Thermal element double oxides

Table III. Thermal Decomposition of Transition Element-Uranium Double Oxides...

All decomposition reactions are endothermal except that of FeU04, presumably because this is the only reaction which involves oxidation of the double oxide. No significant diflFerence was noted in the DTA or TGA curves of the two NiU04 phases. It is interesting to note the alternating pattern in the decomposition reactions of the uranates. The iron, nickel, and zinc double oxides tend to decompose directly into their constituent oxides, while the manganese, cobalt, and copper compounds decompose to other double oxides. The pattern is not carried over into the decomposition temperatures. In this instance, the thermal stability of the double oxides appears to vary directly with the characteristic transition element oxidation states Gr(III) > Mn, Go (III, II) > Ni, Zn(II) > Gu(II, I). The iron compounds constitute a definite exception to this pattern. 

Oxygen forms binary compounds with nearly all elements. Most may be obtained by direct reaction, although other methods (such as the thermal decomposition of carbonates or hydroxides) are sometimes more convenient (see Topic B6). Oxides may be broadly classified as molecular, polymeric or ionic (see Topics B1 and B2). Covalent oxides are formed with nonmetals, and may contain terminal (E=0) or bridging (E-O-E) oxygen. Especially strong double bonds are formed with C, N and S. Bridging is more common with heavier elements and leads to the formation of many polymeric structures such as Si02 (see Topics FT and F4). 

The synthesis of hydrogen bromide from the elements is also catalyzed by carbons [147]. A few preliminary experiments were performed with carbon black Corax 3. A hydrogen stream satnrated with Br2 at 273 K was passed over the samples at 423 K. At this temperatnre, thermal dissociation of HBr is negligible, and the conversion is a measnre of catalytic activity. The HBr formed was absorbed in water and titrated with standard alkali [96]. The catalytic activity of the carbon black tripled after treatment of the oxidized black with NH3 at 873 K, and doubled when heat-treated at 1273 K. This behavior agrees very well with that observed in the oxidation reactions described in this section. 

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