Moderating the interaction among Pd, CeO2, and Al2O3 for improved three-way catalysts

Abstract

The Pd distribution and the CeO₂–Al₂O₃ combination are among the decisive factors for the performance of commercial three-way catalysts. Generally, the sufficient doping of Pd into ceria-based oxides and the intimate interaction between CeO₂ and Al₂O₃ could both benefit the three-way catalytic reactions. However, in the present work, the moderate doping of Pd into CeO₂ and less intimate CeO₂–Al₂O₃ interaction were found to be responsible for the much higher catalytic activity (the decrease in T₅₀ was 52, 119, or 55 °C for C₃H₆, CO, or NO) in PdCe/Al₂O₃-CP than PdCe/Al₂O₃-Imp, for which the Pd and Ce species were co-loaded onto Al₂O₃ through the co-precipitation or impregnation method, respectively. It was intriguing to find that the co-precipitated PdCeO_x in PdCe/Al₂O₃-CP showed less sufficient doping of Pd into CeO₂ than the co-impregnated PdCeO_x in PdCe/Al₂O₃-Imp; as a result, both a higher fraction of highly active metallic Pd and a higher Pd dispersion were realized in PdCe/Al₂O₃-CP. Moreover, due to the less intimate CeO₂–Al₂O₃ interaction, specifically the less severe penetration of the Pd and Ce species into Al₂O₃, PdCe/Al₂O₃-CP showed higher Pd dispersion, specific surface area, pore volume and size than PdCe/Al₂O₃-Imp. The presence of more abundant reactive Pd⁰, and the higher accessibility of the active Pd and CeO₂ sites, together with improved redox properties and enriched oxygen vacancies contributed much to the enhanced three-way catalytic activity of PdCe/Al₂O₃-CP. Additionally, simultaneously optimizing the Pd distribution and the CeO₂–Al₂O₃ combination in a single step, as reported in this work, is also highly desirable in industry.