Dynamic Change in Oxygen Storage and Release Mechanisms of Pd/CeO2–ZrO2 Catalyst by Isotope Imaging

Oxygen storage and release on oxygen storage materials such as CeO2–ZrO2 solid solution (CZ) are key phenomena for high-performance automotive three-way catalyst (TWC). In order to clarify the microscopic oxygen dynamics on TWC, oxygen storage distribution in a model catalyst comprising dense CZ substrate with Pd is visualized by combining oxygen isotope labeling and reaction quenching. The effects of temperature, oxygen storage duration, and hydrogen reduction (oxygen release) duration on 18O concentration distribution in the bulk and surface of the model catalysts are systematically investigated. In the temperature range of ≤400 °C, oxygen release and storage originated primarily from the Pd-CZ-gas triple phase boundary (TPB) and Pd surface, while the contribution from the bare CZ surface is limited. Oxygen incorporated from around the TPB diffuses in three directions: toward the CZ depth, CZ surface, and Pd surface. Furthermore, during the oxygen release process, the disappearance of the oxide layer (PdO) on the Pd and the subsequent exposure of metallic Pd accelerate oxygen release from the TPB, expanding the oxygen vacancy region deeper into the CZ directly beneath the Pd. Conversely, in the temperature range of ≥500 °C, in addition to the TPB-mediated process, the oxygen exchange on the entire CZ surface and bulk diffusion are activated, significantly expanding the storage and release regions. This study demonstrates that the OSC (oxygen storage capacity) mechanism in Pd/CZ catalyst is not static; rather, the dominant oxygen release/storage sites and transport pathways dynamically change depending on the temperature and the presence of a PdO layer.