Chromium oxides, decompositions

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

Electrobalances suitable for thermogravimetry are readily adapted for measurements of magnetic susceptibility [333—336] by the Faraday method, with or without variable temperature [337] and data processing facilities [338]. This approach has been particularly valuable in determinations of the changes in oxidation states which occur during the decompositions of iron, cobalt and chromium oxides and hydroxides [339] and during the formation of ferrites [340]. The method requires higher concentrations of ions than those needed in Mossbauer spectroscopy, but the apparatus, techniques and interpretation of observations are often simpler. 

Chromium oxide is deposited by the decomposition of chromium acetyl acetonate, Cr(C5H702)3, in the 520-560°C temperature range.[ 1 It can also be deposited by the decomposition of the carbonyl in an oxidizing atmosphere (CO2 or H2O), at low pressure (< 5 Torr).Dl... 

Methanol is sometimes used to prepare dichromium trioxide by the reduction of chromium oxide (Vi). Since these attempts lead to spontaneous ignition and detonation, it is preferable to use the thermal decomposition of ammonium dichromate, provided it is carried out with caution (see p.200). 

The absorbed chromium (VI) is gradually reduced to chromium (III) as a result of participation in the oxidative decomposition of cystine crosslinks as represented by Scheme 5.18. 

Chromium oxides with a minimal sulfur content are preferred for metallurgical applications. These are obtained by reacting sodium dichromate with ammonium chloride or sulfate in a deficiency of 10 mol% [3.51]. Chromium (III) oxides with a low sulfur content can also be obtained by thermal aftertreatment [3.52], Thermal decomposition of chromic acid anhydride (Cr03) yields high-purity chromium(III) oxide [3.53],... 

The copper chromium oxide (Cu/Cr = 1) has been prepared by coprecipitation of copper and chromium nitrates with ammonium hydroxide, followed by thermal decomposition in flowing nitrogen up to the final temperature (370"C), according to a previously described method (8). The apparatus and the catalytic procedure have also been described elsewhere in case of gas phase reactions (5) and liquid phase reactions (7). 

Catalytic cracking The decomposition of higher alkanes into alkenes and alkanes of lower relative molecular mass. The process involves passing the larger alkane molecules over a catalyst of aluminium and chromium oxides, heated to 500°C. 

A reactive form of copper-chromium oxide suitable for hydrogenations (see Section 2.17.1, p. 87) may be obtained by the decomposition of basic copper ammonium chromate the main reactions may be written as ... 

The shorter-term exposure experiments show that some portion of the organically-bound chlorine, such as trichlorethane or its decomposition products, remains absorbed on a 304 stainless steel surface, even after heating at 35-40°C in a high vacuum. Conversion to an ionic species begins after a short contact period and can be detected using XPS. Formation of the ionic chloride is likely the result of hydrolysis by water also absorbed on the surface, and is perhaps catalyzed by the surface metal oxides. Further atmospheric exposure up to a few months increases the relative amount of the ionic form of chlorine. The composition of the surface oxide layer was altered, with chromium oxide replacing iron oxide as the major species. There was further evidence that chlorine was present as iron chloride, perhaps up to 5% of the surface film. The conditions under which oxidation of such surfaces occurred are quite comparable to those which could occur on steel surfaces in industrial usage. 

Chromium-containing mesoporous silica molecular sieves (Cr-HMS) with tetrahedrally coordinated isolated chromium oxide (chromate) moieties can operate as efficient photocatalysts for the decomposition of NO and the partial oxidation of propane with molecular oxygen under visible light irradiation (Yamashita et al., 2001). 

There is a problem as to the effect of the contaminants upon the development of catalytic activity during heating. We assume that most of the nitrogen is present as potential ammonium nitrate and we note that chromium oxide is an effective catalyst for the decomposition of ammonium nitrate at about 200° (J02). We have heated the catalyst in a tube separated from a mass spectrometer by a valve. After heating to 100° for 30 minutes, the valve was opened and the gases evolved were analyzed. The tube was then pumped out and the temperature increased 50°, held for 30 minutes, analyzed, pumped out, and the temperature raised 50° again. This was continued to 400°. Except at 150- 00°,... 

Chromium oxides The behaviour of chromium(VI) oxide at temperatures below the melting point (470 K, with decomposition) has been described. Sublimation at... 

Glass fiber with and without a catalyst was used to examine effect of the catalyst on the decomposition of SO3. Although a suitable catalyst for the SO decomposition is not known well, a catalyst such as the salts of vanadium and silver, ferric oxide, chromium oxide, and some of rare earths is used by industry for production of H2SO4. The catalyst used for the decomposition of SO3 was ferric oxide or hematite due to its availability. 

Winter 10) relates isotopic exchange of molecular oxygen with magnesium oxide, zinc oxide, chromium oxide, nickel oxide, and iron oxide. He also compares the rates of isotopic exchange of these oxides with oxygen and the rates of adsorption and catalytic activity relating to the oxidation of CO and the decomposition of NgO. 

Copper chromite has been made by the ignition of basic copper chromate at a red heat and by the thermal decomposition of copper ammonium chromate. The procedure given here is a modification of the latter method in which barium ammonium chromate is also incorporated. Copper-chromium oxide hydrogenation catalysts have also been prepared by grinding or heating together copper oxide and chromium oxides, by the decomposition of copper ammonium chromium carbonates... 

Unfortunately, the catalyst can also become deactivated during the calcination, by several processes. Bulk hexavalent chromium oxide, CrC>3, or chromic acid, is unstable at temperatures above approximately 200 °C and begins to decompose into the trivalent oxide Cr2C>3 [39,40,42], On the catalyst, it is only the esterification with silica that stabilizes chromium in the hexavalent form at temperatures up to 900 °C. However, the chromate or dichromate ester can be hydrolyzed by water vapor present in the air used for the catalyst activation, as shown in Scheme 53. When this happens at elevated temperatures, decomposition to Cr(III) occurs. In the presence of water vapor and traces of Cr(VI), large crystallites of a-chromia are formed [74,75,134,135,731-733], which can be very difficult to reoxidize and disperse. 

In studies of the decomposition of methanol alone in the presence of such catalysts as zinc oxide-copper oxide or zinc oxide-chromium oxide methyl formate has been noted in the decomposition products showing that there is some tendency to form this substance even at atmospheric pressure. 

Studies of the decomposition of methanol over various catalysts show that the same catalysts are active toward the decomposition reaction cat pressures of one atmosphere as are active toward the synthesis at the higher pressures. Indeed, to Patart is attributed the statement that the results from the work of Sabatier on the catalytic decomposition of methanol led directly to the use of certain of the methanol synthesis catalysts.188 144 Smith and Hawk 145 found that zinc oxide made by igniting die carbonate, mixtures of zinc and chromium oxides in the atomic proportions of 4 zinc to 1 chromium, mixtures of zinc and uranium oxides,... 

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