Surface State Changes of Pd Three-Way Catalysts under Dynamic Lean/Rich Perturbation Compared with Static Condition

Abstract

In three-way catalytic converters, momentary perturbations of the exhaust gas composition between fuel-rich (reducing) and fuel-lean (oxidizing) conditions near the stoichiometric point affect the catalyst surface and the conversion efficiencies of NOx, CO, and hydrocarbons. However, the specific changes in the surface state of the catalyst under such dynamic conditions have not been elucidated in detail. In this study, in situ diffuse reflectance spectroscopy (DRS) was applied to monitor real-time changes in the surface state of Pd-based catalysts occurring under rich/lean perturbation at intervals of 2 s. The Pd oxidation state was found to fluctuate in synchronism with this perturbation, but its oscillation amplitude was much smaller when supported on CeO2–ZrO2 (CZ) than on Al2O3. Because this buffering effect of CZ due to its oxygen scavenging function mitigates the gradual oxidation of active metallic Pd to less-active Pd oxide, higher reaction rates were achieved especially for NOx and CO under the dynamic perturbation atmosphere. Furthermore, the Pd–CZ interface quickly supplied active oxygen species to prevent coke deposition originating from the decomposition of adsorbed hydrocarbon, which was unavoidable on Pd/Al2O3 under stoichiometric and fuel-rich conditions. This study demonstrates that in situ DRS provides a simple and useful tool for the direct observation of surface state changes under the real reaction atmosphere at the laboratory level. The information obtained is of great use for understanding the role of CZ in transient and unsteady-state catalytic processes and bridging a gap between real on-board TWC performances and laboratory tests.