Improved Low-Temperature CO Oxidation Performance of Pd Supported on La-Stabilized Alumina

Abstract

Simulated diesel oxidation catalysts (DOCs) consisting of 2.5% Pd were prepared on γ-Al₂O₃ and lanthana-stabilized γ-Al₂O₃; 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/Al₂O₃ 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–Al₂O₃ 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 low-temperature CO oxidation reactivity in Pd-based automotive exhaust catalysts