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

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.

On the effect of metal loading on the reducibility and redox chemistry of ceria supported Pd catalysts

The effect of Pd loading on the redox characteristics of a ceria support was examined using in situ Pd K-edge XAS, Ce L₃-edge XAS and in situ X-ray diffraction techniques. Analysis of the data obtained from these techniques indicates that the onset temperature for the partial reduction of Ce(IV) to Ce(III), by exposure to H₂, varies inversely with the loading of Pd. Whilst the onset and completion temperatures of the reduction of Ce(IV) to Ce(III) are different, both samples yield the same maximal fraction of Ce(III) formation independent of Pd loading. Furthermore, the partial reduction of Ce is found to be concurrent with the reduction of PdO and demonstrated that the presence of metallic Pd is necessary for the reduction of the CeO₂ support. Upon passivation by room temperature oxidation, a full oxidation of the reduced ceria support was observed. However, only a mild surface oxidation of Pd was identified. The mild passivation of the Pd is found to lead to a highly reactive sample upon a second reduction by H₂. The onset of the reduction of Pd and Ce has been demonstrated to be independent of the Pd loading after a mild passivation with both samples exhibiting near room temperature reduction in the presence of H₂.

Effect of oxygen storage materials on the performance of Pt-based three-way catalysts

Pt supported on oxygen storage materials (CeO2 and CeO2–ZrO2) as effective three-way catalysts.