A. TiO2
The coordinative and/or dissociative adsorption of various probe mole- cules has been used to characterize the surface properties of TiO2, which finds applications as a catalyst, photocatalyst, and sensor. Among the molecules used as probes, we mention CO (37, 38, 563–576), CO2(563, 565, 577), NO (578, 579), water (580, 581), pyridine (582, 583), ammonia (584, 585), alcohols (586, 587), ethers (including perfluoroethers) (588), ozone (589), nitrogen oxide (590), dioxygen (591), formic acid (592–594), benzene (584), benzoic acid (595), and chromyl chloride (596).
In an investigation of the rutile form of TiO2powder, two CO desorp- tion processes were observed (567) (at 175 and 475 K) and tentatively at- tributed to the C-end interaction of CO with surface oxygen ions and to the O-end interaction with surface defect sites, respectively. The existence of two different adsorbed species was also shown by IR spectroscopy by Tanaka and White (565), who observed bands at 2185 and 2115 cm−1(the lower- frequency band corresponding to a more strongly bonded species). Only the high-frequency component was observed by Morterraet al. (571) for CO on Na-doped rutile. An attempt to correlate the IR spectra of CO adsorbed at 77 K with the structural and morphological transformations (character- ized by HRTEM) accompanying the anatase–rutile phase transition was made by Cerratoet al.(597). On passing from the nearly pure anatase phase
364 A. ZECCHINA et al.
(Titanoxid P25, Degussa) to the pure rutile modification (obtained by firing at 1073 K), a progressive sintering of the TiO2microcrystals was observed, leading to a decrease of the surface area from about 50 to 8 m2g−1. Simulta- neously, the morphology changed from well-defined microcrystals predom- inantly exposing the (010) planes to highly irregular and stepped particles, possibly exposing an appreciable amount of other high-index faces (of un- known nature). These changes were reflected in a significant modification of the IR spectrum, changing from an intense, narrow band at 2179 cm−1 [assigned to Ti4+–CO adducts formed on the (010) faces], typical of the anatase phase to a spectrum including two much broader absorptions, at 2175 and 2156 cm−1 (the second band being more intense). The reduced CO stretching frequency found for CO on rutile is a clear indication of the reduction of the polarizing tendency of the Ti centers, which in turn is sug- gestive of a tendency of rutile to expose surfaces with increasing homopolar character upon treatment under vacuum at high temperature (22).
The CO/anatase system was investigated at room temperature, when only about 10–15% of the total surface Lewis acid sites were covered by CO (38, 41). The IR spectra depend strongly on the degree of hydration of the surface and the presence of contaminants. Two different families of surface Lewis acid sites were identified, one deriving from the removal of molec- ularly adsorbed water and the second (stronger) originating from removal of surface OH groups (41). The two families of sites give rise to reversibly formed adducts with CO bands at about 2187 and 2206 cm−1, respectively.
The first family (2187 cm−1) is by far the predominant one and is plausibly associated with Ti4+ions on extended faces. The CO extinction coefficients of the two species were found to be 2.6 ±0.3 × 106mol−1cm for the band at 2187 cm−1and 3.8 ± 0.2×106mol−1cm for the band at 2206 cm−1(571).
Removal of OH groups at high temperature also yields a third (weak) family of weaker cationic sites revealed by pyridine but not by CO.
Experiments performed at room temperature suffer from the limitation that only a small fraction of the surface sites are detected by adsorption of CO. Therefore, measurements at lower temperature (e.g., 77 K) are neces- sary to permit probing of all the surface sites. Spectra recorded at 77 K for CO on Titanoxide P25 anatase are summarized in Fig. 36. As discussed pre- viously, this material consists of morphologically well-defined microcrystals that preferentially expose the (010) face. The spectrum of CO adsorbed at maximum coverage is dominated by a band at 2179.5 cm−1 (37). The ex- tremely narrow band (FWHM<1.5 cm−1) of this component suggests that the respective Ti–CO complexes are formed on regular surface planes and that the observed band atθ = 1 is the collective in-phase stretching mode of an ordered 2D layer of parallel CO molecules polarized by fivefold coor- dinated ions exposed on the predominant (010) planes. The Ti4+–Ti4+dis- tance on this plane is approximately 0.3 nm, and strong repulsive interactions
FIG. 36. IR spectra of12CO adsorbed at∼77 K on TiO2samples outgassed at 873 K at coverages ranging fromθ=1 (5.33 kPa) toθ→0. (Inset) Spectra in the OH region before (—) and after (ã ã ã ã) contact with CO [reproduced from Spotoet al.(37) with permission from Pergamon Press].
between CO molecules adsorbed on the first nearest neighbors are therefore expected. Consequently, the maximum occupation of the surface sites is only 50% (corresponding to∼2.8 CO/nm2).
Decreasing CO coverage leads to a blue shift of the main peak from 2179.5 to 2192.5 cm−1 (singleton frequency) (ν˜=13 cm−1), accompanied by a change of the band half-width from∼1.5 to∼13 cm−1. These effects indi- cate the gradual disappearance of the ordered phase of adsorbed CO with formation of a more diluted phase with reduced adsorbate–adsorbate inter- actions. On the basis of12CO/13CO isotopic substitution experiments, the contributions of dynamic (ν˜dyn) and static (ν˜stat) effects to the total shift (ν˜tot=13 cm−1) were estimated to be 3.5 and −16.5 cm−1, respectively.
These values and the value of the dynamic polarizability, αv = 0.025 ˚A3, suggest that the surface Ti4+–CO bonds are dominated by electrostatic (po- larization) forces.
Much less intense features were observed at 2211.5 (shifted to 2207 cm−1 at low coverage)—approximately 2175, 2166, 2153, and 2138 cm−1. The first
366 A. ZECCHINA et al.
band clearly corresponds to the band observed at 300 K and assigned to Ti4+– CO complexes on the stronger Lewis acid sites (fourfold coordinatedαsites located on different high-index planes and on defects). The bands at∼2175 and 2166 cm−1were assigned to more weakly bonded species formed during the subsequent occupation of the sites on the (010) face; a tilted configura- tion of these species was suggested. The 2153- and 2138-cm−1absorptions are attributed to hydrogen-bonded species (formed between CO and the residual surface OH groups responsible for the absorptions in the range 3800–3600 cm−1; Fig. 36, inset) and physically adsorbed CO, respectively.
Similar spectra were reported separately (39) and interpreted analogously, except for the band at 2166 cm−1, which was assigned to CO adsorbed on 5-coordinated Ti4+ions in acid–base pairs and, hence, having very low elec- trophilicity.
The major conclusions are sumarized as follows:
1. The extended faces behave as a 2D array of weak Lewis acid centers (Ti45c+). These faces appear to be stable and unreactive.
2. Stronger Lewis acid sites associated with Ti44c+ centers exist only on higher-index faces or on defects such as edges and steps.
3. CO probes did not detect any strongly basic sites.
4. Activationin vacuoat high temperature (needed to eliminate most of the hydroxyl groups) is usually accompanied by a loss of oxygen and of sto- ichiometry (with the accompanying optical effects preventing spectroscopic observations).
The properties of TiO2surfaces can be selectively modified by incorpora- tion of, for example, sulfates, phosphates, peroxides, and vanadium pentox- ide. The resulting tailored products may have catalytic properties superior to those of TiO2. This is a broad and important subject, which does not fit within the limited scope of this review.
B. ZrO2
1. Pure ZrO2
Several groups (598–602) investigated the surface properties of a highly sintered monoclinic (ZrO2) by IR spectroscopy of CO adsorbed at 77 K.
Morterraet al.reported a stepwise formation of five different bands (la- beled A–E in Ref.600) upon increasing the CO equilibrium pressure. This behavior is similar to that observed for CO on ZnO (Section V.A.1) and can be attributed to the development of lateral interactions between CO oscillators with increasing coverage. The different bands correspond to dif- ferent local geometries around a given Zr4+ã ã ãCO adduct. The main differ- ences between CO/ZrO2and CO/ZnO are two narrow bands, at 2172 and
2167 cm−1, atθ=θmax (HWFM ∼=3–3.5 cm−1) on ZrO2, in contrast to a single band for CO on ZnO. The spectrum probably reflects the presence of two well-developed and defect-free faces on the microcrystals of monoclinic ZrO2. As was observed for CO on ZnO as the coverage decreased, new bands appear and disappear, indicating a complex behavior corresponding to different CO environments on the two faces. The peak at 2167 cm−1has been tentatively ascribed to CO adsorbed on (111) faces, where the Zr4+
ions are separated by 0.35 nm and form a rhombohedral array. With di- luted isotopic mixtures, the dynamic and static shifts have been evaluated:
ν˜dyn=3 cm−1andν˜stat= −25 cm−1, respectively. From theν˜ value,αv
was calculated to be 0.024 ˚A, which suggests that electrostatic polarization forces are mainly involved. The static shift is close to that observed for CO on ZnO.
The structural and morphological properties of monoclinic ZrO2samples calcined at increasing temperatures were investigated by XRD, HRTEM, and IR techniques (603, 604). The data show that the (111) face is pre- dominant, and the (001) and (011) faces were observed only for samples sintered at low temperatures. Two types of surface OH groups and undis- sociated, strongly coordinated H2O molecules were detected by IR spec- troscopy. Computer simulations of the (111), (001), and (011) faces were used to explain the experimental results.
A detailed investigation of the surface properties of the tetragonal phase of ZrO2by HRTEM and IR techniques has also been reported (605). The tetragonal phase of zirconia is metastable at low temperatures. Therefore, yttria-stabilized samples were prepared and characterized (606–608). The structural, morphological, and surface hydration features of tetragonal ZrO2 were investigated by XRD, HRTEM, and IR techniques (606), and the interactions of the surfaces with CO (607) and CO2 (608) were reported.
2. Surface-Modified ZrO2
The surface properties of ZrO2can be selectively modified with additives such as these mentioned for TiO2. Sulfates markedly improve the catalytic properties (609–613); this broad and important subject deserves a separate review and is beyond the scope of this article.