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Diesel oxidation catalysts (DOCs), which decrease the amount of harmful carbon monoxide (CO), nitrogen oxide (NO), and hydrocarbon (HC) emissions in engine exhaust, typically utilize Pt and Pd in the active phase. There is universal agreement that the addition of Pd improves both the catalytic performance and the durability of Pt catalysts. However, the mechanisms by which Pd improves the performance of Pt are less clear. Because these catalysts operate under oxidizing conditions, it is important to understand these catalysts in their working state. Herein, we report the microstructure of PtPd catalysts that are aged in air at 750 °C. After 10 h of aging, EXAFS and XANES analysis show that the Pt is fully reduced but that almost 30 % of the Pd species are present as an oxide. HRTEM images show no evidence of surface oxides on the metallic PtPd particles. Instead, the PdO is present as a separate phase that is dispersed over the alumina support. Within the metallic particles, Pt and Pd are uniformly distributed and there is no evidence of core–shell structures. Therefore, the improved catalytic performance is likely associated with the co‐existence of metallic Pt and Pd on the catalyst surface.

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Simulated diesel oxidation catalysts (DOCs) consisting of 2.5% Pd were prepared on γ-Al 2 O 3 and lanthanastabilized γ-Al 2 O 3 ; it was found that the La-containing catalyst had higher CO conversion and lower onset temperature for CO oxidation (∼100 °C). Aberration-corrected STEM showed that the La−alumina support helped to stabilize Pd in smaller particles and clusters, increasing dispersion from 17 to 26%. The higher dispersion was responsible, in part, for the improved CO oxidation rate; at 140 °C, the turnover frequency (TOF) was improved from 0.0019 to 0.0095 s −1 with the addition of La. This TOF increase appears to be tied to facile redox behavior of the Pd/La−alumina catalyst, which was evident in the results of in situ X-ray absorption spectroscopy (XAS) and FTIR spectroscopy. In these experiments, both catalysts were calcined at 500 °C to form PdO and then reduced to Pd metal at 140 °C in the presence of CO. When the CO-covered catalyst was exposed to CO oxidation reaction conditions at 140 °C, the 2.5% Pd/Al 2 O 3 catalyst remained nearly fully reduced, and the surface coverage of CO did not change, indicating irreversible CO adsorption and very low reactivity toward oxygen. On the other hand, the more active 2.5% Pd/La−Al 2 O 3 catalyst was more reactive toward oxygen, with a portion of the Pd becoming oxidized when the gas phase was switched from pure CO to the reaction mixture. There was a drop in surface coverage of CO when switching from pure CO to the reaction mixture on the Pd/La−alumina. The results suggest that the role of the La−alumina support is 2-fold, increasing the dispersion of Pd by forming small, stable Pd particles and allowing a portion of the Pd to exhibit facile redox behavior at low temperatures, making the Pd less susceptible to poisoning by CO. This work provides insights into factors that could lead to improved lowtemperature CO oxidation reactivity in Pd-based automotive exhaust catalysts.