Active lattice oxygen in Co-doped SrTiO₃ in CO oxidation based on the Mars-van Krevelen mechanism

Currently, most commercial environmental catalysts depend on large amounts of platinum-group metals (PGMs) to achieve high purification efficiency against environmental pollutants such as CO and NO. However, such catalysts are not readily available and have high cost, while PGMs exhibit significant price volatility. Toward the development of PGM-free environmental catalysts, we focus on the Mars-van Krevelen mechanism in which lattice oxygen constituting transition metal oxides participates in the catalytic reaction. This study demonstrates the contribution of the lattice oxygen constituting the SrTi_1-xCo_xO₃ perovskite to the CO oxidation activity. The catalytic activity of SrTi_1-xCo_xO₃ increased with increasing Co substitution and reached a maximum at x = 0.2. By contrast, Co substitution at x ≥ 0.4 resulted in decreased CO oxidation activity. H₂ temperature-programmed reduction and in situ X-ray diffraction (XRD) under a H₂ atmosphere suggested that the catalytic activity of SrTi_1-xCo_xO₃ corresponded well with the lattice oxygen release behavior; that is, among the SrTi_1-xCo_xO₃ samples, Ti-rich SrTi_0.8Co_0.2O₃ released lattice oxygen at the lowest temperature under the H₂ atmosphere. As the amount of substituted Co increased, the lattice oxygen release temperature increased, indicating that the Co-O-Ti bond was more reactive than the Co-O-Co bond. The states of the lattice oxygen in SrTi_1-xCo_xO₃ were investigated in detail through synchrotron XRD and O K-edge X-ray absorption spectroscopy. Notably, in the samples with x ≥ 0.4, which had a large number of Co-O-Co bonds, partial electron donation from lattice oxygen to Co species occurred, suggesting the strengthening of the Co-O-Co bonds. These results indicate that lattice oxygen, which forms relatively weak Co-O-Ti bonds in SrTi_1-xCo_xO₃, acts as an effective active site for CO oxidation, providing valuable guidance for the rational design of PGM-free oxide catalysts.