Bulk metal oxides structures

The dehydrated surface metal oxide species are not coordinated to water and, therefore, their molecular structures are not related to those present in aqueous solutions. Consequently, the pH at PZC model cannot be employed to predict the dehydrated surface metal oxide structures. The molecular structures of the dehydrated surface metal oxide species, however, possess similarity to the structural inorganic chemistry of bulk metal oxides because of the absence of water ligands in both systems [60-62]. Instead of being solvated by coordinated water in the aqueous solution complexes, the bulk metal oxide structures are coordinated to various cations (e.g., K, Na, Ca, Mg, Fe, Al, Ce, Zr, H, etc.). Prior to discussing the current understanding of the molecular structures of the dehydrated surface metal oxide species, a brief review of the structural inorganic chemistry of bulk metal oxides and their determination methods are presented to highlight the molecular structural similarities, as well as differences, between these two- and three-dimensional metal oxide systems. 

The bulk metal oxide structures have been determined with extensive and highly accurate x-ray diffraction crystallographic studies [60]. Unfortunately, the structural inorganic chemistry of dehydrated surface metal oxides on oxide supports cannot be determined with x-ray diffraction crystallography because of the absence of long-range order (>4 nm) in the surface metal oxide overalyers. Information about the local structures of the dehydrated surface metal oxides, however, can be obtained with in situ molecular spectroscopic techniques of dehydrated supported metal oxides Raman [31,63], IR [64], UV-Vis [44,50,65,66], XANES/EXAFS [46-51,67,68], chemiluminescence [69], and solid stateNMR for certain nuclei (e.g., Mo, H, etc.) [32,33,70,71]. UV-Vis, XANES/EXAFS,... 

As an initial example of using DFT calculations to describe phase stability, we will continue our discussion of bulk metal oxides. In this chapter, we are interested in describing the thermodynamic stability of a metal, M, in equilibrium with gas-phase O2 at a specified pressure, Po2, and temperature, T. We will assume that we know a series of candidate crystal structures for the metal... 

The first Raman spectra of bulk metal oxide catalysts were reported in 1971 by Leroy et al. (1971), who characterized the mixed metal oxide Fe2(MoC>4)3. In subsequent years, the Raman spectra of numerous pure and mixed bulk metal oxides were reported a summary in chronological order can be found in the 2002 review by Wachs (Wachs, 2002). Bulk metal oxide phases are readily observed by Raman spectroscopy, in both the unsupported and supported forms. Investigations of the effects of moisture on the molecular structures of supported transition metal oxides have provided insights into the structural dynamics of these catalysts. It is important to know the molecular states of a catalyst as they depend on the conditions, such as the reactive environment. 

Beyond providing bulk structural information about 3-D metal oxide phases, Raman spectroscopy can also provide information about the terminating (and thus 2-D) surface layers of bulk metal oxides. For example, surface Nb=O, V = O, and Mo=O functionalities were detected by Raman spectroscopy for bulk Nb2Os, and for vanadium-niobium, molybdenum-vanadium, molybdenum-niobium, and vanadium-antimony mixed oxide phases (Guerrero-Perez and Banares, 2004 Jehng and Wachs, 1991 Zhao et al., 2003). 

The inherent complexity of bulk metal oxides also makes the study of oxide surfaces a difficult undertaking. Substantive issues include surface stoichiometry and termination, geometric and electronic structure, and the role of defects on surface properties. Accordingly, activity in oxide surface science has exhibited exponential growth over the past decade as the fascinating scientific issues and varied technological importance of oxide surfaces become more apparent to the international surface science community. Much of the... 

Polyoxometalates (POMs) are molecular nanosized polyanionic scaffolds, with multi-metal oxide structure and thus with a general motif being at the interface between molecular complexes and extended oxides.They offer well-defined models for the reactivity of metal bulk oxides, displaying, in addition, the typical tunability of the molecular species. 

In summary, the Raman studies have provided a deeper understanding of the molecular structure and reactivity properties of bulk metal oxide catalysts during selective oxidation reactions. However, the fundamental insights have primarily been limited to the bulk properties of the bulk metal oxide catalysts. In order to obtain surface information about metal oxide catalysts with Raman spectroscopy (essentially a bulk characterization technique), it is necessary to look at chemisorbed species on the surface of bulk metal oxides (see Sec. VIII) or highly dispersed metal oxide systems such as supported metal oxide catalysts. 

Fein et al. [65] also investigated the TOFs of bulk metal oxides toward formic acid oxidation through the dissociative chemisorption of the HCOOH to surface formate species HCOO-M. The authors obtained similar structure-activity relationships as observed for methanol and isopropanol. 

The fundamental goal of nanoparticle research is to assemble atoms in a controllable way and design nanostructured materials with the desired physical and chemical properties. A major part of the research in the field of nanoscience is dedicated to the development of synthesis routes to nanoparticles and nanostructures. Conventionally, solid-state reactions between powders have successfully been employed for the low-cost production of bulk metal oxides. However, to obtain metal oxide nanoparticles with well-defined shape, size, and composition, these solid-state routes are unsuitable. In contrast to these high-temperature processes, liquid-phase synthesis routes, and in particular sol-gel routes, offer better possibility to control the variation of structural, compositional, and morphological features of the final nanomaterials [1,2]. 

Structure, electronic states, and binding states of supported metal oxides are considered to be in different states compared with bulk metal oxides because of their interaction with supports. These factors may affect strongly the catalytic performances. 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

The reactions occurring at reacting metal electrodes are associated with structural changes lattice destruction or formation of the metal and, in certain cases, of other solid reaction components (oxides, salts, etc.). One should know the metal s original bulk and surface structure in order to analyze the influence of these structural changes. 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

The chemistry of metal oxides can be understood only when their crystal structure is understood. Knowledge of the geometric structure is thus a prerequisite to understanding the properties of metal oxides. The bulk structure of polycrystalline solids can usually be determined by x-ray... 

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