Nonstoichiometry reductive

Partial pressure of oxygen controls the nature of defects and nonstoichiometry in metal oxides. The defects responsible for nonstoichiometry and the corresponding oxidation or reduction of cations can be described in terms of quasichemical defect reactions. Let us consider the example of transition metal monoxides, M, 0 (M = Mn, Fe, Co, Ni), which exhibit metal-deficient nonstoichiometry. For the formation of metal vacancies in M, 0, the following equations can be written ... 

Partial oxidation or reduction leading to nonstoichiometry in solids such as TiOo, Fe304, or Mn02 may be expected to shift the IEP(s) toward that characteristic of the oxidation state produced (72). 

The tuning of electron counts is one of the strategies of the solid state chemists. Elements can be substituted, atoms intercalated, nonstoichiometries enhanced. Oxidation and reduction, in solid state chemistry as in ordinary molecular solution chemistry, are about as characteristic (but experimentally not always trivial) chemical activities as one can conceive. The conclusions we reached for the Pt-Pt chain were simple, easily anticipated. Other cases are guaranteed to be more complicated. The COOP curves allow one, at a glance, to reach conclusions about the local effects on bond length (will bonds be weaker, stronger) upon oxidation or reduction. 

Fluorite type oxides are particularly prone to nonstoichio-metric effects. This most commonly occurs in the form of cation nonstoichiometry induced by partial reduction of the cation or by replacement of a portion of the oxide by flnoride. Anion excess phases can occur as a result of cation oxidation or by replacement with higher valence impurities. The dominant defect in this structure involves the migration of oxygen to the large cuboidal interstice resulting in the formation of a vacancy at a normal lattice site. A vacancy of this type is called a Frenkel defect. 

As seen from Fig. 10.11, the value of (3-8) in Lai xSrxCo03 falls off with decreasing oxygen activity much more rapidly than for the other compounds shown. The general trend at which the perovskites become nonstoichiometric follows that of the relative redox stability of the late transition metal ions occupying the B-site, i.e. Cr " > Fe > Mn > Co ". The reductive nonstoichiometry of the cobaltites increases further by partial B-site substitution with copper and nickel. 

The reductive (and oxidative) nonstoichiometry and the stability in reducing oxygen atmospheres of perovskite-type oxides was reviewed by Tejuca et al. [174]. Data from temperature programmed reduction (TPR) measurements indicate that... 

Gibb et al. (98) studied the series of substituted perovskites SrFe,-Ru,-,03-x by Mdssbauer spectroscopy. The incorporation of Fe in the structure takes place exclusively as Fe3+. Substitution of Ru4+ by Fe3+ for x < 0.3 leads to the appearance of oxygen deficiency. For x > 0.3 there is an increasing proportion of Ru5+ and a parallel decrease in oxygen deficiency. Thus for x = 0.5 the reductive nonstoichiometry appears to be... 

Voorhoeve et al. (30) reported oxidative nonstoichiometry for the perovskites LaM03+x (M = Cr, Mn, Fe) and reductive nonstoichiometry for LaCo03-x. XPS measurements carried out on a similar series of LaM03 oxides after heat treatment in air at 900°C indicated a surface nonstoichiometry that changes from oxidative (M = Cr, Mn) to reductive (M = Fe, Co, Ni, Rh) (102). This nonstoichiometry was found to be much more marked than that observed in the bulk. The more oxygen deficient perovskites were those that are more easily reducible (M = Co, Ni, Rh). Tabata et al. (103) found also significant differences between the chemical composition of the surface (determined by XPS) and of the bulk (determined by X-ray fluorescence spectroscopy) in a series of Sr Lai- CoOs oxides. These results indicate a very different behavior of the surface with respect to the bulk in these compounds. This is an important factor to be considered when trying to correlate the composition of a perovskite with its catalytic performance. 

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