Transition-metal oxides, decompositions

Transition-metal oxides are particularly effective decomposition and burning-rate catalysts. The metal elements can demonstrate variable valence or oxidation states. 

To finish with another trend for NO removal consisting in NO direct decomposition, we would like to depict the infrared study of NO adsorption and decomposition over basic lanthanum oxide La203 [78], In this case, the basic oxygens are proposed to lead to N02 and N03 spectator species, whereas the active sites for effective NO decomposition are described as anion vacancies, which are often present in transition metal oxides. This last work makes the transition with the study of DeNO, catalysts from the point of view of their ability to transfer electrons, i.e. their redox properties. 

The decomposition rates of several glycol nitrates and glyceryl trinitrate are greatly enhanced by the presence of transition metal oxides or chelates. 

The importance of transition-metal mediated decomposition of ligands has been reviewed by Garrou [30] with an emphasis on oxidative addition as the mechanism. 

Iridium dioxide — Iridium oxide crystallizes in the rutile structure and is the best conductor among the transition metal oxides, exhibiting metallic conductivity at room temperature. This material has established itself as a well-known - pH sensing [i] and electrochromic [ii] material (- electrochromism) as well as a catalytic electrode in the production of chlorine and caustic [iii]. The oxide may be prepared thermally [iv] (e.g., by thermal decomposition of suitable precursors at temperatures between 300 and 500 °C to form a film on a substrate such as titanium) or by anodic electrodeposition [v]. 

Decomposition of a mixture of nitrates is often a convenient method for the preparation of mixed transition metal oxides. The success of the method depends upon the fact that typically, the decomposition of the nitrates results in an intimate oxide mixture that readily reacts at higher temperatures to form a homogeneous, complex oxide. Oftentimes nitrate mixtures can be formed as precnrsors by dissolving a stoichiometric mixture of the oxides or carbonates in nitric acid and evaporating to dryness. This method is one of the options used to prepare the superconducting YBa2Cn30y and isotypes. 

In the case of transition metal oxides, mixed-valence compounds may resnlt if the decomposition is carried out in air. Thus, highly pure NiO and MnO can be obtained by heating the respective carbonates in vacno while nearly stoichiometric FeO is obtained by the decomposition of its oxalate but it will rapidly pickup oxygen when exposed to air. Conversely, decomposition of Fe2(Ox)3 yields Fe203. 

Figure 6.5 shows the yields ([wt%]) of the reaction of PET using several transition metal oxide catalysts under the following conditions a temperature of 500°C, a time factor (the ratio of the mass of the catalyst W, to the PET feed rate F) of 0.317 h, and a particle size of 0.21-0.25 mm. Fc203 did not show activity, hence these results have been omitted. With respect to the reduction of terephthalic acid, FeOOH, nickel hydroxide and nickel oxide showed the decomposition activity of terephthahc acid. However, a large amount of benzoic acid, which is also a sublimate material (sublimation point 100°C), was produced over nickel hydroxide and nickel oxide. Because these nickel compounds are more expensive than FeOOH, FeOOH was considered to be a suitable catalyst for the decomposition of terephthalic acid. 

FIGURE 18.14 The decomposition of hydrogen peroxide, H2O2, to water and oxygen is catalyzed by adding a very small amount of transition metal oxide (a), (b) The water evolves as steam because of the heat given off in the reaction. 

The semiconducting forms of the transition metal oxides are almost never fully oxidized but often have nearly Daltonide stoichiometry. Oxidation of the metal powder, decomposition of the carbonates, and oxidation of a volatile precursor are all employed. Metal carbonyls, such as Ni(CO)4 and Fe(CO)s, are very useful, since distillation is a facile method of purification for these volatile liquids. Caution Ni(CO)4 is very toxic, and generates CO upon decomposition. 

Brittain et al. [112] have reported studies of the decompositions of several sulfates using a torsion efhision detection technique to identify the primary gaseous decomposition products. Zn804 and Zn0.2ZnS04 yielded SO3 as the sole volatile product between 800 and 900 K, together with finely divided residual solids. Mg804 reacted similarly between 900 and 1000 K, but in the presence of additives expected to promote breakdown of SO3 (platinum group metals and transition metal oxides) the products were consistent with the equilibrium mixture ... 

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