Cobalt-promoted palladium as a three-way catalyst

Fifteen catalysts were prepared by intermittently impregnating alumina washcoats with water solutions containing La³⁺, Co²⁺ and PdCl^2?₄ ions/complex and calcining them at 550–820°C. The catalysts were evaluated with respect to light-off performance, at stationary and transient feed gas stoichiometry, respectively, and redox characteristics, using NO/CO/C₃H₆/O₂/N₂ gas mixtures to simulate car exhaust. Alumina supported Pd exhibited three-way activity, i.e., simultaneous oxidation of CO and C₃H₆ and reduction of NO in a narrow interval around stoichiometric composition of the feed gas. Compared to Pd alone, addition of La or Co caused a widening of the interval under net reducing conditions. Addition of Co to Pd caused a significant increase in the activities for oxidation of CO and C₃H₆ under stoichiometric conditions. The conversions of CO and C₃H₆ started at about 100 degrees lower temperatures over Co-promoted Pd compared to unpromoted Pd. A marked increase in the activity for the reduction of NO at transient conditions was observed over Co-promoted Pd compared to unpromoted Pd. The catalysts were characterized by X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy combined with energy-dispersive spectroscopy analysis, X-ray photoelectron spectroscopy (XPS), and specific surface area measurements. Only Co²⁺ could be detected by XPS in the surface layers of the Co-containing sample. A significant part of the cobalt is present in forms which can be oxidized and reduced under synthetic car exhaust conditions. These oxidizable/reducible cobalt sites are predominantly active for oxidation of CO and C₃H₆, hence promoting the reduction of NO over Pd by initiating these exothermic reactions in the catalyst.