis Acid Sites on AlzOs sample in the IR beam, the isosteric heat values are intrinsically limited by the fact that they represent the energy values averaged over all the sites, with the most important contribution deriving from the most abundant family of sites (the species (CO)A in the present case). Unlike that, with the direct calorimetric procedure adopted here, a broad and continuous range of CO coverages is explored, and the evolution of the heat of adsorption with coverage is obtained: this allows the description of the energetic heterogeneity of the surface sites. If the energetic behavior of the CO/Al2O3system is somehow anomalous (and, indeed, it turns out to be quite different from that of most dometal oxides),this peculiarity is bound to escape detection whenever the (averaged) adsorption enthalpy is calculated from spectroscopicdata. Still, a basic contradiction remains: a t high CO pressures, the adsorption heat declines to values that are incompatible with a chemisorption observable a t 300 K. This also implies that either the YCO frequency remains much higher than expected on the basis of the adsorption heat or the adsorption heat becomes much lower than expected on the basis of the vco frequency. Should the observed CO stretching frequency be higher than suggested by the adsorptionenthalpy,the correlation frequency/enthalpy would not hold in the case of Al203. Recently, Pacchioni et aL30 have described a large vco upward shift for some adsorbed CO species, also in the virtual absence of a-charge release, as due to a "wall effect" through which the lone pair of the vibrating C atom goes very close to the surface so as to increase the Pauli repulsion; effects like this might be active at the surface of alumina. Even so, it is difficult to admit that a CO adspecies, whose adsorption enthalpy turns out to be only of the order of =10 kJ mol-', has a vco as high as -2195 cm-l, when CO adsorbed onto much weaker Lewis acid centers dispersed on alumina (like, for instance, Ce4+- or Ca2+cue) exhibit vco frequencies at -2175-2180 cm-l and adsorption enthalpies of the order of =35 k J m0l-l.~6*3l From our correlation curve between vco and AJf, a vco of -2195 cm-l should correspond to an enthalpy of 40-50 kJ mol-'. Thus, it is necessary to invoke the occurrenceof different phenomena at the surface of the CO/Al2O3system. The only tentative explanation that we can propose is that, when adsorbing onto regular crystal planes of transition aluminas, CO does not simply coordinate onto cus Al surface ions, but induces some reversible surface modifications through which shielded surface Al ions become available for CO adsorption. This process, though caused by a fairly soft ligand like CO, would represent (at least in part) the reverse of the surface reconstruction phenomena that have been invoked to explain some of the data reported in the previous sections. Should this be the case, the reversible surface modifications induced by CO uptake would imply a (reversible) endothermic step. As a consequence, the low heats measured would actually be the combination of two contributions of oppositesign: a (small)term due tosurface modifications and the (larger) heat evolved during the formation of the a-coordinated (CO)Acomplex. This hypothesis, proposed to justify the very low adsorption heat measured at high CO coverages with all the transition alumina preparations examined, is speculative and virtually impossible to demonstrate experimen(30) Pacchioni, G.; Cogliandro, G.; Bagus, P. S. Surf. Sci. 1991,255, 144.
(31) Morterra, C.; Bolii, V.; Magnacca, G.; Cerrato, G. J. Electron Spectrosc. Relat. Phenom. 1993,64165,235-240.
a
A
ii50
1050 wavenumber cm-'
Figure 10. Absorbance differential spectra obtained by subtracting the spectrum of alumina, on which increasing amounts of CO are being adsorbed, from the corresponding spectrum of the bare solid: (a) CO uptake at 4 7 K onto An31023 (pmvaries, from top to bottom, between 1.0 X 1 V and 4.0 X 10' Torr; (b) CO uptake at 4 7 K onto Am31023 @CO varies, from top to bottom, between 1.0 X 10-9 and 4.0 X 10' Torr; (c) CO uptake at =300 K onto Ala31023 (pmvaries, from top to bottom, between 1.5 X le2and 1.4 X 102 Torr. The ordinate scale of part c underwent a 2-fold magnification. tally. The hypothesis is certainly supported by the spectroscopicdata shown in Figure 1 0 the low-temperature adsorption of CO on y-Al2O3 (Figure loa) and on 6,6-Al2O3 (Figure lob) brings about the reversible elimination of two discrete bands centered a t =lo70 and =lo60 cm-l. This effect has been observed also by Marchese et al." with the adsorptionof CO onto Alon C,and can be ascribed, 3 ~the elimination of some spinel following Lavalley et ~ 1 . , to AlIV-O stretching modes localized a t the surface and upward shifted with respect to the regular bulk "-0 stretching mode a t =950 cm-l 33 by the surface increase of covalencyand decrease of the Madelung energy brought about by crystal truncation and surface dehydration. Figure 1Ocshowsthat, already a t -300 K, COuptake starts eliminatingthe bands ascribed to surface localized AP-0 vibrations. The elimination of the bands proceeds with CO coverage and implies that most of the effect (i.e., the elimination of the strongest component a t -1060 cm-l) ought to be ascribed to the slow-growing (CO)Aadspecies. The interpretation is as follows: upon CO adsorption, both on defects and on (reconstructed)regular planes, the coordination of surface ions increases, the covalency of the surface decreases, and the surface-localized AP-0 vibrational states shift downward toward the regular spectral position of bulk A P - O vibrations. This is a surface relaxation process, to which should correspond a (32) Lavalley,J. C.; Benaisaa, M. Adsorption and Catalysis on Oxide Surfaces. In Studies in Surface Science and Catalysis; Che, M., Bond, G. C., Eds.; Elsevier Science Publishers: Amsterdam, 1985, Vol. 21, p 251. (33) Serna, C. J.; Rendon,J. L.; Iglesias,J. E.Spectrochim. Acta, Part A 1982.38.797.
1824 Langmuir, Vol. 10, No. 6,1994
discrete enthalpic contribution, that balances in part the heat released during the adsorption process. It is here anticipated that, when CO adsorbs on Ca2+and/or Ce4+ doped aluminas, the q vs Bco plots tend at high CO coverages to a more reasonable value of =30 kJ mol-l, and the bands due to localized (defective) AIVLO surface vibrations are no longer observed: the presence of foreign ions has modified the surface layer, and probably removed the “anomaly” of the A1203s ~ r f a c e . ~ ~ ~ ~ ~
Conclusions The combined use of IR spectroscopy and adsorption microcalorimetry, applied to the ambient temperature adsorption of CO, allowed the surface acidity of two different microcrystalline phases of transition A1203to be characterized. The results reported here are thought to contribute to a better and more complete understanding of the complex nature and behavior of the surface of these materials. In particular, (i) IR spectroscopic data show that up to three types of CO adspecies can form in variable and odd amounts at the surface of transition aluminas, depending on the temperature of calcination (which determines the crystal phase) and on the temperature of vacuum activation (which determines the extent of surface dehydration),and (ii) parallel calorimetric/gas-volumetric measurements allow the population and, in principle, the energy distribution of the acidic sites at which the three CO adspecies form to be determined. Some adsorptive features peculiar to the transition A1203/C0systems can be pointed out. (i) There is a strict correlation between the population of surface acidic sites and the extent of surface dehydroxylation, as already postulated by other researcher^.^ Still,the population of cus cations formed on dehydration and sufficiently acidic to a-coordinate CO at ambient temperature is quite low, as compared with other metal oxides, and is very small compared with the amounts of OH groups eliminated upon vacuum activation. Surface ion mobility and shielding effects must be thus invoked. (ii) At high CO coverages, the experimental heat of adsorption falls to values surprisingly low (=lo kJ mol-’) for a-coordinated CO complexes. This occurs in all samples studied, confirming previous data2 that originated some confusion in the comprehension of the energy of the COi A1203system. The crude experimentalheat of adsorption becomes, at relatively high coverages, not compatible with a plain a-coordination of CO on cus AP+ centers and cannot be correlated any longer with the observed CO stretching frequencies. (34) Morterra, C. Data to be published shortly.
Morterra et al. Other proLesses besides CO coordination are thought to occur at the surface of aluminas (they may actually occur also ofi other oxides, but with much less evident effects). Most likely, the contact with CO induces reversible modifications in the structure of the surface, through which shielded A13+ ions become coordinatively available for CO uptake. Acidic sites iormed at the surface of transition aluminas possess a l a i t heterogeneity, which is primarily due to structural rraacms: there are different types of cus AP+ cations thai have higher coordinative unsaturation (i.e., higher acidit>)when they are located at the edges of the microcrystal>dnd/or within the narrow pores than in the case of sit*? located on flat portions of regular crystal planes. Each strLtcturally different family of Lewis acidic sites exhibits some heterogeneity, which is minimal and mostly due to weak adsorbate-adsorbate (inductive)interactions in the case of the sites in crystallographically defective configuratloris (weak induced heterogeneity), whereas it is quite vast and mainly due to geometrically different situations in the case of the far predominant sites located on regular crystal planes (strong intrinsic heterogeneity). The zero-coverage heat values (i.e., the maximum energy of CO adsorption) could be estimated for the various samples studied and for the different CO adspecies formed. These value:. correlate quite well with the relevant stretching i‘reyuency values (and, in particular, with the zero-coveruge CO stretching figures), and follow the nonlinear rz!dtionship previously proposed to exist between adsorption enthalpy and CO stretching frequency. Both observables can be thus considered as reasonable measures lit tkip Lewis acidity of cus do surface cations. The moat acidic CO adspecies ((C0)c; zero-coverage frequency Z 2 2 3 0 cm-’, zero-coverage adsorption heat 430 kJ mol-’) is mmt insensitive to lateral adsorbate-adsorbate effects, and t3: ns out to be peculiar to cusAIIVsiteslocated in some structurally defective configurations most commonly met ai the surface and within the pore system of the -y-Al,O pnase. Acknowledgment. This research was partly financed by the CXK (Rome), Progetto Finalizzato Materiali Speciali. ‘The authors are indebted to Dr. M. Baricco (Turin University), who obtained and interpreted the XRD data, and t b Dr, M. Fucale (Centro Ricerche Fiat), who performed the BET measurements and pore-size distributions.
