Modulation of the Surface Properties of Sulfated Zirconia by Controlled

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Modulation of the Surface Properties of Sulfated Zirconia by Controlled Addition of Calcium Oxide E. Escalona Platero* and M. Pen˜arroya Mentruit Departamento de Quı´mica, Universidad de las Islas Baleares, 07071 Palma de Mallorca, Spain

M. J. Torralvo Ferna´ndez and M. R. Alvarez Lo´pez Departamento de Quı´mica Inorga´ nica, Universidad Complutense de Madrid, 28040 Madrid, Spain

D. Scarano Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Via Pietro Giuria 7, 10125 Torino, Italy Received August 9, 1996. In Final Form: March 21, 1997X Several CaO-ZrO2 samples have been prepared by impregnation of sulfated zirconia with calcium nitrate solutions followed by calcination at 1000 K for 3 h. The addition of calcium oxide to the sulfated zirconia contributes to the stabilization of the tetragonal form of zirconium oxide, although the major phase present in the CaO-ZrO2 samples is the monoclinic one. Calcium oxide causes the transformation of the surface sulfate groups of sulfated zirconia from covalent to ionic and the shift of the O-H stretching bands to higher wavenumbers, reaching values higher than those corresponding to pure ZrO2. In this way, calcium oxide modifies the acid character of sulfated zirconia; the strongest Lewis acid sites were observed to disappear progressively with increasing calcium content. Brønsted acidity was also found to disappear when the molar concentration of calcium oxide reaches 3%.

1. Introduction Sulfated zirconia has been widely investigated in the last years because of the increased acidity, when compared to pure zirconia, due to the presence of sulfate groups.1-13 The sulfated oxide is able to catalyze highly acidic or superacidic catalyst-demanding processes, for instance n-alkane isomerization. The precise nature of the acid centers having enhanced catalytic activity has been discussed by many researchers, but no unanimous opinion was reached yet. Some authors attribute a role to the Lewis acid sites,14,15 others to the Brønsted acid centers,16 and a third group to the synergistic effect of both types of sites.6 Intermediate formation of carbenium ions17,18 X

Abstract published in Advance ACS Abstracts, May 1, 1997.

(1) Tanabe, K. Mater. Chem. Phys. 1985, 13, 347. (2) Bensitel, M.; Saur, O.; Lavalley, J. C.; Mabilon, G. Mater. Chem. Phys. 1987, 17, 249. (3) Bensitel, M.; Saur, O.; Lavalley, J. C.; Morrow, B. A. Mater. Chem. Phys. 1988, 19, 147. (4) Komarov, V. S.; Sinilo, M. F. Kinet. Katal. 1988, 29, 605. (5) Arata, K. Adv. Catal. 1990, 37, 165. (6) Nascimento, P.; Akratopoulou, C.; Oszagyan, M.; Coudurier, G.; Travers, C.; Joly, J. F.; Vedrine, J. C. Proceedings of the 10th International Congress on Catalysis; Elsevier: Amsterdam, 1993. (7) Chen, F. R.; Coudurier, G.; Joly, J. F.; Vedrine, J. C. J. Catal. 1993, 143, 616. (8) Iglesia, E.; Soled, S. L.; Kramer, G. M. J. Catal. 1993, 144, 238. (9) Morterra, C.; Cerrato, G.; Emanuel, C.; Bolis, V. J. Catal. 1993, 142, 349. (10) Riemer, T.; Spielbauer, D.; Hunger, M.; Mekhemer, G. A. H.; Kno¨zinger, H. J. Chem. Soc., Chem. Commun. 1994, 1181. (11) Kustov, L. M.; Kazansky, V. B.; Figueras, F.; Tichit, D. J. Catal. 1994, 150, 143. (12) Corma, A.; Martı´nez, A.; Martı´nez, C. J. Catal. 1994, 149, 52. (13) Comelli, R. A.; Vera, C. R.; Parera, J. M. J. Catal. 1995, 151, 96. (14) Morterra, C.; Cerrato, G.; Pinna, F.; Signoretto, M.; Strukul, G. J. Catal. 1994, 149, 181. (15) Pinna, F.; Signoretto, M.; Strukul, G.; Cerrato, G.; Morterra, C. Catal. Lett. 1994, 26, 339. (16) Babou, F.; Bigot, B.; Sautet, P. J. Phys. Chem. 1993, 97, 11501. (17) Kazansky, V. B. Acc. Chem. Res. 1991, 24, 379. (18) Wan, K. T.; Khouw, C. B.; Davis, M. E. J. Catal. 1996, 158, 311.

S0743-7463(96)00789-5 CCC: $14.00

has also been postulated to contribute to catalytic processes mediated by sulfated zirconia. Sulfated zirconia is conventionally prepared by impregnation of zirconium oxide or hydroxide with sulfuric acid or ammonium sulfate solutions followed by calcination at the appropriate temperature. It can also be prepared by treating the pure oxide with SO2 and O2.2 Recently some of us have described an alternative and more direct route consisting in the thermolysis of zirconium sulfate.19 The resulting sulfated zirconia is similar to those prepared by conventional procedures; however, the method based on thermolysis of zirconium sulfate presents important advantages with respect to conventional procedures, among them: (i) the high surface area of the resulting sulfated zirconia even after calcination at high temperature,19 (ii) the avoidance of doping the pure material, since sulfate groups from the precursor are retained on the surface of the oxide,20 and (iii) the high thermal stability of the sulfate groups, which require a temperature higher than 973 K to begin the thermolysis process.20 The acid strength of ex-sulfate zirconia, determined by FTIR spectroscopy of adsorbed CO and pyridine, is higher than that shown by the pure oxide.21 The strongest Brønsted acid sites are characterized by an OH stretching band at 3650 cm-1, and they are linked to the simultaneous presence of disulfate species and of trace amounts of molecular water. The Lewis acid sites with the highest strength are Zr4+ cations under the inductive effect of adjacent sulfate groups. For some specific applications, it can be interesting to consider the possibility of modulating the acidity of (19) Escalona Platero, E.; Pen˜arroya Mentruit, M. Mater. Lett. 1992, 14, 318. (20) Escalona Platero, E.; Pen˜arroya Mentruit, M. Catal. Lett. 1995, 30, 31. (21) Escalona Platero, E.; Pen˜arroya Mentruit, M.; Otero Area´n, C.; Zecchina, A. J. Catal. 1996, 162, 268.

© 1997 American Chemical Society

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sulfated zirconia by adding controlled quantities of CaO. This basic oxide is able to form solid solutions with ZrO2,22 or it can simply stay on the surface, depending on calcination temperature. The CaO-ZrO2 system has been thoroughly studied from the structural point of view because the addition of calcium oxide to zirconia stabilizes the cubic phase at temperatures below its range of existence in the pure oxide, thus improving its behavior as a ceramic material.23 Regarding surface properties of the CaO-ZrO2 system there is, however, very little information. It is important to mention the investigations carried out by Feng et al.,24 who showed that ZrO2 doped with calcium has increased basic character, measured by temperature-programmed desorption of carbon dioxide. The increased basicity of the CaO-ZrO2 system results in improved selectivity in the formation of isobutane and isobutene from synthesis gas.24 The main aim of the present work is to investigate the influence of CaO on the acid strength of sulfated zirconia. However, crystalline structure, textural parameters (surface area and porosity), and the behavior of surface species (OH and sulfate groups) will also be investigated as a function of CaO content. The results obtained will be compared with those corresponding to both pure and sulfated zirconia. 2. Experimental Section 2.1. Sample Preparation. Several CaO-ZrO2 samples with different CaO concentration were prepared by impregnation of ZrO2 with Ca(NO3)2‚4H2O solutions (Merck, analytical grade), followed by calcination at 1000 K for 3 h in a free atmosphere. The thermal treatment was interrupted twice to grind the samples, in order to attain a greater degree of homogeneity. The nominal Ca content was 1, 3, 5, 10, and 15 mole %, and the corresponding samples will be called hereafter SZx, where x denotes the nominal Ca content. The parent ZrO2 (SZ sample) was obtained by thermolysis at 1000 K of zirconium sulfate (BDH, analytical grade). Structural, textural, and infrared characterization of this material was reported elsewhere.19-21 Briefly, the ZrO2 was found to be mainly in the monoclinic form with about 15% of the tetragonal phase, and it showed a BET surface area of 90 m2 g-1 and a most frequent pore radius of 8 nm. Chemical analysis showed a sulfur content of 1.1% (from the precursor), which was present in the form of both isolated and polynuclear sulfate groups at the zirconium oxide. 2.2. Sample Characterization. The SZx samples were characterized by several techniques: X-ray diffraction, Raman spectroscopy, N2 adsorption-desorption at 77 K, and FTIR spectroscopy of the free samples and of adsorbed probe molecules (CO and pyridine). X-ray Diffraction. Powder X-ray diffractograms of the different samples were obtained using a Siemens D5000 diffractometer equipped with a primary beam monochromator and Cu KR1 radiation. All the diffractograms were obtained in the 20-70° (2θ) interval by step scanning at 0.01° for 3 s. Raman Spectroscopy. Raman spectra were obtained, at room temperature in a free atmosphere, with a Jobin-Yvol Ar+ laser (Model U1000) spectrometer by using ca. 30 mW of the 514.5 nm line for excitation. The spectra were obtained in the 100-900 cm-1 interval. N2 Adsorption-Desorption at 77 K. Specific surface area, pore volume, and pore size distribution were determined from the corresponding nitrogen adsorption-desorption isotherms (at 77 K), which were obtained using a volumetric apparatus of conventional design; nitrogen (N39) was used as the adsorbate, and helium (N45) for calibration of dead volumes. Prior to determination of each isotherm, the corresponding sample was (22) Garvie, R. C. J. Am. Ceram. Soc. 1968, 51, 553. (23) Bansal, G. K.; Heuer, A. H. Am. Ceram. Soc. Bull. 1970, 49, 386. (24) Feng, Z.; Postula, W. S.; Erkey, C.; Philip, C. V.; Akgerman, A.; Anthony, R. G. J. Catal. 1994, 148, 84.

Figure 1. X-ray diffraction patterns of the SZx samples. outgassed at 400 K for 2 h, in a dynamic vacuum of 10-4 Torr (1 Torr ) 133.3 N m-2). FTIR Spectroscopy. For IR studies, thin self-supporting wafers of each sample (about 0.03 g cm-2) were prepared and activated under a dynamic vacuum, inside IR cells which allowed in situ high-temperature treatments, gas dosage, and room-25 or lowtemperature26 measurements to be performed. Spectra were collected, at 3 cm-1 resolution, on an FTIR Bruker 66 spectrometer. They were obtained at low temperature for adsorbed CO (sample wafer cooled with liquid nitrogen) and at room temperature for adsorbed pyridine. For each sample the spectrum taken before the dosage of the probe molecule has been used as background; all the spectra of adsorbed molecules shown in this work are background subtracted. High-purity CO (Air products, C30) was used without further purification while pyridine (Aldrich, 99%) was distilled in vacuum and turned gas-free by several freeze-pump-thaw cycles.

3. Results and Discussion 3.1. Structural Studies. Figure 1 shows the X-ray diffraction patterns of the SZx samples in the 20-47° (2θ) interval. Most of the observed diffraction lines could be assigned to the monoclinic phase of ZrO2. The agreement with the corresponding ASTM card27 was excellent. Samples having the highest CaO content also showed minor diffraction lines at 22.2° and 25.6° (2θ) which could be assigned to CaZrO328 and CaZr4O9,29 respectively. The diffraction line at 30.3° (2θ), whose relative intensity increases with Ca content, can equally well be assigned to either the cubic30 or the tetragonal31 phase of ZrO2. Distinction between these two phases rests on the splitting (25) Boccuzzi, F.; Coluccia, S.; Ghiotti, G.; Zecchina, A. J. Phys. Chem. 1978, 82, 1298. (26) Marchese, L.; Bordiga, S.; Coluccia, S.; Martra, G.; Zecchina, A. J. Chem. Soc., Faraday Trans. 1993, 89, 3483. (27) ASTM, Powder diffraction file, card No. 13-307. (28) ASTM, Powder diffraction file, card No. 35-790. (29) ASTM, Powder diffraction file, card No. 28-887. (30) ASTM, Powder diffraction file, card No. 7-337. (31) ASTM, Powder diffraction file, card No. 17-923.

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Figure 2. Raman spectra of the SZx samples.

of the (200, 002) and (311, 113) lines of the tetragonal modification. This criterion is hard to apply to the SZx samples on account of the low crystallinity of these materials and the simultaneous presence of diffraction lines from the monoclinic phase. The tetragonal and cubic phases can be distinguished by Raman spectroscopy. On the basis of factor-group analysis32-34 and experimental results10,35,36 tetragonal zirconia is expected to yield a spectrum consisting of six Raman bands (at about 148, 263, 325, 472, 608, and 640 cm-1) while cubic zirconia is characterized by a single band centered around 490 cm-1. The Raman spectra of the different samples are shown in Figure 2. The most intense bands could be assigned to the monoclinic phase. The presence of a band at 270 cm-1 (marked with a T), which is almost negligible in the samples with less calcium content, allows us to assign the minor phase to the tetragonal modification of zirconium dioxide. Therefore, the progressive growth of the maximum at 30.3° in the diffractograms of SZx samples is due to the progressive formation of the tetragonal phase. The monoclinic modification of zirconium oxide is the thermodynamically stable phase at room temperature. However, the cubic and tetragonal modifications can be stabilized at room temperature under several conditions. For instance, the presence of calcium oxide or magnesium oxide allows the cubic phase to be stabilized,22,37-39 while (32) Keramidas, V. G.; White, W. B. J. Am. Ceram. Soc. 1974, 57, 22. (33) Srinivasan, R.; Simpson, S. F.; Harris, J. M.; Davis, B. H. J. Mater. Sci. Lett. 1991, 10, 352. (34) Gulino, A.; La Delfa, S.; Fragala`, I.; Egdell, R. G. Chem. Mater. 1996, 8, 1287. (35) Philippi, C. M.; Mazdiyasni, K. S. J. Am. Ceram. Soc. 1971, 54, 254. (36) Mercera, P. D. L.; van Ommen, J. G.; Doesburg, E. B. M.; Burggraaf, A. J.; Ross, J. R. H. Appl. Catal. 1990, 57, 127; 1991, 71, 363; 1991, 78, 79. (37) Duwez, P.; Odell, F.; Brown, F. J. Am. Ceram. Soc. 1952, 35, 107. (38) Dietzel, A.; Tober, H. Ber. Dtsch. Keram. Ges. 1953, 30, 47.

Platero et al.

Figure 3. (a) Nitrogen adsorption-desorption isotherms, at 77 K, on samples SZ3, SZ10, and SZ15. (b) Adsorbed volume (V) versus multilayer thickness (t) for the same samples. (c) Pore size distribution curves: rp ) pore radius; Vp ) pore volume.

the presence of sulfate groups contributes to the stabilization of the tetragonal phase.5,7,13,40 The existence of the tetragonal phase in SZx samples can be related to the presence of the sulfate groups. Therefore, the effect of these groups on the crystalline structure of the samples SZx appears to dominate over the effect of calcium oxide. Note, however, that under the experimental conditions used in this work it is likely that CaO is not uniformly distributed in the bulk of the solid. Surface segregation of a calcium-rich phase is likely to have taken place. In any case, the tetragonal phase is only a minor component in SZx samples. 3.2. Surface Area and Porosity. N2 adsorption isotherms up to P/P0 ) 0.8 were obtained on SZ1 and SZ5 samples, and full N2 adsorption-desorption isotherms were obtained on the remaining samples. All of the full isotherms were found to be akin to type IV of the BDDT classification,41 and the hysteresis loops correspond to type H1,42 as shown in Figure 3a. The shape of the isotherms (including the hysteresis loops) suggests that the materials are mesoporous solids, the pore texture of which conforms to a geometric model of tubular pores with widened parts. The absence of significant microporosity, suggested by the shape of the nitrogen adsorption isotherms, has been confirmed by using the de Boer Va-t method.43 Plots of multilayer thickness (t) versus volume of adsorbed nitrogen (Va) could be extrapolated through the origin (Figure 3b). (39) Barbariol, I. Ann. Chim. 1965, 55, 321. (40) Morterra, C.; Cerrato, G.; Pinna, F.; Signoretto, M. J. Phys. Chem. 1994, 98, 12373. (41) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723. (42) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (43) Lippens, B. C.; de Boer, J. H. J. Catal. 1965, 4, 319.

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Table 1. Textural Parameters of SZ and SZx Samples sample

SBET (m2 g-1)

St (m2 g-1)

Vp (cm3 g-1)

SZ SZ1 SZ3 SZ5 SZ10 SZ15

91 75 63 59 52 56

90 79 65 59 50 58

0.515

8

0.371

8

0.303 0.292

8 7.5

rp (nm)

The observed increasing slope of this t-plot in the high-t region supports the above assignment of pore shape.43 The Va-t plots were obtained using the standard isotherm n2, taken from the work of Lecloux and Pirard.44 This standard isotherm was chosen so as to match the CBET parameter of the experimental isotherms analyzed. BET and t surface area were calculated from the corresponding adsorption isotherms and Va-t plots, respectively, using the value of 0.162 nm2 for the surface covered by one nitrogen molecule. The results are given in Table 1. The close similarity found between SBET and St values is remarkable; this again indicates the absence of microporosity in the studied materials. Also shown in Table 1 is the total pore volume (Vp) and the most frequent pore radius (rp). The pore volume was obtained by multiplying the amount of adsorbed nitrogen, directly read from the isotherms at P/P0 ) 0.9975, by the conversion factor 0.001 56.45 The most frequent pore radius was calculated following the Pierce method46 with the equation of Halsey for multilayer thickness.47 The monolayer thickness was taken as 354 pm. The analysis was applied to the adsorption branch of each isotherm, which must be preferred to the desorption branch for type IV isotherms.48 The pore size distribution curve was found to be unimodal in all cases, with a maximum at rp ≈ 8 nm, as shown in Figure 3c. For comparison, Table 1 includes the textural parameters for the parent material (SZ sample) which were published elsewhere.19 It can be observed in this table that increasing amounts of Ca do not modify significantly the most frequent pore radii (rp), which maintain the value of ∼8 nm for the different samples. However, there is an initial decrease of specific surface area from 90 m2 g-1 (SZ sample) which then tends toward a final equilibrium value of ∼55 m2 g-1. Pore volume changes in a similar way from 0.515 cm3 g-1 (SZ sample) down to 0.300 cm3 g-1 (SZ10 and SZ15 samples). The textural parameters of SZx samples indicate that the presence of calcium oxide favors the sintering process of zirconium oxide. The role of calcium in this process can be explained in general terms, as a consequence of the anionic vacancies created in the crystal lattice when two Ca2+ ions substitute one Zr4+ ion. The presence of these vacancies should favor anion diffusion. 3.3. FTIR Studies. Samples Activated at 873 K for 1 h. Figure 4 shows the IR spectra in the 1600-850 cm-1 region of the different SZx samples. For comparison, the corresponding spectra of the parent material, SZ sample, and CaSO4‚2H2O are also depicted in this figure. It can be observed that the spectrum of SZ1 is very similar to that of SZ, while the spectrum of SZ15 resembles that of CaSO4‚2H2O. The spectrum of sample SZ has been described elsewhere,21 and it has been assigned to different types of covalent sulfate groups present at the surface of the (44) Lecloux, A.; Pirard, J. P. J. Colloid Interface Sci. 1979, 70, 265. (45) Sing, K. S. W. Chem. Ind. 1967, 829. (46) Pierce, C. J. Phys. Chem. 1953, 57, 149. (47) Halsey, G. D. J. Chem. Phys. 1948, 16, 931. (48) Mata Arjona, A.; Parra Soto, J. B.; Otero Area´n, C. Stud. Surf. Sci. Catal. 1982, 10, 175.

Figure 4. FTIR spectra, in the sulfate region, of samples SZ, SZx, and CaSO4‚2H2O. The SZ sample is mainly monoclinic.

zirconia sample. The spectrum shows an intense band with several components in the 1450-1340 cm-1 region (SdO stretching) and several bands in the 1150-850 cm-1 region (SsO stretching). The SdO stretching band has a pronounced maximum at 1398 cm-1 and a broad tail on the low-frequency side. In agreement with several authors,3,9,49 we have assigned the 1398 cm-1 band to disulfate species ([(ZrO)2sS(dO)s]2sO) and the bands at lower frequency to monosulfate species ((ZrO)3sSdO). The multiplicity of components in the 1385-1340 cm-1 region can be explained in terms of surface heterogeneity, which must yield monosulfate species with slightly different spectroscopic manifestations. The spectrum of CaSO4‚2H2O shows a complex band at ∼1150 cm-1 assigned to the T2 mode of ionic sulfate groups with Td symmetry.50 The IR spectra of SZx samples indicates that the addition of calcium oxide to sulfated zirconia causes the transformation of covalent into ionic sulfate groups. There are both types of covalent sulfate groups (di- and monosulfates) and a small proportion of ionic species in sample SZ1. In SZ3, disulfate groups have disappeared and the proportion of ionic species has increased. These ionic species continue to grow with calcium content at the expense of the covalent ones, and they become dominant in the SZ15 sample. Figure 5 shows the IR spectra of SZx samples in the O-H stretching region; the corresponding spectra of SZ and pure ZrO2 are also included in this figure. The IR spectrum of the SZ sample shows two bands at 3750 and 3650 cm-1. These bands shift to higher wavenumbers with calcium content, reaching higher values than in the pure oxide: 3770 and 3665 cm-1 (assigned respectively to terminal hydroxyls and to bridged Zr(OH)Zr groups2,51,52). The band at higher frequency in the IR spectra of SZx samples has always the lowest intensity, and it has almost disappeared in SZ10 and SZ15. The Brønsted acidity of the SZ sample is characterized by the presence of the O-H stretching band at 3650 cm-1.21 The progressive hypso(49) Morterra, C.; Cerrato, G.; Bolis, V. Catal. Today 1993, 17, 505. (50) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley: New York, 1986; Chapter 2. (51) Yamaguchi, T.; Nakano, Y.; Tanabe, K. Bull. Chem. Soc. Jpn. 1978, 51, 2482. (52) Morterra, C.; Aschieri, R.; Volante, M. Mater. Chem. Phys. 1988, 20, 539.

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Figure 5. FTIR spectra, in the O-H stretching region, of samples SZ, SZx, and pure ZrO2. The SZ sample is mainly monoclinic.

chromic shift of this band is related to the decrease of Brønsted acid strength, as shown below. CO Adsorbed at Low Temperature. Carbon monoxide is frequently used as a molecular probe for Lewis acid sites. The stretching frequency of CO adsorbed on coordinatively unsaturated cations (via the carbon atom) exhibits hypsochromic shifts from the value of 2143 cm-1 corresponding to the free molecule,53 and the magnitude of the shift can be used to probe the strength of the corresponding Lewis acid center. Carbon monoxide can also interact with OH groups, forming H-bonded adducts. These adducts lead to characteristic shifts of the O-H stretching mode to lower frequency, the magnitude of the shift (∆νO-H) being a measure of the H-bond energy and therefore of the Brønsted acid strength of the corresponding OH group.54 Figure 6 shows the FTIR spectra in the C-O stretching region of carbon monoxide adsorbed (at liquid nitrogen temperature) on SZ1-SZ10 samples, previously outgassed for 1 h at 873 K. CO adsorbed on sample SZ1 gives rise to IR spectra very similar to those obtained on sample SZ;21 they consist of five bands at 2212, 2202, 2196, 2188, and 2165 cm-1, which saturate in succession upon increasing CO equilibrium pressure. The spectra of sample SZ3 show only two bands, which for small CO doses appear at 2208 and 2197 cm-1. Two bands are also shown in the spectra of sample SZ5 but at lower wavenumbers: 2206 and 2192 cm-1. The spectra of sample SZ10 (as well as those of sample SZ15, not included in the figure) show only a wide band centered at 2187 cm-1. The frequencies of CO adsorbed on the different samples, read the first time they were observed in the spectra, are collected in Table 2; this table also includes the corresponding features for samples SZ and pure ZrO2. The bands at 2212, 2202, 2196, and 2188 cm-1, present in the IR spectra of samples SZ and SZ1, are assigned to CO molecules interacting with Lewis acid centers of different strength: Zr4+ ions with varying degree of Lewis acidity. The three bands with lower wavenumbers were also observed for CO adsorbed at low temperature on pure zirconium dioxide.2,55 The band at 2212 cm-1 (characteristic of sulfated zirconia) is assigned to CO adsorbed (53) Kno¨zinger, H. In Elementary Reaction Steps in Heterogeneous Catalysis; Joyner, R. W., van Santen, R. A., Eds.; Kluwer: Amsterdam, 1993; p 267. (54) Paukshtis, E. A.; Yurchenko, E. N. React. Kinet. Catal. Lett. 1981, 16, 131. (55) Morterra, C.; Bolis, V.; Fubini, B.; Orio, L.; Williams, T. B. Surf. Sci. 1991, 251/252, 540.

Figure 6. Low-temperature FTIR spectra of CO adsorbed on samples SZ1-SZ10 at increasing equilibrium pressure (from ca. 10-2 to 20 Torr). The inset shows the O-H stretching region of sample SZ1 before and after dosing with CO (20 Torr). Table 2. Position of IR Absorption Maxima for CO Adsorbed (at Low Temperature) on Samples SZ, SZx, and Pure ZrO2 νC-O (cm-1)

sample SZa SZ1 SZ3 SZ5 SZ10 SZ15 ZrO2b a

2212 2212 2208

2202 2202

2196 2196 2197

2206 2201

2197

2188 2188

2165 2165

2192 2187 2187 2190

b

Reference 21. References 2 and 55.

on Zr4+ ions with enhanced Lewis acidity, caused by neighboring covalent sulfate groups. The addition of calcium oxide causes the progressive disappearance of the highest wavenumber CO absorption bands, which indicates the progressive destruction of the strongest Lewis acid sites. This destruction is likely brought about by both the intrinsic basic character of calcium oxide and the transformation that this oxide causes in the sulfate groups present on the surface of zirconium oxide (from covalent to ionic character; see Figure 4). Regarding this last point, it is known that the enhanced acidity of sulfated zirconia is directly linked with the presence of disulfate groups;21,49 therefore, the absence of these groups, even when calcium oxide is not present, causes the acidity of sulfated zirconia to decrease.21,49 The formation of an adduct between Ca2+ and CO (Ca2+‚‚‚CO) will give rise to an IR absorption band at 2180 cm-1.56 Such a band is not distinctly observed in the spectra of Figure 6; however, due to the low proportion of calcium oxide in the SZx samples, its presence cannot be

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Table 3. O-H Stretching Frequencies and Downward Shift (∆νOH) upon Interaction with Adsorbed CO at Low Temperature sample

νO-H (cm-1)

SZ SZ1 SZ3 SZ5 SZ10 SZ15 ZrO2

3750, 3650 3764, 3655 3770, 3674 3779, 3677 3780, 3687 3689 3770, 3665

∆νO-H (cm-1) low-frequency band 140 140 125 108 90

ref 21 present work present work present work present work present work 58

discarded, since it could be overshadowed by other bands present in the IR spectra. For high CO equilibrium pressure, the IR spectrum of the sample SZ1 shows an additional band at 2165 cm-1. The presence of this band is accompanied by changes in the O-H stretching region, as observed in the inset of Figure 6. The most pronounced effect is observed in the 3655 cm-1 band, which is enlarged and shifted to lower wavenumbers. This is the characteristic behavior of H-bond interaction,57 indicating the formation of adducts of the OH‚‚‚CO type. The magnitude of the shift, which can be considered as an indication of the acid strength,54 is ∼140 cm-1 (the same observed for sample SZ). In the IR spectra of the remaining SZx samples, the band at 2165 cm-1 is not clearly observed; however, in the O-H stretching region similar effects as those described for the sample SZ1 are observed, i.e. an enlargement and shift to lower wavenumbers of the O-H stretching band, although the magnitude of these effects decreases with increasing calcium content. Table 3 collects the O-H stretching frequencies and the shift (∆νO-H) measured for SZ1-SZ5 samples upon CO adsorption; this table also includes the values obtained for the sample SZ21 and for pure ZrO2.58 It can be appreciated that the magnitude of the shift decreases with the content of calcium, indicating the progressive weakening of the H-bond between OH groups and the CO molecule. Pyridine Adsorbed at Room Temperature and Outgassed at 423 K. Pyridine is a stronger base than CO, and it is used to characterize Lewis and Brønsted acid sites on the surface of solid materials.53,59 The pyridine molecule can coordinate Lewis acid sites through the nitrogen lonepair electrons; it can also form pyridinium ions with acidic OH groups; and finally, it can form H-bonded species on OH groups with less acidic character. The formation of these different species can be sensitively detected by monitoring the ring vibration modes 8a and 19b (named according to the nomenclature adopted by Wilson60). These modes, which appear at 1580 (8a) and 1439 (19b) cm-1 for gas-phase pyridine,53,59 undergo upward shifts upon coordination of the molecule with the different types of acid sites. Specifically, they are observed at 15801600 cm-1 (8a) and at 1400-1447 cm-1 (19b) after formation of H-bonded species; at 1600-1635 (8a) and at 1447-1464 (19b) after coordination to Lewis acid sites; and at ∼1640 (8a) and at 1535-1550 cm-1 (19b) after formation of the pyridinium ion. The greatest values of each range correspond to sites with the highest acid character. Figure 7 shows room-temperature spectra, in the 17001400 cm-1 region, of pyridine adsorbed on the SZ1-SZ5

Figure 7. Room-temperature FTIR spectra of pyridine adsorbed on samples SZ1, SZ3, and SZ5.

samples, which were previously outgassed at 473 K for 1 h. This low outgassing temperature was selected in order to have a great degree of surface hydroxylation and allow the possible formation of pyridinium ions. The spectra were obtained after equilibrating the samples (at room temperature) with pyridine vapor followed by outgassing at 423 K for 5 min, in order to drive off physisorbed species. The assignment of the IR absorption bands (shown in Figure 7) was made according to Parry59 and to Kno¨zinger.53 The 8a mode of pyridine coordinated to Lewis acid sites shifts from 1610 cm-1 (sample SZ1) down to 1607 cm-1 (sample SZ5), reflecting the decrease of Lewis acid strength with increasing calcium content. The Lewis acid strength of sample SZ1 is the same as found for sample SZ, which was also the case for CO adsorbed at 77 K. Sample SZ5 has, however, similar Lewis acid strength as that found for pure zirconia.2,4 Among all SZx samples, and regarding Brønsted acidity, only sample SZ1 is able to give rise to the pyridinium ion by interaction with pyridine (band at 1545 cm-1), which was also the behavior found for sample SZ.21 Samples SZ3 and SZ5 behave as pure zirconia.4,61 This is as expected, because the Brønsted acidity of sample SZ, associated with the 3650 cm-1 band, is directly linked to the presence of disulfate groups; when these groups are no longer present (as seen for samples SZ3 and SZ5 in Figure 4), Brønsted acidity should not be manifested. It is interesting to note in this respect that the 3650 cm-1 band, observed in the IR spectrum of sample SZ, shifts to higher wavenumbers as calcium content increases, reaching 3689 cm-1 for sample SZ15 (see Figure 5). Finally, it is worth mentioning that adsorption of both CO and pyridine was found to shift the peak position of covalent sulfate bands (spectra not shown), thus proving that these are (mainly) surface species. Further details were given elsewhere21 for the SZ sample. 4. Conclusions

(56) Morterra, C.; Mangacca, G.; Cerrato, G.; Del Favero, N.; Filippi, F.; Folonari, C. V. J. Chem. Soc., Faraday Trans. 1993, 89, 135. (57) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman: San Francisco, 1996. (58) Escalona Platero, E.; Pen˜arroya Mentruit, M. Unpublished results. (59) Parry, E. P. J. Catal. 1963, 2, 371. (60) Wilson, E. B. Phys. Rev. 1934, 45, 706.

The addition of small quantities of calcium oxide to sulfated zirconia contributes to the stabilization of the tetragonal phase, although the major phase in all SZx (61) Waqif, M.; Bachelier, J.; Saur, O.; Lavalley, J. C. J. Mol. Catal. 1992, 72, 127.

3156 Langmuir, Vol. 13, No. 12, 1997

samples is the monoclinic one. Textural analysis of the mixed oxides revealed that calcium oxide favors sintering of sulfated zirconia; there is a decrease of SBET from 90 m2 g-1 for sample SZ (which does not contain CaO) down to 55 m2 g-1 for SZ15 (which has the largest amount of CaO). The most frequent pore radius is, however, independent of calcium content and equal to 8 nm. The presence of calcium oxide causes the transformation of sulfate groups at the surface of sulfated zirconia from covalent to ionic and the shift of the O-H stretching bands to higher wavenumbers, reaching values higher than those corresponding to pure zirconia. CO adsorption at low temperature on the SZ1 sample gives rise to an IR spectrum with components at 2212, 2202, 2196, 2188, and 2165 cm-1. The first four bands are assigned to CO molecules interacting through the carbon atom with four types of Lewis acid sites (Zr4+ ions with different degrees of coordinative unsaturation); the band at 2165 cm-1 is assigned to the formation of the OH‚‚‚CO adducts. As calcium content increases, the CO

Platero et al.

absorption bands corresponding to the strongest Lewis acid sites succesively disappear (starting from those at highest wavenumber), thus showing that incorporation of CaO reduces the Lewis acidity of sulfated zirconia. This last point was also stated by pyridine adsorption at room temperature; the 8a mode of pyridine coordinated to Lewis acid sites was observed to shift from 1610 cm-1 (sample SZ1) down to 1607 cm-1 (sample SZ5). IR spectra of adsorbed pyridine also proved that only the sample with the lowest calcium content (SZ1) has OH groups with Brønsted acid character capable of generating pyridinium ions. Acknowledgment. This work has been supported by the Spanish DGICYT, Ref. PB93-0425. La Fundacio´n Joan Montaner is gratefully acknowledged for a fellowship to M.P.M. The authors thank Prof. C. Otero Area´n and Prof. A. Zecchina for helpful discussions. LA960789K