ABO3 type oxides

Fig. 4 Ideal perovskite structure for ABO3 type oxides.

As mentioned in sect. 2.5.1.1 the activity of ABO3 perovskite-type oxides for CH4 combustion is almost not affected by changes in the B-site cations (table 19). The partial substitution of Sr or Ag in the A site (table 21) improved the catalytic activities but not so much as observed for hydrocarbon combustion at low reaction temperatures. (See sect. 2.5.1.2). 

Various substituted lanthanum manganates having the perovskite ABO3-type structure, as well as La2Cu04 and simple copper oxides are studied. In the present study, are reported the synthesis, characterization of the various phases and catalytic test results. 

Since 1970 perovskite-type oxides (ABO3) have been suggested as substitutes for noble metals in automotive exhaust catalysts [1]. These oxides are efficient for oxidation reactions when for reduction the results obtained from the literature are dissimilar [2], mainly due to huge differences in the experimental conditions. The properties of perovskite-based catalysts are a flmction of the spin and the valence state of the metal in the B site cation, which is surrounded octahedrally by oxygen. The A site cation is located in the cavity made by these octahedra. For some perovskite-type oxides, their electronic structures have been pointed out to be similar to those of transition metals on the basis of theoretical... 

One category of dense proton conducting membranes that has received considerable attention in the preceding decade is proton conducting perovskite type oxide ceramics [4-6]. The stoichiometric chemical composition of perovskites is represented as ABO3, where A is a divalent ion (A +) such as calcium, magnesium, barium or strontium and B is a tetravalent ion (B +) such as cerium or zirconium. Although simple perovskites such as barium cerate (BaCeOs) and strontium cerate... 

Since 1970, the perovskite type oxides, typically rare earth oxides with a (ABO3) formula, have been suggested as substitutes for noble metals in automotive exhaust catalysis (1). The most studied perovskites are LaM03 ( M = first row transition metal ) (2,3,4), where M is considered as the active site of the catalyst. The cobaltites show good activity as oxidation catalysts, the reactivity seems to depend on the facility of cobalt to undergo the transition Co Co m, which may be correlated to an oxygen non stoichiometry, and to the spin state of the cation (5). Furthermore, series of LaM03 oxides revealed similar profiles for CO adsorption studies as for NO adsorption, with NO adsorption maxima for M = Mn and Co (6). The reactivity of these catalysts has been shown not only to depend on the surface area, but also on the preparation process (7). 

ABO3 perovskite-type oxides with transition-metal ions at the B-site have high ionic and electronic transport in the form of p or n semi-conductivity (mixed ionic and electronic conductivity), caused by different oxidation states of the transition-metal cation. For dense ceramic membranes, perovskite-type oxides with the following cations are preferred A = Ln (lanthanide ion), Ca, Sr, Ba B = Cr, Mn, Fe, Co, Ni, Cu. 

Perovskite Based Electrolytes In mixed oxides of the perovskite-type oxide, general fomiula ABO3, ion vacancies can be generated by substimtion of both A and B catkms in valence stable oxides. 

General Perovskite-type oxides have the general formula ABO3. The total valence of the A metal and the B metal ions is 6, and various combinations are possible. However, there is a limit to the ionic radius (r) of the constituent metal... 

Application of Microwave and Ultrasound Irradiation in the Synthesis of Perovskite-Type Oxides ABO3... 

Figure 17.2 Ideal models of perovskite-type oxides with ABO3 and A2BO4 structures.

Figure 25.6 Structure of perovskite-type oxides with the generai formula of ABO3 [45].

Perovskite-type oxides are characterized by a great flexibility of the structure to accommodate substitutions of cations or anions vacancies. This is extremely useful in the CO hydrogenation via the stabilization of transition metals by incorporating them into the perovskite structure ABO3. 

Ferrites are ABO3 perovskite-type oxides with high catalytic activity due to optimal defect structure [96]. 

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