Oxygen storage capability in Co- and Fe-containing perovskite-type oxides☆
Abstract In this paper we report on oxygen storage-related properties of selected Co- and Fe-containing perovskite-type oxides, and analyze their advantages and disadvantages in relation to the Mn-based, A-site ordered BaYMn2O5 + δ system. In particular, the crystal structure of reduced and oxidized Ln0.5A′0.5Co0.5Fe0.5O3 − δ (Ln: La, Sm; A′: Sr, Ba) and La0.6Sr0.4Co0.8Fe0.2O3 − δ is given, results of in situ XRD observation of the oxidation process of the reduced materials is presented, as well as oxygen storage capacity and kinetics measured on oxidation/reduction cycles in isothermal and non-isothermal conditions are reported. Rietveld refinement of the crystal structure carried out for reduced compounds revealed the presence of brownmillerite-type phase for La0.6Sr0.4Co0.2Fe0.8O2.42, La0.5Sr0.5Co0.5Fe0.5O2.53 and Sm0.5Sr0.5Co0.5Fe0.5O2.53. Upon oxidation these materials transform to perovskite-type phase. On the contrary, La0.5Ba0.5Co0.5Fe0.5O3 − δ and A-site cation ordered Sm0.5Ba0.5Co0.5Fe0.5O3 − δ possess the same crystal structure in the reduced and oxidized forms. What's more is that the oxidation process causes a significant decrease of the unit cell volume for each studied compound. Rapid in situ XRD studies (1 min scans), performed every 5 °C during oxidation of the materials, allowed to observe ongoing structural changes. TG measurements revealed unusually low onset temperatures of oxidation, with reduced La0.5Sr0.5Co0.5Fe0.5O3 − δ oxidizing at about 40 °C. Isothermal oxidation/reduction cycles measured with changing of the atmosphere between air and 5 vol.% H2 in Ar, performed in 400–600 °C allowed to establish oxygen storage-related properties of the studied materials, and it was found that La0.5Sr0.5Co0.5Fe0.5O3 − δ shows enhanced kinetics of the reduction process, while for La0.6Sr0.4Co0.8Fe0.2O3 − δ the measured reversible oxygen storage capacity can exceed 4.2 wt.%, well above that of the BaYMn2O5 + δ system. While these results are very promising, the main drawback arises from a low stability of the considered Co- and Fe-containing oxides, especially in terms of their long-time performance.