Influence of Pd Morphology and Support Surface Area on Redox Ability of Pd/Ce0.67Zr0.33O2 under CO−He Pulse and Transient CO−O2 Measurements

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.