Transition metal oxides tungsten oxide

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

Adam and Lohray122 have used thianthrene 5-oxide (88) as a mechanistic probe in oxidations with transition metal peroxides. They oxidized 88 with various diperoxo complexes of chromium, molybdenum and tungsten and formulated a plausible mechanism on the basis of the products formed, 89 and 90. 

The reduction of a transition-metal oxide and boron oxide by an electropositive metal such as Al, Mg or an alkali metal has been used as a pathway to titanium, iron, chromium, tungsten and alkali-earth borides . ... 

I am not going to discuss early transition metal oxide or alkoxide chemistry here. Instead I would like to use the few minutes allotted me to describe out work with low valent dimers of tantalum and tungsten. Professor McCarley will no doubt see his influence on our work and forgive this digression from the topic of his excellent presentation. 

As reviewed by Ponec,18 the formation of alcohols is observed when a metal is promoted by a transition metal oxide. Kiennemann et al,19 has associated the presence of anion vacancies at the metal-support interface with the capability to dissociate CO and allow CO insertion to produce higher alcohols. This model can be used to explain our results on tungsten carbides. 

The oxide tetrafluorides and dioxide difluorides of molybdenum and tungsten are the most studied transition-metal oxide fluorides, and their preparation and properties are discussed separately below. 

The group 5-7 supported transition metal oxides (of vanadium, niobium, tantalum, chromium, molybdenum, tungsten, and rhenium) are characterized by terminal oxo bonds (M =0) and bridging oxygen atoms binding the supported oxide to the cation of the support (M -0-MSUpport). The TOF values for ODH of butane or ethane on supported vanadia were found to depend strongly on the specific oxide support, varying by a factor of ca. 50 (titania > ceria > zirconia > niobia > alumina > silica). 

Transition Metal Salts and Oxides on Alumina. Transition metal salts, particularly chlorides and nitrates, are frequently used as starting materials for the preparation of supported transition metal oxides or supported precursors for supported metal catalysts. Also, many catalytic materials, particularly supported molybdenum and tungsten oxide and sulfide catalysts, contain transition metal ions, namely Co, Ni , and Fe " as promoters. Thus, it is interesting to study the spreading and wetting behavior of salts of these transition metals and of their oxides. This is of particular importance for promoted catalyst materials, since in practice the incorporation of the active phase and the promoter should be possible in one step for economic reasons. 

Bulk Mixed Oxide Catalysts. - Raman spectroscopy of bulk transition metal oxides encompasses a vast and well-established area of knowledge. Hie fundamental vibrational modes for many of the transitional metal oxide complexes have already been assigned and tabulated for systems in the solid and solution phases. Perhaps the most well-known and established of the metal oxides are the tungsten and molybdenum oxides because of their excellent Raman signals and applications in hydrotreating and oxidation catalysis. Examples of these two very important metal-oxide systems are presented below for bulk bismuth molybdate catalysts, in this section, and surface (two-dimensional) tungstate species in a later section. 

Whereas a pure molecular substance has a definite stoichiometry, this is not always true for solids. Defects in crystals can include vacancies (atoms missing from their expected sites) and interstitials (extra atoms in sites normally vacant in the unit cell). An imbalance of defects involving different elements can introduce nonstoichiometry. This is common in compounds of transition metals, where variable oxidation states are possible (see Topics D5 and H4). For example, the sodium tungsten bronzes are formulated as NaxW03, where x can have any value in the range 0-0.9. 

The term insertion compound is used for solids where atoms or ions enter a three-dimensional framework without disrupting its essential structure. Many oxide bronzes are of this type, based on transition metal oxides with inserted alkali or other electropositive metals. For example, the sodium tungsten bronzes are of composition NaxW03, where x can range from zero up to about 0.9. Their... 

J. W. Johnson, A. J. Jacobson, S. M. Rich, and J. F. Brody, New layered compounds with transition-metal oxide separated by covalently bound organic ligands. Molybdenum and tungsten trioxide-pyridine, J. Am. Chem. Soc., 103, 5246-5247 (1981). 

Molecular transition metal oxides ab initio and density functional electronic structure study of tungsten oxide clusters " ... 

Hydrogen oxidation catalysis happens to be more difficult to obtain than hydrogen production, if noble metals are excluded. In particular, several nanoparticulate catalysts such as transition metal oxides/sulfides-based nanoparticles catalyze H2 evolution [29-34], while only tungsten carbide has been demonstrated to be active for H2 oxidation [35]. Even in the case of organometallic catalysts, only few complexes have proved to be able to catalyze H2 oxidation rather than evolution (see below). 

Transition metal oxides that do not change their transparency, or color very little, under ion/electron insertion and extraction can also be used as a counter electrode in electrochromic devices anploying tungsten oxide as a cathodic material. There has been particular interest in oxides based on vanadium pentoxide and cerium oxide. Pure V2O5 as well as a mixture of vanadium and titanium oxide are of interest. Cerium-based mixed oxides, in particular cerium-zirconium oxide (Veszelei et al. [1999]), exhibit less optical absorption, but the stability is not sufficient for many applications. 

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