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 Y1-xBaxCoO3-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 Y0.5Ba0.5CoO2.72 (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 t2g6eg0 states, which can translate to a magnetic t2g5eg1 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.

NOx Storage Performance at Low Temperature over Platinum Group Metal-Free SrTiO3-Based Material.

Pt-based catalysts are commonly employed as NOx-trapping catalysts for automobiles, while perovskite oxides have received attention as Pt-free NOx-trapping catalysts. However, the NOx storage performance of perovskite catalysts is significantly inferior at low temperatures and with coexisting gases such as H2O, CO2, and SO2. This study demonstrates that NOx storage reactions proceed over redox site (Mn, Fe, and Co)-doped SrTiO3 perovskites. Among the examined catalysts, Mn-doped SrTiO3 exhibited the highest NOx storage capacity (NSC) and showed a high NSC even at a low temperature of 323 K. Moreover, the high NOx storage performance of Mn-doped SrTiO3 was retained in the presence of poisoning gases (H2O, CO2, and SO2). NO oxidation experiments revealed that the NSC of Co-doped SrTiO3 was dependent on the NO oxidation activity from NO to NO2 via lattice oxygen, which resulted in an inferior NSC at low temperatures. On the other hand, Mn-doped SrTiO3 successfully adsorbed NO molecules onto its surface at 323 K without the NO oxidation process using lattice oxygens. This unique adsorption behavior of Mn-doped SrTiO3 was concluded to be responsible for the high NSC in the presence of poisoning gases.