Chromium ions at the surfaces of inorganic oxides are characterized by a wide variability of the oxidation state, coordination number, and local struc- ture. Consequently, Cr-based materials are especially attractive as catalysts.
Much is known about the catalytic activity of pure Cr2O3for various reac- tions (469), including polymerization of alkenes (470–472), hydrogenation–
dehydrogenation of hydrocarbons (473–481), reduction of NO and decom- position of N2O4(482), and oxidation of organic compounds (483, 484).
α-Cr2O3is an ideal model system because it can be obtained either in the form of microcrystalline powders (22, 23) with well-defined habits and exposing thermodynamically stable and extended faces or in the form of crystalline oxide films (4, 5, 485–487). In the following, the surface properties of α-Cr2O3 polycrystalline samples at different degrees of sintering (and hence with the crystalline habits varied over a wide range) are summarized
338 A. ZECCHINA et al.
on the basis of spectroscopic results, which are compared with those obtained for single crystals.
1. Morphology and Surface Structures: Comparison of Dispersed Materials and Crystal Films
α-Cr2O3microcrystals can be prepared easily by combustion of ammo- nium dichromate, and their surface properties can be characterized by IR spectroscopy with CO and other small molecules (N2, O2, and H2) as probes (488–493). The preparedα-Cr2O3samples have high specific surface areas (about 40–70 m2g−1), and the microparticles have dimensions in the range of 20–40 nm. The particles are irregularly shaped. However, as shown by the presence of interference fringes in the HRTEM images, they have crystalline character (77, 491, 492) (Fig. 23a).
FIG. 23. HRTEM images of (a) high-surface-area, [reproduced from Scaranoet al.(492) with permission of Elsevier Science Publishers] and (b) sinteredα-Cr2O3samples [adapted from Scaranoet al.(491) with permission of Elsevier Science Publishers]; (c) SEM images of highly sinteredα-Cr2O3samples [reproduced from Scaranoet al.(77) with permission of The Royal Society of Chemistry].
FIG. 23. (continued)
In sintered samples, the dimensions of the microcrystals are typically 50–
100 nm, corresponding to specific surface areas of about 5 m2g−1. These assume the shapes of simple polyhedra, the edges of which are often straight without signs of imperfections, suggesting that the intersecting faces are regular. The particles are well-grown single microcrystals, as demonstrated by the interference fringes of the corresponding planes, which are observed when the particles are suitably oriented with respect to the electron beam (Fig. 23b).
340 A. ZECCHINA et al.
FIG. 23. (continued)
The angles formed by the edge traces are statistically grouped into a few narrow intervals, suggesting that the morphologies of these well-grown crys- tals are represented by simple polyhedra that expose only a few planes. On the basis of the detailed analysis of the electron microscopy images, energy considerations, and computer simulations, it was concluded that the exposed planes are (01 ¯12) and, to a lesser extent, ( ¯2116) and (11 ¯20). These faces are all neutral, as expected for samples that have been treated at high temperatures and consequently assume the most thermodynamically stable morphology, as is generally observed for oxides.
Further sintering at high temperature increases the size of the microcrys- tals without significant modifications of the shape, as illustrated by SEM images (Fig. 23c). Such observations are indirect evidence suggesting that the distribution of faces is not seriously altered (77), but this interpretation appears to be inadequate because it does not take account of the accurate analysis of the crossing plane fringes and the simultaneous use of simulation methods.
Although the experimentally determined faces of sintered microcrystals are all neutral, there are many reports indicating that oriented Cr2O3(0001) thin films are preferentially formed by epitaxial growth on metal single crys- tals (281, 494–496); well-ordered, nonpolar (10 ¯12) surfaces have received far less attention.
The structures of the three families of faces, terminating the polyhedra of well-sintered samples, as obtained by simply cutting the crystal lattices along the (01 ¯12), ( ¯2116), and (11 ¯20) planes taking into account surface relaxation have been discussed extensively (22) and are summarized only briefly here (Fig. 24).
First, the (01 ¯12) face contains an array of equivalent Cr3+ions in fivefold (square pyramidal) coordination separated by 0.365 nm (corresponding to 6.8 Cr ions/100 ˚A2). The plane defined by the the four oxygen ions surround- ing each Cr3+ion does not coincide with the surface plane, indicating that not only is the surface slightly rumpled but also the direction, where the electric field associated with positive charge is maximized, is not perpen- dicular to the surface plane. HRTEM images show that these faces are the predominant ones.
FIG. 24. Representation of the most commonly exposedα-Cr2O3faces. The distances (ex- pressed in ˚A) between the Cr3+ions are reported. Cr3+are white and O2−are gray.
342 A. ZECCHINA et al.
Second, the neutral face, belonging to the ( ¯2116) family, contains four- and fivefold coordinated ions, aligned along rows 0.365 nm apart. Within each row, the four- and fivefold coordinated sites are located in pairs only 0.289 nm apart, and the distance between the pairs is 0.574 nm. The density of sites is 7.65 Cr ions/100 ˚A2. The presence of fourfold coordinated ions suggests that these faces should be more reactive and more prone to reconstruction phenomena than the (01 ¯12) faces.
Finally, the (11 ¯20) face is characterized by the highest density of sites (10.88 Cr ions/100 ˚A2). These Cr3+sites are all fivefold coordinated and present in pairs 0.265 nm apart, and the distance between the pairs is 0.415 nm. Because the Cr3+ions are greatly shielded by the surrounding oxygen ions (which are located at an upper level), this surface appears quite homopolar, and it is expected to be less reactive or even unreactive with probe molecules.
2. IR Spectra of Adsorbed CO
The typical sequence of spectra (in the 2200- to 2100-cm−1 range) ob- served at increasing coverages of CO on the three progressively sintered samples described previously (see Fig. 23) is shown in Fig. 25. As mentioned
FIG. 25. IR spectra of12CO adsorbed at∼100 K on progressively sinteredα-Cr2O3samples for CO coverages ranging fromθ =1 (5.33 kPa) toθ→0 on (a) high-surface-area samples, (b) sintered samples, and (c) highly sintered samples (dashed line in b): IR spectrum of12CO/13CO (15 : 85) mixture at maximum coverage [reproduced from Zecchinaet al.(22) with permission of Elsevier Science Publishers].
previously, these samples are characterized by crystallites of increasing size and morphological perfection (from∼20–40 to 50–100 nm up to 0.5–0.8àm), with the sintering procedure having progressively eliminated or restructured the less stable faces (i.e., those containing the most coordinatively unsatu- rated ions). These morphological differences explain the following obser- vations: (i) The intensity of the CO stretching bands gradually decreases with sintering, (ii) the IR spectrum undergoes a progressive simplification, and (iii) the FWHM of the single components gradually decreases (because inhomogeneous broadening effects associated with the surface irregulari- ties are progressively eliminated). These observations further support the strategy of progressive sintering (vide supraFigs. 7, 10, and 12) for the char- acterization of sites located on surface planes of high structural perfection (comparable to that of single-crystal faces). On passing from the sequence of spectra represented in Fig. 25a to those shown in Fig. 25b, the carbonyl bands become more structured, and three intense narrow bands, at 2179, 2172, and 2167 cm−1(measured atθ∼=1), are detected which are not clearly separated in Fig. 25a. These changes are associated with the increasing per- fection of the exposed faces, and they are an indirect consequence of the formation of polyhedra with more defined structure. On the basis of the rel- ative abundance of the faces defining the microcrystals shown in Fig. 1, the 2179, 2172-cm−1doublet (θ∼=1) has been assigned to CO on five- and four- fold coordinated Cr3+ions of the ( ¯2116) face, and the peak at 2167 cm−1can be safely attributed to CO end-on adsorbed on Cr3+ions of the (01 ¯12) face.
Further sintering not only causes a drastic decrease in the overall inten- sity of the CO spectrum but also leads to the preferential disappearance of the peaks associated with the ( ¯2116) faces (Fig. 25c). Simultaneously, a new, broader peak appears at ∼2157 cm−1, which can be ascribed to CO adsorbed on highly shielded Cr ions exposed on the (11 ¯20) faces. In agree- ment with this hypothesis, formation of this new band is highly reversible, and it quickly disappears upon outgassing of the sample at 110–120 K. This result indicates that after severe sintering treatments, the dominant faces are largely homopolar and characterized by Cr3+ ions in highly shielded positions.
A similar broad band (at ∼2158 cm−1) was found for CO on α-Al2O3
obtained by sintering ofγ- andδ-Al2O3at 1373 K (32, 497) and on sintered α-Fe2O3 (498). These observations suggest that the surfaces of all oxides treated at very high temperatures tend to acquire a homopolar character because the small positive ions occupy relaxed positions to minimize the surface energy.
To extract more structural information from the IR spectra of CO ad- sorbed on the different faces, the detailed behavior of the main peak, which
344 A. ZECCHINA et al.
is associated with CO adsorbed through the carbon end on Cr3+ions on the (01 ¯12) faces (Fig. 25c), is discussed.
First, the peak is initially observed at 2181 cm−1(θ ∼=0, singleton) and then gradually moves to 2167 cm−1(θ→1) as a consequence of adsorbate–
adsorbate interactions (ν˜ = −14 cm−1). Simultaneously, the FWHM de- creases from 4 to 1.5 cm−1, thus indicating that the exposed (01 ¯12) faces are extended and nearly defect free.
Second, isotopic substitution experiments using 12CO–13CO mixtures show that theν˜ = −14 cm−1 shift is the result of two opposing effects, namely, dynamic (ν˜dyn=13 cm−1) and static (ν˜st= −27 cm−1). By stan- dard methods, the dynamic polarizabilityαvwas found from theν˜dynvalue to beαv = 0.1007 ˚A.
The stretching frequency of CO adsorbed on Cr3+ions of (01 ¯12) faces is blue shifted relative to the gas-phase CO frequency. Electrostatic forces and σbonding play the most important roles in the adsorption. The Stark effect associated with the positive electric field centered at the Cr3+sites increases the stretching frequency of adsorbed CO (268).
The observed shift with respect the gas phase is similar to that found for CO on other oxides, where nodelectrons are involved. This result seems to suggest thatd–πoverlap does not occur. However, when the dynamic shifts and the dynamic polarizabilities are considered, the observed analogies with nontransition metal oxides are not so straightforward. It is a matter of fact that in the case ofα-Cr2O3the dynamic polarizabilityαv, is definitely higher than expected for a surface complex, where only electrostatic forces and σbonding are operating (ZnO/CO system). This observation has also been reported for other transition metal cations. In view of the great sensitivity of theαvparameter tod–πcontributions, the Cr3+ã ã ã CO bond is inferred to be characterized not only byσdonation but also by somed–πbackdonation contributions (13, 92).
The adsorption of CO on a site is also accompanied by backrelaxation of the surrounding ions. This is a through-solid effect, which must be added to the through-space effects (static dipole–dipole interactions, etc.) to ex- plain the largerν˜statshift (53, 499, 500).
The CO adsorption on (11 ¯20) faces (being predominant after severe sin- tering) is characterized by ˜ν(CO)=2157 cm−1. This frequency is in line with the structure of Cr3+centers, which are more shielded by O2−ions in the nearest positions; hence, they are associated with electric fields significantly lower than those characteristic of (01 ¯12) faces. Consequently, the perturba- tion (and the related adsorption enthalpy) of the CO molecule adsorbed on these shielded Cr3+sites is expected to be small and the corresponding stretching frequency to be only slightly blue shifted relative to that of gas- phase CO. Another consequence of the low interaction strength is a longer
equilibrium distance [∼3.7 ˚A (77)] which will not permit any orbital overlap to occur (i.e.,σandd–πeffects are totally absent).
3. Correlation of the Stretching Frequencies of Adsorbed CO with the Surface Electric Fields at Lewis Acid (Cr3+) Sites
Once the positions of surface cations and anions are determined, the elec- tric fields at the center of mass of adsorbed CO can be calculated by the procedure outlined previously. After this preliminary step, the known elec- tric field– ˜ν(CO) relationships (56) readily give the stretching frequencies of CO adsorbed at the Cr3+centers located on both relaxed and unrelaxed faces. The computed frequencies are then compared with the experimental values. The following observations emerge (77):
1. For prismatic (01 ¯12) and (11 ¯20) faces ofα-Cr2O3, theν(CO) frequencies and the associated electric fields do not appreciably depend on relaxation.
2. For ( ¯2116) faces, the frequencies and electric fields critically depend on the relaxation because the Cr3+ions located in exposed positions undergo a remarkable inward relaxation.
3. The calculated frequencies are in satisfactory agreement with the ex- perimental values for the (01 ¯12) and (11 ¯20) faces. This result again indicates that the Cr3+ ã ã ã CO bond is mainly electrostatic in nature (even if a minor contribution of chemical overlap forces is present).
4. For species adsorbed on ( ¯2116) faces, the experimental frequencies are closer to those calculated for relaxed structures than to those obtained for un- relaxed models. This result is expected because the backrelaxation induced by the reversibly adsorbed CO on this highly coordinatively unsaturated and strongly relaxed surface is quite modest.
In summary CO is a good probe of surface fields and, indirectly, of surface Lewis acidity, as has also been observed for other systems withoutdelec- trons. A close examination of other, more subtle effects (dipole–dipole in- teractions) shows that a smalld–π contribution is present which primarily affects the dynamic polarizability (i.e., the dynamic charge transfer from adsorbed CO to the surface centers and vice versa during the stretching motion).
4. C2H4Adsorption and C2H4/CO Coadsorption
In many chromium-containing oxides, such as Cr/SiO2and Cr/Al2O3for ethene polymerization, the first step of ethene activation is the formation of C2H4–Cr2+(or C2H4–Cr3+) molecular complexes. These molecular com- plexes are then transformed into more strongly adsorbed species of unknown
346 A. ZECCHINA et al.
structure, which play a role in the initiation steps of the polymerization reaction (501–507). The understanding of the interaction of C2H4with struc- turally well-defined Cr3+centers, such as those located on low-index faces of α-Cr2O3, is expected to assist the elucidation of the fundamental chemistry of the Cr3+/C2H4system (493). The main results obtained for polycrystalline samples (Fig. 26) can be briefly summarized as follows: (i) Ethene is molecu- larly adsorbed on a fivefold coordinated Cr3+site of the predominant (01 ¯12) face, giving a coordination complex with local C2vsymmetry in which ethene interacts with the electron-withdrawing Cr3+centers through aπbond per- pendicular to the plane of the alkene; (ii) the ethene molecule oligomerizes on only a few defective sites; and (iii) the polymerization activity of reduced samples is definitely greater than that of stoichiometric samples. It is inferred that the family of active sites is a small fraction of Cr2+centers formed during the activation at high temperature in the presence of hydrocarbon impurities
FIG. 26. IR time-resolved spectra of ethene polymerization reaction on (a) nearly stoichio- metric and (b) reducedα-Cr2O3samples: CH3and CH2stretching mode regions. Continuous curves, IR spectra taken at 10-s intervals (a) and 7-s intervals (b) in the presence ofP= 5.32 kPa of ethene; dashed curves after a total contact time of 30 (a) and 8 min (b) and ethene removal by outgassing at room temperature; curves on the bottom, ethene gas [reprinted with permission from Scaranoet al.(493), Copyright 1994 American Chemical Society].
and presumably located on high-index faces, edges, and steps. Starting from these few reduced sites, the living polymer chains grow and spread out on the flat surfaces and tend to gradually cover the whole microcrystal (Fig. 27;
see color insert). The IR spectra of these polymer chains demonstrate that the CH2groups of the chain interact with the Cr3+centers of the flat surfaces.
Anomalous low-frequency bands in both the (CH2) stretching and bending regions demonstrate agostic-type interactions between –CH and Cr3+sites (Fig. 28). This weak agostic interaction is destroyed by CO because CO is
FIG. 28. Modifications of the IR spectrum of living polymer chains on stoichiometricα-Cr2O3 induced by CO: solid curve, before12CO adsorption; dashed curve, after12CO adsorption (p= 2.66 kPa) (Inset) Expanded view of theδ(CH2) mode region [reprinted with permission from Scaranoet al.(493). Copyright 1994 American Chemical Society].
348 A. ZECCHINA et al.
able to displace the –CH groups from the Cr3+centers (Figs. 27b and Fig. 28).
The ability of Cr3+surface ions to undergo agostic interactions with satu- rated –CH groups has also been demonstrated by IR spectra of adsorbed n-heptane (493).
The main conclusions are that (i) Fivefold coordinated Cr3+sites exposed on the (01 ¯12) faces, although reactive in the formation of weak molecu- lar Cr3+–ethene complexes, are not active in catalytic polymerization, and (ii) polymerization (and oligomerization) activity is attributed only to Cr2+
centers, located at structural defects (such as edges, steps, and corners).
This last conclusion strongly suggests that a highly coordinatively unsatu- rated state is a necessary prerequisite for the polymerization activity of Crx+
centers.
5. H2Adsorption
Because of the similarity between the electronic structure of H–H and C–H bonds in hydrogen and hydrocarbon molecules, respectively, it is ex- pected that Cr3+sites ofα-Cr2O3 can be reactive in hydrogen adsorption.
As shown in Fig. 29, H2 adsorption on sinteredα-Cr2O3 samples leads to the development of a narrow band at about 3936 cm−1. This band, never reported before, is clearly the ˜ν(HH) of hydrogen molecularly adsorbed on Cr3+sites located on predominant (01 ¯12) faces (unpublished results). We in- fer that the hydrogen molecule is adsorbed in end-on form and that the shift
˜ν∼= −225 cm−1is associated with the polarization induced by the electric field centered at the Cr3+ions. This interaction is similar to that described for H3C–H, and the spectroscopic consequences with regard to the stretching frequencies are also very similar (shift to lower ˜ν). That polarization ef- fects can lower the ˜ν(HH) frequency has already been demonstrated for the titanosilicate ETS10/H2system, in which H2interacts with Na+ions (508).
In addition to the bands for H2molecularly adsorbed on extended faces, very weak irreversible bands at 3600 and 1590 cm−1are observed (results not reported here). These bands are the ˜ν(OH) and ˜ν(CrH) modes of hydrogen species dissociatively adsorbed on Cr3+O2−pairs located at less abundant defects (e.g., edges and steps).
6. Interaction ofα-Cr2O3with Other Simple Molecules
The adsorption of NO on morphologically well-defined samples has not been investigated in detail. On high-surface-area materials (509, 510), sta- ble nitrosyl species are formed together with disproportionation products (N2O and NO−2); this complex chemistry renders the interpretation of the resulting (complex) spectra difficult (unpublished results). However, the
FIG. 29. IR spectra of H2adsorbed at 77 K onα-Cr2O3samples in the ˜ν(H–H) region for coverages ranging fromθ = 1 (5.33 kPa) toθ→0 (unpublished).
higher stability of surface nitrosyls relative to the corresponding carbonyl species can be considered to be a general feature of transition metal oxide surface chemistry.
The catalytic activity of chromia for the NO+NH3reaction in the pres- ence of oxygen and activity/morphology relations has been investigated (511, 512). The activities of amorphous and of crystallineα-chromia in various re- action mixtures (NH3+NO+O2, NH3+O2, and NH3+NO) have been compared. The specific activity and selectivity of each system were reviewed (476). Amorphous chromia was found to be more active than crystalline chromia in the typical temperature range (423–473 K) because of its higher density of labile oxygen sites.
7. Hydrogenation/Dehydrogenation Reactions
Because of the importance for industrial production of light alkenes such as propene (the precursor for high-purity polypropylene) and isobutylene