The compounds of SrFe1–x Ti x O3−δ with a robust perovskite structure are synthesized by Ti substitution for Fe sites in SrFeO3−δ, and they display improved oxygen storage properties and chemical stability in a CO2 atmosphere. The oxygen storage rate of SrFe0.8Ti0.2O3−δ is over 5 times faster than that of SrFeO3−δ. Further, in the presence of CO2, SrFe1–x Ti x O3−δ with x ≥ 0.2 maintains its oxygen storage property, whereas, for x ≤ 0.1, the oxygen release rate drastically deteriorates due to the formation of SrCO3. The perovskite structures (space group I4/mmm) of SrFe1–x TixO3−δ (x ≥ 0.2) are preserved even after reduction treatment under a H2 atmosphere at 773 K. In contrast, the material with x ≤ 0.1 undergoes a phase transformation from perovskite (I4/mmm) to brownmillerite structures (Ima2), and the latter easily reacts with CO2 to form a large amount of SrCO3 on the surface. Thus, the robust perovskite structure maintains its original framework despite the reduction treatment, resulting in improved oxygen storage rate as well as the CO2 resistance.

This study proves that a small amount of Pd loading (1 wt%) on SrFeO can dramatically enhance the oxygen-storage properties of SrFeO. The topotactic oxygen intake and release between SrFeO and SrFeO takes place in response to gas switching between an O flow and H flow, regardless of the presence or absence of Pd loading. The effect of Pd loading is significant for the oxygen-release process under H atmosphere; that is, highly dispersed Pd metal nanoparticles sized less than 1 nm formed on Pd/SrFeO to promote H dissociation, resulting in the improvement of the oxygen-release temperature and rate. Pd/SrFeO with a layered perovskite structure has a higher oxygen-release property at lower temperature than Pd/SrFeO with a perovskite phase without the layered structure. These facts indicate that the surface reaction as well as the crystal structure are responsible for the oxide ion mobility in perovskite structure, and also provide guidelines for designing novel oxygen-storage materials.

Environmental catalysts are required to operate highly efficiently under severe conditions in which they are exposed to reductive and oxidative atmospheres at high temperatures. This study demonstrates that SrFeTi O-supported Pd catalysts exhibit high catalytic activities for NO reduction with CH and CO in the presence of O, which is a model reaction for the purification of automotive exhaust gases. Catalytic activity is enhanced with increasing Ti content, and the highest activity is observed for Pd/SrFeTiO among the examined catalysts. The state of the loaded Pd species can be controlled by the Fe/(Fe + Ti) ratio in SrFeTi O, and highly active PdO nanoparticles are properly anchored on SrFeTiO. The Ti-rich Pd/SrFeTiO shows significantly higher catalytic activity and is more thermally stable than the conventional Pd/AlO, which has a high surface area. Since Fe-rich SrFeTi O has the high oxygen storage capacity, its response capabilities to atmospheric fluctuations were evaluated by changing the oxygen concentration during NO reduction. As a result, Fe-rich Pd/SrFeTiO retains its high NO-reduction activity for longer times even under oxidative conditions, when compared to SrFeO or Ti-rich Pd/SrFeTi O. The oxygen storage properties of Pd/SrFeTiO allow it to effectively act as an oxygen buffer for NO reduction. These properties ensure that the SrFeTi O support, with both high thermal stability and oxygen storage capacity, is a very useful environmental-catalyst material.