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The effect of the Pd-support interface on the dynamic oxygen storage capacity (DOSC) and three-way catalytic activities were investigated using Pd chromatographically distributed between Ce 0.7 Zr 0.3 O 2 and Al 2 O 3 . CO-He pulse, H 2 -TPR, and XPS show that the Pd-(Ce, Zr) Ox interface promotes a higher degree of oxygen releasing than the Pd-(Al 2 O 3 ) interface while maintaining the oxidized states of Pd. Pd promotion at the Pd-(Ce, Zr) Ox interface depends on the oxygen species of ceria-zirconia, and promotion takes place on surface oxygen and subsurface oxygen species. Dynamic CO-O 2 transient measurement shows that DOSC is greatly promoted by the Pd-(Ce, Zr) Ox interface. The transient results indicate that the Pd-(Ce , Zr) Ox interface accelerates the oxygen spillover and back-spillover between Pd and the support oxide, but this phenomenon is not obvious for the Pd-(Al 2 O 3 ) interface. Higher CO oxidation activity was obtained over a catalyst with a higher degree of Pd-(Ce, Zr) Ox interface in proportion to its DOSC performance. A two-step CO oxidation mechanism at the Pd-(Ce , Zr) Ox interface includes oxygen migration and its reaction with the adsorbed CO. Higher NO reduction and C 3 H 8 oxidation rates were obtained with a catalyst with more Pd-(Al 2 O 3 ) interface, showing negative correlation with the DOSC performance.

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Effects of Pd-supported interactions toward redox behaviors concerning three-way catalytic reactions and oxygen-buffering effects are investigated through stepwise changing Pd-loading locations over ceria−zirconia and alumina. Through light-off tests and kinetics, texture, and surface studies, discrepant but redistributable Pd−A2O3 and Pd−Ce0.7Zr0.3O x interfaces are defined and analyzed. Pd species are inclined to promote the transformation of Ce4+ to Ce3+ and maintain themselves as fine particles on ceria−zirconia surfaces. Oxygen spillover promoted by Pd n+/Pd0−Ce4+/Ce3+ redox couples benefits the oxygen-buffering effect, but is limited by the increase of reaction temperatures and ceria−zirconia reducibility. This strong oxidative interaction overcomes the possibility of ceria-related anionic vacancies in facilitating NO dissociation and, thus, improves CO conversion only. Stochiometric light-off tests show higher activities for NO reduction and C3H8 oxidation on a Pd−Al2O3 interface, where the different morphologies and redox states of Pd-supported interfaces should be the main contributing factors for efficient molecular bond dissociation.

The influences of Pd sites and specific surface area on oxygen storage capacity (OSC) were investigated over a series of Pd/Ce 0.67 Zr 0.33 O 2 and Ce 0.67 Zr 0.33 O 2 samples by CO-He pulse and dynamic CO-O 2 measurements at 300-550 °C. The results show that the rate and quantity of oxygen storage/release capacity are greatly enhanced by Pd support, showing a more prominent CO 2 slope gradient and CO 2 reduction peak than that of Ce 0.67 Zr 0.33 O 2 . The existence of Pd-(Ce,Zr)Ox (interface between Pd and ceria-zirconia mixed oxides) interaction is confirmed to be important for oxygen reduction. During the CO-He pulse, acceleration of oxygen reduction by Pd promotion is limited by the reducibility of the whole system. At low reducibility, Pd-(Ce,Zr)Ox interaction is evident for OSC. Conversely, when the reducibility reaches above 12%, Pd-(Ce,Zr)Ox interaction is less effective, ascribed to the bulk oxygen migration in ceria-zirconia becoming the rate-determining step of the reduction process. The experiments with dynamic pulses of CO-O 2 reveal that dynamic oxygen storage capacity (DOSC) is closely affected by the temperature range; DOSC is greatly affected by Pd-(Ce,Zr)Ox at low temperature. Considering the effect of Pd deterioration and ceria-zirconia sintering on OSC, after hydrothermal aging at high temperature, no significant difference is related to support sintering. Additionally, deteriorated Pd sites are more effective in decreasing OSC. Pd site evolution under hydrothermal aging may be dominated by Pd sintering, rather than by Pd encapsulation. By calculating the CO 2 production rate, Arrhenius plots are suggested to show that the apparent activation energy increased by Pd site deterioration, rather than by sintering of ceria-zirconia.

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