Visualization of oxygen storage process in Pd/CeO2-ZrO2 three-way catalyst based on isotope quenching technique

Abstract

Elucidation of oxygen dynamics and transport in oxygen storage component of three-way catalyst (TWC) is a crucial issue for clarifying the reaction mechanism and precise kinetic modeling. Herein, we report a direct visualization of the oxygen storage process in a Pd/CeO2-ZrO2 (CZ) via a combined method of oxygen isotope labeling and reaction quenching. 18O2 is stored in a model TWC comprising a dense CZ substrate with Pd at 600 °C, the catalyst is quenched, and the 18O distribution in the surface and bulk is visualized by secondary-ion mass spectrometry with a 100-nm-scale spatial resolution. The results show that the presence of Pd enhances both the surface 18O concentration and the 18O diffusion depth into the CZ bulk. 18O concentration on the Pd surface is higher near the Pd/CZ interface, indicating that oxygen adsorption near the Pd/CZ interface is preferred to that close to the Pd center. The promoting effect of Pd on oxygen vacancy formation in CZ is supported by density functional theory calculations. In addition, the comparison between the 18O distribution and oxygen release/storage simulation based on diffusion equation provides the local oxygen release/storage rates. These are of the same order of magnitude and qualitatively consistent with the trend of conventional oxygen storage capacity measurements. The visualization technique reported here is a promising tool for directly investigating the oxygen dynamics and transport at the metal/oxygen storage material system from both qualitative and quantitative aspects.