Spinels chromate

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

Soluble pigments The most important pigments in this class are the metallic chromates, which range in solubilities from 17 0 to 0-00005 g/1 CrO . An examination has recently been carried out of the mechanism of inhibition by chromate ions and it has been shown by chemical analysis of the stripped film, Mdssbauer spectroscopy and electron microprobe analysis that the air-formed film is reinforced with a more protective material in the form of a chromium-containing spinel (Chapter 17). The situation is, however, complicated by the possibility that some chromates, particularly the basic ones, may inhibit through the formation of soaps. There is evidence that lead chromate can function in this way. 

Copper Chromate Black, pigment for plastics, 7 369t Copper chromite, molecular formula, properties, and uses, 6 563t Copper chromite black spinel, formula and DCMA number, 7 348t Copper citrate, molecular formula, 6V638t Copper compounds, 7 767-783 analytical methods, 7 773-776 economic aspects, 7 773 health and safety factors, 7 776 properties and manufacture, 7 768-773 uses, 7 776-780... 

Fourcroy predicted that this mineral would give chemists the opportunity to make a more thorough study of the properties of chromium and perhaps to discover compounds of it which, because of their rich and varied colors, would be useful in painting and in the manufacture of glass and enamel (90). He also encouraged study of the chromium alloys. The chrome-iron ore is now known as chromite. It is not a chromate, but has the spinel composition, Fe(Cr02)2-... 

Chromic hydroxide is an amphoteric compound and exhibits acidic properties in combining with basic oxides to form chromites, to which the general formula M-jO.CrjOg is given, and which are isomorphous with the corresiiouding aluminium compounds known as spinels. They may be considered as derived from an acid, HCrOj the monohydrate, CrjOj.HjO, has this empirical formula. From a study of the action of sodium hydroxide on chromium hydroxide for prolonged periods and the rate of the formation of chromate by the oxidation of dissolved chromite, it would appear that the chronlic hydroxide acts as a poly basic acid. 

Figure 12.1 Chromatic parameters (<3 6 plane of the CIE Lab coordinates) of oxide (a) and non-oxide (b) perovskites proposed as pigments. The numbered fields represent the color space of industrial Inorganic pigments black Fe-Cr-Co-Ni spinel (1), magenta Cr-malayaite (2), pink hematite-zircon (3),...

In numerous cases, known as reciprocal solutions, the solid solution can be considered to be the product of two pairs of pure compounds, which are called the poles. Thus, the solution between iron sesquioxide (Fc304) and nickel chromate (NiCr204) 5delds spinel (Fe, Ni )i (Cr, Fe )i. However, this solution could also give rise to the following pure compounds iron chromate (FeCr204) and nickel ferrite (NiFe04). We say that the possible poles of spinel are either the iron sesquioxide-nickel chromate pair or the iron chromate-nickel ferrite pair. 

As seen in Chap. 6, the interaction of seal glass with metallic interconnect is much more severe due to chromate formations. The current chapter is a continuation of Chap. 6, which focuses on the chemical compatibiUly of glass/glass-ceramic seals with the mixed alkaline earth glasses. In addition, the spinel-coated interconnects and their interaction with glasses have been discussed. 

The preparation, stability and catalytic activity of non-stoichiometric spinel-type phases used in the synthesis of methanol were investigated as a function of the composition, heating temperature and atmosphere. It was shown that these phases formed mainly via amorphous chromates, especially for copper-rich catalysts. High activities in the synthesis of methanol were observed for zinc-rich samples (with a maximum for a catalyst in which 20% of the zinc ions were substituted by copper ions) and associated with the presence of a non-stoichiometric spinel-type phase, stable also in the reaction conditions. On the other hand, the low activity of copper-rich catalysts was attributed to the instability of the spinel-type phase where much of the copper segregates into well crystallized metallic copper, with a further poisoning effect by zinc and cobalt. 

Figures 2a and b report the XRD powder patterns of the precipitates heated at 653K in air and in a reducing atmosphere (H2 N2= 10 90 v/v), respectively. Calcined samples (Fig. 2a) show the presence only of spinel-type phases, whose XRD patterns become more and more broad as the copper content increases. IR spectra confirm the presence, for all calcined samples, of spinel phases, and also show he presence of dichromate-type phases (25), the amounts of which increase with increasing copper content. In previous papers it was shown that non-stoichiometric Zn/Cr spinel-type phases formed by decomposition of amorphous chromates and that some amounts of residual Cr ions are present in these phases (8,15). Taking into account that copper and zinc may form mixed spinel-type phases (with cubic symmetry for high zinc contents) (20,24), we may hypothesize the formation up to a ratio Cu/Cu-i-Zn= 0.5 of cubic non-stoichiometric spinel-type phases, containing both elements and characterized by an excess of bivalent ions. On the other hand, on the basis of the XRD spectra of Figure 2a, we cannot speculate about the number and/or nature of the phases present in the copper-rich catalysts.

Table 2 confirmes the presence in the calcined samples of increasing amounts of chromates, with a maximum for Cat F in which ca 43% of the total chromium is present as Cr ions. Up to a Cu/Cu+Zn ratio< 0.5, the amount of CuO extracted is lower than both the theoretical value and the chromate content, and does not depend directly on the latter. This is particulary true for Cat B, taking into account also the values of the samples heated at 653K in N2. Therefore, two Cu containing fractions are present in these spinel-type phases, which show different solubilities in the NH4OH/NH4NO3 solution, but... 

On the other hand, the amount of CuO extracted in the copper-rich samples is mainly related to the presence of G ions (compare Cat D heated in air or N2). However, it must be noted that for all samples the amount of CuO overlaps that of chromates (if both are expressed on an atom basis), indicating that also in these samples a consistent fraction of CuO ( more than 33% of the total CuO extracted) may be present as excess of Cu " ions inside a spinel-type phase. Unfortunately, no further support for this hypothesis may be obtained from XRD powder patterns. 

Non-stoichiometric spinel-type phases may be obtained mainly via amorphous chromates and their stability and reactivity are strongly influenced by the composition. Very stable spinel-type phases active in the synthesis of methanol may be obtained at low copper contents, while copper-rich catalysts show a considerable tendency for segregation of metallic copper with a considerable decrease in catalytic activity. 

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