Experimental and Computational Insights into the Catalytic Mechanism of Y 1- x Ba x CoO 3-δ Perovskite Oxides with a Controlled Crystal Structure.

The crystal structure of Co-based perovskite oxides (ACoO 3 ) can be controlled by adjusting the A-site elements. In this study, we synthesized Y 1- x Ba x CoO 3-δ ( x = 0, 0.5, and 1.0) via a coprecipitation method and investigated their CO oxidation performances. YCoO 3 ( x = 0; cubic perovskite oxide; Pbnm ) shows a higher catalytic performance than Y 0.5 Ba 0.5 CoO 2.72 ( x = 0.5; A-site-ordered double perovskite oxide; P 4 /nmm ), which exhibits high oxygen nonstoichiometric properties, and BaCoO 3 ( x = 1.0; hexagonal perovskite oxide; P 6 3 /mmc ), which contains high-valent Co 4+ species. To elucidate the reaction mechanism, we conducted isotopic experiments with CO and 18 O 2 . The CO oxidation reaction on YCoO 3 proceeds via the Langmuir-Hinshelwood mechanism, which is a surface reaction of CO and O 2 gas that does not utilize lattice oxygen. Because of the significantly smaller specific surface area of YCoO 3 compared with that of the reference Pt/Al 2 O 3 , the bulk features of the crystal structures affect the catalytic reaction. When density functional theory is applied, YCoO 3 clearly exhibits semiconducting properties in the ground state with the diamagnetic t 2g 6 e g 0 states, which can translate to a magnetic t 2g 5 e g 1 configuration upon excitation by a relatively low energy of 0.64 eV. We propose that the unique nature of YCoO 3 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.