Experimental and Computational Insights into the Catalytic Mechanism of Y1–xBaxCoO3−δ Perovskite Oxides with a Controlled Crystal Structure

The crystal structure of Co-based perovskite oxides (ACoO₃) can be controlled by adjusting the A-site elements. In this study, we synthesized Y_1-xBa_xCoO_3-δ (x = 0, 0.5, and 1.0) via a coprecipitation method and investigated their CO oxidation performances. YCoO₃ (x = 0; cubic perovskite oxide; Pbnm) shows a higher catalytic performance than Y_0.5Ba_0.5CoO_2.72 (x = 0.5; A-site-ordered double perovskite oxide; P4/nmm), which exhibits high oxygen nonstoichiometric properties, and BaCoO₃ (x = 1.0; hexagonal perovskite oxide; P6₃/mmc), which contains high-valent Co⁴⁺ species. To elucidate the reaction mechanism, we conducted isotopic experiments with CO and ¹⁸O₂. The CO oxidation reaction on YCoO₃ proceeds via the Langmuir-Hinshelwood mechanism, which is a surface reaction of CO and O₂ gas that does not utilize lattice oxygen. Because of the significantly smaller specific surface area of YCoO₃ compared with that of the reference Pt/Al₂O₃, the bulk features of the crystal structures affect the catalytic reaction. When density functional theory is applied, YCoO₃ clearly exhibits semiconducting properties in the ground state with the diamagnetic t_2g⁶e_g⁰ states, which can translate to a magnetic t_2g⁵e_g¹ configuration upon excitation by a relatively low energy of 0.64 eV. We propose that the unique nature of YCoO₃ activates oxygen in the gas phase, thereby enabling the smooth oxidation of CO. This study demonstrates that the bulk properties originating from the crystal structure contribute to the catalytic activity and reaction mechanism.