Structure and Bonding
Here, information about the structures of the most commonly encoun- tered classes of oxides and halides is given. This brief description is intended to facilitate understanding of the following sections and to help the reader understand the geometrical properties that determine the structure and re- activity of the most commonly exposed faces of oxides and halides.
A. OXIDES ANDHALIDES WITHROCKSALTSTRUCTURE
MgO and NaCl are the best examples of this class of ionic solids, which includes NiO, CoO, CaO, BaO, and LiF (24). The morphologies of these solids are represented in Fig. 1, in which the local geometric structures of low-index (100), (010), and (001) faces on edges and corners are illustrated schematically. The morphologies of the microparticles represented in Fig. 1 have been determined on the basis of results obtained experimentally and with computer modeling techniques.
In the bulk, cations and anions are sixfold coordinated. The total energy of the crystal is nearly completely given by the Madelung (electrostatic) energy. On the surfaces of the microcrystals, different local coordinations are encountered, with coordination numbers varying from five on the (100), (010), and (001) faces to four on edges and steps and three on corners.
Threefold coordinated ions are also present on reconstructed (111) faces.
As a consequence of the identical crystallographic structure and the very similar ionic radii, MgO and NiO or CoO form uniform solid sol- utions Mg(1−x)MxO (0 ≤ x ≤ 1), where M is Ni or Co (25). Real crys- tals are often covered by strongly adsorbed water and carbon dioxide, and thermal treatments in vacuo are needed to clean the surfaces and create the surface coordinative unsaturation mentioned previously (in par- ticular the fourfold and threefold coordinated sites are cleaned only at very high temperatures). In this connection, it is evident that the surface chemis- try of clean surfaces is primarily determined by the presence of these
FIG. 1. Representation of particle morphologies as obtained by the combined use of HRTEM and computer modeling.
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coordinatively unsaturated sites, the relative abundance of which is influ- enced by the surface morphology.
B. OXIDES ANDHALIDES WITH THEWURTZITESTRUCTURE
In these systems, the metal and oxygen or halogen ions are fourfold co- ordinated. ZnO and CuCl are the best examples of this group of solids. The tetrahedral coordination is usually associated with a substantial degree of covalency, and consequently the positive and negative centers do not carry the full charge (half of the value is considered a good approximation in many cases).
Electron microscopy results and surface energy calculations show that typical microcrystals of ZnO expose preferentially the (10 ¯10) and (11 ¯20) faces (Fig. 1), where the ions are threefold coordinated; this is a situation only rarely encountered with systems having the rock salt structure. Ions in still lower coordination state can potentially be found on edges, steps, and corners. However, such highly coordinatively unsaturated sites cannot really be found on microcrystals of stoichiometric ZnO because hydroxyl groups permanently saturate these coordinative vacancies. These groups cannot be eliminated even under the most severe thermal treatmentsin vacuowithout altering the stoichiometry of the crystals (26). For this reason, these highly defective structures will not be considered in the case studies for the wurtzite structure.
C. CUPRITE(Cu2O) STRUCTURE
Because of the importance of cuprous oxide and cuprous-based systems in catalysis, the most common shape of the microcrystals of Cu2O, as deter- mined experimentally and theoretically, is reported in Fig. 1. Several investi- gations of Cu2O single crystals have been reported, and these are particularly valuable for purposes of this review.
The cuprite structure consists of a body-centered cubic (bcc) array of oxygen atoms, and the copper atoms occupy centers of four of the eight cubes into which the bbc cell may be divided. In this partially covalent structure, copper has a linear coordination and oxygen a tetrahedral coordination.
D. OXIDES WITH THECORUNDUMSTRUCTURE
The corundum structure is assumed by three oxides of primary importance in catalysis, namely,α-Cr2O3,α-Al2O3, andα-Fe2O3(which,inter alia,form solid solutions in the complete compositional range) (21, 27). The three- dimensional arrangement of the ions in this hexagonal structure consists of a hexagonal close-packed array of anions with the cations occupying two- thirds of the octahedral interstices.
The shape of the microparticles is reported in Fig. 1. Although bulk cations are six-fold coordinated, the ions on regular low-index faces can assume five- fold [(11 ¯20), (01 ¯12), and ( ¯2116) faces] and fourfold [( ¯2116) face] coordina- tion. Higher coordinative unsaturation is encountered only at edges, steps, and corners (usually saturated by hydroxyl groups). Because the radius to charge ratios of Al3+ and Cr3+ are small, there are only small deviations from full ionicity. For the same reason, the effective charges present on the positive and negative ions are smaller than the formal ones.
E. OXIDES WITHHEXAGONALSTRUCTURE: La2O3
Among the oxides with hexagonal structure (La2O3, Y2O3, and Ce2O3), which find application mostly as ceramic materials, lanthanum oxide is the one with the most thoroughly investigated surface structure. Both experimental results (analysis of the surface profile images of the micro- crystals by HRTEM) (28) and theoretical results are available (29). Conse- quently, we focus on La2O3.
In this structure, each La ion is surrounded by seven oxygen ions—four in tetrahedral and three in octahedral coordination. This coordination is associated with the large radius of the lanthanum ion. Experimental and theoretical investigations (28, 29) show that the most stable surfaces are the (001) and the (011) faces; the stability of the (001) face is greater than that of the (011) face. On this basis, it is inferred that the most plausible shape of the microcrystals of well-crystallized La2O3can be represented as in Fig. 1.
F. SPINELS
Spinels form a large class of compounds that are important in catalysis.
The structure of “normal” spinels is related to that of MgAl2O4, the unit cell of which contains 32 oxygen atoms in a nearly perfect cubic array (30, 31).
The Mg and Al cations occupy tetrahedral and octahedral sites, respectively.
In addition to Mg, other divalent cations, including Co, Ni, and Cu, can be incorporated in the spinel structure. Al can be replaced by Ga, In, Cr, and other trivalent ions. In addition to normal spinels, so-called inverse structures are also possible in which half the trivalent cations occupy the octahedral sites, with the other trivalent cations and all the divalent cations in tetrahedral positions. Several factors influence whether a given spinel will adopt the normal or inverse structure, including the following: (i) the relative ionic radii, (ii) the Madelung constants of the normal and inverse structures, (iii) the crystal field stabilization energy of the transition metal ions, and (iv) polarization or covalency effects. Defective spinel structures are also known as acidic supports in catalysts, such asγ-Al2O3andδ-Al2O3(32).
A variety of coordination states (threefold and fivefold) exist on the low- index surfaces (Fig. 1), depending on (i) the local structure of the bulk site
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(tetrahedral or octahedral) from which the surface site has originated and (ii) the face index. Because of the structural complexity of these systems, more details are given in the following sections, in which special cases are considered.
G. PEROVSKITES
Oxides with the perovskite-type structure constitute an important class of inorganic solids having the general formula ABX3, in which A (the large cation) may be an alkali, alkaline earth, or rare earth ion; B represents a small transition metal or main group ion; and X is commonly oxygen or a halide. The perfect structure is cubic and shows a network of corner-sharing BX6 octahedra, centered on the transition metal ions B. All the oxygen atoms are in topologically equivalent 2-coordinate positions, bridging the two adjacent B sites. The dodecahedral interstices of the anionic sublattice are occupied by a second cation A, and the cationic charge can be distributed in different ways between octahedral and dodecahedral sites so as to ensure electroneutrality. Examples are KNbO3(K+, Nb5+), CaTiO3(Ca2+, Ti4+), and LaCrO3(La3+, Cr3+).
Regarding the morphology, a polyhedron terminated by the (001) face is expected for cubic II–IV perovskites, i.e., ABO3perovskites in which A and B are divalent and tetravalent, respectively. In these perovskites, two non- polar (001) surface terminations are possible (AO and BO2). On an A–O terminated surface, the cation A is octa-coordinated, whereas on the BO2ter- minated surface the cation B is penta-coordinated. III–III perovskites, bulk structures with lower symmetry, are more stable (orthorombic or rhombohe- dral) than II–IV perovskites, and the nonpolar low-index faces are more com- plex and show a different coordinative environment for both A and B cations.
More than 90% of the natural metallic elements of the periodic table form perovskites; the wide range of cations, the possibility of partial substi- tution of A or B cation sites, and the remarkable capacity to accommodate a multitude of different kinds of defects result in a wealth of properties of these solids leading to applications ranging from superconductors (33) to oxidation catalysts (34).
H. OXIDES WITH THEAB2STRUCTURE: TiO2
TiO2is an important oxide with a broad range of applications in catalysis (as a catalyst or a support) (6), photocatalysis (35, 36), and sensor technol- ogy; it is also used as a pigment. Of the three titanium dioxide polymorphs (rutile, anatase, and brookite), rutile and anatase have been most widely investigated; they are the only ones reviewed here.
Both rutile and anatase crystallize in a tetragonal lattice, and their bulk structure can be described basically in terms of a three-dimensional ar- rangement of TiO6 octahedra. The two polymorphs differ by the degree of distortion of each octahedral unit and by the manner in which the TiO6
building blocks are spatially assembled. The structural differences described previously result in different physicochemical properties, such as density (4.250 g cm−3for rutile and 3.894 g cm−3for anatase) and stability (rutile is more stable than anatase by about 4.9 kJ mol−1).
Most of the data available for polycrystalline TiO2concern the anatase modification, for which correlations of adsorption properties with the mi- crocrystal morphology (as determined by electron microscopy) have been realized. It has been shown (37) that TiO2 obtained by flame hydrolysis of TiCl4 (anatase containing 25% rutile, Titanoxid P25 from Degussa) is made of well-shaped hexagonal prisms, with crystallite dimensions in the 10–50-nm range, predominantly exposing the (010) face and, to a lesser extent, the (101) and (001) faces. A simplified model illustrating the mor- phology of the anatase P25 microcrystals is represented in Fig. 1. The (010) (or the isostructural (100)) and the (101) (or the isostructural (011)) termi- nations have also been found to be predominant on TiO2samples prepared by other routes; (110), (111), (112), and (113) faces have been also observed, although to a lesser extent (38, 39).
As shown in Fig. 1, on both the (010) and (001) faces the exposed Ti4+
ions are fivefold coordinated. Molecular dynamics calculations (40) indicate the occurrence of some relaxation of the (001) surface, which does not, however, change the coordination of the metal ions. Unlike the (010) and (001) terminations, the (101) face is polar in nature, and extensive surface reconstruction is therefore to be expected.
We mention that the surface of prepared polycrystalline TiO2samples is strongly hydrated because of the presence of both surface hydroxyl groups and coordinated molecular water, and that small amounts of residual OH groups are still present even after outgassingin vacuoat temperature as high as 973 K (38, 39, 41).
I. AMORPHOUSSiO2 WITH AND WITHOUTTRANSITIONMETALIONS
Silicas are amorphous materials formed by irregularly corner-linked rigid tetrahedra [SiO4] forming flexible rings and chains. The [SiO4] unit, which is the building block of all siliceous materials, is able to form a peculiar Si–O–Si bond with other [SiO4] moieties characterized by Si–O–Si angles ranging from 130 to 180◦with very low energy cost (42), as shown by cal- culations adopting different basis sets (43). Because of this flexibility, various models of amorphous silica have been constructed as networks
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of SiO4 building blocks with a random distribution of the Si–O–Si angle centered around 140◦.
On the external surfaces, valences are saturated by hydrogen forming hydroxyl groups. In addition to siloxane bridges, four types of surface hy- droxyl groups are distinguished: isolated, geminal, vicinal, and interacting.
29Si CP/magic angle spining nuclear magnetic resonance (NMR) spectra con- firm this picture, showing three silicon signals (bulk silicon, silicon carring one hydroxyl, and silicon linked to two hydroxyls) (44). Surface hydroxyl groups are widely used to anchor foreign species (transition metal ions) that are highly dispersed reactive species. One of the silica-based systems most widely investigated, because of its importance as an ethene polymerization catalyst (Phillips or Union Carbide processes), is represented by Crn+grafted on silica (45). We describe this system in detail later. The surface properties of amophous silica are not reviewed here because they have already been described extensively (46).