In Situ Time-Resolved Redox Dynamics of Pd Catalysts Under Oscillating A/F Conditions

Abstract

The redox dynamics of Pd catalysts under lean/rich perturbation conditions was analyzed by in situ time-resolved reflectance spectroscopy. The real-time redox of Pd/Al2O3 was monitored using the reflectance at 650 nm at every second under simulated exhausts (CO–C3H6–NO–O2), which were switched between air-to-fuel ratio of 14.1 (rich) and 15.0 (lean). Compared to the fast oxidation of metallic Pd (Pd0) upon rich-to-lean switching, the reduction of Pd oxide (Pd2+) upon reverse (lean-to-rich) switch started after a long induction period and proceeded slowly. Because the similar result was also observed for a Pd metal foil, the unequal redox rates under the lean/rich perturbation condition is characteristic of Pd. The temperature dependence of the redox rate demonstrated that the activation energy for the reduction of Pd oxide is greater than that of the Pd oxidation. The faster oxidation rate of Pd surface is considered as the primary reason for steep decrease of NO reduction efficiency in a slightly lean A/F region.