Bulk structural complexity

One of the most significant results from the advent of these surface science studies on oxides relevant for the present catalytic applications is the fact that oxides can be multiply terminated and that they are not terminated [154, 180, 186-190] in cuts through the bulk structure. This is not unexpected in general [98,156,179] but it is of great value to know this in attempts to understand the mechanisms that activate oxides for catalysis. These rigorous studies must be differentiated from more empirical studies carried out on termination issue with qualitative methods and without predictive power but with the still invaluable advantage that they can be applied [97,191-193] to complex MMO catalyst systems. Such studies can be used to probe the surface reactivity, to address the issue of segregation of, for example, vanadium out of an MMO system and to compare different qualities of the nominally same material with speculative assumptions about the influence of defects. 

Selective oxidation materials fall into two broad categories supported systems and bulk systems. The latter are of more practical relevance although one intermediary system, namely vanadia on titania [92,199-201], is of substantial technical relevance. This system is intermediary as titania may not be considered an inert support but rather as a co-catalysts [202] capable of, for example, delivering lattice oxygen to the surface. The bulk systems [100, 121, 135, 203] all consist of structurally complex oxides such as vanadyl phosphates, molybdates with main group components (BiMo), molybdo-vanadates, molybdo-ferrates and heteropolyacids based on Mo and W (sometimes with a broad variation of chemical composition). The reviews mentioned in Table 1.1 deal with many of these material classes. 

Regarding the morphology, a polyhedron terminated by the (001) face is expected for cubic II-IV perovskites, i.e., AB03 perovskites in which A and B are divalent and tetravalent, respectively. In these perovskites, two nonpolar (001) surface terminations are possible (AO and BO2). On an A-O terminated surface, the cation A is octa-coordinated, whereas on the BO2 terminated surface the cation B is penta-coordinated. Ill—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 complex and show a different coordinative environment for both A and B cations. 

Spectroscopic and bulk aqueous complexation studies of major lichen acids, including derivation of the structures and stabilities of metal-organic complexes and precipitates involving a wide range of elements. 

Figure 9. Crystal packing arrangement of tetralinchromium tricarbonyl complexed with TOT. The TOT units are not shown for clarity. The view illustrates the polar arrangement of the organometallic in the bulk structure.

In general, borates are structurally complex, since the boron atoms can be in 3 and/or 4 coordination and oligomer, ring, and chain polymers are all found (Christ and Clark, 1977 Wells, 1975). We shall not attempt to describe fully the complexity of these structures but will concentrate on the fundamental polyhedral units. The molecular geometric and electronic structures of these materials can be studied using many of the site-specific spectroscopies previously discussed. The bulk properties of the materials also change, of course, depending upon the molecular structure. 

For a discrete molecule with a simple structure, a microstructure is sufficient to characterize the given molecule. For a complex system such as that of asphaltene, the information required for characterization has to include association as well as micelle formation. The microstructure has been chosen arbitrarily to refer to short-range bonding, that is, distances between 0.5 A-2.0 A whereas the macrostructure (bulk structure) pertains to molecular interactions or orders at larger distances (20 A-2000 A). 

These examples are typical of molecular imprinting with metal complexes in bulk polymers. The bulk structures depend highly on the source of monomer and... 

With the advent of synthetic methods to produce more advanced model systems (cluster- or nanoparticle-based systems either in the gas phase or on planar surfaces), we come to the modern age of surface chemistry and heterogeneous catalysis. Castleman and coworkers demonstrate the large influence that charge, size, and composition of metal oxide clusters generated in the gas phase can have on the mechanism of a catalytic reaction. Rupprechter (Chap. 15) reports on the stmctural and catalytic properties of planar noble metal nanocrystals on thin oxide support films in vacuum and under high-pressure conditions. The theme of model systems of nanoparticles supported on planar metal oxide substrates is continued with a chapter on the formation of planar catalyst based on size-selected cluster deposition methods. In a second contribution from Rupprecther (Chap. 17), the complexities of surface chemistry and heterogeneous catalysis on metal oxide films and nanostructures, where the extension of the bulk structure to the surface often does not occur and the surface chemistry is often dominated by surface defects, are discussed. 

The concept of active sites has helped explain catalysis by enzymes and coenzymes. Although surface functional groups are less specific than enzymes, they form an array of surface complexes whose reactivities determine the mechanism of many surface-controlled processes. Many mechanisms can be described readily in terms of Br0nsted acid sites or Lewis acid sites. Of course, the properties of the surfaces are influenced by the properties and conditions of the bulk structure, and the action of special surface structural entities will be influenced by the properties of both surface and bulk. List I gives an overview of the major concepts and important applications. 

While the effective alloy Hamiltonian described above allowed us to consider structural competitors in bulk alloys, we have also seen that the structural complexity on crystal surfaces can be tamed using the idea of effective Hamiltonians. Indeed, our discussion of the surface reconstructions on both the Au and W surfaces culminated in effective Hamiltonians. For example, in our discussion of the reconstructions on the (100) surface of fee Au we invoked an effective Hamiltonian of the form... 

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