Calcination oxides

A dichromate or chromate solution is reduced under pressure to produce a hydrous oxide, which is filtered, washed, and calcined at 1000°C. The calcined oxide is washed to remove sodium chromate, dried, and ground. Sulfur, glucose, sulfite, and reducing gases may be used as reducing agent, and temperatures may reach 210°C and pressures 4—5 MPa (600—700 psi). 

Changes in catalysts during preparation, which often involves thermal calcination, oxidation, and reduction, can also be followed by recording the associated variations in sample weight, as in normal thermogravimetry (TG) or differential thermogravimetiy (DTG) or in sample temperature,... 

The individual techniques used to characterize molybdena catalysts are now considered. Table II presents a listing of articles concerning the characterization of molybdena catalysts. Unless otherwise specified, we implicitly refer to Mo and/or Co supported on an activated alumina, commonly y-AlaOs. Most work has been done on the calcined (oxidized) state of the catalyst because of ease of sample handling. Reduced and sulfided catalysts are more difficult to work with since for meaningful results, exposure of these samples to air or moisture should be rigorously avoided. Therefore, sample transfer or special in situ treatment facilities must be provided. 

The most common reactor of this type is the lime kiln. This is a noncatalytic reaction where gas reacts with calcium carbonate moving down the kiln. Other reactions performed in the rotary kiln include calcination, oxidation, and chloridization. 

Carlino, S., Hudson, M. J., Husain, S. W. and Knowles, J. A. (1996). The reaction of molten phenylphosphonic acid with a layered double hydroxide and its calcined oxide. Solid State Ionics 84, 117. 

Oxidation always reduces the lEP for an oxide, for example ferric (+3) versus ferrous (+2). For M0O2 through M02O5, the lEP decreases from pH 12 to pH 0.5. The lEP is alwa hi er for hydrated oxides than for freshly calcined oxides. 

The platinum particle sizes found in our calcined oxidation catalysts are much larger than those observed [6] in the reduced microemulsion catalysts moving in the range 0.5 -3.5 nm. Calcination in air of platinum catalysts including microemulsion catalysts apparently evolves a growth of primary platinum particles by a sintering process. 

A number of organochromium compounds form highly active polymerization catalysts when deposited onto an already calcined oxide carrier. Usually the carrier plays an essential role, because without it such compounds rarely exhibit any activity. In many respects, the organochromium catalysts are quite different from their chromium oxide counterparts. Examples of organochromium compounds which form active catalysts include those with chromium in any of the formal chromium valences from Cr(0) to Cr(IV). 

Chlorination of nitrate-calcined oxide has been carried out in a fluidized bed at S00°C. Oxide from oxalate calcination has been chlorinated in a continuous screw calciner at 250 to 3S0°C. Because many impurities form volatile chlorides tmder these conditions, relatively good decontamination from impurities results. Consequently, this is a logical conversion step to follow the direct calcination of Pu(N03)4. 

The powder X-ray diffraction patterns for the dried precursors correspond to a well crystallized material with a hydrotalcite-like structure. This fact indicates that highly homogenous precursors are obtained. The XRD results for the calcined oxides (Fig. 1) show diffraction peaks corresponding as much to a spinel-like phase as to metallic oxides such as nickel oxide. The relative proportion of these depends to a large extent on the catalyst composition. As these solids are poorly crystallized (except for the Ni-Co-Zn-AI catalyst), it is not possible to identify through this technique which divalent cation is present in the crystalline net, since there is hardly Table 1. Nominal composition and Sbet of the samples. any difference between the diffraction angles of the spinels corresponding to the different metallic cations. The tendency of Co and Zn to form a spinel-like structure, in which cobalt would occupy the octahedral sites, is very... 

Depending on the conditions of preparation, Th(N03)4 crystallizes from solutions of thorium hydroxide (or from HNO3 solutions of moderately calcined oxide) with varying contents of water of crystallization. When a not too acid solution is concentrated by evaporation, Th(N03)4 crystallizes in the cold with 12 moles of HgO. A solution evaporated at IS C yields the penta-hydrate, which is stable to 80°C if heated in an atmosphere free of COg. At higher temperatures, it converts to the trihydrate, and between 125 and 150°C, to the hemihydrate. Above 150°C the remaining water is split off, together with nitrogen oxides. 

A common, carefully dried bomb tube is charged with 2.7 g. of NbgOs and 10 ml. of SOClg. Care should be taken in the preparation of Nb s (from precipitated hydrated oxide) not to exceed 400°C, since excessively calcined oxide is inactive and reacts incompletely. 

J. L. Lassaigne, and K. Briiokner found that chromic oxide is not attacked by sulphur vapour at a white-heat. H. Moissan observed that the oxide which has not been heated to a high temp, forms chromic sulphide when heated to 440° in a current of hydrogen sulphide but the calcined oxide is not attacked by this gas. 

H. N. Warren, some nitride as well 0. Schmidt studied the adsorption of nitrogen by chromic oxide, and also of ammonia. J. E. Ashby found that heated chromic oxide favours the combustion of ammonia in air. D. Maneghini studied it as a catalyst in the oxidation of ammonia. F. Ephraim observed that chromic oxide is attacked by SOdium amide. M. Z. Jovitschitsch found that chromic oxide dissolves when digested for 10 hrs. with fuming or cone, nitric acid the calcined oxide does not dissolve in nitric acid R. Weber found that if strongly heated with phosphorus pentachloride chromic oxide furnishes the chloride. C. Lefevre studied the action of alkali arsenates on chromic oxide. 

In the Pechini method, metal ions from starting materials, such as carbonates, nitrates, and alkoxides, are complexed in an aqueous solution with a-carboxylic acids, such as citric acid. When heated with a polyhydroxy alcohol, such as ethylene glycol, polyesterification takes place. Once excess liquid is removed, a transparent resin is formed. The resin is then heated to decompose the organic constituents. After milling and calcination, oxide powders can be obtained accordingly. Various oxide nanopowders have been synthesized by using Pechini or modified Pechini methods [269-276], although less information is available on the fabrication of transparent ceramics with these powders. 

In the as-synthesized CrAPO-5, the chromium(III) is octahedrally coordinated within the framework (four framework oxygen atoms and two water molecules are in the Ciflll) coordination sphere). During calcination, oxidation of chromium(III) to chromiumCVI) occurs leading to the formation of diox-ochromium(VI), which is still bonded to the internal aluminophosphate lattice. The acidic P-OH groups derived from the decomposition of template balance the charges (216). 

CoHMA Four-coordinated framework Co + on A1 sites UV-vis Presence of Br0nsted and Lewis sites no leaching of Co with calcination Oxidation of cycloalkanes (cyclohexane, cyclooctane, cyclododecane), mild conditions (296- 298)... 

---