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FTIR Study of Low-Temperature CO Adsorption on Pure and Ammonia-Precovered TiO2 (Anatase) Konstantin Hadjiivanov,*,† Jean Lamotte,‡ and Jean-Claude Lavalley‡ Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria; and Laboratoire Catalyse et Spectrochimie, URA CNRS D0414, ISMRA, 6, Bld. du Mare´ chal Juin, 14050 Caen Cedex, France Received December 16, 1996. In Final Form: March 12, 1997X Low-temperature CO adsorption on TiO2 (anatase) has been investigated by FTIR spectroscopy on (i) a pure sample, (ii) a sample on which the sites for CO adsorption at room temperature are blocked by ammonia, and (iii) anatase whose surface is covered by ammonia. Adsorption of small amounts of CO at 100 K on anatase leads to the appearance of two bands at 2210 and 2192 cm-1 due to CO adsorbed on two kinds (R and β′, respectively) of Ti4+ sites (the same species are also observed when adsorbing CO at room temperature). The increase of the introduced CO amount involves sites (β′′-sites) that are inert at room temperature. The CO molecules adsorbed on β′′-sites interact with CO molecules preadsorbed on β′ sites, as a result of which the two adsorption forms produce a common absorption band whose maximum is shifted to 2179 cm-1 at higher coverage. With increasing amount of adsorbed CO, one more kind (γ) of Ti4+ site with very weak acidity is detected, the corresponding absorption band being at 2165 cm-1. Under CO equilibrium pressure two additional reversible adsorption forms appear: CO H-bonded to surface hydroxyl groups (ν(CO) at 2155 cm-1) and physically adsorbed CO (band at 2138 cm-1). Simultaneously with the appearance and increase in intensity of the band at 2155 cm-1, a broadening and shift by about -115 cm-1 of the bands for the surface hydroxyl groups occur. Adsorption of a 13CO shows that the shifts of the bands at 2210 and 2192 cm-1 are mainly of a static type (-4 and -17 cm-1, respectively), the dynamic components being only +4 cm-1 for the β-carbonyls and not measurable for the R-carbonyls. CO adsorption on reduced anatase indicates the formation of the same types of carbonyls. However, in this case, part of the introduced CO probably dissociates and oxidizes the Ti3+ ions into Ti4+. Low-temperature CO adsorption on anatase on which the sites for CO adsorption at room temperature (the R and β′ sites) are preliminary blocked by ammonia leads to the formation of carbonyls on β′′ and γ sites (ν(CO) at 2177 and 2156 cm-1, respectively), as well as of CO adsorbed on OH groups and physically adsorbed CO. In this case the shift of the OH stretching modes is still -115 cm-1. Low-temperature CO adsorption on anatase fully precovered with ammonia shows the appearance of weakly bound forms only: a part of the carbonyls on γ-sites (ν(CO) shifted to 2151 cm-1), CO adsorbed on hydroxyl groups, and physically adsorbed CO. However, in this case the shift of the ν(OH) stretching modes is only -65 cm-1. The nature of the different kinds of active sites for adsorption of ammonia and CO on anatase is discussed.
Introduction Among the numerous papers dealing with the surface properties of transition metal oxides, a large number are devoted to titania, especially to its anatase modification. This is due to its interesting properties as a catalyst support.1-7 Thus, anatase-supported vanadia catalysts have found industrial application for partial oxidation of o-xylene to phthalic anhydride2 and are also active in other partial oxidation reactions,3 in the ammoxidation of aromatic hydrocarbons,4 and in the selective catalytic reduction (SCR) of nitrogen oxides.5,6 Titania-supported metals are typical for the observation of the so-called “strong metal-support interaction” (SMSI), which is one †
Bulgarian Academy of Sciences. URA CNRS D0414. X Abstract published in Advance ACS Abstracts, June 1, 1997. ‡
(1) Matsuda, S.; Kato, A. Appl. Catal. 1993, 8, 149. (2) Bond, G. C. J. Catal. 1989, 116, 531. Nikolov, V.; Klissurski, D.; Anastasov, A. Catal. Rev.sSci. Eng. 1991, 33, 319. Pinaeva, L.; Lapina, O.; Mastikhin, V.; Nosov, A.; Balzhinimaev, B. J. Mol. Catal. 1994, 88, 311. (3) Bond, G. C.; Sarkany, J.; Parfitt, G. D. J. Catal. 1979, 57, 476. (4) Sanati, M.; Anderson, A. J. Mol. Catal. 1990, 59, 233. Andreikov, E. I. React. Kinet. Catal. Lett. 1983, 22, 351. (5) Schneider, H.; Tschudin, S.; Schneider, M.; Wokaun, A.; Baiker, A. J. Catal. 1994, 147, 5. Kantcheva, M.; Bushev, V.; Klissurski, D. J. Catal. 1994, 145, 96. (6) Centi, G.; Nigro, C.; Perathoner, S.; Stella, G. Catal. Today 1993, 17, 159. Hilbrig, F.; Schmelz, H.; Kno¨zinger, H. In New Frontiers in Catalysis; Guczi, L., Solymosi, F., Teteni P., Eds.; Elsevier: Amsterdam, 1993; p 1351. (7) Haller, G. L.; Desasco, D. F. Adv. Catal. 1989, 36, 173. Vannice, M. A. Catal. Today 1992, 12, 255. Bond, G. C. In Metal-Support and Metal-Additive Effects in Catalysis; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1982; p 1.
S0743-7463(96)02104-X CCC: $14.00
of the most studied phenomena in catalysis.7 In addition, TiO2 belongs to the small number of oxides which exhibit a superacidity after modification by sulfates.8 Titania itself is an efficient catalyst in the Claus reaction1,9 and in a lot of photocatalytic processes such as water decomposition, hydrogenolysis and hydrogenation of alkenes and alkynes, isomerization of butenes, and deposition of noble metals from solutions containing their ions.10 The surface properties of anatase have been investigated by IR spectroscopy in a large number of studies.11-30 Most authors agree that two kinds of surface hydroxyl groups exist on the anatase surface; they are characterized by (8) Hadjiivanov, K.; Davydov, A.; Klissurski, D. Izv. Khim. (Bulg. Akad. Nauk.) 1988, 21, 516. Tanabe, K. Mater. Chem. Phys. 1987, 17, 1. Waqif, M.; Bachelier, J.; Saur, O.; Lavalley, J.-C. J. Mol. Catal. 1992, 72, 127. (9) Steijus, M.; Mars, P. Ind. Eng. Chem. Prod. Res. Dev. 1977, 16, 35. (10) Anpo, M. Res. Chem. Intermed. 1989, 11, 67. Kalyanasundaram, K. In Energy Resources through Photochemistry and Catalysis; Gra¨tzel, M., Ed.; Mir: Moscow, 1986. Hermann, J. M.; Disdier, J.; Pichat, P. J. Catal. 1988, 113, 72. Borgarello, E.; Serpone, N.; Emo, G.; Harris, R.; Pelizzetti, T. E.; Minero, C. Inorg. Chem. 1986, 25, 4499. (11) Ramis, G.; Busca, G.; Lorenzelli, V. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1591. (12) Yi, L.; Ramis, G.; Busca, G.; Lorenzelli, V. J. Mater. Chem. 1994, 4, 175. (13) Busca, G.; Saussey, H.; Saur, O.; Lavalley, J. C.; Lorenzelli, V. Appl. Catal. 1985, 14, 245. (14) Hadjiivanov, K.; Davydov, A.; Klissurski, D. Kinet. Katal. 1988, 29, 161. Hadjiivanov, K.; Klissurski, D.; Davydov, A. J. Chem. Soc., Faraday Trans. 1 1988, 84, 34. Hadjiivanov, K.; Klissurski, D. Chem. Soc. Rev. 1996, 25, 61. (15) Hadjiivanov, K.; Klissurski, D.; Busca, G.; Lorenzelli, V. J. Chem. Soc., Faraday Trans. 1991, 87, 174.
© 1997 American Chemical Society
Pure and Ammonia-Precovered TiO2 (Anatase)
bands at 3675-3665 and 3720-3715 cm-1.13,17-19 However, evacuation at high temperatures results in a more complicated spectrum, indicating the presence of a series of hydroxyl groups in the 3740-3640 cm-1 spectral region.15,16 In general, the surface acidity of anatase is of the Lewis type. Indeed, ammonia and pyridine adsorptions lead to the appearance of IR bands typical of coordinatively bound species. Some authors13,20 have also reported the existence of protonic acid sites on anatase, but in most cases, they result from the presence of impurities such as surface sulfates8,13 or silica.13,15 The existence of some amount of strong acid-basic pairs on anatase has recently been evidenced by the observation of dissociative ammonia adsorption species.16 Carbon monoxide is the most used probe molecule for sensitive determination of the strength of the aprotonic acid sites. Almost all authors studying the CO adsorption on anatase at room temperature (rt) report the existence of two types of adsorption sites differing in enthalpy of adsorption.12-14,16-19,22-24 Carbon monoxide adsorption on the most energetic sites (R-sites) is characterized by a band at 2208 cm-1, which is shifted to 2206 cm-1 when the equilibrium pressure increases. Carbon monoxide adsorption on the less energetic sites (β-sites) gives rise to an absorption band at 2190 cm-1 shifted to 2185 cm-1 with increasing coverage. Both types of species desorb after evacuation. As a rule, the concentration of β-sites is much higher than that of R-sites. It has been established that there was no direct relationship between the degree of anatase dehydroxylation and the concentration of the different Lewis acid sites.23 A study on the coadsorption of CO and NH3 at rt has shown that only part of the ammonia adsorption sites are able to coordinate CO at ambient temperature.16 When CO is adsorbed with formation of a σ-bond only and the adsorption is weak, as is the case of Ti4+ ions, performing the experiments at a low temperature permits the detection of a larger number of adsorption sites. Lowtemperature CO adsorption on anatase or on a fumed titania from Degussa (which contains some rutile) has been investigated in several papers.26-31 It is established that at high coverage one band with a maximum at ca. 2180 cm-1 predominates in the spectrum. In most papers this band is assigned to CO adsorbed on β-sites (its initial position at low coverages is at about 2190 cm-1). Soltanov et al.30 have observed a change in slope of the adsorption isobar of these species and have proposed the existence of two adsorption forms of CO differing in energetic (16) Hadjiivanov, K.; Saur, O.; Lamotte, J.; Lavalley, J.-C. Z. Phys. Chem. (Munich) 1994, 187, 281. (17) Yates, D. J. C. J. Phys. Chem. 1967, 65, 746. (18) Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chem. 1971, 75, 1216 and 1221. (19) Tanaka, K.; White, J. M. J. Phys. Chem. 1984, 88, 4708. (20) Morishige, K.; Kanno, F.; Ogawara, S.; Sasaki, S. J. Phys. Chem. 1985, 89, 4404. (21) Kno¨zinger, H. Z. Phys. Chem. (Frankfurt) 1975, 69, 119. (22) Bolis, V.; Fubini, B.; Garrone, E.; Morterra, C.; Ugliengo, P. J. Chem. Soc., Faraday Trans. 1992, 88, 391. Bolis, V.; Fubini, B.; Garrone, E.; Morterra, C. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1383. (23) Morterra, C. J. Chem. Soc., Faraday Trans. 1 1988, 84, 1617. (24) Davydov, A. A. Adsorpt. Adsorbents 1977, 5, 83. (25) Went, G. T.; Bell, A. T. Catal. Lett. 1991, 11, 111. (26) Spoto, G.; Morterra, C.; Marchese, L.; Orio, L.; Zecchina, A. Vacuum 1990, 41, 37. (27) Tsyganenko, A. A.; Denisenko, L. A.; Zverev, S. M.; Filimonov, V. N. J. Catal. 1985, 94, 10. Zverev, S. M.; Smirnov, K. S.; Tsyganenko, A. A. Kinet. Katal. 1988, 29, 1439. (28) Lange, F.; Hadjiivanov, K.; Schmelz, H.; Kno¨zinger, H. Catal. Lett. 1992, 16, 97. (29) Kantcheva, M.; Hadjiivanov, K.; Davydov, A.; Budneva, A. Appl. Surf. Sci. 1992, 55, 49. (30) Soltanov, R.; Paukshtis, E.; Yurtchenko, E. Kinet. Katal. 1982, 23, 164. (31) Zaki, M. I.; Kno¨zinger, H. Mater. Chem. Phys. 1987, 17, 201.
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characteristics but having very close ν(CO) values. When an equilibrium pressure is maintained, other bands for adsorbed CO also appear, their maxima being situated at about 2175, 2165, 2155, and 2140 cm-1.26,28-31 The latter band characterizes physically adsorbed CO, while the band at 2155 cm-1 corresponds to CO adsorbed on surface hydroxyl groups.31 The assignment of the other bands is less obvious. In most papers, the presence of two kinds of Lewis acid sites as detected on the anatase surface by ammonia or CO adsorption has been explained by the existence of fourfold and fivefold coordinated titanium cations situated on different faces (edges, corners, steps) of the oxide crystallites.13,14,16,19 However, the analysis of the literature data reveals a significant structural heterogeneity of titanium dioxide which cannot be ascribed to a difference in the coordination number of the titanium cations only. Recent data show that ammonia is not a sufficiently sensitive probe molecule to distinguish between the different types of Lewis acid sites on anatase and that some titanium cations are inert toward CO adsorption at room temperature.16 It is not clear whether these ions can be monitored by CO adsorption at low temperature. The purpose of the present paper is a detailed study of the structural and induced heterogeneity of the anatase surface including the effect of adsorbed molecules on the properties of the unoccupied adsorption sites and on the surface hydroxyl groups. For this purpose, CO adsorption has been investigated on (i) pure anatase, (ii) anatase partly covered with ammonia (to a degree of coverage sufficient to poison the sites active in CO adsorption at room temperature), and (iii) anatase fully covered with ammonia. Experimental Section Titania was prepared by hydrolysis of titanium tetrachloride with ammonia, followed by calcination at 400 °C for 1 h. The sample thus obtained consisted of anatase and had a specific surface area of 63 m2 g-1. The IR spectra were recorded by a Nicolet-MX-1 FTIR spectrophotometer having a resolution of 4 cm-1. The number of the scans used was usually 512. However, in some cases when kinetic effects were studied, only 56 scans were collected. An IR cell was specially constructed for the low-temperature measurements. This cell was equipped with a gas burette, allowing the introduction of well-known amounts of adsorbate. The vacuum/ adsorption apparatus used ensured a vacuum better than 10-5 Torr. Self-supporting disks were prepared from the sample and activated in the IR cell by treatment under oxygen (100 Torr, 1 h, 400 °C) followed by 1 h of evacuation at the same temperature. The specific surface area was measured by low-temperature nitrogen adsorption. X-ray phase analysis was performed with a DRON-3 diffractometer using Cu KR radiation.
Results IR Spectrum of Anatase. The IR spectrum of the activated sample is similar to the anatase spectra reported in the literature. Below 1100 cm-1 the sample is opaque to the IR beam due to the self-absorption of anatase. Above 1300 cm-1 the transmittance decreases again due to the significant light-scattering. Two main bands at 3720 and 3675 cm-1 are observed within the ν(OH) region. They characterize OH stretching modes of two types of different surface hydroxyl groups.13-20,22-24,26 Ammonia Adsorption. Since the room temperature adsorptions of NH3 and CO on our anatase sample have been investigated in detail,16 only the main results directly associated with the aim of this study will be reported here. Successive introductions of ammonia portions on the sample leads to the appearance and increase in intensity of bands characterizing coordinatively bound ammonia:
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Figure 1. FTIR spectra of CO adsorbed on anatase at 100 K. Introduction of 0.5 (a), 1 (b), 2 (c), 3 (d), 4 (e), 5 (f), 6 (g), 7 (h), 10 (k), 11 (l), and 13 (m) µmol of CO. 14-16,32,33 ν (NH ) at 3390 cm-1, ν (NH ) at 3260 cm-1, δ as 3 s 3 as (NH3) at 1607 cm-1, δs(NH3) at 1170 cm-1, and 2δas(NH3) (in a Fermi resonance with νs(NH3)) at 3150 cm-1. With increasing coverage the band at 1170 cm-1 is shifted to 1147 cm-1. When coverage reaches its maximum, a band appears at 1220 cm-1; simultaneously a new type of hydroxyl group, characterized by ν(OH) at 3568 cm-1 (ν(18OH) at 3558 cm-1) is also observed. On the basis of the behavior of the bands, we have proposed that they are due to the formation of dissociative forms of adsorbed ammonia.16 When equilibrium ammonia pressures are maintained, the OH band at 3568 cm-1 disappears. Concomitantly, a weak band at 1465 cm-1 appears. This band is typical of ammonium ions32,33 (δas(NH4+)) and shows that the hydroxyl groups that disappeared exhibit a protonic acidity. It should be noted that the intensity of the bands characterizing coordinatively bound ammonia species is considerably higher than that of the other bands. Adsorption of CO at Room Temperature. The adsorption of CO on activated anatase under different pressures leads to the formation of two IR bands which correspond to two types of Ti4+-CO complexes differing in enthalpy of adsorption.12-19 The band at 2208 cm-1 (corresponding to R-carbonyls) characterizes the more energetic form. Its intensity increases up to a maximum reached under an equilibrium pressure of about 2 Torr. Increasing the equilibrium pressure shifts it from 2208 cm-1 (at 0.5 Torr of CO) to 2206 cm-1 (at 32 Torr of CO). The weaker form is characterized by a band at 2192 cm-1 at low coverages, its maximum being situated at 2186 cm-1 at high pressures. The corresponding sites do not seem fully occupied even under an equilibrium pressure of 64 Torr of CO. Low-Temperature CO Adsorption. The introduction of 0.5 µmol of CO into the cell results in the appearance of two IR bands with maxima at 2210 and 2191 cm-1 (Figure 1). These bands can be assigned to carbonyls formed on R- and β-Ti4+ sites, respectively. The increase in amount of introduced CO leads to a broadening and a weak intensity increase of the higher frequency band and a gradual shift of its maximum to the lower frequencies. This shift does not depend on the band intensity and is rather a function of the total amount of adsorbed CO.
(32) Davydov, A. A. IR Spectroscopy Applied to Surface Chemistry of Oxides; Nauka: Novosibirsk, 1984. (33) Nakamoto, K. IR Spectra of Inorganic and Coordination Compounds; Mir: Moscow, 1966.
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Thus, when 6 µmol of CO is introduced, the corresponding maximum is situated at 2208 cm-1, whereas upon the introduction of 12 µmol of CO it is observed at 2206 cm-1. In contrast to this band, the intensity of that at 2191 cm-1 continues to increase until the introduction of about 10 µmol of CO. Simultaneously, its maximum is shifted to 2179 cm-1. It is interesting to note that at lower coverages (less than 6 µmol of CO) each preceding band is included in the contours of the next band (obtained after introduction of a new CO portion), suggesting that the shift may be interpreted as due to the overlapping of each preceding band with a new band situated at a lower frequency. However, at higher coverages there is a real shift, i.e. the adsorption of each new CO portion is associated with a loss of a high-frequency component of the preceding band. This real shift results from the induced frequency change of the preadsorbed carbon monoxide species. The integral absorption of the 2179 cm-1 band (detected at low temperature) considerably exceeds that of the 2188 cm-1 band resulting from CO adsorption at room temperature upon saturation. This evidences that additional types of sites are involved in the adsorption at low temperature. Further on, these sites will be denoted as β′′ sites in contrast to the β′ sites that adsorb CO at room temperature. CO adsorbed on β′′ sites interacts with CO molecules preliminarily adsorbed on the β′-sites. As a result, both types of carbonyls produce a common absorption band. Extrapolation shows that at zero coverage the band characteristic of CO adsorbed on R-sites has a maximum at 2210 cm-1, whereas, for β′-sites, this value is 2192 cm-1. Along with the intensity increase of the band at 21912179 cm-1, the appearance and intensity rise of a weak band with a maximum at about 2127 cm-1 are also observed. This band, in good agreement with the expected isotopic shift, may be attributed to 13CO molecules (natural abundance in CO) adsorbed on β-sites. The increase in the amount of CO introduced beyond 10 µmol is accompanied by the appearance of a new absorption band with a maximum at 2165 cm-1. This band cannot be attributed to CO adsorbed on surface hydroxyl groups because no concomitant changes in the OH stretching region can be established. The formation of dicarbonyls is also excluded, since (i) no other band can be associated with it and (ii) the intensity increase of the 2164.5 cm-1 band does not occur at the expense of another band.34 This indicates that the corresponding CO adsorption species are formed on another kind of site from the anatase surface. Further on, these sites will be denoted as γ-sites. Under an equilibrium pressure of 1 Torr of CO, two additional bands at 2155 and 2138 cm-1 are observed (Figure 2). The intensity change of the former band with the change in CO equilibrium pressure occurs simultaneously with a shift of the bands characterizing surface hydroxyl groups (Figure 3). Hydrogen bonding occurs, explaining also the broadening and the higher integral absorption of the new bands due to hydroxyl groups. The frequency shift can be estimated to ca. -115 cm-1. As expected, evacuation of the sample results in the disappearance of the 2155 cm-1 band and restoration of the spectrum of the unperturbed hydroxyl groups. The 2155 cm-1 band can be ascribed to CO adsorbed on OH groups.31 As for the 2138 cm-1 band, it drops in intensity with the equilibrium pressure and vanishes after short evacuation, which confirms its assignment to physically adsorbed CO.26-29 (34) Hadjiivanov, K.; Klissurski, D. React. Kinet. Catal. Lett. 1991, 44, 229.
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Figure 4. Dependence of the integral absorption of the bands for R-, β′-, and β′′-Ti4+ carbonyls (for the notations see the text) on the amount of CO introduced into the IR cell at 100 K.
Figure 2. FTIR spectra of CO adsorbed on anatase at 100 K. Equilibrium pressure of 40 (a), 1 (b), and 0.1 (c) Torr of CO and time evolution under dynamic vacuum (d-f).
Figure 3. Changes in the ν(OH) region after low-temperature CO adsorption on anatase samples differently precovered by ammonia. Pure anatase sample in the presence of 4 Torr of CO (a) and after evacuation (b); anatase with preadsorbed 5.0 µmol of NH3 in the presence of 4 Torr of CO (c) and after evacuation (d); anatase fully covered by ammonia in the presence of 4 Torr of CO (e) and after evacuation (f).
Evacuation at low temperature leads also to an essential decrease in intensity of the 2179 cm-1 band and a back shift of its maximum to higher frequencies. The intensity decrease of this band reaches a limit of ca. 45% of its initial integral absorption, an intensity close to that of the band due to CO adsorbed on β′-sites at room temperature and high equilibrium pressures. Hence, even at 100 K, the CO adsorption on the β′′-sites is reversible. Figure 4 illustrates the dependence of the integral absorption between 2220 and 2170 cm-1 on the amount of CO introduced into the IR cell. Evidently, up to ca. 3.5 µmol of adsorbed CO, the variation is linear. This indicates close values of the extinction coefficients of the carbonyls formed on the R- and β′-sites. The change in shape of the curve above this value is due to the reversibility of the CO adsorption at high coverages, especially when carbonyls adsorbed on β′′-sites are formed. Adopting the same molar absorptivity coefficients for all bands in the 2210-2175 cm-1 spectral region, the total amount of CO adsorbed
under high pressure of CO on R-, β′-, and β′′-sites can be estimated to be ca. 6.5 µmol. This corresponds to concentrations of 0.2 R-Ti4+ nm-2 and 3.4 β-Ti4+ nm-2. Low-Temperature CO Adsorption on Reduced Anatase. Anatase reduction with hydrogen (1 h, 400 °C, 100 Torr of H2, followed by 1 h evacuation at the same temperature) leads to blue coloring of the sample, which proves the formation of Ti3+. Simultaneously the absorbance at 1300 cm-1 (the wavenumber of the highest transmittance) significantly increases (from 1.05 to 1.8), which is typical for reduced n-type semiconductors. Adsorption of 0.5 µmol of CO produces weak bands characteristic of R- and β′-Ti4+-CO carbonyls with maxima at 2208 and 2190 cm-1, respectively. Simultaneously, the total transmittance of the sample increases. Introduction of a second portion of CO causes an intensity increase of the bands due to titanium carbonyls and a new increase in transmittance. Additional CO portions mainly lead to an enhanced intensity of the Ti4+-CO bands. No other bands due to surface carbonyls are observed at low coverage. It is worth noting that during CO adsorption the sample turns white again. The dependence of the integral absorption on the amount of CO introduced is not linear in this case. Extrapolation shows that at least one CO portion has been consumed for other purposes than for the formation of Ti4+-CO carbonyls. Low-Temperature Adsorption of 13CO. The adsorption of 13CO (containing 93% 13CO, 5% 12CO, and 2% 13C18O as a contaminant) has also been studied. Similarly to the previous case, the first portions lead to the formation of two absorption bands, the maxima being now at 2160 and 2141 cm-1 (Figure 5), which is in good agreement with the expected value of the isotopic shift.33 The increase in amount of adsorbed CO leads to a quick saturation of the band at 2160 cm-1 and to an enhanced intensity of that at 2141 cm-1. The initial wavenumber of the β′Ti4+-13CO complexes, at 2141 cm-1, shifts to 2131 cm-1 after adsorption of 3 µmol of CO. With further addition of CO, the intensity of this band increases, but no substantial shift of its maximum is observed. At relatively low concentrations of adsorbed CO, a band at 2175 cm-1, which characterizes 12CO adsorbed on β-sites, is registered. At an equilibrium pressure of 2 Torr of CO, this band is found at 2175 cm-1. A very weak band at 2206 cm-1 characterizing 12CO adsorbed on R-sites has also been detected. At higher coverages 13CO adsorbed on γ-sites is also visible at 2116 cm-1. The adsorption of isotope mixtures of 12CO and 13CO is generally used to determine the values of the dynamic and static shifts of the CO bands.27 It is known that the shift of a band due to CO adsorbed on a definite kind of site upon an increase in coverage is caused by lateral (adsorbate-adsorbate) interactions, which are generally
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Figure 6. FTIR spectra of CO (16 Torr) adsorbed at rt on pure anatase (a) and on a sample with preadsorbed 1 (b), 2 (c), 3 (d), 4 (e), and 5 (f) µmol of NH3. The spectra in the B-part of the figure are recorded after evacuation of the carbon monoxide. Figure 5. FTIR spectra of 13CO adsorbed on anatase at 100 K: amount of CO introduced into the IR cell, 0.5 (a), 1 (b), 1.5 (c), 2 (d), 3 (e), 4 (f), 9 (g), 14 (h) µmol of CO; equilibrium CO pressure, 2 Torr (k).
decomposed into (i) dynamic interactions essentially of dipole-dipole type and (ii) static interactions (chemical shift).27,35 The former one results in an increase of the CO stretching frequency, whereas the latter leads to the opposite effect. Dilution of 12CO with 13CO hinders the dipole-dipole interactions, eliminating the dynamic shift. From our experiments it can be concluded that the dynamic shift for 12CO adsorbed on the (β′ + β′′)-sites is +4 cm-1. This is in good agreement with the reported value of 3 cm-1.27 Taking into account the overall and the dynamic shift, the static shift of the latter band was calculated to be -17 cm-1. The dynamic shift for CO adsorbed on the R-sites, if any, is within the framework of the experimental error, whereas the static shift is -4 cm-1. It was not possible to estimate the shifts for CO adsorbed on the γ-sites because of the overlapping of the γ-Ti4+-12CO band with the band due to 13CO adsorbed on R-sites. As for the weak bands at 2108, 2091, and 2075 cm-1 observed in Figure 5k, they can be assigned to 13CO adsorbed on hydroxyl groups, physisorbed 13CO, and the 13 18 C O contaminants in our isotopic mixture, respectively. In order to establish the mobility of the adsorbed carbonyls, the following experiment was performed. Some amount of 12CO was introduced to a sample whose surface was covered by strongly adsorbed 13CO species (2 Torr at 100 K, followed by evacuation), and then the sample was evacuated. The resulting spectrum (not shown for brevity) showed almost equal intensities of the sets of bands due to 13CO and 12CO adsorbed on the R- and β-sites. These results evidence the mobility of all of the CO adsorbed species. Low-Temperature CO Adsorption on Anatase Partially Covered by Ammonia. The purpose of the further experiments is to study the adsorption of CO on anatase on which both kinds of sites involved in CO adsorption at room temperature (R- and β′-sites) are blocked by ammonia. We had established earlier16 that the occupation of these sites was achieved before saturation of the surface by coordinatively bound ammonia. To determine the amount of NH3 necessary to prevent CO adsorption at rt, small doses of ammonia have been adsorbed successively at room temperature and then CO was introduced (Figure 6). It is found that, with increasing amounts of preadsorbed ammonia, the intensity of the CO bands gradually decreases. After the fifth ammonia (35) Scarano, D.; Zecchina, A.; Reller, A. Surf. Sci. 1988, 198, 11.
Figure 7. FTIR spectra of CO adsorbed at 100 K on anatase, precovered with 5 µmol of NH3: amount of introduced CO, 1 (a), 2 (b), and 5 (c) µmol of CO; at an equilibrium CO pressure of 0.1 (d), 2 (e), and 10 (f) Torr; and after a short evacuation (g).
portion (5 µmol of NH3 introduced), no CO bands are practically detected. Thus, we can conclude that under these conditions almost all of the R- and β′-sites are blocked by ammonia. Introduction of successive CO portions at low-temperature on this sample partially covered by ammonia leads initially to the appearance of a band at 2177 cm-1 followed by another at 2156 cm-1 (Figure 7). The former band is attributed to CO adsorbed on β′′-sites. The band at 2156 cm-1 coincides in position with ν(CO) of H-bonded CO species. However, the respective adsorption form is relatively strong and no shift of the ν(OH) stretching modes occurs at low CO pressure. This suggests that the 2156 cm-1 band results from CO adsorption on γ-sites somewhat affected by preadsorbed ammonia. Indeed, no band at 2165 cm-1 characteristic of CO adsorption on unperturbed γ-sites is detected, and it is unlikely that the low-acidic γ-sites would be occupied by ammonia before the occupation of the stronger β′′-sites. At an equilibrium pressure of 10 Torr of CO, the intensity of the band at 2156 cm-1 increases and its maximum was detected at 2155 cm-1. Simultaneously, the bands characterizing anatase hydroxyl groups are blue-shifted (see Figure 3). Similarly to the case of the pure surface, the average value of this shift is about -115 cm-1. We conclude that when a CO equilibrium pressure is kept in the cell, the band at 2155 cm-1 is due to the overlapping of two bands: one caused by CO adsorbed on γ-sites and the other by CO adsorbed on surface hydroxyls. Low-Temperature CO Adsorption on Anatase Fully Covered by Ammonia. For these experiments, 5 Torr of ammonia was put in contact with an activated
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containing Ti3+ ions, did not lead to the formation of any carbonyl bands at about 2115-2110 cm-1.36 Results from CO adsorption on reduced anatase evidence an oxidation of the sample after the introduction of a small amount of CO. We suggest it results from CO dissociation according to the reaction
Ti3+-0 -Ti3+ + CO f Ti4+-O-Ti4+ + C
Figure 8. FTIR spectra of CO adsorbed on anatase fully covered with ammonia. Equilibrium CO pressure of 4 (a), 2 (b), 1 (c), 0.2 (d), and 0.1 (e) Torr, and after a short evacuation (f).
anatase and then pumped out for 15 min, all this at rt. Low-temperature CO adsorption on such a sample results in two absorption bands with maxima at 2152 and 2141 cm-1 (Figure 8). The latter band decreases faster in intensity with the decrease of the equilibrium CO pressure, and both bands disappear after evacuation. Simultaneously with the decrease of the equilibrium pressure, the maxima of the bands due to the surface hydroxyl groups are blue-shifted (Figure 3). However, in contrast to the previous two cases, this shift is now much smaller (-65 cm-1). As in the previous case, the behavior of the band at 2152 cm-1 (it appears before perturbation of the OH groups) shows that it is due to CO adsorbed on γ-sites strongly affected by ammonia. At higher equilibrium pressures, CO adsorbed on surface hydroxyls participates also in the formation of this band. Comparison of the spectra obtained after CO adsorption on anatase surfaces with different ammonia coverages shows the following peculiarities: The β′′-sites (which are not involved in CO adsorption at room temperature) exhibit a weaker acidity than the β′-sites and are not poisoned on a partially ammoniacovered surface. However, on anatase fully covered by ammonia both β′′- and β′-sites are blocked. The frequency shift of the ν(OH) vibrations for pure and partially ammonia-poisoned surfaces is the same. In the case of surfaces completely covered with ammonia, the ∆ν(OH) shift is lower (see Figure 3). The band at 2165 cm-1 characterizing CO adsorbed on γ-sites is gradually shifted to lower frequencies when the surface is covered by ammonia. Discussion The heterogeneity of the anatase surface has been widely discussed in the literature. The two major problems under consideration are (i) the variety of Lewis acid sites and (ii) the variety of surface hydroxyl groups. It has been established that, in general, the concentration of the Lewis acid sites increases with sample dehydroxylation, but no direct relationship between these two parameters has been found.23 Lewis Acidity of Anatase. There is a general consensus that the Lewis acid sites on anatase detected by adsorption of ammonia or CO are coordinatively unsaturated Ti4+ cations.12-26 Some authors report the existence of Ti3+ ions (CO absorption band at ca. 2115 cm-1).12,13 However, according to Tanaka and White,19 this band is rather associated with an oxidized surface probably containing (bi)carbonates. We observed no CO bands around 2115 cm-1. Accordingly, CO adsorption on a TiO2/SiO2 sample, reduced at 500 °C with hydrogen and
(1)
It should be noted that we have no direct evidence of carbon formation, since it cannot be detected by IR spectroscopy. However, many papers on the SMSI effect considered CO dissociation on titania-supported metal catalysts reduced at a high temperature and containing Ti3+ ions.7 They assumed that CO is dissociated, the carbon atom being localized on the supported metal, while the oxygen atom oxidizes the Ti3+ ions. This mechanism is supported by the well-known promoting action of titania during the hydrogenization of CO over noble metal catalysts.7 Our results suggest that CO can dissociate on reduced anatase even without the participation of a supported metal. We have also shown that partly reduced anatase is oxidized in the presence of oxygen at room temperature, which restores its white color.16 The present results evidence an even stronger reducing ability of partially reduced anatase. This is in agreement with the recent observation of Lu et al.,37 who reported that at rt water oxidized a reduced 110 TiO2 (rutile) plane, evolving hydrogen. On the basis of the above considerations, we can state that the Lewis acid sites monitored by CO adsorption on anatase represent only coordinatively unsaturated Ti4+ ions. Some authors propose that the two types of Lewis acid sites detected by ammonia are identical to those monitored by CO. Primet et al.18 assumed that the stronger sites are formed during desorption of hydroxyl groups, while the weaker sites result from the desorption of water molecules. Moreover, many authors are of the opinion that the strong sites represent four-coordinated titanium cations, while the weaker ones are five coordinated.13-16,19,23 One more assumption has been put forward: depending on the state of the second coordination sphere, five-coordinated ions can differ essentially in electrophilicity and part of them may be inert with respect to CO adsorption at room temperature.14,16 Our recent data,16 as well as the results presented here, show that the sites involved in CO and ammonia adsorption at room temperature are not identical: the CO adsorption sites are only part of the sites for coordinative adsorption of ammonia. This means that the difference in coordination number of the titanium cations is not sufficient to explain the heterogeneity of the Lewis acid sites. The results obtained in the present work evidence a considerable surface heterogeneity of anatase. At least four kinds of CO adsorption forms (on R-, β′-, β′′-, and γ-sites) have been detected. The sites of the highest electrophilicity (R-sites) have to be assigned to highly coordinatively unsaturated titanium cations. According to our calculations, the concentration of these sites is low (about 0.2 site per nm2). Previous estimations done by other methods also led to values of about 0.2-0.3 sites per (36) Fernandez, A.; Leyrer, J.; Gonzalez-Elipe, E. P.; Munuera, G.; Kno¨zinger, H. J. Catal. 1988, 112, 489. (37) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Phys. Chem. 1994, 98, 11733.
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nm2.14,16,38 It should be noted that these values strongly depend on the sample morphology. Thus, some fumed anatase samples presenting a relatively regular crystal shape are characterized by a negligible concentration of R-sites.8,26 In previous works13-14,16 we proposed that the R-sites corresponded to four-coordinated Ti4+ cations on some crystal planes not exposed to a high extent, such as 110 and 113, as well as on some edges (e.g. 100 × 010, 100 × 110, and 101 × 110). The edge localization of part of the R-sites accounts for the shift of the R-Ti4+-CO band wavenumber with the filling of the β-sites. Moreover, monocarbonyls formed on R-sites (having two coordinative vacancies each) will not be parallel, which would lead to the absence of a dynamic shift, as experimentally observed here. The second type of sites detected by CO adsorption are the β′-sites. At room temperature they are not completely occupied even at relatively high pressures. The estimation of the concentration of these sites carried out by different methods (blocking with ammonia; blocking with transition metal cations adsorbed from solutions; measurement of the IR band intensities) indicates that it ranges from 0.6 to 1.2 sites per nm2 according to the sample studied.14,38 The sites occupied by CO after the R- and β′-sites, but only at low temperature, are the β′′-sites. Their existence has been proved in the present paper by CO adsorption on the partially ammonia-poisoned surface. The appearance of a common band for the β′ and β′′ carbonyls could be the reason that many researchers have not distinguished between the two kinds of sites up to now. Let us now discuss the localization of the β′- and β′′sites. According to literature data,35 the observation of a dynamic shift for the 2191-2179 cm-1 band means that the following conditions are fulfilled: (i) the CO molecules vibrate with equal frequencies and (ii) the oscillators are parallel and close to one another. Thus, it appears that the β′ and β′′ sites are located on regular crystal faces. Their difference can be structural or induced. We prefer the latter possibility for two reasons: (i) if the sites are structurally heterogeneous, the CO frequencies on both kinds of sites would differ, which would lead to the absence of a dynamic interaction, and (ii) the Ti4+ cations from the most abundant anatase planes (Figure 9) are identical. Thus, the following model can be proposed. The β′- and β′′-sites are structurally homogeneous. They represent five-coordinated titanium cations situated on the crystal faces that are most characteristic for anatase: 100 (isostructural with 010) and 101 (isostructural with 011). Adsorption of CO at room temperature occurs with occupation of a part of the possible adsorption sites (the β′-sites). This leads to a decrease in acidity of the other half of the sites (the β′′-sites). Adsorption of a CO molecule (at low temperature) on a β′′-site leads to an interaction with the CO preadsorbed on the near β′-sites, and all sites become equivalent. As a result, the CO molecules vibrate with the same frequencies. The concentration of the β′and β′′-sites would be almost equal, as estimated in this study. When adsorbed on β′-sites, ammonia also weakens the acidity of the titanium cations left free but does not prevent CO adsorption on them, as shown by the lowtemperature CO adsorption on anatase partially covered by ammonia. In addition to the R- and β-sites, the results also show the presence of γ-titanium cations. The stretchings of CO adsorbed on these sites is strongly affected by preadsorbed ammonia. Some of these sites are detected (38) Hadjiivanov, K.; Vassileva, E.; Kantcheva, M.; Klissurski, D. Mater. Chem. Phys. 1991, 28, 367. Hadjiivanov, K.; Klissurski, D.; Kantcheva, M.; Davydov, A. J. Chem. Soc., Faraday Trans. 1991, 87, 907.
Hadjiivanov et al.
Figure 9. Scheme of an anatase crystal with exposed 101, 011, and 001 planes. The black circles denote c.u.s. Ti4+ and c.u.s. O2-, whereas the ions from the subsurface layers and the coordinatively saturated surface O2- are presented by gray circles.
even on a surface covered by NH3. This is in agreement with our earlier observation that a small part of the coordinatively unsaturated titanium cations on anatase coordinate ammonia only reversibly at room temperature and therefore are liberated after evacuation.16 Evidently, the γ-sites are of very low acidity and the low stability of the carbonyls formed on them suggests that in this case CO is bonded to the cations as a result of an electrostatic interaction rather than by a σ-bond. Similar bonding has been observed when CO was adsorbed at low temperature on alkali metal cations.39 The concentration of γ-Ti4+ ions on our sample (calculated assuming the same coefficient of molar absorptivity for all carbonyls) is ca. 0.8 site per nm2. The γ-sites can be attributed to five-coordinated Ti4+ ions on the 001 plane and some edges such as 101 × 011 (Figure 9). The acidity of these ions is weaker than the acidity of the β-Ti4+ ions because the γ-titanium cations participate in acid-base rows with coordinatively unsaturated surfaces (cus’s). O2- anions and thus form strong bonds not with one but with two oxygen anions. The irreversibly coordinated ammonia species occupy almost all of the c.u.s. Ti4+ sites, the first to be filled in being the R- and β′-sites followed by the β′′-sites. On partially ammonia precovered surfaces, CO adsorbed on the β′′-sites shows stretches at a frequency similar but lower than that observed with pure surfaces. In contrast, the acidity of the γ-sites is strongly perturbed by preadsorbed ammonia. In other words, the weaker sites are more strongly influenced by preadsorbed molecules. This fact may be due to the stronger bond between the weaker Ti4+ sites and neighboring O2- ions, which favors the transmittance of electrons, and to the electrostatic nature of the Ti4+-CO bond. In some publications26,27 the reported total concentration of CO adsorbed at low temperatures is about 0.5 monolayer. For the different anatase faces the concentration of coordinatively unsaturated titanium cations varies between 4.5 and 7 ions per nm2.14 The concentration of the R- and β-sites is found to be 3.6 Ti4+ per nm2 and, therefore, not far from 0.5 monolayer. The rest of the surface contains γ-sites (where CO probably forms a semilayer) and is occupied by surface hydroxyl groups. Hydroxyl Coverage on Anatase. In some earlier papers, the existence of different hydroxyl groups on (39) Zecchina, A.; Bordiga, S.; Lamberti, C.; Spoto, G.; Carnelli, L.; Ossoe-Arean, C. J. Phys. Chem. 1994, 98, 9577.
Pure and Ammonia-Precovered TiO2 (Anatase)
anatase has been explained by the possibility of their localization on different crystal faces or on different sites of the same plane, however without specifying the surface forms.13,19 However, Primet et al.18 proposed a model based on the prevailing exposition of the 001 face. The IR spectra exhibit two types of surface hydroxyls depending on whether in the neighborhood there is or is not another hydroxyl. Busca et al.13 suggested that the hydroxyl groups on anatase are localized on crystallite edges and corners, but no further details are given. Hadjiivanov et al.14 proposed an anatase model according to which both types of hydroxyl groups are bonded by five-coordinated titanium cations which differ in the state of the second coordination sphere. The interaction between the surface Ti-OH groups and CO is of the hydrogen-bond type. In general, this type of interaction is used for quantitative characterization of the acidity of surface hydroxyl groups. The adsorption of weak bases leads to the formation of a hydrogen bond with the H+ of the hydroxyl group, as a result of which the ν(OH) wavenumber is shifted to lower frequencies. The higher the proton mobility, the larger the shift. Benzene and olefins are mostly used as probe molecules.13,15,40 However, the application of CO at low temperatures has also been proposed.31 Carbon monoxide has some advantages because larger molecules may be adsorbed on aprotonic sites situated in the neighborhood, which hinders their direct interaction with the hydroxyl groups, thus modifying the results.15 Our results show that the shift of the OH stretching modes after low-temperature CO adsorption on both pure and partially ammonia-covered surfaces is the same, i.e. ammonia adsorbed on R- and β′-sites does not affect the acidity of the OH groups. However, on the surface completely covered by ammonia, there is another situation: the ν(OH) shift is much smaller. This would indicate a direct effect of ammonia adsorbed on β′′-sites upon the acidity of the surface hydroxyls. In this case the acidity sharply drops. A similar effect has been observed earlier with benzene adsorption on surfaces completely covered with ammonia.15 This may be interpreted as a proof that ammonia cannot give electrons to distant sites but only to sites in its vicinity. There are two possibilities to explain the above results: (i) the surface hydroxyl groups of anatase are relatively far away from the R- and β′-sites but are closer to the β′′-sites, and (ii) ammonia is coordinated to the same site where the surface OH group is bound, thus decreasing its acidity. The latter proposition would explain the higher uptake coverage of coordinated ammonia (7.5 µmol) as compared with the amount of CO adsorbed on the R- and β-sites (6.5 µmol). (40) Efremov, A.; Davydov, A. React. Kinet. Catal. Lett. 1980, 15, 327.
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It is interesting to discuss the possibility of a direct interaction between adsorbed ammonia and CO. Our experiments do not show any shift of the ν(NH) stretching bands during CO adsorption (very small changes have been observed in the difference spectra only). However, it should be noted that (i) the protons of the NH groups must possess a much lower acidity than that of the OH group and (ii) the NH vibrations themselves have an almost five times lower sensitivity to hydrogen bond formation.41 This means that a direct interaction between CO and NHx groups is also possible, but for the above reasons its spectral performance is negligible and cannot be detected. However, we are of the opinion that CO is physically adsorbed on the ammonia-covered part of the surface, since the bands due to physically adsorbed CO are more intense when CO is adsorbed on ammoniacovered samples. Conclusions (1) CO adsorption on anatase leads to formation of two kinds of surface Ti4+-CO carbonyls, characterized by ν(CO) at 2210 and 2192 cm-1. These carbonyls are formed with the participation of the R- and β′-Ti4+ sites, respectively. In addition to these species, at 100 K, CO forms successively: (i) carbonyls on β′′-Ti4+ sites that produce a common band with CO adsorbed on the β′-sites at 2179 cm-1, (ii) carbonyls on γ-Ti4+ sites with a frequency at 2165 cm-1, (iii) species adsorbed on surface hydroxyl groups (2155 cm-1), (iv) and physically adsorbed CO (2140 cm-1). (2) The structural heterogeneity of the anatase surface includes the existence of three types of Lewis acid sites: R, β, and γ. The heterogeneity of the β sites (β′ and β′′) is believed to be induced. (3) Ammonia adsorbed on R- and β′-sites does not affect the acidity of the anatase surface hydroxyls. However, it slightly decreases the electrophilicity of the β′′-sites and, to a higher extent, the electrophilicity of the γ-sites. (4) When anatase is covered with ammonia, the R-, β′-, and β′′-sites are blocked. The acidity of the γ-sites and the surface hydroxyls strongly decreases. Acknowledgment. The financial support from the European Economic Community (Program PECO) is highly acknowledged. Thanks are also due to Prof. A. Tsyganenko for some helpful discussion. LA962104M (41) Nakamoto, K.; Margoshes, M.; Rundle, R. E. J. Am. Chem. Soc. 1955, 77, 6480.
