Structure of Bulk Oxides

Combined Rotation and Multiple Pulse Spectroscopy (CRAMPS) is a technique in which the dipolar interaction is averaged through a multiple-pulse sequence [54, 55]. The simultaneous spinning around the magic angle, as in MAS NMR, averages the chemical shift anisotropy. Under appropriate conditions, CRAMP spectra can be of greater resolution than MAS NMR spectra. While CRAMPS is not exclusively a surface-sensitive technique, the majority of catalytic applications have focused on the study of adsorbed species, and the information on surface structure that can be extracted from their spectra. 

Metal oxide-based materials are widely employed as catalysts for a wide number of applications, particularly in processes such as dehydrogenation and oxidation, where redox chemistry is important The structure of metal oxides facilitates these reactions through the transfer of oxygen, or the removal of hydrogen. In order to fully understand the structural dependence of these processes, and hence to refine existing catalysts and catalytic processes and to develop new active materials, it is 

Vanadium catalysts are among the most significant metal oxide catalysts. For instance, considering only supported catalysts, between 1967 and 2000, 28% of all published papers were concerned with vanadium oxide-based materials [57]. This represents a greater fraction than for any other metal or metal oxide. One reason for this is the wide range of applications in which vanadium oxide catalysts may be employed, examples of which are outlined below. 

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An STM image of the two-dimensional Pd O surface oxide is displayed in Fig. 17.12a, together with the proposed atomic structure (Fig. 17.12b) [67]. Studies of oxidized nanoparticles by HRTEM and electron diffraction have reported surface oxides that did not match the structures of bulk oxides (e.g., for Rh [68-70]). With access to advanced imaging methods, such as STM combined with first-principles theoretical modeling using DFT, the exact structure has been determined ([63] and references therein). 

The release and uptake of oxygen without essentially changing the basic bulk structure of the oxides are desirable in catalysis (Gai 1993) and efforts are in progress to use these oxides as catalysts in selective oxidation reactions of hydrocarbons. 

To sum up, the available data relative to oxygen adsorption or catalysis on nickel oxide show the existence of two types of chemisorbed oxygen, one of them being related to the ability of this oxide to accommodate excess oxygen. The evidence concerning this latter property will now be reviewed with emphasis on the defect structure of bulk nickel oxide. 

Recent studies of supported vanadium oxide catalysts have revealed that the vanadium oxide component is present as a two-dimensional metal oxide overlayer on oxide supports (1). These surface vanadium oxide species are more selective than bulk, crystalline V2O5 for the partial oxidation of hydrocarbons (2). The molecular structures of the surface vanadium oxide species, however, have not been resolved (1,3,4). A characterization technique that has provided important information and insight into the molecular structures of surface metal oxide species is Raman spectroscopy (2,5). The molecular structures of metal oxides can be determined from Raman spectroscopy through the use of group theory, polarization data, and comparison of the... 

Enhancement of reactivity of incendiary components has been claimed by the introduction of impurity states, particularly into metallic oxides (Refs 56 86). Impurity states have a twofold effect they disturb the lattice structure of the oxide (and[ so increase the diffusivity of the reactants), and they disturb the electronic distribution on the surface as well as in the bulk. The argument is made that by doping the oxide, or by controlling the formation temp, one may change an oxide from an n-type to a p-type semiconductor and hence cause it to become a better electron acceptor, and vice versa... 

The bulk crystal structures of many oxides are discussed at length in Refs. 9 and 19. Our interest here is in determining the geometric structure of various surfaces that can exist on bulk oxide crystals. 

One important information coming from calculations is the structure of the oxide surface. Oxide surfaces are often heavily reconstructed or simply relaxed compared to the truncated bulk, and the experimental determination of the surface structure is often not easy. To this end, reliable classical potentials have been developed, in particular for the study of ionic crystals and of covalent solids. Nowadays, first principle band structure calculations making use of large supercells can also be used. These methods, although quite expensive from the point of view of the size of the calculations, provide results which are in excellent agreement with the experimental determinations. Band structure calculations, usually based on plane waves basis sets and on the density functional (DFT) approach represent the most appropriate eomputational... 

The electronic structure of bulk VO has been calculated by different band structure methods [110-114] and using correlated electron procedures [115]. This Mott-Hubbard metal, which forms a rocksadt type lattice, is the simplest of all single valence oxides of vanadium and has been treated theoretically already a long time ago. As an example, Neckel et al. [114] have published results from self-consistent APW calculations for the experimentally known lattice geometry... 

Raman chemical imaging can be employed to access the homogeneity and the structural stability in terms of oxidation rates, onset of hematite, and organic contamination of as-precipitate and oxidized iron oxide samples. Oxidation-related Raman features have been established by comparative study of bulk oxides and nanoparticles attained in two different oxidation states, suggesting that the solid nanophase synthesized had a mixed magnetite-maghemite composition [52]. 

For all phenomena mentioned, the characterization of the oxide bulk, surface/ interface and defect structure of metal oxides is most important, along with stmcture including geometric (crystal) and electronic structure, as well as composition. The variability in catalyst structure and chemical composition (which typically depend... 

Strength (FLS) empirical approach are discussed in Section 3 as methods for determining the molecular structures of metal-oxide species from their Raman spectra. The state-of-the-art in Raman instrumentation as well as new instrumental developments are discussed in Section 4. Sampling techniques typically employed in Raman spectroscopy experiments, ambient as well as in situ, are reviewed in Section S. The application of Raman spectroscopy to problems in heterogeneous catalysis (bulk mixed-oxide catalysts, supported metal-oxide catalysts, zeolites, and chemisorption studies) is discussed in depth in Section 6 by selecting a few recent examples from the literature. The future potential of Raman spectroscopy in heterogeneous catalysis is discussed in the fmal section. 

Important information coming from calculations is the structure of the oxide surface. Oxide surfaces are often heavily reconstructed or relaxed compared to the truncated bulk, and the experimental determination of the surface... 

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