Defect perovskite oxides a case study

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. 

Anion-deficient perovskites and vacancy-ordered structures. Anion-vacancy 

One of the best-characterized perovskite oxides with ordering of anion vacancies is the brownmillerite stmctme exhibited by Ca2Fe205 and Ca2FeA105 (Grenier et al, 1981). The compositions could be considered as anion-deficient perovskites with one-sixth of anion sites being vacant. The orthorhombic unit cell of the brownmillerite structure (a = 5.425, b = 5.598 and c = 14.768 A for Ca2Fe205) arises because of vacancy-ordering and is related to the cubic perovskite as a - c- 

OTOOTOTOOT Other ordered intergrowth phases reported are Ca7Fe6TiO,8 

Anderson, J. S. (1972a) in Surface and Defect Properties of Solids (eds Roberts, M. W. Thomas, J. M.) Vol. 1, The Chemical Society, London. 

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The defect perovskites that have been more studied in heterogeneous catalysis were those having in position A an alkaline, alkaline-earth, or lanthanide element and in position B a first-row transition metal. We will discuss here some examples of nonstoichiometric perovskites, paying attention preferentially to the concentration and type of defects that are formed. The influence of these defects in the catalytic performance of these oxides has been clearly established in a number of cases. Some relevant examples will be discussed in Section VII. 

Only few reports are specifically devoted to the preferential oxidation of CO in the presence of H2 in the open literature. However, since this reaction is very dose to the oxidation of CO in the absence of hydrc en, many of the prindpal observations extracted from the very popular and reviewed above studies dedicated to CO oxidation may be adapted for studying the potential application of perovskites in the PROX reaction. In both cases, the key point is the contribution of the structure to defects of such networks. For instance, Pena and Fierro [9] showed the comparison of the CO oxidation reaction carried out with O2 or with N2O on BaTiOs solids. Differences in the activation eneigy of these reactions near the Curie temperature of the catalyst and a very slow reaction rate in the temperature range from 373 to 473 K were observed, being limited by the desorption of CO2 under steady-state conditions. These features were considered for proposing that the CO oxidation proceeds via surfece defects, confirming that the defects into the perovskite structures are crucial for their apphcation in the oxidation of CO. 

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