Ammonium chromate, decomposition

This catalyst is prepared by the decomposition of basic copper ammonium chromate the main reactions may be written as ... 

The first step in the decomposition of ammonium chromate is conversion to the dichromate [981] ( 375 K), the rate of this process being reduced by oxygen [982] (1—100 Torr). Thermoanalytical measurements have identified four stages in the decomposition [983], At 508 K... 

Catalysts suitable specifically for reduction of carbon-oxygen bonds are based on oxides of copper, zinc and chromium Adkins catalysts). The so-called copper chromite (which is not necessarily a stoichiometric compound) is prepared by thermal decomposition of ammonium chromate and copper nitrate [50]. Its activity and stability is improved if barium nitrate is added before the thermal decomposition [57]. Similarly prepared zinc chromite is suitable for reductions of unsaturated acids and esters to unsaturated alcohols [52]. These catalysts are used specifically for reduction of carbonyl- and carboxyl-containing compounds to alcohols. Aldehydes and ketones are reduced at 150-200° and 100-150 atm, whereas esters and acids require temperatures up to 300° and pressures up to 350 atm. Because such conditions require special equipment and because all reductions achievable with copper chromite catalysts can be accomplished by hydrides and complex hydrides the use of Adkins catalyst in the laboratory is very limited. 

The kinetics of the ammonium chromate, bichromate and trichromate decomposition were investigated by Fischbeck and Spingler [48] who measured the increase of pressure with time in an evacuated vessel. The activation energy is 40.0, 49.0 and 34.0 kcal/mole, respectively. 

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 ... 

Because the decompositions of many ammonium salts evolve NHj and HjO simultaneously (or conseeutively) in a 2 1 molar ratio, it is often convenient to represent the formula of such salts in the form m(NH4)20.n(metal oxide).xH20, where m, n and x are integers (see ammonium chromates, vanadates, tungstates and molybdates below). 

The first step in the thermal decomposition of ammonium chromate is dissociation to the dichromate [95] at about 375 K. The rate of this process is decreased by the presence of oxygen [96]. The deammination and dehydration reaction ... 

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... 

AMMONIUM CHROMATE (7788-98-9) Not combustible but will enhance an existing fire. A powerful oxidizer. A heat- and shock-sensitive explosive. Contact with strong reducing agents such as hydrazine, alcohols, or ethers can cause explosion. Contact with water produces an alkaline solution, with evolution of free ammonia. Violent reaction with combustible materials, finely divided metals, organic substances. Aqueous solution is incompatible with organic anhydrides, acrylates, alcohols, aldehydes, alkylene oxides, substituted allyls, cellulose nitrate, cresols, caprolactam solution, epichlorohydrin, ethylene dichloride, isocyanates, ketones, glycols, nitrates, nitromethane, phenols, vinyl acetate. Exothermic decomposition with maleic anhydride. 

A copper chromite catalyst can be prepared merely by mechanically mixing copper oxide and chromium oxide or by thermal treatment of such mixtures.188 However, the best method for laboratory practice has proved to be thermal decomposition of copper ammonium chromate. 

Chromia—alumina catalysts are prepared by impregnating T-alumina shapes with a solution of chromic acid, ammonium dichromate, or chromic nitrate, followed by gentie calciaation. Ziac and copper chromites are prepared by coprecipitation and ignition, or by thermal decomposition of ziac or copper chromates, or organic amine complexes thereof. Many catalysts have spiael-like stmctures (239—242). 

Catalytic elfects on the thermal decomposition and burning under nitrogen of the nitrate were determined for ammonium dichromate, potassium dichromate, potassium chromate, barium chloride, sodium chloride and potassium nitrate. Chromium(VI) salts are most effective in decomposition, and the halides salts during burning of the nitrate [1]. The effect of chromium compounds soluble in the molten nitrate, all of which promote decomposition of the latter, was studied (especially using ammonium dichromate) in kinetic experiments [2],... 

Sodium chromate is produced as part of the process for obtaining Cr203, and it is probably the most important chromium compound. Although there are other oxides of chromium, Cr203 is very important because of its catalytic properties. One way to obtain this oxide is by the decomposition of ammonium dichromate,... 

J. Taylor and Sillitto found that ammonium nitrate mixed with 3% of potassium chromate or bichromate undergoes spontaneous decomposition ( burning ) on being heated to about 400°C. J. Taylor et al. [5, 27] suggest utilizing mixtures made from ammonium nitrate and potassium chromate as the main component of rocket solid fuels. This is described in more detail in Vol. III. 

Chromates are usually yellow or red in colour, and, except those of ammonium, the alkali metals, calcium, strontium, and magnesium, are practically insoluble in w ater. They are obtained by oxidation of chromites, by fusion of chromium sesquioxide with the appropriate base in presence of air or of an oxidising agent by oxidation of chromium salts in solution or by double decomposition. Normal, di-, and tri-clrromates, etc., are derived from one and the same acid oxide KaCrOj behaves like an alkali torvards CrOg, since it is quantitatively converted into dichromate. A large number of complex double chromates are known. 

The compound is soluble in ammonia and in water (though with partial decomposition), and explodes on moderately heating. With alkalies it yields chromates with acids, chromic salts and hydrogen peroxide are formed. In both cases gaseous oxygen is evolved. Its properties being those of an ammine and not of an ammonium salt, it may be formulated thus ... 

One of the simplest methods of preparation is by decomposition of a thermally unstable compound. The nitrate or chloride is often preferred, sulphates tend to decompose at higher temperatures. Where the presence of residual traces of anion is to be avoided, the metal salts of organic acids are particularly useful. Formates, oxalates, acetates etc, decompose at low temperatures and often reduce the metal at the same time. For the preparation of catalysts from anions, the ammonium salt is frequently used. Metallic salts of complex acids can be used as a source of metal oxide mixtures. Decomposition of the appropriate chromate, tungstate, molybdate or vanadate will produce the mixed oxide. 

A common feature of the decompositions of many ammonium compounds is the identification of the first step in reaction as proton transfer. The consequent accumulation of protons with the residual oxy-anions may be followed by the elimination of water, accompanied by condensation (or continued condensation) of the anions, see chromates and phosphates above, ultimately leading to residual oxide formation. Alternatively the acid may be volatilized, many ammonium salts sublime... 

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