Structure studies metal oxides

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... 

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. 

We shall be largely concerned here with the structures of metal oxides in the crystalline state since nearly all these compounds are solids at ordinary temperatures. We shall mention a number of suboxides, but we shall exclude peroxides and superoxides (and ozonates), for these compounds, in which there are 0-0 bonds, are included in Chapter 11. Little is known of the structures of metal oxides in the liquid or vapour states, though several have been studied as vapours (Table 12.1). The structures of the oxides of the semi-metals and of the B subgroup elements are described in other chapters. 

A good surfactant can also serve as an adjuvant to the lubrication process itself. The adsorption of stearate ion on metal oxide films has been studied [54, 55]. It was shown that the equilibrium for stearate adsorption on a metal surface was high, and the kinetics reasonably fast. Stearate, along with the appropriate cation, has even been shown to form multilayered structures on metal oxides. This added adsorption gives additional protection to the substrate. Adsorbed surfactants alone were able to reduce the friction of one system by -80% [56]. This was rationalized by the extended chains and their ability to easily orient along the direction of shear force. 

From this work, we can understand the reactivity order of different Ti02 surfiices it also proved the surface property and structure of metal oxide surfaces are key factors to near-surface water behavior on surface. It will be important to understand how the property of water near surface changes quantitatively on different metal and metal oxide surfaces. How this change in the near surface affects the interaction between surface and adsorbents should be studied systematically. More efforts are needed from both ab initio quantum mechanics (QM) calculations. ReaxFF development for a fundamental understanding of complicated Ti02 systems is important. 

The present review is concerned mainly with the electrochemical formation and redox behavior of the hydrous oxides of those transition metals centered within and around Group VIII of the periodic table. There have been a number of recent reviews of monolayer oxide growth on these metals so that this area will not be treated here in an exhaustive manner. Structural data for many of the systems (especially direct evidence obtained by investigation of hydrous films themselves) are very sparse at the present time. However, some idea of the type of material involved can be obtained from structural studies of oxide battery materials a useful introduction to the structural complexities in this area in general is Alwitt s account of the aluminium oxide system. An important feature of hydrous oxides, not normally as evident with their anhydrous analogs, is their acid-base behavior and in particular the influence of the latter on the redox properties of the hydrous material. Because of its central role in many oxide (especially hydrous oxide) processes, and its relative neglect in the electrochemistry of these systems until quite recently, this add-base character of oxide systems will be reviewed here in some detail. 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

Catalytic A catalytic-membrane reactor is a combination heterogeneous catalyst and permselective membrane that promotes a reaction, allowing one component to permeate. Many of the reactions studied involve H9. Membranes are metal (Pd, Ag), nonporous metal oxides, and porous structures of ceran iic and glass. Falconer, Noble, and Speriy [in Noble and Stern (eds.), op. cit., pp. 669-709] review status and potential developments. 

Investigations based on equation (a) are indirect. Direct structural studies using diffraction techniques (X-ray or neutron), or electron microscopy, while they cannot detect the low concentrations of defects present in NiO or CoO are indispensible to the study of grossly non-stoichiometric oxides like FeO, TiOj, WOj etc., and particularly electron microscopes with a point-to-point resolution of about 0.2 nm are widely used. The first direct observation of a point defect (actually a complex of two interstitial metal atoms, and two oxygen atoms in Nb,2029) was made" using electron microscopy. 

The only crystalline phase which has been isolated has the formula Pu2(OH)2(SO )3(HaO). The appearance of this phase is quite remarkable because under similar conditions the other actinides which have been examined form phases of different composition (M(OH)2SOit, M=Th,U,Np). Thus, plutonium apparently lies at that point in the actinide series where the actinide contraction influences the chemistry such that elements in identical oxidation states will behave differently. The chemistry of plutonium in this system resembles that of zirconium and hafnium more than that of the lighter tetravalent actinides. Structural studies do reveal a common feature among the various hydroxysulfate compounds, however, i.e., the existence of double hydroxide bridges between metal atoms. This structural feature persists from zirconium through plutonium for compounds of stoichiometry M(OH)2SOit to M2 (OH) 2 (S0O 3 (H20) i,. Spectroscopic studies show similarities between Pu2 (OH) 2 (SOO 3 (H20) i, and the Pu(IV) polymer and suggest that common structural features may be present. 

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. 

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

Understanding and controlling oxide surfaces are the key issues for the development of industrial oxide catalysts, but oxide surfaces are in general heterogeneous and complicated, and hence have been little studied so as to put them on a scientific basis by traditional approaches. While studies of the structure of surfaces have focused on metals and semiconductors over the past thirty years, the application of surface science techniques to metal oxides has blossomed only within the last decade[l-3]. 

A highly detailed picture of a reaction mechanism evolves in-situ studies. It is now known that the adsorption of molecules from the gas phase can seriously influence the reactivity of adsorbed species at oxide surfaces[24]. In-situ observation of adsorbed molecules on metal-oxide surfaces is a crucial issue in molecular-scale understanding of catalysis. The transport of adsorbed species often controls the rate of surface reactions. In practice the inherent compositional and structural inhomogeneity of oxide surfaces makes the problem of identifying the essential issues for their catalytic performance extremely difficult. In order to reduce the level of complexity, a common approach is to study model catalysts such as single crystal oxide surfaces and epitaxial oxide flat surfaces. 

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