Transition metal oxides chromium oxide

This work is devoted to the synthesis of Zr02 by various methods, the synthesis of zirconium-containing pentasils and Zr02 - -zeolite based binary carriers. These materials were used as carriers of transition metal oxides (chromium, cobalt) and their catalytic properties were characterized in the selective reduction of NO by methane and propane-butane mixture, the acidic properties of the samples were investigated by thermoprogrammed desorption and IR-spectroscopy methods. 

It has been established from these studies that the different catalytic properties of transition metal oxides (chromium, cobalt) on zirconium dioxide are attributed to their different acidic properties determined by TPDA and IR-spectroscopy. The most active catalyst is characterized by strong acidic Bronsted centers. The cobalt oxide deposited by precipitation on the zirconium-containing pentasils has a considerable oxidative activity in the reaction N0+02 N02, and for SCR-activity the definite surface acidity is necessary for methane activation. Among the binary systems, 10% CoO/(65% H-Zeolite - 35% Z1O2)... 

Vigorous oxidation leads to the fonnation of a carboxylic acid, but a number of methods pemnit us to stop the oxidation at the intemnediate aldehyde stage. The reagents most commonly used for oxidizing alcohols are based on high-oxidation-state transition metals, particularly chromium(VI). 

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 . ... 

The acidity and the covalent nature of transition metal oxides generally increases with increasing oxidation state of the metals. This is shown by the oxides of chromium. 

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). 

Several high-temperature procedures have been described in the literature for the preparation of the transition-metal dioxides. Direct oxidation of the metals, lower oxides, chlorides, or nitrate precursors provides a convenient route to the dioxides of several metals Ti, Mn, Ru, Rh, Os, Ir, and Pt.1,3-5 (Syntheses of the rutile forms of rhodium and platinum dioxides by direct oxidation requires application of high pressures.5) Reduction of higher oxides is the most common method of synthesis for these dioxides V02, Nb02, Mo02, W02, and /3-Re02.4,6-8 Stoichiometry in these reactions is most readily controlled by use of the respective metal or a lower oxide as reductant. Chromium dioxide is normally synthesized by hydrothermal reduction of the trioxide.9... 

Transition metal oxidants such as permanganate, rathenium tetroxide and chromium(VI) oxide are convenient and efficient reagents for routine cleavage reactions. The use of phase transfer catalysts (quaternary ammonium and phosphonium ions, primarily) has made it possible to solubilize transition metal oxides such as permanganate and chromate in nonaqueous solvents, and to thereby increase the scope of these reactions substantially. ... 

In addition to the antimony fluorides, silver, mercury, thallium, aluminium, zinc, zirconium, chromium and other fluorides [7] such as mercury(II) fluoride, vanadium pentafluoride [24] and various transition metal oxide fluorides [25] have been used in exchange processes, although much less widely. 

Among the transition metals from chromium through zinc, iron remains the only element for which no double oxide formation with uranium oxide has been reported. Both the l.T and 1 3 compounds of mainganese, cobalt, and copper have been prepared, while only the 1 1 compound of chromium, and the 1 3 compound of nickel and zinc are known. 

Describe typical oxidation states of the transition metals Describe some oxides, oxyanions, and hydroxides of chromium... 

Examples of supports modifying the properties of transition metal oxides have also appeared in the literature. Recent work points to iron oxide phases as important species in Fischer-Tropsch synthesis (3 ). Iron oxide supported on SiO2 (4 ) and TiO ( ) resist reduction under conditions in which bulk iron oxide easily reduces. Thus supported iron oxide catalysts are potentially interesting Fischer-Tropsch catalysts. The extensive studies on ethylene polymerization catalysts suggests that chromium (VI) species exist on a SiOp surface at temperatures above which bulk chromic anhydride (CrOg) decomposes ( ). 

The active phase of transition metals (Co, Cr) oxides was deposited by the precipitation method, ionic exchange from nitrate salts solutions and ionic exchange in solid phase as well [8,10], The catalysts were obtained by impregnation of zirconia obtained by both methods with aqueous solutions of the corresponding salts (cobalt or chromium nitrate), subsequent drying at 100 °C, and roasting at 320 °C for 6 h. The McxOy/ZrOz samples containing 5-10 wt% metal oxides on the support (relative to the metal) were prepared by this method. 

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. 

Analogous oxidative-addition reactions are exhibited by certain complexes of the early transition metals, notably chromium(II) (high spin d"), as exemplified by... 

Some transition metal oxides in which the metal has a high oxidation number act as acidic oxides. Two examples are manganese(Vll) oxide (Mn207) and chromium(Vl) oxide (Cr03), both of which react with water to produce acids ... 

The catalytic oxidation of alcohols selectively to carbonyl compounds is one of the more important transformations in the synthetic organic chemistry. A large number of oxidants have been reported in the literature and most of them are based on transition metal oxides such as chromium and manganese [1-3]. A serious drawback to these reagents is the need to use them in large amounts, very... 

Some high oxidation state transition metal oxides are acidic oxides that is, they dissolve in water to give solutions of ternary acids. Manganese(VII) oxide, Mn207, and chromium(VI) oxide, Cr03, are the most common examples. 

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