Lewis acid and base sites at the surface of microcrystalline TiO2 anatase: relationships between surface morphology and chemical behaviour

Applied Catalysis A: General 200 (2000) 275–285

Lewis acid and base sites at the surface of microcrystalline TiO2 anatase: relationships between surface morphology and chemical behaviour Gianmario Martra∗ Dipartimento di Chimica IFM dell’Università degli Studi di Torino, Via P. Giuria 7, I–10125 Torino, Italy Received 21 December 1999; received in revised form 29 March 2000; accepted 1 April 2000

Abstract The relationships between morphology and Lewis acid and base character of surface sites of two types of titania powders (TiO2 P25 and TiO2 Merck) were studied by HRTEM and FTIR spectroscopy of adsorbed molecules. Electron micrographs revealed that TiO2 P25 microcrystals have a prismatic shape, mainly exposing (0 0 1) and (0 1 0) surface planes. TiO2 Merck powder, which exhibits a significantly lower specific surface area, appeared constituted by large roundish microcrystals. FTIR spectra of adsorbed CO indicated that Ti4+ ions exposed on (0 0 1) and (0 1 0) faces of TiO2 P25 particles are Lewis acid centres significantly stronger than those present on the surface of TiO2 Merck microcrystals. As in both cases the exposed cations are coordinated to five oxygen anions, the observed differences in Lewis acidity are ascribed to some difference in the geometric arrangement of the O2− ligands. Such difference in structure affects the basicity of these centres also. In fact, a fraction of O2− ions on the surface of TiO2 P25 behave as basic centres toward CO2 linearly adsorbed on neighbour Ti4+ centres, and then Lewis acid–base pairs can be recognised. By contrast, no basic activity towards CO2 was detected for the TiO2 Merck sample. The two titania powders exhibited different chemical behaviour in condition of high surface hydration also. Hydroxyl groups on the surface of hydrated TiO2 P25 are able to transform benzaldehyde molecules in hemiacetalic-like species, whereas C6 H5 CHO molecules are only weakly perturbed by interaction with the OH groups on TiO2 Merck particles. This feature could be related to the different photocatalytic behaviour in the oxidation of toluene in gas phase, where benzaldehyde was found as a relevant intermediate species. © 2000 Elsevier Science B.V. All rights reserved. Keywords: TiO2 morphology; Lewis acid and base sites; HRTEM; FTIR spectroscopy

1. Introduction Microcrystalline TiO2 powders are widely employed in the field of heterogeneous catalysis. In 1970s, a TiO2 -based catalyst was first applied commercially in air pollution control equipment as support ∗ Fax: +39-11-670-7855. E-mail address: [email protected] (G. Martra).

for redox oxides and became a subject of many scientific studies, reviewed in the following decade [1,2]. More recently, model V2 O5 /TiO2 systems [3] and industrial V2 O5 –WO3 /TiO2 SCR catalysts [4] for the abatement of NOx from waste gases from incinerator plants were studied by a pool of research European laboratories in the frame of a program devoted to the standardisation of heterogeneous catalysts produced and employed in Europe.

0926-860X/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 6 4 1 - 4

276

G. Martra / Applied Catalysis A: General 200 (2000) 275–285

TiO2 is also used as support for metal catalysts active in CO hydrogenation [5] and oxidation with NO [6] and O2 [7]. Noticeably, strong-metal support interaction (SMSI) was first reported for noble metals supported on TiO2 [8,9]. However, TiO2 is an actual catalyst as such also, active in hydrocarbon selective oxidation processes [10] and dimerisation of formaldehyde to methyl formate [11]. Titanium dioxide plays a relevant role in several electrochemical and photoelectrochemical applications, where TiO2 based thin films are largely employed in photoelectrolytic cells, also intended to be active under solar radiation [12–14]. Since the discovery of the photocatalytic splitting of water on TiO2 [12] a relevant studies of the photocatalytic behaviour of titania has been carried out. In particular, attention have been focused on the photo-oxidation processes of organic pollutants in waste waters and in air [15–17], inspired by the potential application of TiO2 based photocatalysts for effective, environmental friendly purification processes of water and air. Such a variety of uses needed an optimisation of the performance of the material for each specific application. This stimulated researches in the field of the preparation of TiO2 in different forms, such as nanoparticles [18,19,62], thin films [20,63] and, very recently, mesoporous sieves [21]. However, the morphology does not result in different textural properties (i.e. specific surface area and porosity) only. The various form of titania exhibit different surface structural arrangements, with different surface reactivity. As most of the TiO2 based processes involve complex phenomena with at least one step occurring on the surface of TiO2 particles, the knowledge of their surface chemical behaviour in dependence of their surface structure can be very useful. For instance, surface-dependent chemical pathways were evidenced for the reactivity of acetic acid [22], formaldehyde [23] and water [24] on single crystal surfaces (mostly of the rutile type). Several studies on the interaction of water with microcrystalline TiO2 rutile appeared [25–31], suggesting that (1 1 0) surface planes are more reactive for water dissociation with respect to the other exposed crystal faces. In all cases, these structure-sensitive reactions occurred on acid–base pairs, indicating that the chemical behaviour of both Ti4+ (Lewis acid) and O2− (Lewis base) ions depends on the surface structure.

In the case of microcrystalline TiO2 anatase, which is the titania form most widely employed in catalysis and photocatalysis, relationships between surface morphology and chemical properties were investigated also, but essentially focusing on the features of Lewis acid Ti4+ centres [32–34]. In this paper, the behaviour of surface Ti4+ –O2− acid–base pairs of two anatase TiO2 powders in dependence on their different morphology is reported and discussed. Besides the behaviour towards molecular probes, such as CO and CO2 , the reactivity of hydrated surfaces towards benzaldehyde is also considered. This molecule is involved in the reactive pathway of the photocatalytic oxidation of gaseous toluene, which was recently found strongly dependent on the type of microcrystalline TiO2 used as photocatalyst [35,36].

2. Experimental Two commercial microcrystalline TiO2 powders were studied: (i) titanoxide Degussa P25 (80% anatase, 20% rutile, BET specific surface area 50 m2 g−1 ), and (ii) TiO2 Merck (pure anatase phase, BET specific surface area 10 m2 g−1 ). Both samples were of a high cationic purity. As for anions, TiO2 Merck contained traces of sulphate species (<0.005 wt.%), while in TiO2 P25 a higher amount of chloride impurities (<0.08 wt.%) was present. Such level of Cl− ions does not affect the Lewis surface properties of stoichiometric TiO2 , as resulting from a study on a similar Cl-free TiO2 powder [37]. Size and morphology of the TiO2 microcrystals were detected by HRTEM measurements carried out with a Jeol 2000EX microscope equipped with polar piece and top entry stage. Before the introduction in the instrument, the samples were ultrasonically dispersed in isopropyl alcohol, and a drop of the suspension was deposited on a copper grid covered with a lacey carbon film. Chemical features of the surface centres were investigated by FTIR spectroscopy of adsorbed species. TiO2 powders were pressed in form of self-supporting pellets (ca. 20 mg cm−2 ). For measurement at room temperature (r.t.), the samples were placed in a conventional IR quartz cell equipped with KBr windows, permanently connected to a vacuum line (residual pressure: 1.0×10−6 Torr; 1 Torr=133.33 Pa) allow-

G. Martra / Applied Catalysis A: General 200 (2000) 275–285

277

3.1. Surface morphology of TiO2 particles

correspond to the more abundant faces exposed at the surface of the microcrystals. Both types of surfaces exhibit five coordinated Ti4+ cations (hereafter referred to as Tiv 4+ ) having a single coordinative unsaturation with respect to the full six coordination in the bulk, and oxygen anions bonded to three cations, as in the bulk, and bridged over two cations (hereafter referred to as OII 2− ) [41–44]. In most cases, lateral faces of these prismatic particles, which correspond to a lower fraction of the exposed surface, were assigned to (1 0 1) and (1 1 0) planes on the basis of their parallelism with the fringes pattern lying in the image plane. Furthermore, some roughness of the edges was observed at high magnification (Fig. 1), and then, taking into account the small dimension of the crystallites, the presence of a not negligible amount of defect sites (on edge, corner and step positions) must be considered also. In the case of the TiO2 Merck powder, larger particles were observed, with size in the 40–300 nm range (mean size ca. 140 nm), exhibiting both sharp and roundish edges (Fig. 2A). The prevalence of roundish border prevented a reasonable crystallographic indexing of a large fraction of the surface of the particles. On the other hand, sharp edges were mainly found parallel to fringes pattern 0.353 nm apart (Fig. 2B), corresponding to the interplanar spacing of (1 0 1) planes

TEM images showed that Degussa P25 powder is constituted by plate-like particles, with irregular polygonal contours, with size in the 20–80 nm range (mean size ca. 40 nm). In most cases, interference fringes were detected at high resolution. An example is reported in Fig. 1, where the image of a microcrystal exhibiting two series of fringes 0.134 nm apart and perpendicular to one another is reported. These fringes are related to (1 1 0) planes of anatase [39], and the fact that the crossing angle is 90◦ indicate that these planes are actually observed perpendicularly to the (0 0 1) face (i.e. along the (0 0 1) direction), which then corresponds to the particle face lying in the plane of the image. In other cases the analysis of fringe patterns revealed that also (0 1 0) planes are exposed at the surface of the microcrystals, in agreement with a previous study [40]. In all cases, (0 0 1) and (0 1 0) planes were observed as basal, and more extended, faces of thin prismatic particles, and then should

Fig. 1. Electron micrograph of TiO2 P25. Original magnification: ×1,000,000

ing all thermal adsorption–desorption experiments to be carried out in situ. IR measurements at liquid nitrogen temperature were carried out in a suitable home-designed cell [38]. To study the surface properties at different degrees of dehydroxylation, the samples were first outgassed at the desired temperature for 1 h, and then treated in O2 (150 Torr) at the same temperature for 1 h, cooled at room temperature in O2 and finally outgassed. After this procedure the samples appeared white in colour, as expected for fully oxidised, stoichiometric, TiO2 . Samples treated in such way will be referred to as ‘activated’ at the indicated temperature. FTIR spectra (2 cm−1 resolution) were run with a Bruker IFS 48 spectrometer, equipped with a MCT detector. The spectra of adsorbed molecules were reported in absorbance, having subtracted the spectrum of the sample prior the adsorption as background. High purity O2 , CO and CO2 gases from Matheson and benzaldehyde from Carlo Erba were employed. O2 and CO were used without any further purification except liquid nitrogen trapping, while CO2 and benzaldehyde were admitted onto the samples after several freeze–pump–thaw cycles. 3. Results and discussion

278

G. Martra / Applied Catalysis A: General 200 (2000) 275–285

Fig. 2. Electron micrographs of TiO2 Merck. (A) View of an ensemble of microcrystals, original magnification ×80,000; (B) particular of a straight border region, original magnification ×600,000; the darker spots are the consequence of damage occurred to the material under the electron beam.

of anatase [39], and then they should be assigned to this type of face. Also this surface plane exhibits Tiv 4+ and OII 2− ions. 3.2. CO adsorption at 77 K: acid sites The features of the Lewis acid Ti4+ centres exposed at the surface of the two types of TiO2 particles were

investigated by IR spectroscopy of CO adsorbed at 77 K. Fig. 3A reports spectra of CO adsorbed, at decreasing CO coverage, on TiO2 P25 previously outgassed at 873 K to obtain a nearly totally dehydroxylated sample. The main sharp peak, centred at 2178 cm−1 at high CO coverage can be assigned to parallel CO oscillators ␴-bonded to highly acidic five coordinated Ti4+ on regular (0 1 0) and (0 0 1) faces [40]. By decreasing the CO coverage the peak gradually shifts to 2191 cm−1 , the frequency of isolated CO oscillators, owing to the progressive vanishing of the dynamic and static adsorbate–adsorbate interactions as the amount of adsorbed CO decreases [40]. Noticeably, the large shift observed well agrees with the relative large extension and regularity of (0 0 1) and (0 1 0) surface planes observed by TEM, as the overall entity of the shift depends on the number of coupled oscillators and, therefore, on the extension of the planes which accommodate the adsorbed layers [45–47]. The weak band at 2153 cm−1 , highly reversible by decreasing the CO coverage, is assigned to CO molecules hydrogen bonded to residual OH groups [48]. As a consequence of this interaction the weak bands at 3720 and 3675 cm−1 observed in the spectrum of the sample before CO adsorption (Fig. 3A, inset (a)), due to the stretching mode of hydroxyl

Fig. 3. IR spectra of CO adsorbed at 77 K on TiO2 P25 (A) and TiO2 Merck (B) activated at 873 K (see Section 2). Curves in the main frame of both sections are spectra taken in the presence of decreasing CO pressure: (a) 20; (b) 10; (c) 5; (d) 2; (e) 1; (f) 0.5; (g) 0.1 Torr CO and (i) after outgassing for 1 min at 77 K. In the insets the spectra of the samples: (a) before CO adsorption; (b) after admission of 20 Torr CO and (c) after decreasing the CO pressure to 2 Torr are reported.

G. Martra / Applied Catalysis A: General 200 (2000) 275–285

groups left on the surface after activation at 873 K, are shifted to lower frequency, producing a broad and more intense component centred at 3570 cm−1 (Fig. 3A, inset (b)). The original spectral profile of the hydroxyl groups is fully restored by decreasing the CO pressure in equilibrium with the sample to 2 Torr (Fig. 3A, inset (c)), while at the same time the 2153 cm−1 band in the CO stretching region disappears (Fig. 3A, main frame, d). Both features indicate that in this condition a complete desorption of CO from OH groups occurs. Noticeably, a weak band is observed at 2212 cm−1 , assigned to CO adsorbed on more acidic Ti4+ ions of high Lewis acidity with very low coordination, located on edges, steps and corners [34,40]. The same experiment was carried out on TiO2 Merck, and the results are reported in Fig. 3B. At high CO coverage the spectrum is dominated by a broad peak at 2156 cm−1 (Fig. 3B (a)). By decreasing the amount of adsorbed CO this band progressively decrease in intensity and shifts to 2159 cm−1 , and a weak shoulder at ca. 2170 cm−1 is detected (Fig. 3B (b–i)). Finally, a very weak band at ca. 2210 cm−1 is present also, more evident at high CO coverage. The inspection of the spectrum of the sample before CO adsorption revealed the presence of a very weak component at 3670 cm−1 (Fig. 3B, inset (a)), due to residual OH groups left on the surface after activation at 873 K. By admission of CO onto the sample, this band shifts to low frequency, originating a broader and more intense component at 3575 cm−1 (Fig. 3B, inset (b)), due to the interaction through hydrogen bonding of such groups with CO. The corresponding band of CO molecules adsorbed in this form may contribute to the main component at 2156 cm−1 observed in the CO stretching region (Fig. 3B (a)). The original band of hydroxyl groups at 3670 cm−1 is fully restored by decreasing the CO pressure in equilibrium with the sample to 2 Torr (Fig. 3B, inset (c)), indicating that CO molecules are desorbed form OH groups. In this condition, in the CO stretching zone a peak at 2157 cm−1 is still present (Fig. 3B (d)), only due to CO molecules interacting with surface Ti4+ cations. These adducts may be present in larger extent at higher CO coverage, contributing to the main part of the peak at 2156–2157 cm−1 in the presence of higher CO pressure. This component dominates the spectra of adsorbed CO by far, and then can

279

be assigned to CO molecules adsorbed on the most common type of Ti4+ ions, which should be cationic centres on the roundish surfaces of the TiO2 Merck microcrystals, which were found the most abundant surface terminations by HRTEM. The small shift underwent by this band by decreasing the CO coverage indicates that adsorbate–adsorbate interactions are not so effective for the TiO2 Merck sample. This suggests that small crystal planes are present on the surface of TiO2 Merck particles, exposing cationic centres quite far each other. Electron microscopy showed that TiO2 Merck microcrystals exhibit a minor fraction of (1 0 1) faces also. The weak shoulder at ca. 2170 cm−1 , better observed at low CO coverage (Fig. 3B (i)), could then be assigned to CO molecules adsorbed on Ti4+ ions exposed on this less common type of planes. Finally, the weak component at ca. 2210 cm−1 is assigned to CO adsorbed on a very limited number of Ti4+ ions in low coordination, scarcely present on roundish and large microcrystals. Both the band at 2156 cm−1 and the shoulder at ca. 2170 cm−1 are located at lower frequency than the bands due to CO adsorbed Ti4+ cations on the most abundant surface planes of TiO2 P25 (see Fig. 3A). Noticeably, an even larger difference in frequency occurs between the weak signals due to isolated CO molecules adsorbed at very low coverage on the most common type of surface terminations of the two materials, located at 2191 cm−1 for TiO2 P25 (Fig. 3A (i)) and at 2159 cm−1 for TiO2 Merck (Fig. 3B (i)). As in this condition adsrobate–adsorbate interactions are in both cases negligible, the position of these components mainly results from the interactions of CO molecules with the adsorbing centres. The upward shift of the stretching mode of CO molecules adsorbed on surface cations with respect to CO in gas phase (2143 cm−1 ) increases as the Lewis acid strength of the adsorbing centres increases [49], and then the differences in the position of the stretching band of CO adsorbed on the two types of TiO2 powders clearly indicates that both types of Ti4+ ions exposed on roundish surfaces and (1 0 1) faces of TiO2 Merck exhibit a significantly lower Lewis acidity than those exposed on the (0 0 1) and (0 1 0) surfaces of TiO2 P25 microcrystals. As previously commented on, the roundish shape of the TiO2 Merck microcrystals prevent the indexing

280

G. Martra / Applied Catalysis A: General 200 (2000) 275–285

of most part of their surface. Then, the possibility of a discussion of the difference in Lewis acid strength of the surface Ti4+ ions of the two types of TiO2 powders by assuming reasonable different models for their surface planes must be discharged at present. However, it must be considered that Lewis acidity of surface cations depends on the number and geometric arrangement of oxygen anions coordinated to the cationic centres. Ti4+ ions emerging on both the most abundant (0 0 1) and (0 1 0) faces of the TiO2 P25 microcrystals are five-coordinated. Ti4+ centres on the surface of TiO2 Merck particles exhibit a lower Lewis acidity, but this cannot be ascribed to a lower degree of coordinative unsaturation. More likely, they should be described in terms of Ti4+ ions coordinated to five O2− ligands, the arrangement of which results in a more efficient shielding of the positive electric fields associated to the cationic centres. Such arrangement could results from some relaxation phenomena, involving a displacement of the cations inward the surface. In order to better distinguish the features related to the different acid centres on the P25 microcrystals a study of the surface sites as a function of the degree of dehydroxylation was carried out. The comparison of IR spectra of CO adsorbed at 77 K on a fully dehydroxylated TiO2 P25 sample activated at 873 K with the spectra of CO adsorbed on a partially hydroxylated sample pre-treated at a milder temperature (423 K) revealed that in the latter case the large majority of Ti4+ ions on flat faces were exposed and able to adsorb CO. By contrast no band at 2212 cm−1 was detected for such treated sample, indicating that the coordination vacancy of Ti4+ cations on defect sites are still filled by hydroxyl groups (spectra not reported).

obtained for the TiO2 P25 sample, and these must be accompanied by basic O2− centres in similar low coordination. On the basis of previous studies on CO adsorption [32–34,40,48] it can be argued that these sites are not so basic to carry out a nucleophilic attack to the carbon atom of carbon monoxide molecules, as observed for acid–base pairs in low coordination in alkaline-earth oxides [50–52] and, more recently, zirconium oxide [53]. However, some activity might be exhibited towards stronger acid probe molecules, such as CO2 . Furthermore, this acid probe could react with O2− ions on regular planes also. In order to better distinguish the role of sites with different coordination, the characterisation of basic centres of TiO2 P25 was carried out on samples activated at 423 (where only centres on regular faces are exposed) and 873 K (where centres in defect position are exposed also). 3.3.2. IR spectra of adsorbed CO2 By admitting increasing amounts of CO2 form 0.08 up to 100 Torr on TiO2 P25 activated at 423 K an intense peak at 2351 cm−1 , accompanied by a weak satellite at 2280 cm−1 , and a complex pattern characterised by a series or related bands in the 1750–1000 cm−1 range were observed (Fig. 4).The main band at 2351 cm−1 (maximum out of scale at high CO2 coverage, Fig. 4a and b) is assigned to the Σu+ mode of 12 CO linearly adsorbed on Ti4+ ions on regular faces 2 of TiO2 P25 microcrystals [54], while its weak partner at 2280 cm−1 is the corresponding absorption for

3.3. CO2 adsorption at RT: basic sites 3.3.1. Preliminary remarks Data commented on in the previous section clearly indicate that the two TiO2 powders with different morphology exhibit Ti4+ centres of very different Lewis acidity. Of course, these cationic centres are accompanied by O2− counter-anions, and then the presence of different types of Lewis basic centres can be expected in the two cases, actually resulting in cation–anion pairs with different Lewis acid–base character. Evidence of the presence of strong Lewis acid Ti4+ ions in defect positions (edges, steps, corners) was

Fig. 4. IR spectra of CO2 adsorbed under increasing pressure on TiO2 P25 activated at 423 K (see Section 2): (a) 0.08; (b) 0.8; (c) 3.0; (d) 10; (e) 40 and (f) 100 Torr CO2 .

G. Martra / Applied Catalysis A: General 200 (2000) 275–285 13 CO

molecules present in natural abundance. Both these components appear quite sharp and symmetric in shape, suggesting a high homogeneity of the features of the cationic centres, in agreement with the finding provided by the FTIR spectra of CO adsorbed at 77 K discussed above. The complex pattern at lower frequencies is mainly due to various types of carbonate-like species, namely mono- (bands at 1578 and 1359 cm−1 ) and bidentate(bands at 1672, 1243 and 1053 cm−1 ) carbonate groups, produced by nucleophilic attack of basic O2− ions exposed on regular faces to CO2 molecules which, in a first step of interaction, had been adsorbed linearly adsorbed on neighbour cations [64,54]. Furthermore, components at 1630, 1430, 1408 and 1221 cm−1 are assigned to bicarbonate species, which should be produced by reaction of CO2 with some basic-OH groups left on sites in defect position [64,54]. Traces of water admitted onto the sample together with CO2 can also partly contribute to the 1630 cm−1 peak. The presence of bands due to carbonate-like species witnesses the presence of Ti4+ –O2− pairs where the anionic moiety is basic enough to react with CO2 , indicating that Lewis acid–base couples are actually exposed on the regular surface planes of TiO2 P25 microcrystals. However, these couples correspond only to a fraction of surface sites, as CO2 molecules are largely adsorbed in linear form also (band at 2351 cm−1 ), indicating that O2− ions unable to carry out a nucleophilic attack to the CO2 molecules adsorbed on neighbour Ti4+ cations are also present. Such a difference in chemical behaviour could be ascribed to some difference in the geometric arrangement of cations and anions of the various planes exposed at the surface of TiO2 P25 microcrystals. For instance, TiV 4+ –OII 2− bonds were reported to be shorter on (0 0 1) faces than on (0 1 1) [32], and this can strongly affect the reactivity of such acid–base pairs. These structural features were suggested to play a key role in the dissociation of water on Ti4+ –O2− pairs on TiO2 rutile single crystal surfaces under ultrahigh vacuum condition [24]. A similar reactivity towards CO2 was found for TiO2 P25 activated at 873 K (spectra not reported). However, bands due to bicarbonate-like species appeared strongly reduced in intensity, because of the high degree of dehydroxylation attained, and components due to bidentate carbonate species appeared

281

less intense, as they are formed on surface anions, the basicity of which can be affected by the presence of neighbour hydrated centres [54]. New components grew up at ca. 1595 and 1315 cm−1 , due to the asymmetric and symmetric stretching modes of monodentate carbonate species formed on Lewis acid–base pairs in defect position (edges, steps, corners). These components exhibit a slightly larger splitting than the monodentate carbonate species on regular planes, suggesting that carbonate species formed on defect position may exhibit a higher distortion with respect to the D3h symmetry of bulk carbonate species [55]. CO2 adsorption was carried out on TiO2 Merck also. Very similar results were obtained by admitting CO2 on samples activated at 423 and 873 K, and then, for the sake of brevity, only the spectra obtained in the case of the sample outgassed at the highest temperature are reported in Fig. 5. Only a peak at 2347 cm−1 due to carbon dioxide molecules linearly adsorbed on Ti4+ ions was observed, progressively increasing in intensity as the amount of CO2 admitted on the sample is increased form 0.08 up to 100 Torr (Fig. 5a–f). This band appeared located at lower frequency than in the case of TiO2 P25 (2351 cm−1 ). As the shift of the Σu+ band of adsorbed CO2 molecules with respect to the gas phase (where it is observed at 2343 cm−1 ) increases together with the Lewis acid strength of the cationic adsorbing sites increases [56], this difference

Fig. 5. IR spectra of CO2 adsorbed under increasing pressure on TiO2 Merck activated at 873 K (see Section 2): (a) 0.08; (b) 0.8; (c) 3.0; (d) 10; (e) 40 and (f) 100 Torr CO2 .

282

G. Martra / Applied Catalysis A: General 200 (2000) 275–285

in position well agrees with the difference in Lewis acid strength of the surface Ti4+ ions of the two types of TiO2 powders evidenced by the IR spectra of CO adsorbed at 77 K. Noticeably, no bands due to carbonate-like species were detected, indicating that no O2− centres able to react with CO2 linearly adsorbed on Ti4+ ions are exposed on the surface of the roundish microcrystals of the TiO2 Merck sample. This indicated that the different morphology and surface structure of this type of material affects the properties of both surface Ti4+ and O2− ions forming acid–base pairs. 3.4. Reactivity of hydrated surfaces As commented in Section 1, TiO2 is largely employed as heterogeneous photocatalyst for the removal of pollutants from water and air. In these processes the surface centres are not longer Ti4+ and O2− ions, as the surface is covered by a full monolayer of water molecules and hydroxyl groups. These last species were recognised to play a key role in the first steps of the photo-degradation process by producing highly reactive OH radical through interaction with photo-generated holes, h+ [57,58,65]. Furthermore, they act as adsorbing centres for reactants, intermediate species and products. In the frame of a research project devoted to the study of the photo-degradation of aromatic pollutants on TiO2 in aqueous medium and in air, the photo-oxidation of toluene in gas phase in the presence of water vapour on TiO2 Merck and TiO2 P25 was carried out [35,36]. Benzaldehyde was found in gas phase as the main product of the toluene partial oxidation when using TiO2 Merck, and the catalyst activity was preserved for very long time of stream. By contrast, for the runs carried out with TiO2 P25 as photocatalyst, no significant amount of products of toluene conversion was found in gas phase, and the catalyst underwent a progressive deactivation in few hours. On this basis, a comparative study of the features of the surface hydroxyls groups of the two types of TiO2 powders and their adsorptive behaviour towards benzaldehyde has been carried out. 3.4.1. IR spectra of hydroxyl groups Fig. 6 reports the IR spectra in the ␯OH region of TiO2 P25 (Fig. 6a) and TiO2 Merck (Fig. 6b)

Fig. 6. IR spectra in the 3800–2600 cm−1 range of TiO2 P25: (a) outgassed at room temperature for 45 min and (a0 ) activated at 873 K, and of TiO2 Merck: (b) outgassed at r.t. for 45 min and (b0 ) activated at 873 K.

outgassed at room temperature, both significantly more intense than the corresponding spectra recorded after activation at 873 K (Fig. 6, curves a0 and b0 , respectively), which resulted in an extensive dehydroxylation. By contrast, after outgassing at room temperature an almost full monolayer of hydroxyl groups and water molecules coordinated to surface cations is left [26,28,33]. These conditions were assumed representative of the hydration state of the catalyst surface during the photo-oxidative process, carried out in the presence of water vapour at mild temperature (413 K) [35]. The spectrum of TiO2 P25 (Fig. 6a) is characterised by a series of narrow components in the 3800–3600 cm−1 domain, exhibiting a dominant peak at 3630 cm−1 with a series of shoulders, heavily overlapped, on the high frequency side, due to the stretching mode (␯OH ) of different types of free hydroxyl groups [33,34,59], and by an intense and broad absorption in the 3600–3200 cm−1 range, with two maxima at 3415 and 3260 cm−1 , resulting from the superposition of the ␯OH mode of bonded hydroxyl groups and of the symmetric and antisymmetric ␯OH modes of molecular water coordinated to Ti4+ ions [27,33,60], as cations on (1 1 0) face of microcrystalline rutile [24]. The variety of components in the 3800–3600 cm−1 region witnesses for a large heterogeneity of hydroxyl

G. Martra / Applied Catalysis A: General 200 (2000) 275–285

283

groups. The origin of this heterogeneity can be ascribed to the different types of planes exposed at the surface of the TiO2 P25 microcrystals and to the presence of sites in defect position (steps, edges, corners), as reported in the literature [33,34,59]. Interestingly, it may be noticed that Ti4+ ions on the most abundant, regular faces of TiO2 P25 microcrystals monitored by CO adsorption appeared highly homogeneous, while at least two types of O2− were suggested on these faces on the basis of the CO2 adsorption data. Apparently, OH groups, which are formed by dissociation of water molecules on Ti4+ –O2− pairs, reflect the heterogeneous character of the system. By contrast, the IR spectrum of TiO2 Merck appeared quite simpler. A single peak at 3665 cm−1 , slightly asymmetric on the low frequency side, is observed, and a weaker broad band is present in the 3500–2800 cm−1 range (Fig. 6b). The main component at 3665 cm−1 and the ill resolved shoulder on its low frequency side should correspond to the stretching vibration of two kinds of free hydroxyl groups, indicating a higher homogeneity of the Ti4+ –O2− pairs for this type of material. Furthermore, the relatively weak intensity of the broad band in the 3500–2800 cm−1 range, mainly due to the symmetric and asymmetric ␯OH modes of coordinated water molecules, indicates that Ti4+ ions able to coordinated H2 O in a undissociate form occur in a very low extent at the surface of the TiO2 Merck particles.

sites for benzaldehyde, through hydrogen bonding between the oxygen atom of the carbonylic group and the H atom of the OH species [61]. The adsorption of benzaldehyde on TiO2 P25 produced a significantly different spectral pattern. In this case the components due to benzaldehyde molecules adsorbed in an unperturbed form were observed as minor features, and the IR spectrum appeared dominated by new component, such as an intense peak at 1650 cm−1 and a series of bands at 1518, 1495, 1451, and 1413 cm−1 (Fig. 8). As for the TiO2 Merck,

3.4.2. Infrared spectra of adsorbed benzaldehyde To understand the striking different photocatalytic results obtained with the two TiO2 sample the adsorption of benzaldehyde on TiO2 Merck outgassed at r.t. produced an IR spectrum characterised by a main peak at 1700 cm−1 and bands at 1657, 1601, 1586, 1447 and 1312 cm−1 (Fig. 7a). This spectral pattern is very similar to that exhibited by benzaldehyde in CCl4 solution, suggesting that aldehyde molecules are adsorbed on the surface of the catalyst essentially in a weakly perturbed form. As a consequence of the benzaldehyde adsorption, the peak at 3665 cm−1 observed in the spectrum of the catalyst prior the contact with benzaldehyde (Fig. 7, inset, curve a) completely disappears, being transformed into a broad and complex band in the 3650–3200 cm−1 range (Fig. 7, inset, curve b). This behaviour clearly indicates that hydroxyl groups act as effective Lewis acid adsorption

Fig. 8. IR spectra of benzaldehyde adsorbed under increasing pressure on TiO2 P25 outgassed at RT for 45 min: (a) 0.5; (b) 1.0; (c) 2.0 and (d) 3 Torr. Inset: IR spectra of the catalyst in the ␯OH region: (a) before and (b) after admission of 3 Torr of benzaldehyde vapour.

Fig. 7. IR spectra of benzaldehyde adsorbed under increasing pressure on TiO2 Merck outgassed at RT for 45 min: (a) 0.5; (b) 1.0; (c) 2.0 and (d) 3 Torr. Inset: IR spectra of the catalyst in the ␯OH region: (a) before and (b) after admission of 3 Torr of benzaldehyde vapour.

284

G. Martra / Applied Catalysis A: General 200 (2000) 275–285

bands due to the ␯OH mode of hydroxyl groups are completely depleted by admitting benzaldehyde onto the sample (Fig. 8, inset), indicating that OH species are involved in the adsorption process. However, the main peak at 1650 cm−1 and the other new bands at lower frequency could be assigned to hemiacetalic-like species formed by nucleophilic attack of basic oxygen of hydroxyl species to the carbon atom of the carbonyl group of adsorbed aldehyde molecules, as in the first step of the Cannizzaro reaction. Apparently, differences in morphology and surface structure of the two types of TiO2 powders investigated affect also the reactivity of the hydroxyl groups present in high hydration conditions, which behave as electron acceptor (through the H atom) in the case of TiO2 Merck, while exhibit a nucleophilic character (through the O atom) for TiO2 P25. This different chemical behaviour could well account for the different photocatalytic behaviour exhibited by the two types of TiO2 powders. In the case of TiO2 Merck benzaldehyde molecules resulting from the photo-oxidation of toluene weakly interact with the catalyst surface, and can be released in gas phase. By contrast, hydroxyl groups on the surface of TiO2 P25 are able to react with photo-produced benzaldehyde, which is retained on the catalyst surface and can be converted in other products strongly adsorbed on the TiO2 microcrystals, leading to the progressive deactivation of the catalyst.

4. Conclusions HRTEM investigations and FTIR studies of adsorbed CO, CO2 and benzaldehyde evidenced significant differences in the chemical features of the surface centres of TiO2 microcrystals with different morphology. Ti4+ ions of high Lewis acidity are exposed on regular (0 0 1) and (0 1 0) surface planes of prismatic plate-like TiO2 P25 microcrystals. On these faces also O2− ions basic enough to carry out a nucleophilic attack to CO2 molecules originally adsorbed on neighbour cations are present, and then Lewis acid base pairs can be actually recognised. By contrast, only weak Lewis acid Ti4+ ions were found on the surface of larger roundish TiO2 Merck particles. As in both cases cation centres resulted five coordinated to the

surrounding oxygen anions, it can be inferred that differences in Lewis acid strength are related to a different geometric arrangement of their surface O2− ligands. Furthermore, no basic reactivity towards CO2 was found for TiO2 Merck, indicating that the different morphology and surface structure of the microcrystals affects the properties of surface anions also. Finally, these two morphologically different TiO2 powders exhibited different surface reactivity in the hydrated form also. Hydroxyl groups on the surface of TiO2 P25 were observed to behave as nucleophilic agents towards benzaldehyde molecules, which are transformed into hemiacetalyc species. By contrast, benzaldehyde is adsorbed in a weakly perturbed form by the OH groups on TiO2 Merck. These features could account for the different photocatalytic behaviour in the oxidation of toluene observed for the two TiO2 powders.

Acknowledgements Prof. S. Coluccia, Prof. L. Palmisano and Prof. M. Schiavello are gratefully acknowledged for fruitful discussions. The author also acknowledges the Ministero della Ricerca Scientifica e Tecnologica (MURST) for financial support. References [1] [2] [3] [4] [5] [6] [7] [8]

[9]

[10] [11] [12] [13]

S. Matsuda, A. Kato, Appl. Catal. 8 (1983) 149. H. Bosch, F. Janssen, Catal. Today 2 (1988) 369. J.C. Védrine (Ed.), Eurocat oxide, Catal. Today, 20 (1994) 1. J.C. Védrine (Ed.), Eurocat oxide, Catal. Today, 56 (2000) 329. M.A. Vannice, J. Catal. 74 (1982) 199. F. Boccuzzi, E. Guglielminotti, G. Martra, G. Cerrato, J. Catal. 146 (1994) 449. M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301. W. Conner Jr., G.M. Pajonk, S.J. Teichner, D.D. Eley, H. Pines, P.B. Weisz (Eds.), Advance in Catalysis, Vol. 34, Academic Press, San Diego, 1986, Chapter 1, p. 1. G.L. Haller, D.E. Resasco, in: D.D. Eley, H. Pines, P.B. Weisz (Eds.), Advance in Catalysis, Vol. 36, Academic Press, San Diego, 1989, Chapter 3, p. 173. M.S. Wainwright, N.R. Forster, Catal. Rev. 19 (1979) 211. M. Ai, J. Catal. 83 (1983) 141. A. Fujishima, K. Honda, Nature 238 (1972) 37. Y. Matsumoto, J. Kurimoto, Y. Amagasaki, E. Sats, J. Electrochem. Soc. 127 (1980) 2360.

G. Martra / Applied Catalysis A: General 200 (2000) 275–285 [14] W.A. Badawy, J. Mater. Sci. 32 (1997) 4979. [15] M. Schiavello (Ed.), Photocatalysis and Environment. Trends and Applications, Nato ASI Series C: Mathematical and Physical Sciences, Vol. 237, Kluwer Academic Publishers, Dordrecht, 1988. [16] E. Pelizzetti, N. Serpone (Eds.), Photocatalysis: Fundamental and Applications, Wiley, New York, 1993. [17] F. Ollins, H. Al-Ekabi (Eds.), Photocatalytic Purification and Treatment of Water and Air, Elsevier, New York, 1993. [18] Z. Zhu, L.Y. Tsung, M. Tomkiewicz, J. Phys. Chem. 99 (1995) 15945. [19] L. Shi, N.B. Wong, K.C. Tin, C.Y. Chung, J. Mater. Sci. Lett. 16 (1997) 1284. [20] W.A. Badaway, F. Decker, K. Doblhofer, Solar Energy Mater. 8 (1983) 363. [21] D. Qing, L. Ping, M. Yuging, H. Nonque, Y. Chunwari, Cuihua Xuebao 19 (1998) 483. [22] K.S. Kim, M.A. Barteau, J. Catal. 125 (1990) 353. [23] H. Idriss, K.S. Kim, M.A. Barteau, Surface Sci. 262 (1992) 113. [24] M.A. Henderson, Langmuir 12 (1996) 5093, and references therein. [25] P. Jones, J.A. Hockey, Trans. Faraday Soc. 67 (1971) 2669. [26] G. Munuera, F.S. Stone, Discuss. Faraday Soc. 52 (1971) 205. [27] M. Primet, P. Pichat, M.-V. Mathieu, J. Phys. Chem. 75 (1971) 1216. [28] D.M. Griffits, C.H. Rochester, J. Chem. Soc., Faraday Trans. 1 73 (1977) 1510. [29] C.H. Rochester, Colloid Surface 21 (1986) 205. [30] Y. Suda, T. Morimoto, Langmuir 3 (1987) 786. [31] D.D. Beck, J.M. White, C.T. Ratcliffe, J. Phys. Chem. 90 (1986) 3123. [32] G. Busca, H. Saussey, O. Saur, J.C. Lavalley, V. Lorenzelli, Appl. Catal. 14 (1985) 245. [33] C. Morterra, J. Chem. Soc., Faraday Trans. 84 (1988) 1617. [34] G. Cerrato, L. Marchese, C. Morterra, Appl. Surface Sci. 70/71 (1993) 200. [35] V. Augugliaro, S. Coluccia, V. Loddo, L. Marchese, G. Martra, L. Palmisano, M. Schiavello, Appl. Catal. B. Environ. 20 (1999) 15. [36] G. Martra, V. Augugliaro, S. Coluccia, E. Garc´ıa-Lopez, V. Loddo, L. Marchese, L. Palmisano, M. Schiavello, in: Proceedings of the 12th International Congress on Catalysis, Granada, Spain, July 2000, Studies in Surface Science and Catalysis, Elsevier, Amsterdam, in press. [37] C. Morterra, G. Ghiotti, E. Garrone, E. Fisicaro, J. Chem. Soc., Faraday Trans. 1 76 (1980) 2102.

285

[38] L. Marchese, S. Bordiga, S. Coluccia, G. Martra, A. Zecchina, J. Chem. Soc., Faraday Trans. 89 (1993) 3483. [39] Powder diffraction File JCPDS 21-1272. [40] G. Spoto, C. Morterra, L. Marchese, L. Orio, A. Zecchina, Vacuum 41 (1990) 37. [41] H.P. Bohem, Adv. Catal. 16 (1966) 179. [42] M. Primet, P. Pichat, M.V. Mathieu, C.R. Acad. Sci. Ser. B 267 (1968) 799. [43] M.A. Enriquez, C. Dorémoieux-Morin, J. Fraissard, J. Solid State Chem. 40 (1981) 233. [44] K.I. Khadziivanov, A.A. Davydov, D.G. Klisurski, Kinet. Catal. 29 (1988) 140. [45] R. Disselkamp, H. Chang, G.E. Ewing, Surface Sci. 240 (1990) 193. [46] P. Hollins, J. Pritchard, Surface Sci. 89 (1979) 486. [47] P. Hollins, J. Pritchard, in: R.F. Willis (Ed.), Vibrational Spectroscopy of Adsorbate, Springer, Berlin, 1980, p. 125. [48] A.A. Tsyganenko, L.A. Denisenko, S.M. Zverev, V.N. Filimonov, J. Catal. 94 (1985) 10. [49] D.A. Seanor, C.H. Amberg, J. Chem. Phys. 42 (1965) 2967. [50] A. Zecchina, F.S. Stone, J. Chem. Soc., Chem. Commun. 582 (1974). [51] S. Coluccia, E. Garrone, E. Guglielminotti, A. Zecchina, J. Chem. Soc., Faraday Trans. 1 77 (1981) 1063. [52] M. Anpo, S.C. Moon, K. Chiba, G. Martra, S. Coluccia, Res. Chem. Intermed. 19 (1993) 495. [53] M. Anpo, S.C. Moon, Res. Chem. Intermed. 25 (1999) 1. [54] C. Morterra, A. Chiorino, F. Boccuzzi, E. Fisicaro, Z. Phys. Chem., Neue Folge 124 (1981) 211. [55] G. Busca, V. Lorenzelli, Mater. Chem. 7 (1982) 89. [56] C. Morterra, G. Cerrato, C. Emanuel, Mater. Chem. Phys. 29 (1991) 447. [57] K. Okamoto, Y. Yamamoto, H. Tanaka, M. Tanaka, A. Itaya, Bull. Chem. Soc. Jpn. 58 (1985) 2015. [58] V. Augugliaro, L. Palmisano, A. Scalfani, C. Minero, E. Pelizzetti, Toxicol. Environ. Chem. 16 (1988) 89. [59] K. Tanaka, J.M. White, J. Phys. Chem. 86 (1982) 4708. [60] G. Munuera, F. Moreno, J.A. Prieto, Z. Phys. Chem., Neue Folge 78 (1972) 113. [61] M. Allian, E. Borello, P. Ugliengo, G. Spanò, E. Garrone, Langmuir 11 (1995) 4811. [62] Z. Ahu, M. Lin, G. Dagan, M. Tomkiewicz, J. Phys. Chem. 99 (1995) 15950. [63] W.A. Badawy, E.L. -Taher, Thin Solid Films 158 (1988) 277. [64] M. Primet, P. Pichat, M.-V. Mathieu, J. Phys. Chem. 75 (1971) 1221. [65] K. Okamoto, Y. Yamamoto, H. Tanaka, A. Itaya, Bull. Chem. Soc. Jpn. 58 (1985) 2023.