Promotion of A-Site Ag-Doped Perovskites for the Catalytic Oxidation of Soot: Synergistic Catalytic Effect of Dual Active Sites

Understanding the active sites in the catalyst is essential for the design of efficient redox catalysis. A series of La(1–x)AgxCoO3 (x = 0, 2.5, 5.0, and 7.5%) perovskite catalysts were synthesized by sodium EDTA–citric acid complexation. La97.5Ag2.5CoO3 shows the best catalytic activity with the solubility range at the perovskite A site, with T90, T50, and T10 values of 448, 358, and 302 °C, respectively. Moreover, with the presence of 5% steam, these values decreased to 404, 320, and 276 °C, respectively. H2-TPR and O2-TPD characterization confirmed that after the A site was partially replaced by Ag+, the improvement of catalytic performance was attributed to the easier formation of oxygen vacancies and the enhancement of lattice oxygen transportation at low temperatures. According to the in situ DRIFTS study, the La97.5Ag2.5CoO3 catalyst had two active centers, including Co3+ and oxygen vacancies. Density functional theory calculations verified that after adding Ag, the formation energy of surface oxygen vacancies was decreased from 0.629 to 0.383 eV, and the oxygen vacancies began to participate in the reaction as an active site. In addition, the stable Co–O–N–Olatt adsorption structure on LaAgCoO3 indicates that there was a synergistic effect between the Co3+ and oxygen vacancy sites. From the experimental and theoretical results, the cyclic redox mechanism of NO-assisted soot oxidation over the LaAgCoO3 catalyst is proposed. Our work reveals the activity evolution of oxygen vacancies in perovskites and the interaction mechanism between the oxygen vacancies and the adjacent metal sites, which provides a promising strategy for the rational design of high-performance perovskite oxidation catalysts.