Palladium/Ferrierite versus Palladium/SSZ‐13 Passive NOx Adsorbers: Adsorbate‐Controlled Location of Atomically Dispersed Palladium(II) in Ferrierite Determines High Activity and Stability**

Ultrasmall Pd Clusters in FER Zeolite Alleviate CO Poisoning for Effective Low-Temperature Carbon Monoxide Oxidation

Ultrasmall Pd4 clusters form in the micropores of FER zeolite during low-temperature treatment (100 °C) in the presence of humid CO gas. They effectively catalyze CO oxidation below 100 °C, whereas Pd nanoparticles are not active as they are poisoned by CO. Using catalytic measurements, infrared (IR) spectroscopy, X-ray absorption spectroscopy (EXAFS), microscopy, and density functional theory calculations, we provide the molecular-level insight into this previously unreported phenomenon. Pd nanoparticles get covered with CO at low temperatures, which effectively blocks O2 activation until CO desorption occurs. Small Pd clusters in zeolites, in contrast, demonstrate fluxional behavior in the presence of CO, which significantly increases the affinity for binding O2. Our study provides a pathway to achieve low-temperature CO oxidation activity on the basis of a well-defined Pd/zeolite system.