Copper chromate catalysts

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). 

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. 

Copper(ll) chromite is obtained by heating copper chromate, CuCr04 at 400°C. The Adkin catalyst, a mixture of copper oxide and copper chromite, is prepared by mixing aqueous solutions of copper nitrate, sodium dichromate and ammonium hydroxide the orange precipitate of copper ammonium chromate formed is dried and then heated below 400°C. 

Thus it is not copper chromate which acts directly as a catalyst but it first gets converted to CuO or Cu20 which actually catalyzes the reaction [263]. Rastogi et al. have reported that CaC03 is a better catalyst in comparison to CuO in PS/AP composite propellants [264]. 

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... 

Dr. S. L. Stafford (Alfa Inorganics) writes that I am unable to tell the difference from the method of preparation between the Lazier catalysts and the Adkins catalysts. They seem to be essentially identical and both are made in the same way as the material which we offer. Our copper chromate is a fine black powder of the formula indicated plus small amounts of barium chromate which may or may not be essential as the activator. The catalyst is stable to both air and moisture. ... 

Chromium copper arsenate as a wood preservative (uses 62% of the chromic acid in the United States) Catalyst for polymerization of ethylene Copper chromite catalyst for hydrogenation Zinc chromate near the zinc anode gives batteries 50-80% more shelf life... 

Adkins and his co-workers187 prepared very active copper chromite catalysts by decomposing copper ammonium chromate at a low temperature (100°) and reducing the product in methanol by a stream of hydrogen at 100-200 atm. 

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. 

Many catalyst chemical formulations and geometric shapes are used to promote oxidation-reduction reactions. Chemical types used for VOC oxidation include platinum, platinum alloys, copper chromate, copper oxide, cobalt oxide, chromium oxide, manganese oxide, and nickel. The catalysts are often categorized as platinum metal group (PMG) and base metal (or metal oxide) types. The active catalyst is often supported on an inert carrier such as gamma alumina. Catalyst forms include metal ribbons, mesh and gauze honeycomb monoliths and small beads or particles that can be used in a fixed, fluidized, or moving bed. 

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.

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. 

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

Usually the nitric acid/amine interaction is more dangerous when impurities that can play a catalytic role are present. This goes for metal oxides such as copper oxides, iron (III) oxide and divanadium pentoxide. Salts such as sodium or ammonium metavanadates, iron trichloride, alkaline chromates and dichromates, cyanoferrates and alkaline or nitrosopentacyanoferrates can also act as catalysts. 

Recently, a novel process for the preparation of chromia promoted skeletal copper catalysts was reported by Ma and Wainwright (8), in which Al was selectively leached from CuA12 alloy particles using 6.1 M NaOH solutions containing different concentrations of sodium chromate. The catalysts had very high surface areas and were very stable in highly concentrated NaOH solutions at temperatures up to 400 K (8, 9). They thus have potential for use in the liquid phase dehydrogenation of aminoalcohols to aminocarboxylic acid salts. 

The effects of various metal oxides and salts which promote ignition of amine-red fuming nitric acid systems were examined. Among soluble catalysts, copperQ oxide, ammonium metavanadate, sodium metavanadate, iron(III) chloride (and potassium hexacyanoferrate(II) with o-toluidine) are most effective. Of the insoluble materials, copper(II) oxide, iron(III) oxide, vanadium(V) oxide, potassium chromate, potassium dichromate, potassium hexacyanoferrate(III) and sodium pentacyanonitrosylferrate(II) were effective. 

Promoter deposition through different mechanisms can account for different catalyst properties. In particular, chromate depositing as chromia does not easily redissolve but, zinc oxide does redissolve once the leach front passes and the pH returns to the bulk level of the lixiviant. Therefore, chromate can provide a more stable catalyst structure against aging, as observed in the skeletal copper system. Of course, promoter involvement in catalyst activity as well as structural promotion must be considered in the selection of promoters. This complexity once again highlights the dependence of the catalytic activity of these materials on the preparation conditions. 

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