FTIR Study of Low-Temperature CO Adsorption on MgAl-Hydrotalcite

A well-defined two-stage weight loss pattern was observed, as already disclosed in earlier reports.29The first weight loss is generally ascribed to wa...
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FTIR Study of Low-Temperature CO Adsorption on MgAl-Hydrotalcite and Its Calcined Forms S. Kannan,*,† D. Kishore,† K. Hadjiivanov,‡ and H. Kno¨zinger*,§ Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, GB Marg, Bhavnagar 364 002, India, Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria, and Department Chemie, Physikalische Chemie, LMU Mu¨ nchen, Butenandtstrasse 5-13 (Haus E), 81377 Germany Received January 21, 2003. In Final Form: March 31, 2003 The evolution of the Lewis acidity of a MgAl hydrotalcite sample (Mg/Al ratio of 4) during thermal treatment in vacuo at different temperatures was followed by means of FTIR spectroscopy of CO adsorbed at 85 K. The materials were also characterized by XRD, TG-DTA, and BET measurements. It was found that samples activated up to 623 K are characterized by weakly acidic protonic sites, the density of which sharply decreases at higher activation temperatures. This is associated with the water loss of the sample around this temperature. Lewis acid sites are detected on the surface after activation at 573 K. Their concentration increases with the activation temperature up to 873 K and then slightly decreases due to the sample sintering. A separate MgO phase was detected only after 923 K activation. 12CO-13CO coadsorption experiments evidence a small dynamic shift of ν(CO) of 3.5 cm-1, which indicates that the Lewis acid sites are cations located on regular crystal planes.

1. Introduction Hydrotalcite-like compounds constitute a class of twodimensional materials, receiving increasing attention because of their diverse applications.1-3 Structurally, they consist of alternating positively charged mixed metal hydroxide sheets and negatively charged interlayers containing anions for charge compensation. They are represented by the general formula [M(II)1-xM(III)x(OH)2] [Ax/nn-]‚mH2O, where M(II) is a bivalent metal ion, M(III) is a trivalent metal ion, A is the interlayer anion, and x can have values between 0.2 and 0.4. Among various systems studied, MgAl hydrotalcite is explored extensively for various base-catalyzed organic transformations for which the participation of an acid-base functionality is well discussed.4-7 It is also reported that both Lewis acidity and basicity are responsible for mediating selectively some catalytic transformations.6 Although there are reports available on characterization of basicity of MgAl hydrotalcites,8,9 no study has been made to characterize the Lewis acidity. The purpose of the present investigation is to follow the evolution of Lewis acidity for MgAl hydro* Corresponding author. E-mail: [email protected] † Central Salt and Marine Chemicals Research Institute. ‡ Bulgarian Academy of Sciences. § Physikalische Chemie, LMU Mu ¨ nchen. (1) Trifiro, F.; Vaccari, A. In Comprehensive Supramolecular Chemistry, Solid State Supramolecular Chemistry: Two and Threedimensional Inorganic Networks; Atwood, J. L., Dasvies, J. E. D., MacNicol, D. D., Vogtle, F., Lehn, J.-M., Aberti, G., Bein, T., Eds.; Pergamon: Oxford, Vol. 7, 1996; p 251. (2) Rives, V., Ed. Layered Double Hydroxides: Present and Future; Nova Sci. Pub., Inc.: New York, 2001. (3) Basile, F., Campanati, M., Serwicka, E., Vaccari, A., Eds. Special Issue on Hydrotalcites. Appl. Clay Sci. 2001, 18, 1-101. (4) Reichle, W. T. J. Catal. 1985, 94, 547. (5) Rao, K.; Gravelle, M.; Valente, J. S.; Figueras, F. J. Catal. 1998, 173, 115. (6) Yamaguchi, K.; Ebitani, K.; Yoshida, T.; Yoshida, H.; Kaneda, K. J. Am. Chem. Soc. 1999, 121, 4526. (7) Kishore, D.; Kannan, S. Green Chemistry (in press, published online on 31st Oct 2002). (8) Corma, A.; Fornes, V.; Rey, F. J. Catal. 1994, 148, 205. (9) Tichit, D.; Lhouty, M. H.; Guida, A.; Chiche, B. H.; Figueras, F.; Auroux, A.; Bartalini, D.; Garrone, E. J. Catal. 1995, 151, 50.

talcite during thermal treatment in a vacuum by means of low-temperature CO adsorption. For better interpretation of the results, the sample was also characterized by independent techniques. CO adsorption on different Mg-containing systems results in the appearance of carbonyl bands in the 22132144 cm-1 region.10 However, bands below 2190 cm-1 have been recorded only with MgO11-18 and CoO-MgO,19 mainly at low temperatures. The strong basic character of the MgO surface is responsible for the formation of carbonite anions11,20 or similar polymeric structures11,16-18,20 upon CO adsorption. The Al3+-CO carbonyls for aluminumcontaining systems have been detected in the region 22452142 cm-1.10 Here again, bands below 2190 cm-1 (lowfrequency or LF bands) are recorded at low temperatures only and characterize weak Lewis acid sites. With aluminum oxides the carbonyl bands were observed (i) at high frequencies (HF) at 2245-2215 cm-1, and with samples activated at temperatures higher than 770 K,21-24 (ii) at intermediate frequencies (MF) of about 2200 (10) Hadjiivanov, K.; Vayssilov, G. Adv. Catal. 2002, 74, 307. (11) Babaeva, N.; Tsyganenko, A. J. Catal. 1990, 123, 396. (12) Coluccia, S.; Baricco, M.; Marchese, L.; Martra, G.; Zecchina, A. Spectrochim. Acta A 1993, 49, 1289. (13) Coluccia, S.; Marchese, L. Catal. Today 1998, 41, 229. (14) He, J. W.; Estrada, C. A.; Corneille, J. S.; Jason, S.; Wu, M. C.; Goodman, D. W. Surf. Sci. 1992, 261, 164. (15) Li, C.; Li, G.; Xin, Q. J. Phys. Chem. 1994, 98, 1933. (16) Tashiro, T.; Ito, J.; Siny, R.; Miyazawa, K.; Hamada, E.; Toi, K.; Kobayashi, H.; Ito, K. J. Phys. Chem. 1995, 99, 6115. (17) Zaki, M. I.; Kno¨zinger, H. In Recent Trends in Chemical Reactor Engineering; Kulkarni, B., Mashelkar, R., Sharma, M., Eds.; Wiley: New Delhi, 1987; p 19. (18) Zaki, M.; Kno¨zinger, H. Spectrochim. Acta A 1987, 43, 1455. (19) Zecchina, A.; Spoto, G.; Coluccia, S.; Guglielminotti, E. J. Phys. Chem. 1984, 88, 2575. (20) Babaeva, A.; Tsyganenko, A. React. Kinet. Catal. Lett. 1987, 34, 9. (21) Zecchina, A.; Platero, E.; Otero Arean, C. J. Catal. 1987, 107, 244. (22) Tsyganenko, A.; Denisenko, L.; Zverev, S.; Filimonov, V. J. Catal. 1985, 94, 10. (23) Morterra, C.; Bolis, V.; Magnacca, G.; Cerrato, G. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 235. (24) Marchese, L.; Bordiga, S.; Coluccia, S.; Martra, G.; Zecchina, A. J. Chem. Soc., Faraday Trans. 1993, 89, 3483.

10.1021/la030019q CCC: $25.00 © 2003 American Chemical Society Published on Web 06/06/2003

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cm-1,18,21-27 and (iii) occasionally at low frequencies (LF), around 2165 cm-1, however, at low temperature.21 The above consideration shows that it is difficult to discriminate unambiguously between Mg2+-CO and Al3+CO species on the basis of the CO stretching frequency only. However, discrete formation of separate alumina and magnesia phases could be followed. 2. Experimental Section 2.1. Samples and Reagents. MgAl hydrotalcite with carbonate as interlayer anion was synthesized by coprecipitation under low supersaturation. Two solutions containing metal nitrates of appropriate concentration (with a Mg/Al atomic ratio of 4.0) and precipitants (NaOH and Na2CO3) were added slowly and simultaneously while maintaining the pH around 9-10 at room temperature under continuous stirring. The resulting gel was allowed to age in the mother liquor at 338 K for 18 h and was filtered, washed thoroughly with distilled water (till no traces of sodium and nitrates were detectable), and dried in an oven at 373 K for 12 h. The elemental analysis of this sample using ICPES showed a Mg/Al atomic ratio of 3.5, reasonably coincident with the ratio in the starting solutions; the small deviation observed is rather common. The sample is denoted MgAl4-HT in the figure captions. Carbon monoxide (99.997) was supplied by Linde. Labeled carbon monoxide (13CO) was provided by Aldrich and had a 13C isotopic purity of 99.0 at %. It contained about 10 mol % 13C18O. 2.2. Methods. In situ powder X-ray diffraction (PXRD) was carried out on a Philips X’Pert MPD system connected to an Anton-Paar high temperature XRK assembly using Cu KR radiation (λ ) 1.540 Å). The sample was mounted in a high temperature cell and heated at 5 K min-1 in steps of 50 K and stabilized for 5 min before measurements. The operating voltage and current were 40 kV and 40 mA, respectively. The step size was 0.05° with a step time of 1 s. Identification of the crystalline phases was made by comparison with the JCPDS files.28 FTIR spectroscopy studies were carried out with a Bruker IFS-66 apparatus at a spectral resolution of 2.0 cm-1, accumulating 128 scans. Self-supporting wafers (∼10 mg cm-2) were prepared from the sample powders and heated directly in a purpose-made IR cell. The latter was connected with a vacuum/ sorption apparatus with a residual pressure less than 10-3 Pa and allowed measurements at low temperatures. Thermogravimetry-differential thermal analysis (TG-DTA) was carried out on a Netzsch thermobalance. Analysis was done from 323 to 973 K at a heating rate of 10 K min-1 under nitrogen (30 cm3 min-1). The gases evolved during thermal treatment were simultaneously analyzed by an on-line quadruple mass spectrometer (Balzers 420) with a focus on m/e 18 (H2O) and 44 (CO2).

3. Results 3.1. In Situ Powder X-ray Diffraction. PXRD of the parent sample (Figure 1, pattern a) showed a pattern similar to that of hydrotalcite (JCPDS: 41-1428), exhibiting sharp and symmetric reflections at low diffraction angles and broad and asymmetric reflections at higher diffraction angles, characteristic of layered materials. With an increase in the temperature of calcinations, the intensity of the basal reflections decreased and shifted to higher diffraction angles with a concomitant increase in broadening. This is more clearly noticeable for higher diffraction angles (observed around 2θ g 30°) (Figure 1, patterns b-e). This is caused by alterations in interlayer spacing due to removal of water from the interlayer space along with partial dehydroxylation, likely occurring at (25) Daniell, W.; Schubert, U.; Glo¨cker, R.; Meyer, A.; Noweck, K.; Kno¨zinger, H. Appl. Catal. A 2000, 196, 247. (26) Morterra, C.; Bolis, V.; Magnacca, G. Langmuir 1994, 10, 1812. (27) Busca, G.; Lorenzelli, V.; Sanchez-Escribano, V. Chem. Mater. 1992, 4, 595. (28) Joint Committee on Powder Diffraction Standards, International Centre for Diffraction Data, Pennsylvania, 1977.

Figure 1. In situ PXRD of MgAl4-HT calcined at 373 (a), 423 (b), 473 (c), 523 (d), 573 (e), 623 (f), 673 (g), 723 (h), 773 (i), 823 (j), 873 (k), 923 (l), 973 (m), 1023 (n), and 1073 K (o).

Figure 2. TG-DTA traces of MgAl4-HT (inset is QMS analysis of H2O and CO2 evolution).

the edges of the sheets, and this process is pronounced till 573 K. With a further increase of the calcination temperature, a phase transformation occurs, as indicated by the disappearance of the diffraction pattern of the HTlike network and the growth of the diffuse MgO diffractions (JCPDS: 30-794) (Figure 1, patterns f-o). However, lattice parameter calculations of MgO showed a deviation from those of pure MgO, suggesting a possible dissolution of aluminum. No significant change in the phase was noticed when the sample was calcined at higher temperatures up to 1073 K; however, the crystallinity of the phase improved with increasing calcination temperature, as evidenced by the increase in intensity and sharpness of the reflections. 3.2. Thermogravimetry-Differential Thermal Analysis. TG-DTA analysis of the sample is shown in Figure 2. A well-defined two-stage weight loss pattern was observed, as already disclosed in earlier reports.29 The first weight loss is generally ascribed to water loss from the interlayer space, while the second weight loss is attributed to dehydroxylation of the hydroxide network as well as decomposition of interlayer carbonate anions accompanied by the destruction of the layered structure. A continuous weight loss of less than 5% was noted up to 973 K, indicating a continuous loss of CO2, which might have been held by or occluded in the calcined oxide phase. This is consistent with the small diffuse peak observed in the mass spectrometric analysis of CO2 at temperatures near 923 K. DTA analysis substantiated the weight loss pattern, showing two defined endotherms corresponding to the observed weight losses. A well-defined twin peak pattern was observed (see Figure 2) of the mass signal of (29) Kannan, S.; Rives, V. J. Mater. Chem. 2000, 10, 489.

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Figure 3. FTIR spectra of MgAl4-HT activated in a vacuum at 473 (a), 573 (b), 673 (c), 773 (d), 873 (e), and 973 K (f).

water during the second weight loss, the interpretation of which, however, remains unclear. 3.3. In Situ FT-IR Spectroscopy: Evolution of the Samples during Vacuum Thermal Treatment. To follow the creation of Lewis acidity on the sample surface, we have activated the sample at different temperatures in the range 473-973 K prior to low-temperature CO adsorption. The spectrum of the sample activated at 473 K showed three intense and broad bands at ∼1540, 1350, and 1040 cm-1, which originate from carbonate ions in the sample structure (Figure 3, spectrum a).30 In addition, a sharp band at 3716 cm-1 and a broad absorption at ∼3200 cm-1 are indicative of the presence of isolated (3716 cm-1) and H-bonded (3200 cm-1) hydroxyls.31 Heating the sample under vacuum at 573 K leads to a slight modification of the spectrum (Figure 3, spectrum b). The carbonate and OH bands are sharpened and slightly reduced in intensity, and the bands due to H-bonded hydroxyls are shifted to 3290 cm-1. These results are indicative of the onset of sample decomposition starting at this temperature. This is consistent with the TG results, which showed a removal of water molecules from the interlayer, a partial dehydroxylation, and a decomposition of carbonate counteranions beginning at around this temperature. Activation at 673 K leads to dramatic changes in the sample background spectrum (Figure 3, spectrum c). The OH bands have drastically decreased in intensity, and only two low-intensity bands are now detected at 3730 and 3670 cm-1. This is indicative of a nearly complete sample dehydroxylation. In addition, the intensity of the carbonate bands has also drastically decreased, which is evidence for a significant conversion of carbonate to oxide phases. This is substantiated by in situ PXRD measurements (Figure 1, pattern f), which showed a complete transformation of the HT-like phase to oxide phases at 673 K, and by the TG-DTA results (Figure 2). Activation at higher temperatures (Figure 3, spectra d-g) only leads to a gradual decrease in intensity of the residual hydroxyl and carbonate bands, but they do not disappear completely from the spectrum even after activation at 973 K (Figure 3, spectrum f). 3.4. Low-Temperature CO Adsorption. Adsorption of CO (900 Pa equilibrium pressure) at 85 K on the sample activated at 473 K leads to the appearance of a very weak band at 2152 cm-1 (Figure 4, spectrum a). The intensity of this band decreases with decreasing CO equilibrium pressure, and it disappears upon evacuation. The very (30) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89. (31) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; J. Wiley & Sons: New York, Chichester, Brisbane, Toronto, Singapore, 1986.

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Figure 4. FTIR spectra of CO adsorbed at 85 K on MgAl4-HT activated under vacuum at 473 K for 1 h: equilibrium pressure of 900 Pa CO (a) and gradual decrease of the coverage during evacuation down to ∼10-2 Pa (b-h).

Figure 5. FTIR spectra of CO adsorbed at 85 K on MgAl4-HT activated under vacuum at 573 K for 1 h: equilibrium pressure of 400 Pa CO (a) and gradual decrease of the coverage during evacuation down to ∼10-2 Pa (b-k).

low stability of these carbonyls as well as the band position is indicative of a very weak adsorption form. The band can be assigned to CO polarized by surface hydroxyl groups of the sample.18 When an analogous experiment was performed with a sample activated at 573 K, the intensity of the carbonyl bands formed increased. Introduction of CO (400 Pa equilibrium pressure) to the sample at 85 K results in the appearance of a band at 2152 cm-1, however, with a pronounced high-frequency shoulder (Figure 5, spectrum a). The band at 2152 cm-1 decreases in intensity with decreasing CO pressure (Figure 5, spectra b-d), thus revealing a well-shaped component at 2173 cm-1, which is attributed to CO adsorption on Lewis acid sites. The intensity of this component also decreases with decreasing CO pressure and disappears completely only under dynamic vacuum. The band at 2152 cm-1 has already been assigned to H-bonded CO. At first sight, the development of this band with an increase in the temperature of activation seems surprising. However, independent BET measurements showed an increase in the specific surface area upon thermal activation of the sample (see Figure 6), which provides more OH groups accessible for CO. The spectra obtained after low-temperature CO adsorption on a sample activated at 623 K are reported in Figure 7. They are similar to the spectra obtained after CO adsorption on the sample activated at 573 K. In this case, however, the band characterizing CO adsorbed on Lewis acid sites is more pronounced. At higher coverage, the band is detected at 2170 cm-1 and is gradually blueshifted to 2175 cm-1 with decreasing coverage. These

CO Adsorption on MgAl-Hydrotalcite

Figure 6. Variation of BET surface area with calcination temperature for MgAl4-HT.

Figure 7. FTIR spectra of CO adsorbed at 85 K on MgAl4-HT activated under vacuum at 623 K for 1 h: equilibrium pressure of 300 Pa CO (a) and gradual decrease of the coverage during evacuation down to ∼10-2 Pa (b-j).

Figure 8. FTIR spectra of CO adsorbed at 85 K on MgAl4-HT activated under vacuum at 673 K for 1 h: equilibrium pressure of 300 Pa CO (a) and gradual decrease of the coverage during evacuation down to ∼10-2 Pa at 115 K (b-q).

results show a pronounced static shift, which suggests that the Lewis acid sites on which CO is adsorbed are located on preformed crystal planes. If the sites were isolated, no significant shift of the CO band would be expected. The picture changes when CO was adsorbed on a sample activated at 673 K (Figure 8). In this case the intensity of the OH-CO band was negligible, which is consistent with a nearly complete surface dehydroxylation, as suggested above (cf. PXRD and DTA results). A band at 2169 cm-1 is dominating the spectrum. This band is gradually blue-shifted with decreasing coverage and reaches 2179 cm-1 at the lowest coverages. The little lower CO frequency at saturation, as compared to the sample

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Figure 9. FTIR spectra of CO adsorbed at 85 K on MgAl4-HT activated under vacuum at 873 K for 1 h: equilibrium pressure of 400 Pa CO (a) and gradual decrease of the coverage during evacuation down to ∼10-2 Pa at 145 K (b-r).

activated at 623 K, is indicative of a stronger interaction between the adsorbed molecules. The results can be rationalized assuming that the average area of the crystal microfaces has increased after thermal treatment at the higher temperature. As a result, the average number of CO molecules adsorbed on separate faces has also increased, which leads to an enhancement of the static interaction between them. Similar spectra of adsorbed CO were observed on samples activated at 723 and 773 K. In the latter case, the band characterizing CO adsorbed on Lewis acid sites is located at 2164 cm-1 at saturation. A quality difference in the spectra is observed with the sample activated at 873 K (Figure 9). In this case, the band characterizing CO adsorbed on Lewis acid sites is more intense as compared to that of the sample activated at 773 K. The band position at saturation is 2160 cm-1 and 2179 cm-1 at low coverages. As discussed above, this evidences an enhancement of the interaction between the adsorbed molecules which was interpreted by an increase of the average area of the microcrystal planes on which CO is adsorbed. Another peculiarity of the sample activated at 873 K is the appearance of a very weak band at ∼2195 cm-1 which characterizes another family of Lewis acid sites. This band is more stable toward evacuation than the band at 2179-2160 cm-1. Indeed, it is expected for carbonyls without back π-donation that the higher the frequency, the higher is the stability of the species.10 The experiments with the sample activated at 973 K revealed similar results (Figure 10). In this case the principal CO band is located at 2159 cm-1 at saturation coverage and its intensity appears to be slightly lower than that measured for the sample activated at 873 K. This suggests initial stages of sintering at this temperature consistent with the observed decrease of the BET surface area (see Figure 6). The higher-frequency shoulder is more intense, and computer deconvolution reveals the band maximum to be located at 2199 cm-1. Another peculiarity of the spectrum of this sample is the appearance of two very weak bands at 2099 and 2082 cm-1. These bands may be assigned to carbonite structures and/or species derived from these, such as the trimer (C3O4),2-11,16-18,20 and evidence the formation of a separate MgO phase on the sample thus treated, suggesting partial segregation of both oxide phases. 3.5. Coadsorption of 12CO and 13CO. To obtain more information on the nature of the CO adsorption sites, 12 CO-13CO coadsorption experiments were carried out. It is well-known for CO adsorbed on oxide surfaces that the red shift of carbonyl bands with the increasing coverage

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metal surfaces. However, a similar shift has been reported with oxide systems if no π-back-donation occurred.10 Thus, for an anatase sample, the dynamic shift of the CO band at 2180 cm-1 was reported to be +3-4 cm-1.32 For the band at 2190 cm-1 observed on ZrO2, a dynamic shift of +3 cm-1 was reported,33 and for the band at 2178 cm-1 on Fe2O3, a shift of +2 cm-1 was reported.34 However, a greater dynamic shift (+5 cm-1) was measured with alumina for the band at 2202 cm-1.35 The observation of dynamic shifts with CO adsorbed on our sample evidences that the adsorption sites are located on flat crystal planes, as proposed above. 4. Discussion Figure 10. FTIR spectra of CO adsorbed at 85 K on MgAl4HT activated under vacuum at 973 K for 1 h: equilibrium pressure of 450 (a), 100 (b), and 50 Pa CO (c) and gradual decrease of the coverage during evacuation down to ∼10-2 Pa (d-q).

Figure 11. FTIR spectra of 12CO and 13CO coadsorbed at 85 K on MgAl4-HT activated under vacuum at 973 K for 1 h: equilibrium pressure of 130 Pa 12CO (a) and gradual increase of the partial 13CO pressure at a total equilibrium pressure of ∼130 Pa (b-e).

could be a result of two opposing effects: a static (chemical) shift and a dynamic shift.10 The static red-shift occurs as a result of a gradual decrease in the Lewis acid strength caused by the donation of electron density from the adsorbate to the solid. The dynamic blue-shift results from a dipole-dipole interaction between neighboring CO oscillators. This effect only occurs if several requirements are fulfilled: (i) CO molecules must vibrate with the same intrinsic frequency, (ii) they have to be oriented parallel, and (iii) they must be adsorbed on a flat plane. If some of the CO molecules are labeled, they will vibrate with different frequency, and hence, no dynamic shift is expected. In general, the experimental frequencies are compared for adsorbed 12CO-13CO isotopic mixtures enriched in one of the two components with those of the pure isotope at saturation. Some spectra of 12CO-13CO coadsorption on the sample activated at 973 K are presented in Figure 11. The 12CO band in the presence of 100 Pa CO equilibrium pressure is located at 2162.5 cm-1 (Figure 11, spectrum a). Addition of small doses of 13CO and subsequent dilution (so that the total CO pressure is around 100 Pa) result in a shift of the 12CO band maximum to 2159 cm-1 (Figure 11, spectrum e). The higher frequency observed after adsorption of pure 12CO is due to the dipole-dipole interaction between adsorbed molecules. This interaction is hindered when the amount of coadsorbed 13CO increases. Thus, the dynamic shift calculated here is around +3.5 cm-1. This shift is small as compared to the case of CO adsorbed on

Performing CO adsorption at low temperature, Coluccia et al.12,13 have detected three kinds of Mg2+ Lewis acid sites. The weakest sites are attributed to pentacoordinated cations on crystal planes and produce the principal carbonyl band at 2152 cm-1. The carbonyl species are easily removed by evacuation. In addition, a weaker band at 2159 cm-1, assigned to CO bonded to four-coordinated sites, and a very weak band at 2200 cm-1 (CO bonded to three-coordinated Mg2+ ions) have been reported. A peculiarity of CO adsorption on MgO is the formation of monomeric and polymeric carbonite structures even at low temperatures.11,16-18,20 It should be noted that a band at 2033 cm-1 is supposed to characterize OdCdCO22species20 while a band at 2106-2095 cm-1 should be caused by trimeric chains.16 It has already been mentioned (vide supra) that the bands arising after CO adsorption on aluminas can generally be divided into three groups: HF bands at 22452215 cm-1; MF bands around 2200 cm-1; and LF bands around 2165 cm-1.18,21-27 There are two general opinions on the assignments of the above bands. Some authors believe that the HF band characterizes CO on (Al3+)tet sites, whereas the MF bands correspond to CO on (Al3+)oct sites due to the differences in the electrophilicity of the Al3+ cation in the two different coordination environments.25,27,28 Another group of authors21,23,24 proposes that the HF bands characterize (Al3+)def defect sites, whereas the MF and LF bands characterize CO on (Al3+)tet and (Al3+)oct sites, respectively. On the basis of the above considerations, we assign the principal carbonyl band (∼2160 cm-1) on our sample to CO adsorbed on Mg2+ sites. Some of these sites are detected on the surface even after activation at 573 K. Note, however, that the acid strength of the sites appears to be somewhat higher than that observed with pure MgO. This phenomenon can be explained by the hypothesis proposed by Tanabe et al.36 for the increase of the Lewis acid strength in mixed oxides. The suggestion that the Mg2+ cations may be located on the surface of a mixed phase is also supported by the absence of carbonites after CO adsorption on the samples activated at temperatures up to 873 K. Note, however, that the absence of carbonites can alternatively be explained by a blocking of the most basic surface sites by residual carbonates, which in fact are still detectable, as indicated by spectrum f in Figure 3. (32) Hadjiivanov, K.; Lamotte, J.; Lavalley, J.-C. Langmuir 1997, 13, 3374. (33) Morterra, C.; Bolis, V.; Fubini, B.; Orio, L.; Williams, T. B. Surf. Sci. 1991, 251/252, 540. (34) Zecchina, A.; Scarano, D.; Reller, A. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2327. (35) Tsyganenko, A.; Zverev, S. React. Kinet. Catal. Lett. 1988, 36, 269. (36) Tanabe, K. Solid Acids and Bases; Kodansha: Tokyo, Academic Press: New York, London, 1970.

CO Adsorption on MgAl-Hydrotalcite

At higher activation temperatures some free Al3+ sites appear on the sample surface and are monitored by CO stretching bands at 2200-2195 cm-1. Finally, it should be noted that the thermal treatment of our sample at temperatures up to 873 K creates Lewis acidity of the sample without the appearance of strong surface basicity, typically characterizing MgO.

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Acknowledgment. This work was financially supported by the Deutsche Forschunggemeinschaft (SFB 338) and the Fonds der Chemischen Industrie. S.K. and K.H. are indebted to the Alexander-von-Humboldt Foundation, Bonn, Germany, for research grants. LA030019Q