Chromates structures

Figure 5.2 Surface chromate structures resulting from treatment of silica with CrO. Calcination at high temperatures in air (or Op insures that Cr remains primarily in the +6 oxidation state and results in Phillips catalyst for polyethylene. Final activation occurs in reactor through reactions with ethylene (see text for details).

Organic sources of Cr(VI) have also been investigated as the chromium source. Baker and Carrick [148] first investigated bis(triphenylsilyl) chromate as a homogeneous model for the surface chromate structures postulated to exist on the Phillips catalyst. This chromate ester is quite stable, but like Cr(VI) /silica, it can also be reduced by olefins under polymerization conditions to give the corresponding aldehyde and Cr(II) or Cr(III). Thus, it mimics the behavior of Cr(VI)/silica in many respects [149]. Bis(triphenylsilyl) chromate does catalyze ethylene polymerization,... 

Ikemura T. and Aota S. (1988). Alternative chromatic structure at CpG islands and quinacrine-brightness of human chromosomes. Global variation in G-tC content along vertebrate genome DNA. Possible correlation with chromosome band structures. J. Mol. Biol. 60 909-920. 

Chromates III). Mixed oxides, e.g. FeCr204, having spinel structures and prepared by solid state reactions. 

Chromium dioxide. Cr02 (HjO plus O2 on Cr03 at high temperature). Black solid with the rutile structure forming chromates(IV) in solid stale reactions. Used in magnetic lap>es. 

The chromates of the alkali metals and of magnesium and calcium are soluble in water the other chromates are insoluble. The chromate ion is yellow, but some insoluble chromates are red (for example silver chromate, Ag2Cr04). Chromates are often isomorph-ous with sulphates, which suggests that the chromate ion, CrO has a tetrahedral structure similar to that of the sulphate ion, SO4 Chromates may be prepared by oxidising chromium(III) salts the oxidation can be carried out by fusion with sodium peroxide, or by adding sodium peroxide to a solution of the chromium(IIl) salt. The use of sodium peroxide ensures an alkaline solution otherwise, under acid conditions, the chromate ion is converted into the orange-coloured dichromate ion ... 

Fig. 8.98 Threshold stress intensities from pre-cracked specimen tests, and threshold stresses from plain specimen tests, for a Mg-7A1 alloy in various structural conditions tested in chromate-chloride solution (after Wearmouth ei al )...

Strong electrolyte A compound that is completely ionized to ions in dilute water solution, 37 Strontium, 543 Strontium chromate, 434 Structural formula A formula showing the arrangement of bonded atoms in a molecule, 34,579-580,586,590,593, 597... 

There are also other unsolved problems. For instance, we have sufficient knowledge neither about the structures of chromium(V) and chromium(IV) species nor about the mechanism of striping of the covalently-bonded oxygen atoms from the chromate ion. 

Herbach, K.M. et ah. Structural and chromatic stability of pnrple pitaya (Hylocereus polyrhizus [Weber] Britton Rose) betacyanins as affected by the jnice matrix and selected additives. Food Res. Int, 39, 667, 2006. 

The examination of the vco bands in the 2200-2179 cm region at room temperature reveals that Cr(II) sites are distributed in two basic structural configurations, namely CrA and Cre. These results confirm fhe view illus-frafed before concerning the structural complexity of the Cr(ll) system. CrA sites seem to correspond to the first family of chromates represented in Scheme 4, while Cre sites correspond to a family characterized by a larger aocro bond angle. It is important to underline here that, when we speak about CrA and Cre sites, we are referring to two families of structures instead of simply to two different well-defined sites. 

The Fourier transforms for both films are again quite similar, but as for the ex situ measurements, the chromate-passivated films appear to have a more glassy structure. It should be mentioned that these studies employed a rather limited data range which makes spectral differentiation difficult. 

Fig. 9 (a) Molecular structures of novel ESIPT dyes, 2,5,-bis[5-(4-t-butylphenyl)-[l,3,4]oxadia-zol-2-yl]-phenol (SOX), and 2,5-bis[5-(4-t-butylphenyl)-[l,3,4]oxadiazol-2-yl]-benzene-l,4,-diol (DOX). (b) Emission colors in the Commission Internationale de L Eclariage (CEE) chromaticity diagram. The inner oval and the filled circle at coordinate (x,y) of (0.33, 0.33) indicate the white region and the ideal color, respectively. Note that PS and PVK denote polystyrene and poly (N-vinylcarbazole) film (reprint from ref. [91], Copyright 2005 Wiley-VCH)... 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

The high activity of Cr-Cl catalyst may result probably from the formation of more active species such as Cr02+ complex cation during the ammoxidation reaction. Some authors [7] assumed that Cr(V) species occurred inside the zeolite structure as complex cation such as Cr02+ coordinated to two framework oxygen atoms and suggest the following two step process as the most probable pathway of chromate formation ... 

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