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End-on surface coordinated (adsorbed) CO2: a specific ligand for surface Lewis acidic centres
MateriaLT
Chemistry
and Physics,
29 (1991)
447
447-456
End-on surface coordinated (adsorbed) CO,: a specific ligand for surface Lewis acidic centres C. Morterra,
G. Cerrato and C. Emanuel
Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica UniversitiL di Torino, via P. Giuria 7, I-10125 Turin [Italy)
dei Materiali,
Abstract Carbon dioxide interacts at ambient temperature at the surface of most solids, yielding several families of chemisorbed species. One such family corresponds to the end-on Ucoordination of CO2 to coordinatively unsaturated surface cationic centres, and in this coordination the molecule preserves the linear shape and loses the center of symmetry =J. End-on coordinated C02, which does not seem to be so common as a ligand (D,,,+C in coordination compounds, turns out to be a probe adspecies very sensitive to the features of surface Lewis acidic sites: a) in systems towards which other probe molecules, like CO, are not active at all (e.g., Al hydroxides, Al fluoride); b) in systems where the presence of surface adsorbate-adsorbate interactions produces differences of acidic strength, towards which other test molecules are not so sensitive (e.g., sulfate-doped ZrO,).
Introduction The surface Lewis acidity of adsorbing solids is brought about by the creation, upon surface dehydration, of coordinatively unsaturated (cus) cationic centres, whose acidic features are determined and/or influenced by several structural, chemical, and coordinative factors. One of the main goals in surface chemistry is the understanding of as many such factors as possible. To do so, one needs to use the adsorption of selected test molecules, some physical features of which ought to depend on most (or possibly all) of the factors determining the Lewis acidity. IR spectroscopy has been long recognized as one of the most suitable tools to reveal (some of) the physical changes brought about in the probe molecules by the test adsorption processes. As for the test adsorption process itself, it is required to be as sensitive and as selective as possible towards the physical properties of the adsorbing centres, and to produce weak (and possibly reversible) perturbations in the centres. For all these reasons, the IR spectra of the adsorption of carbon monoxide at 300 K and, less frequently, at lower temperatures have been studied quite extensively. In fact the adsorption of CO yields surface carbonyl-like species, whose spectral features are easily comparable to those available in the vast
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literature of homogeneous coordination compounds. Moreover, in all cases in which there are no d-electrons available in the adsorbing system (systems of this kind are often quite interesting in the fields of materials science and catalysis), CO adsorbs via a plain a-coordination, which is most frequently weak and reversible, and possesses spectral features that can be readily related to the strength of the a-donation process (i.e., to the acidity of the Lewis centres). Still, there are cases in which CO adsorption either does not occur at all, or turns out not to be as selective as needed, so that other adsorbing probe molecules must be resorted to. The present contribution proposes some cases in which carbon dioxide seems to behave as one such alternative probe adsorbate for the characterization of surface Lewis acidity. It is well known that, on most metal oxides, the adsorption of CO2 involves surface cus anions and/or cus cation/anion pairs, and forms several types of surface carbonate-like species, whose nature yields information on the physico-chemical features of the adsorbing surface. It is also well known that, on most oxidic and nonoxidic solids, CO2 interacts yielding also another adsorbed species, in which the molecule loses the center of symmetry and preserves the linear shape [ Dm,,+ C,,], so that the YtUj mode absorbs strongly in the 2400-2300 cm-’ range (the range of gaseous COe) and the IR-forbidden X+c(9j mode becomes weakly active at = 1370 cm-‘. The linear CO2 adsorbed species is reversible at ambient temperature (or slightly above it), and was thus long considered a physisorbed one. Opposite to that, it can be easily demonstrated (on the basis of the PO value of COB at 300 K, and by dehydration/rehydration experiments) that the linear COz species is a genuine chemisorbed one: it is a molecule v-coordinated in an end-on fashion, via one of the 0 lone-pairs, to surface Lewis acidic centres and, in this respect, it seems to lack a comparable coordination mode in the field of homogeneous coordination compounds. The end-on adsorption of CO2 is fairly similar to the coordination of CO onto cus cationic centres, and the information deriving from the two probe molecules should be, and very often is, comparable. This contribution reports some cases in which the IR spectroscopic study of the end-on coordination of CO2 turns out to be more useful than, or complementary to, the use of CO. Experimental Materials 1) Microcrystalline y-AlOOH (boehmite), hereafter referred to as ALO, was prepared as reported by Lippens and Steggarda [l] and, according to these authors, its crystalline order is somewhat intermediate between that of well-crystallized boehmite and that of pseudo-boehmite; the BET surface area is 185 m2 g-l. 2) A specimen of y-AlF,, hereafter referred to as ALFA, was prepared by the thermal decomposition of ammonium fluoroaluminate in an inert
449
atmosphere, and by XRD resulted to be up to 98% pure; the BET surface area is 18 m2 g-‘. 3) Another specimen of y-AlF,, hereafter referred to as ALFB, was prepared by HF fluoridation of r-AlaO,, and resulted to be over 97% gamma phase, and to contain = 2% residual oxygen; the BET surface area is 28 m2 g-l. 4) Pure monoclinic ZrOa, hereafter referred to as ZRPeTO, was prepared as reported and discussed elsewhere [ 2, 31, and was calcined in air at 870 K; the BET surface area is 78 m2 g - ’ . 5) Sulfate-doped ZrO,, hereafter referred to as ZRSBTO,was prepared by dosing, by the method of incipient wetness, 100 pmol g-’ HaSO, onto ZRPsTO; the BET surface area of ZRSsTO is equal to that of the starting ZRP,,,. Samples 1 and 2 were prepared and kindly supplied by Montedipe S.p.A. (Bollate); sample 3 was prepared and kindly supplied by Montefluos S.p.A. (Port0 Marghera). Methods All samples for IR experiments were prepared either in the form of thin self-supporting pellets (20-35 mg cmm2), or as thin layer depositions (4-12 mg cmW2) over a pure Si plate. The samples were transferred to a vacuum system (residual pressure < 10e5 torr), where they underwent all thermal treatments and in situ IR measurements. The temperature (K) at which each sample was vacuum activated before the adsorption experiments is reported in the text and figures as a numeral following the symbol of the sample. IR spectra were run, at 2 cm- ’ resolution, on FTIR spectrometers (Bruker, mod. IFS 113~ and IFS 88) equipped with MCT detectors.
Results
and discussion
A - The Lewis acidity of octahedrally coordinated Al (Alv’) y-AlOOH Although a great deal of work has been reported on the structural features and on the vibrational spectra and relevant assignment of crystalline Al hydroxides (see [4-71 and references therein), very little, if anything at all, is known on their surface properties. In all crystalline phases, Al hydroxides have their Al ions in the octahedral coordination [ I], and the tetrahedral coordination for Al ions (Air”) is known to become possible only further to the thermal transformation(s) of the hydrates into spine1 phases, characteristic of the so called transition aluminas. It follows that, at the surface of Al hydroxides, the presence of Lewis acidic centres should be ascribable to cus AIW sites. Previous work on cu-A1,Oa(the corundum phase, in which again only the octahedral coordination is available for Al ions), it was shown that surface cus Al”’ centres are too ‘soft’ as acidic sites to coordinate CO at 300 K [S],
450
and also at 78 K they seem to be very little active towards the u-coordination of co [9]. No CO adsorption should thus occur at the surface of Al hydroxides and, consistently, no CO uptake at 300 K can be observed by IR either on Gibbsite ((U-AI(OH),) or Boehmite (y-AlOOH). Figure 1 reports some IR spectra of COz adsorbed at 300 K on AL0 vacuum activated at various temperatures, up to the bulk phase transformation into y-A1203. It can be noted that: i) A plain evacuation at ambient temperature is sufficient to allow the adsorption of COz and the formation of a sharp band (curve l), centered at = 2342 cm-’ and slightly asymmetric on the high V side. The band is ascribed to the end-on a-coordination of CO2 onto surface cus Al”’ sites.
AL0
0
2380
2360 WAVENUMBER
2340
2320
21
cm-’
Fig. 1. IR spectra of 12 torr CO2 adsorbed at 300 K on y-AlOOH (ALO), vacuum activated at the temperatures (K) reported on the curves. (Curve 4 corresponds to the adsorption of CO2 on the sample of curve 3, onto which pyridine was pre-adsorbed at 300 K).
451
ii) Vacuum activation at up to = 473 K leads to a stronger band for the end-on COz surface complex (curves 2-s), while spectral position and shape of the band remain virtually unchanged. The liberation of more cus Aim centres upon surface dehydration is thus deduced. Pre-adsorption of pyridine (py) on the AL0 473 sample allows the formation of a much lower amount of coordinated COz (curve 4), while the spectral position of the residual band is shifted downwards by inductive effects from the strong charge-releasing adsorption of py. iii) When vacuum activation is carried out at T 2 650 K, the background IR spectrum (not shown) indicates that the transformation to r-Al,O,, has begun [5], and the band of linearly coordinated CO, changes in position and shape (curve 5). In particular, a new end-on CO, surface complex is formed, absorbing at ~2360 cm-‘, which is ascribable, on the basis of previous work on transition aluminas [lo], to the coordination onto cus Ali” centres.
AlF, is another system in which Al possesses only the octahedral coordination [ 1 l] and in which, on the basis of what is discussed above, CO should not adsorb, at least at 300 K. Two different preparations of yAlF, have been studied, and on both of them no appreciable CO uptake at 300 K can be detected by IR. Figure 2 reports the spectra of CO2 adsorbed onto the ALFA (section a) and ALFB (section b) preparations activated at temperatures up to 573 K. It can be noted that: i) On both preparations, activation at 300 K leads to a band centered at = 2348 cm-i, slightly different for the two preparations and asymmetric on the high V side. The band is ascribed to the end-on coordination to cus AIW centres created during the incipient surface dehydration, which is monitored by the fair decrease of a band at = 1625 cm-’ (not shown), due to the &ii mode of undissociated coordinated water. The position of the CfcU1 mode of end-on CO2 is higher on y-AlF, than on y-AlOOH, due to inductive effects, i.e., to a stronger charge-withdrawing effect of the more electronegative anions surrounding the coordinating AIW centers in the former systems. ii) When the sample ALFA is activated at higher temperatures (see Fig. 2a), first the band of CO2 end-on coordinated to AIW becomes more distinctly asymmetric on the high V side, so that the presence of slightly different AIW centres can be postulated, and then weak partner bands form at higher V, among which there is a well resolved peak at = 2363 cm- ‘. The components at V> 2360 cm- ’ can be ascribed to the presence on ALFA of minor amounts of coordinatively more unsaturated centres, most likely Al’” sites, located in small defective regions of the surface. iii) On the sample ALFB (see Fig. 2b), an activation at 420 K is sufficient to yield appreciable amounts of a linearly coordinated CO, species absorbing at V:>2360 cm-‘. Further to activation at higher temperatures, the heter-
452
0a ALFA
2400
2380 WAVENUMBER
2360
2340
cm-’
Fig. 2. IR spectra of 12 torr CO2 adsorbed activated at the temperatures (K) reported
2400
2380 WAVENUMBER
2360
2340
cm-’
at 300 K on two preparations on the curves.
of yAlF,,
vacuum
ogeneous nature of the Alv’/COa complexes (postulated above for ALFA) becomes more evident, while two COa bands at fi> 2360 cm-’ are formed (= 2370 and = 2380 cm-‘) and become the species by far predominant in the 2400-2300 cm-’ spectral range. The latter observation indicates that, when prepared by fluoridation of AlaOa, y-AlFs, though crystallographically pure in the bulk, still possesses at the surface oxidic (alumina-like) islands, in which the Lewis acidic activity of cus Aln’ centres, made available by dehydration and made stronger by the presence of the fluoride network, are readily revealed by the end-on a-coordination of COa. The bands of COa end-on coordinated onto the fluoride modified Alrv centres of ALFB are reminiscent of the bands observed by Peri [ 121 on a = 6% surface-fluorided alumina. The spectral features of the Alrv/C02 and AIW/COa end-on complexes and their relative resistance to evacuation are better seen in Fig. 3, where the desorption patterns of COa from ALFB 300 (a) and 573 (b) are reported. B - The Lewis acidity of sulfate-doped ZrOz Cus Zr4+ sites at the surface of monoclinic ZrOa are sufficiently ‘hard’
as Lewis acidic centres to be revealed by the adsorption of CO, which occurs with some peculiar differences both at 300 K [ 131 and at 78 K [ 141.
453
0a
? z
‘2 ALFB 300 z
380
2360 WAVENUMBER
2340 cm-’
2320
2400
@
z
ALFB 573
R I
2380 WAVENUMBER
2360
23a0
cm-’
Fig. 3. IR spectra of CO2 desorption at 300 K from ALFB 300 (a) and ALFB (torr): 1, 12; 2, 6; 3, 3; 4, 1; 5, 10e2.
573 (b). P,,,
When ZrOa is surface doped with small amounts of sulfates, some of the properties of the solid are modified, and the system becomes extremely interesting from the catalytic point of view (super acid catalyst, [ 15 1). The use of CO as a probe molecule for the characterization of sulfate-doped ZrO, has been proposed by Bensitel et al. [ 2 1: the overall IR absorption of CO o-coordinated to cus Zr4+ centres is shifted upwards by some 15 cm-‘, due to the inductive effects produced, on the charge-releasing adsorbed CO, by the strong charge-withdrawing sulfate groups. More details on the ZRS/CO system will be reported elsewhere [IS]. IR spectra of the adsorption of COa on the sulfate-free ZRP,,, system activated at various temperatures (up to the complete dehydration) are shown in Fig. 4a. It can be noted that: i) The earliest stages of dehydration (curves l-2) allow the formation of one linearly coordinated CO, species absorbing weakly at = 2343 cm-‘. ii) Activation at Tr 473 K yields, in the 2380-2340 cm- ’ range, a spectrum of end-on coordinated COz, which is much stronger and progressively more complex. In fact, in the case of the fully dehydrated ZRP,,, 920 sample (curve S), the presence of at least three components can be postulated,
454
2380
2360 WAVENUMBER
23h0 cm-’
23’20
2380
2360 WAVENUMBER
2340
2320
cm-’
Fig. 4. IR spectra of 12 torr COz adsorbed at 300 K on ZRP 670 (a) and ZRSB7,, (b) vacuum activated at the temperatures (K): 1, 300; 2, 373; 3, 473; 4, 573; 6, 673; 6, 773; 7, 873; 8, 920; 9, 973.
among which there is a main component centered at = 2362 cm-’ and a broad absorption centered at = 2355 cm- ‘, best observed in curves 4-5. Corresponding spectra, relative to the adsorption of COa on the sulfate doped ZRSs7,, system, are shown in Fig. 4b, from which it can be noted that: i) For activation at up to 470 K (temperature at which important structural changes occur in the surface sulfate groups [ 16, 17]), there is little activity and only a weak band forms at ~2346 cm-’ (curve 3). This indicates that the presence of sulfates inhibits to some extent the creation of cus Zr4 + centres available for COa coordination. ii) For activation in the 470-670 K temperature interval, in which surface dehydration is the predominant process, the activity towards COz increases
455
markedly (curves 4-6, Fig. 4b), and the overall absorption of end-on coordinated COz is modified in respect of ZRP,,,. In fact, there is a broadish cm-’ (much as there was one on and probably complex band at ~2355 ZRP&, but there is no evidence at all for the band observed on ZRPsTo at = 2362 cm- I, whereas two well resolved bands are formed at = 2385 and = 2370 cm- ‘, where no bands formed on ZRP870. The higher V of the latter two bands indicates that, at these dehydration stages, two definitely stronger Lewis acidic centres are present at the surface of sulfate doped ZrO,, which might be related to the super-acidity of the system. iii) For vacuum activation at T > 773 K, surface dehydration is no longer the predominant process, as a thermal decomposition of surface sulfates occurs gradually at these temperatures [ 171. The spectra of end-on coordinated CO2 clearly indicate that the lower of the two high V components (= 2370 cm-‘) is selectively eliminated upon activation at 870 K (curve 7), whereas the higher component (= 2385 cm- ‘) is eliminated after the complete collapse of surface sulfates at T> 900 K. The elimination of the two high I; CO, components gradually restores the spectrum of the ZRP,,,/CO, system and, in particular, brings back the sharp and strong component at = 2362 crr~ ‘, which was absent in the presence of sulfates (curves 8-9). Conclusions This contribution is part of a systematic work devoted to the study, by various means, of the surface Lewis acidity of solids, and the adsorbing systems proposed here are just a few selected examples of the versatility of COz, rr-coordinated in the linear end-on mode, in revealing some of the features of surface Lewis acidic sites. The chosen examples indicate that the suitability of CO2 is particularly important when other weakly coordinating probe molecules, like CO, turn out to be inactive, or when strong adsorbateadsorbate interactions act at the surface and introduce, among the Lewis acidic centres, spectral differences that other probe molecules are less sensitive to. Acknowledgements Two of the authors are grateful for a fellowship made available by Montefluos S.p.A. (G.C.) and by Centro Ricerche Fiat (C.E.). This research was carried out within the Progetto Finalizzato Materiali Speciali, Consiglio Nazionale delle Ricerche (Roma). References 1 B. C. Lippens and .J. J. Steggerda, in B. G. L&en, Steggerda (eds.), Physical and Chemical Aspects Press, London, 1970.
M. H. Fortuin, C. Okkersee and J. J. of Adsorbents and Catalysts, Acad.
456 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
M. Bensitel, 0. Saur, J. C. Lavalley and G. Mabilon, Mater. Chem. Phys., 17 (1987) 249. C. Morterra, R. Aschieri and M. Volante, Muter. Chem. Phys., 20 (1988) 539. A. B. Kiss, G. Keresztury and L. Farkas, Spectrochim. Acta 36A (1980) 653. P. P. Mardilovich, A. I. Trokhimets and M. V. Zaretskii, Zhur. Prikl. Spektrosk., 40 (1984) 295 (Engl. edn.). P. P. Mardilovich, A. I. Trokhimets, M. V. Zaretskii and G. G. Kupchenko, Zhw. Prikl. Spektrosk., 42 (1985) 659 (Egnl. edn.). D. G. Lewis and V. C. Farmer, Clay Miner., 21 (1986) 93. G. Della Gatta, B. Fubini, G. Ghiotti and C. Morterra, J. Cutal., 43 (1976) 90. A. Zecchina, E. Escalona Plater0 and C. Otero Arean, J. Catal., 107 (1987) 244. C. Morterra, A. Zecchina, S. Coluccia and A. Chiorino, J. Chem. Sot., Faraday Trans. I, 73 (1977) 1544. D. B. Shinn, D. S. Crocket and H. M. Haendler, Inorg. Chem., 5 (1966) 1927. J. B. Per-i, J. Phys. Chem., 72 (1968) 2917. C. Morterra, L. Orio and C. Emanuel, J. Chem. Sot., Faraday Trans., 86 (1990) 3003. C. Morterra, V. Bolis, L. Orio and P. Ugliengo, Muter. Chem. Phys., 29 (1991) 457. K. Tanabe, Mater. Che7n. Phys., 13 (1985) 347. C. Morterra, work in progress. M. Bensitel, 0. Saur, J. C. Lavalley and B. A. Morrow, Mater. Chem. Phys., 19 (1988) 147.
Chemistry
and Physics,
29 (1991)
447
447-456
End-on surface coordinated (adsorbed) CO,: a specific ligand for surface Lewis acidic centres C. Morterra,
G. Cerrato and C. Emanuel
Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica UniversitiL di Torino, via P. Giuria 7, I-10125 Turin [Italy)
dei Materiali,
Abstract Carbon dioxide interacts at ambient temperature at the surface of most solids, yielding several families of chemisorbed species. One such family corresponds to the end-on Ucoordination of CO2 to coordinatively unsaturated surface cationic centres, and in this coordination the molecule preserves the linear shape and loses the center of symmetry =J. End-on coordinated C02, which does not seem to be so common as a ligand (D,,,+C in coordination compounds, turns out to be a probe adspecies very sensitive to the features of surface Lewis acidic sites: a) in systems towards which other probe molecules, like CO, are not active at all (e.g., Al hydroxides, Al fluoride); b) in systems where the presence of surface adsorbate-adsorbate interactions produces differences of acidic strength, towards which other test molecules are not so sensitive (e.g., sulfate-doped ZrO,).
Introduction The surface Lewis acidity of adsorbing solids is brought about by the creation, upon surface dehydration, of coordinatively unsaturated (cus) cationic centres, whose acidic features are determined and/or influenced by several structural, chemical, and coordinative factors. One of the main goals in surface chemistry is the understanding of as many such factors as possible. To do so, one needs to use the adsorption of selected test molecules, some physical features of which ought to depend on most (or possibly all) of the factors determining the Lewis acidity. IR spectroscopy has been long recognized as one of the most suitable tools to reveal (some of) the physical changes brought about in the probe molecules by the test adsorption processes. As for the test adsorption process itself, it is required to be as sensitive and as selective as possible towards the physical properties of the adsorbing centres, and to produce weak (and possibly reversible) perturbations in the centres. For all these reasons, the IR spectra of the adsorption of carbon monoxide at 300 K and, less frequently, at lower temperatures have been studied quite extensively. In fact the adsorption of CO yields surface carbonyl-like species, whose spectral features are easily comparable to those available in the vast
0254-0584/91/$3.50
0 199 1 -
Elsevier
Sequoia, Lausanne
448
literature of homogeneous coordination compounds. Moreover, in all cases in which there are no d-electrons available in the adsorbing system (systems of this kind are often quite interesting in the fields of materials science and catalysis), CO adsorbs via a plain a-coordination, which is most frequently weak and reversible, and possesses spectral features that can be readily related to the strength of the a-donation process (i.e., to the acidity of the Lewis centres). Still, there are cases in which CO adsorption either does not occur at all, or turns out not to be as selective as needed, so that other adsorbing probe molecules must be resorted to. The present contribution proposes some cases in which carbon dioxide seems to behave as one such alternative probe adsorbate for the characterization of surface Lewis acidity. It is well known that, on most metal oxides, the adsorption of CO2 involves surface cus anions and/or cus cation/anion pairs, and forms several types of surface carbonate-like species, whose nature yields information on the physico-chemical features of the adsorbing surface. It is also well known that, on most oxidic and nonoxidic solids, CO2 interacts yielding also another adsorbed species, in which the molecule loses the center of symmetry and preserves the linear shape [ Dm,,+ C,,], so that the YtUj mode absorbs strongly in the 2400-2300 cm-’ range (the range of gaseous COe) and the IR-forbidden X+c(9j mode becomes weakly active at = 1370 cm-‘. The linear CO2 adsorbed species is reversible at ambient temperature (or slightly above it), and was thus long considered a physisorbed one. Opposite to that, it can be easily demonstrated (on the basis of the PO value of COB at 300 K, and by dehydration/rehydration experiments) that the linear COz species is a genuine chemisorbed one: it is a molecule v-coordinated in an end-on fashion, via one of the 0 lone-pairs, to surface Lewis acidic centres and, in this respect, it seems to lack a comparable coordination mode in the field of homogeneous coordination compounds. The end-on adsorption of CO2 is fairly similar to the coordination of CO onto cus cationic centres, and the information deriving from the two probe molecules should be, and very often is, comparable. This contribution reports some cases in which the IR spectroscopic study of the end-on coordination of CO2 turns out to be more useful than, or complementary to, the use of CO. Experimental Materials 1) Microcrystalline y-AlOOH (boehmite), hereafter referred to as ALO, was prepared as reported by Lippens and Steggarda [l] and, according to these authors, its crystalline order is somewhat intermediate between that of well-crystallized boehmite and that of pseudo-boehmite; the BET surface area is 185 m2 g-l. 2) A specimen of y-AlF,, hereafter referred to as ALFA, was prepared by the thermal decomposition of ammonium fluoroaluminate in an inert
449
atmosphere, and by XRD resulted to be up to 98% pure; the BET surface area is 18 m2 g-‘. 3) Another specimen of y-AlF,, hereafter referred to as ALFB, was prepared by HF fluoridation of r-AlaO,, and resulted to be over 97% gamma phase, and to contain = 2% residual oxygen; the BET surface area is 28 m2 g-l. 4) Pure monoclinic ZrOa, hereafter referred to as ZRPeTO, was prepared as reported and discussed elsewhere [ 2, 31, and was calcined in air at 870 K; the BET surface area is 78 m2 g - ’ . 5) Sulfate-doped ZrO,, hereafter referred to as ZRSBTO,was prepared by dosing, by the method of incipient wetness, 100 pmol g-’ HaSO, onto ZRPsTO; the BET surface area of ZRSsTO is equal to that of the starting ZRP,,,. Samples 1 and 2 were prepared and kindly supplied by Montedipe S.p.A. (Bollate); sample 3 was prepared and kindly supplied by Montefluos S.p.A. (Port0 Marghera). Methods All samples for IR experiments were prepared either in the form of thin self-supporting pellets (20-35 mg cmm2), or as thin layer depositions (4-12 mg cmW2) over a pure Si plate. The samples were transferred to a vacuum system (residual pressure < 10e5 torr), where they underwent all thermal treatments and in situ IR measurements. The temperature (K) at which each sample was vacuum activated before the adsorption experiments is reported in the text and figures as a numeral following the symbol of the sample. IR spectra were run, at 2 cm- ’ resolution, on FTIR spectrometers (Bruker, mod. IFS 113~ and IFS 88) equipped with MCT detectors.
Results
and discussion
A - The Lewis acidity of octahedrally coordinated Al (Alv’) y-AlOOH Although a great deal of work has been reported on the structural features and on the vibrational spectra and relevant assignment of crystalline Al hydroxides (see [4-71 and references therein), very little, if anything at all, is known on their surface properties. In all crystalline phases, Al hydroxides have their Al ions in the octahedral coordination [ I], and the tetrahedral coordination for Al ions (Air”) is known to become possible only further to the thermal transformation(s) of the hydrates into spine1 phases, characteristic of the so called transition aluminas. It follows that, at the surface of Al hydroxides, the presence of Lewis acidic centres should be ascribable to cus AIW sites. Previous work on cu-A1,Oa(the corundum phase, in which again only the octahedral coordination is available for Al ions), it was shown that surface cus Al”’ centres are too ‘soft’ as acidic sites to coordinate CO at 300 K [S],
450
and also at 78 K they seem to be very little active towards the u-coordination of co [9]. No CO adsorption should thus occur at the surface of Al hydroxides and, consistently, no CO uptake at 300 K can be observed by IR either on Gibbsite ((U-AI(OH),) or Boehmite (y-AlOOH). Figure 1 reports some IR spectra of COz adsorbed at 300 K on AL0 vacuum activated at various temperatures, up to the bulk phase transformation into y-A1203. It can be noted that: i) A plain evacuation at ambient temperature is sufficient to allow the adsorption of COz and the formation of a sharp band (curve l), centered at = 2342 cm-’ and slightly asymmetric on the high V side. The band is ascribed to the end-on a-coordination of CO2 onto surface cus Al”’ sites.
AL0
0
2380
2360 WAVENUMBER
2340
2320
21
cm-’
Fig. 1. IR spectra of 12 torr CO2 adsorbed at 300 K on y-AlOOH (ALO), vacuum activated at the temperatures (K) reported on the curves. (Curve 4 corresponds to the adsorption of CO2 on the sample of curve 3, onto which pyridine was pre-adsorbed at 300 K).
451
ii) Vacuum activation at up to = 473 K leads to a stronger band for the end-on COz surface complex (curves 2-s), while spectral position and shape of the band remain virtually unchanged. The liberation of more cus Aim centres upon surface dehydration is thus deduced. Pre-adsorption of pyridine (py) on the AL0 473 sample allows the formation of a much lower amount of coordinated COz (curve 4), while the spectral position of the residual band is shifted downwards by inductive effects from the strong charge-releasing adsorption of py. iii) When vacuum activation is carried out at T 2 650 K, the background IR spectrum (not shown) indicates that the transformation to r-Al,O,, has begun [5], and the band of linearly coordinated CO, changes in position and shape (curve 5). In particular, a new end-on CO, surface complex is formed, absorbing at ~2360 cm-‘, which is ascribable, on the basis of previous work on transition aluminas [lo], to the coordination onto cus Ali” centres.
AlF, is another system in which Al possesses only the octahedral coordination [ 1 l] and in which, on the basis of what is discussed above, CO should not adsorb, at least at 300 K. Two different preparations of yAlF, have been studied, and on both of them no appreciable CO uptake at 300 K can be detected by IR. Figure 2 reports the spectra of CO2 adsorbed onto the ALFA (section a) and ALFB (section b) preparations activated at temperatures up to 573 K. It can be noted that: i) On both preparations, activation at 300 K leads to a band centered at = 2348 cm-i, slightly different for the two preparations and asymmetric on the high V side. The band is ascribed to the end-on coordination to cus AIW centres created during the incipient surface dehydration, which is monitored by the fair decrease of a band at = 1625 cm-’ (not shown), due to the &ii mode of undissociated coordinated water. The position of the CfcU1 mode of end-on CO2 is higher on y-AlF, than on y-AlOOH, due to inductive effects, i.e., to a stronger charge-withdrawing effect of the more electronegative anions surrounding the coordinating AIW centers in the former systems. ii) When the sample ALFA is activated at higher temperatures (see Fig. 2a), first the band of CO2 end-on coordinated to AIW becomes more distinctly asymmetric on the high V side, so that the presence of slightly different AIW centres can be postulated, and then weak partner bands form at higher V, among which there is a well resolved peak at = 2363 cm- ‘. The components at V> 2360 cm- ’ can be ascribed to the presence on ALFA of minor amounts of coordinatively more unsaturated centres, most likely Al’” sites, located in small defective regions of the surface. iii) On the sample ALFB (see Fig. 2b), an activation at 420 K is sufficient to yield appreciable amounts of a linearly coordinated CO, species absorbing at V:>2360 cm-‘. Further to activation at higher temperatures, the heter-
452
0a ALFA
2400
2380 WAVENUMBER
2360
2340
cm-’
Fig. 2. IR spectra of 12 torr CO2 adsorbed activated at the temperatures (K) reported
2400
2380 WAVENUMBER
2360
2340
cm-’
at 300 K on two preparations on the curves.
of yAlF,,
vacuum
ogeneous nature of the Alv’/COa complexes (postulated above for ALFA) becomes more evident, while two COa bands at fi> 2360 cm-’ are formed (= 2370 and = 2380 cm-‘) and become the species by far predominant in the 2400-2300 cm-’ spectral range. The latter observation indicates that, when prepared by fluoridation of AlaOa, y-AlFs, though crystallographically pure in the bulk, still possesses at the surface oxidic (alumina-like) islands, in which the Lewis acidic activity of cus Aln’ centres, made available by dehydration and made stronger by the presence of the fluoride network, are readily revealed by the end-on a-coordination of COa. The bands of COa end-on coordinated onto the fluoride modified Alrv centres of ALFB are reminiscent of the bands observed by Peri [ 121 on a = 6% surface-fluorided alumina. The spectral features of the Alrv/C02 and AIW/COa end-on complexes and their relative resistance to evacuation are better seen in Fig. 3, where the desorption patterns of COa from ALFB 300 (a) and 573 (b) are reported. B - The Lewis acidity of sulfate-doped ZrOz Cus Zr4+ sites at the surface of monoclinic ZrOa are sufficiently ‘hard’
as Lewis acidic centres to be revealed by the adsorption of CO, which occurs with some peculiar differences both at 300 K [ 131 and at 78 K [ 141.
453
0a
? z
‘2 ALFB 300 z
380
2360 WAVENUMBER
2340 cm-’
2320
2400
@
z
ALFB 573
R I
2380 WAVENUMBER
2360
23a0
cm-’
Fig. 3. IR spectra of CO2 desorption at 300 K from ALFB 300 (a) and ALFB (torr): 1, 12; 2, 6; 3, 3; 4, 1; 5, 10e2.
573 (b). P,,,
When ZrOa is surface doped with small amounts of sulfates, some of the properties of the solid are modified, and the system becomes extremely interesting from the catalytic point of view (super acid catalyst, [ 15 1). The use of CO as a probe molecule for the characterization of sulfate-doped ZrO, has been proposed by Bensitel et al. [ 2 1: the overall IR absorption of CO o-coordinated to cus Zr4+ centres is shifted upwards by some 15 cm-‘, due to the inductive effects produced, on the charge-releasing adsorbed CO, by the strong charge-withdrawing sulfate groups. More details on the ZRS/CO system will be reported elsewhere [IS]. IR spectra of the adsorption of COa on the sulfate-free ZRP,,, system activated at various temperatures (up to the complete dehydration) are shown in Fig. 4a. It can be noted that: i) The earliest stages of dehydration (curves l-2) allow the formation of one linearly coordinated CO, species absorbing weakly at = 2343 cm-‘. ii) Activation at Tr 473 K yields, in the 2380-2340 cm- ’ range, a spectrum of end-on coordinated COz, which is much stronger and progressively more complex. In fact, in the case of the fully dehydrated ZRP,,, 920 sample (curve S), the presence of at least three components can be postulated,
454
2380
2360 WAVENUMBER
23h0 cm-’
23’20
2380
2360 WAVENUMBER
2340
2320
cm-’
Fig. 4. IR spectra of 12 torr COz adsorbed at 300 K on ZRP 670 (a) and ZRSB7,, (b) vacuum activated at the temperatures (K): 1, 300; 2, 373; 3, 473; 4, 573; 6, 673; 6, 773; 7, 873; 8, 920; 9, 973.
among which there is a main component centered at = 2362 cm-’ and a broad absorption centered at = 2355 cm- ‘, best observed in curves 4-5. Corresponding spectra, relative to the adsorption of COa on the sulfate doped ZRSs7,, system, are shown in Fig. 4b, from which it can be noted that: i) For activation at up to 470 K (temperature at which important structural changes occur in the surface sulfate groups [ 16, 17]), there is little activity and only a weak band forms at ~2346 cm-’ (curve 3). This indicates that the presence of sulfates inhibits to some extent the creation of cus Zr4 + centres available for COa coordination. ii) For activation in the 470-670 K temperature interval, in which surface dehydration is the predominant process, the activity towards COz increases
455
markedly (curves 4-6, Fig. 4b), and the overall absorption of end-on coordinated COz is modified in respect of ZRP,,,. In fact, there is a broadish cm-’ (much as there was one on and probably complex band at ~2355 ZRP&, but there is no evidence at all for the band observed on ZRPsTo at = 2362 cm- I, whereas two well resolved bands are formed at = 2385 and = 2370 cm- ‘, where no bands formed on ZRP870. The higher V of the latter two bands indicates that, at these dehydration stages, two definitely stronger Lewis acidic centres are present at the surface of sulfate doped ZrO,, which might be related to the super-acidity of the system. iii) For vacuum activation at T > 773 K, surface dehydration is no longer the predominant process, as a thermal decomposition of surface sulfates occurs gradually at these temperatures [ 171. The spectra of end-on coordinated CO2 clearly indicate that the lower of the two high V components (= 2370 cm-‘) is selectively eliminated upon activation at 870 K (curve 7), whereas the higher component (= 2385 cm- ‘) is eliminated after the complete collapse of surface sulfates at T> 900 K. The elimination of the two high I; CO, components gradually restores the spectrum of the ZRP,,,/CO, system and, in particular, brings back the sharp and strong component at = 2362 crr~ ‘, which was absent in the presence of sulfates (curves 8-9). Conclusions This contribution is part of a systematic work devoted to the study, by various means, of the surface Lewis acidity of solids, and the adsorbing systems proposed here are just a few selected examples of the versatility of COz, rr-coordinated in the linear end-on mode, in revealing some of the features of surface Lewis acidic sites. The chosen examples indicate that the suitability of CO2 is particularly important when other weakly coordinating probe molecules, like CO, turn out to be inactive, or when strong adsorbateadsorbate interactions act at the surface and introduce, among the Lewis acidic centres, spectral differences that other probe molecules are less sensitive to. Acknowledgements Two of the authors are grateful for a fellowship made available by Montefluos S.p.A. (G.C.) and by Centro Ricerche Fiat (C.E.). This research was carried out within the Progetto Finalizzato Materiali Speciali, Consiglio Nazionale delle Ricerche (Roma). References 1 B. C. Lippens and .J. J. Steggerda, in B. G. L&en, Steggerda (eds.), Physical and Chemical Aspects Press, London, 1970.
M. H. Fortuin, C. Okkersee and J. J. of Adsorbents and Catalysts, Acad.
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