A-Chromium oxide

The proposed mechanism includes a reductive epoxide opening, trapping of the intermediate radical by a second equivalent of the chromium(II) reagent, and subsequent (3-elimination of a chromium oxide species to yield the alkene. The highly potent electron-transfer reagent samarium diiodide has also been used for deoxygenations, as shown in Scheme 12.3 [8]. 

AFM has been used to study surface molecules under different conditions. Colloidal system studies by AFM AFM has allowed scientists to be able to study molecular forces between molecules at very small (almost molecular size) distances. Further, it is a very attractive and sensitive tool for such measurements. In a recent study, the colloidal force as a function of pH of Si02 immersed in the aqueous phase was reported using AFM. The force between an Si02 sphere (ca. 5 mm diameter) and a chromium oxide surface in the aqueous phase of sodium phosphate were measured (pH from 3 to 11). The Si02 sphere was attached to the AFM sensor as shown in Figure 10.3. 

The workup is then carried out as described above. A chromium oxide pigment obtained by this process typically contains (wt%) ... 

In a more recent study of the dehydrogenation of cyclohexane to benzene over a chromium oxide catalyst at 450°C., Balandin and coworkers (Dl) concluded that benzene was formed by two routes. One of these, the so-called consecutive route, involves cyclohexene as a gas phase intermediate, while the other proceeds by a direct route in which intermediate products are not formed in the gas phase. It was concluded that the latter route played a larger role in the reaction than did the former. These conclusions were derived from experiments on mixtures of cyclohexane and Cl4-labeled cyclohexene, which made it possible to evaluate the individual rates Wi, BY, Wt, and Wz in the reaction scheme... 

It was noted that sodium hydroxide-precipitated catalysts were quite active in contrast with comparatively inactive ammonia-precipitated catalysts. Investigation of a series of catalysts with varying sodium content proved that the sodium content of a chromium oxide catalyst has a... 

Treatment of butadiene or 1,4-butanediol with hydrogen sulfide over an alumina catalyst, or an iron sulfide/alumina catalyst, leads to the formation of thiophene. This method has been more useful in the benzothiophene series. Styrene with four equivalents of hydrogen sulfide, when passed over an iron sulfide/aluminum oxide catalyst at 600 °C for 20 seconds, gave a 60% mole conversion to benzo[6]thiophene (47JA2008). Similar treatment of ethylbenzene over a chromium oxide-alumina catalyst gave an 18% yield of benzo[6]thiophene, accompanied by the evolution of hydrogen (48JA2495). 

The details of the mechanism are not well understood yet. Reasonable speculations based on evidence from ESR measurements have been published by van Reijen and Cossee (17) and by Pecherskaya and Kazan-skii (15). Van Reijen and Cossee speculate that the active site in a chromium oxide-silica catalyst is a tetrahedrally coordinated chromium ion. They picture the Cr04 tetrahedron linked to the SiC>2 network by... 

Table V gives pertinent experiments for a chromium oxide-silica catalyst.

Chromates and molybdates protect the steel by passivation and form a chromium oxide-iron oxide in the case of chromate. While the oxide film of chromium may be several monolayers, molybdenum oxide film is of the order of a monolayer.73... 

The most common failures are associated with oxidation, carburization and metal dusting, sulfidation, chlorination, and nitridation. The most common high-temperature degradation mode is oxidation, and the protection against oxidation, in general, is given by the formation of a chromium oxide scale. The presence of a small amount of aluminum or silicon in the alloy may improve the resistance against oxidation of a... 

This condition is satisfied for the Ni-Ti alloy because interactions between O and Ti solutes are very strong (eJ1 = —100), but not for Ni-Cr alloys (e = —25). Thus, additions of Cr up to 20 at.% in Ni do not cause formation of a chromium oxide, the only effect is to increase the dissolution of A1203 in the alloy. As shown in Figure 6.34, Cr additions in Ni significantly decrease the contact angle of Ni on A1203 and this could result from two effects, both of which require that e < 0 ... 

Butadiene, a substance used industrially to make polymers, is prepared by thermal cracking of butane over a chromium oxide/aluminum oxide catalyst, but this procedure is of little use in the laboratory. 

Oh Ni85Cri5(l 11) different stages of oxide formation are found by Jeng et al [68]. At 200-300 °C, initially a chromium oxide is formed and only at higher O2 exposures is NiO also observed. On Ni-Ru [69], XPS and UPS unambiguously indicated the formation of an oxide overlayer. 

An inorganic solid lubricant has been described as a self-lubricating composite comprising a chromium oxide (e.g. chromic oxide, Cr203), a metal binder comprising Cr/Ni or Cr/Co alloy, a metal fluoride, and, optionally, a metal lubricant [60]. A milled powder mix of the composite is deposited onto a substrate, e.g. by plasma spraying. 

These experiments were repeated with a catalyst made by depositing dibenzenechromium(O) on AIPO4, and similar results were obtained, except that the poisoning was not as severe. Preexposure of the catalyst to CO reduced the activity by about 40%, and again decreased the size of the low-MW peak. A quick preexposure to dry air reduced the activity very little, but it did diminish the GPC peak associated with phosphate, a result that again suggests selective oxidation to a chromium oxide species. 

The first industrial plant for the dehydrogenation of butane to butenes was built by COP I Universal Oil Products) on the iCl (Imperial Chemical Industries) complex at Billingham (United Kingdom) in 1939/1940. The UOP process featured a multitube reactor operating with a chromium oxide/aloinma catalyst, at 570°C and 0.8.10 Pa absolute at the inlet, with a pressure drop of 0l5.10 Pa absolute in the tubes (5 m long, 7.5 cm diameter). Once-through conversion was 215 per cent with a molar selectivity of 80 to 90 per cent... 

Balandin and Brussow (20) also prepared a chromium oxide catalyst active in the decomposition, and suggested that in this case reaction proceeded in stages via an edgewise adsorption on a duplet, as in the diagram, where the ring is perpendicular to the surface plane. 

Table II shows the magnetic susceptibility of a chromium oxide gel compared with that of crystalline chromic oxide. The gels always contain some water, hence a more striking comparison is made by calculating the susceptibility of the chromium ions in each substance.

These fibers can be used from room temperature to 1000 C, but because of their ultralow strength, only in non-load bearing end uses. Pristine fiber strength deaeases from room temperature (0.4 GPa) to 1000°C (0.1 GPa). Application of a chromium oxide coating... 

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