Oxygen-Excess Oxides

Potassium superoxide is produced commercially by spraying molten potassium iato an air stream, which may be enriched with oxygen. Excess air is used to dissipate the heat of reaction and to maintain the temperature at ca 300°C. It can also be prepared ia a highly pure state by oxidizing potassium metal that is dissolved ia Hquid ammonia at —50° C. 

Tertiary amine N-oxides may also be used to convert sulphoxides to sulphones16. The reaction proceeds by initial attack by the N-oxide oxygen atom on the sulphoxide moiety, followed by subsequent elimination of the amine. In order to obtain good yields, the reaction must be carried out at 190°Cfor 20 hours with a 20-fold excess of N-oxide in the presence of acid catalysts. The sulphone must then be separated by chromatography, thus making the method less attractive than other procedures and so it has not been employed synthetically. 

Fig. 29. Differential heats of interaction of carbon monoxide, at 200°C, with samples of nickel oxide containing excess oxygen, preadsorbed rapidly (A) or slowly (B).

Li and Armor reported that Co-exchanged zeolites present a very high catalytic performance for the CH4-SCR, even in oxygen excess conditions [1, 3], Bimetallic Pt-and Pd-Co zeolites have revealed an increase of activity, selectivity towards N2 and stability, when compared with monometallic Co catalysts [4-8] even in the presence of water in the feed. Recent works show that these catalytic improvements are due to the presence of specific metal species as isolated metal ions, clusters and oxides and their location inside the cavities or in the external surface of zeolite crystallites [9, 10],... 

The presence of 0.3 wt.% Pd on Co-HFER (3 wt.%) catalyst results on a very important increase of low-temperature interaction of CH4 with N02, as a consequence of both the presence Pd species (Pd2+ and PdO) and the cobalt oxides redistribution (formation of Co oxo-cations and decrease of cobalt oxide). With bimetallic catalyst, under oxygen excess conditions, an increase of 30 % in the NOx conversion to N2 is attained. 

Nickel oxide is a classical nonstoichiometric oxide that has been studied intensively over the last 30-40 years. Despite this, there is still uncertainty about the electronic nature of the defects present. It is well accepted that the material is an oxygen-excess phase, and the structural defects present are vacancies on cation sites. Although it is certain that the electronic conductivity is by way of holes, there is still hesitancy about the best description of the location of these charge carriers. 

The analysis of oxygen-excess oxides is similar to that for metal-rich phases just given. For example, the creation of oxygen excess by cation vacancies can be written ... 

Various defect models have been proposed to explain the defect structure of the doped LaMn03 oxides particularly in the oxygen-excess region. At high oxygen... 

The oxide is stable above 575°C. Thus, it can be prepared by heating iron with oxygen under appropriate pressure at 575°C. Also, iron(ll) oxide has been prepared by saturating the fused triiron tetroxide with iron, powdering the mixture, followed by magnetic separation of the oxide from excess iron (Sidgwick, N.V. 1950. The Chemical Elements and Their Compounds, Vol.2, pp 1328, Oxford Clarendon Press). 

In the Cap2 structure, a typical feature is present a large octahedral hole (sited at (1/2,1/2,1/2)) which corresponds to a favorable potential position in which other (interstitial) anions may be accommodated. It is the site in which oxygen excess is thought to be located in hyperstoichiometric oxides. 

Perovskites constitute an important class of inorganic solids and it would be instructive to survey the variety of defect structures exhibited by oxides of this family. Nonstoichiometry in perovskite oxides can arise from cation deficiency (in A or B site), oxygen deficiency or oxygen excess. Some intergrowth structures formed by oxides of perovskite and related structures were mentioned in the previous section and in this section we shall be mainly concerned with defect ordering and superstructures exhibited by these oxides. 

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