Subject transition-metal oxides

Chromylchloride, Cr02Cl2, the main subject of the publication which led to the original discussion about the mechanism [12], shows a very different reactivity compared to the other transition metal oxides discussed above. Even in the absence of peroxides, it yields epoxides rather than diols in a complex mixture of products, which also contains cis-chlorohydrine and vicinal dichlorides. Many different mechanisms have been proposed to explain the great variety of products observed, but none of the proposed intermediates could be identified. Stairs et al. have proposed a direct interaction of the alkene with one oxygen atom of chromylchloride [63-65], while Sharpless proposed a chromaoxetane [12] formed via a [2+2] pathway. 

Vanadium oxide dispersed on supporting oxides (Si02 Al Oo, Ti02, etc.) are frequently employed as catalysts in reactions like partial oxidation and ammoxidation of hydrocarbons, and NO reduction. The modifications induced on the reactive properties of transition metal oxides like V20 when they are supported on an oxide carrier has been the subject matter of recent study. There is much evidence showing that the properties of a thin layer of a transition metal oxide interacting with the support are strongly modified as compared to the properties of the bulk oxide (1-3). In the recent past, increasing attention has been focussed... 

This section does not attempt a survey of this large subject. Its aim is only to show briefly how metallic Ni, Co and Fe differ from the transitional-metal oxides that are of particular interest to us in connection with the metal-insulator transition. The former are in our view in a sense more complicated, because the number of electrons in the d-band is in all cases non-integral, while in metallic oxides (such as V203 under pressure and Cr02) this is not the case. 

Among the phase transitions where electronic factors play a major role, the most well-known are the metal-insulator transitions exhibited by transition-metal oxides, sulfides, and so on. This subject has been discussed at length.2,23,24 A recent observation26 of some interest is that the metal-nonmetal transition occurs at a critical electron concentration as given by the particular form of the Mott criterion, = 0.26 ... 

Nonaqueous solvents can form electrolyte solutions, using the appropriate electrolytes. The evaluation of nonaqueous solvents for electrochemical use is based on factors such as -> dielectric constant, -> dipole moment, - donor and acceptor number. Nonaqueous electrochemistry became an important subject in modern electrochemistry during the last three decades due to accelerated development in the field of Li and Li ion - batteries. Solutions based on ethers, esters, and alkyl carbonates with salts such as LiPF6, LiAsly, LiN(S02CF3)2, LiSOjCFs are apparently stable with lithium, its alloys, lithiated carbons, and lithiated transition metal oxides with red-ox activity up to 5 V (vs. Li/Li+). Thereby, they are widely used in Li and Li-ion batteries. Nonaqueous solvents (mostly ethers) are important in connection with other battery systems, such as magnesium batteries (see also -> nonaqueous electrochemistry). 

Oxides of the lanthanide rare earth elements share some of the properties of transition-metal oxides, at least for cations that can have two stable valence states. (None of the lanthanide rare earth cations have more than two ionic valence states.) Oxides of those elements that can only have a single ionic valence are subject to the limitations imposed on similar non-transition-metal oxides. One actinide rare-earth oxide, UO2, has understandably received quite a bit of attention from surface scientists [1]. Since U can exist in four non-zero valence states, UO2 behaves more like the transition-metal oxides. The electronic properties of rare-earth oxides differ from those of transition-metal oxides, however, because of the presence of partially filled f-electron shells, where the f-electrons are spatially more highly localized than are d-electrons. 

This volume provides a view of some of the main areas of development and of recent progress in the study of well-characterised oxide surfaces. The first chapter by Henrich, one of the pioneers of modem surface studies of oxides, and co-author of the first text on the subject, provides an overview of the subject and relates the remaining chapters to this overview. Chapters 2 to 4, by Noguera, by Pacchioni and by Hermann and Witko, are concerned with the theory of oxides surfaces they cover a range of materials from simple rocksalt structures such as MgO through to the complexity of transition metal oxides, and also present some complementary methods of modelling and calculation. These theoretical studies also address the key issue of surface defects, and cover some aspects of adsorption at oxide surfaees. fri some ways oxide surfaces is a topic in which theory was, for some years, ahead of experiment, and hence unchallenged. This was especially tme in the predictions and... 

Silica aerogels are highly porous solids with specific surfaces up to 1000 m g [1]. The doping of aerogels with transition metal oxides like vanadia to form efficient catalysts has been a subject of great interest [2-5]. Vanadia doped silica gels show in addition colour changes upon adsorption of small molecules such as water, ammonia or formaldehyde [6] and may therefore used as optical sensors. 

The bulk crystal form of the alkaline earth oxides is the NaCl structure. The bulk termination at the (100) surface consists of a stack of coplanar bilayers each containing one cation and an anion. CaO(lOO) has been the subject of a LEED study which found a contraction of top interlayer spacing by —1.2% (Prutton et al., 1979). Buckling of the top bilayer was not investigated. Buckling of the top bilayer was found in a recent LEED study of MgO(lOO) (Blanchard et al. 1990). In this case, the oxygen atom moves out of the surface by 0.05 0.025 A whilst the Mg atom sinks into the surface the same distance. The center-of-mass plane of the top MgO bilayer is unrelaxed. The (100) surface of transition metal oxide CoO, which also has... 

Transition metal oxidants such as manganese and chromium oxidants have been widely used in the chemical industry over the years. They have a major disadvantage in that they produce large volumes of effluent containing the transition metals which are subject to more and more strenuous controls on discharge levels. Supported reagents or effluent recycle could be considered but neither is easy on an industrial scale and they are cures rather than prevention of the problem. 

Some of the transition metal-oxide systems have become a subject of intensive research in the last two decades. The relation between the parabolic oxidation kinetics and the predominating point defect in the oxide was verified. To discuss the high-temperature oxidation mechanism of non-noble metals it is appropriate to start with a brief survey of some of the literature on the point defect dependent properties of, for example, nickel oxide. 

The oxidative functionalization of olefins mediated by transition metal oxides leads to a variety of products including epoxides, 1,2-diols, 1,2-aminoalcohols, and 1,2-diamines [1]. Also the formation of tetrahydrofurans (THF) from 1,5-dienes has been observed, and enantioselective versions of the different reactions have been developed. Although a lot of experimental data has been available, the reaction mechanisms have been a subject of controversial discussion. Especially, osmium (VIII) complexes play an important role there, as the proposal of a stepwise mechanism [2] for the dihydroxylation (DH) of olefins by osmium tetroxide (OSO4) had started an intense discussion about the mechanism [2—11],... 

The crystalline transition metal oxides Ti02, SrTiO, SrZrOs considered above are classified as d insulators with quite wide bandgap, being diamagnetic with no unpaired electrons. This means that these crystals have no inherent magnetization, but when subjected to an external field develop magnetization that is opposite to the field (the transition atom spins tend to be oriented in the direction opposite to the field). 

In general, the ammoxidation reactions are carried out in the presence of a heterogeneous catalysts at elevated temperatures, slightly increased pressure and in the gas phase. Multicomponent oxidic catalysts, mainly containing transition metal oxides are in use. Most catalysts for olefin conversions consist of bismuth and molybdenum oxides. They can be doped by a wide variety of further transition metals. In addition, oxide components like silica, alumina and titania are incorporated as inert diluents or active constituents. The catalysts for the reactions of aromatics and hetero-aromatics mainly contain vanadium, molybdenum and other transition metals. In addition, the reaction can also be carried out in liquid phase. The reported literature on this subject is rather scanty. 

---