COBALT-PROMOTED PALLADIUM AS A 3-WAY CATALYST

Fifteen catalysts were prepared by intermittently impregnating alumina washcoats with water solutions containing La3+, Co2+ and PdCl42- ions /complex and calcining them at 550-820 degrees C. The catalysts were e valuated with respect to light-off performance, at stationary and tran sient feed gas stoichiometry, respectively, and redox characteristics, using NO/CO/C3H6/O-2/N-2 gas mixtures to simulate car exhaust. Alumin a supported Pd exhibited three-way activity, i.e., simultaneous oxidat ion of CO and C3H6 and reduction of NO in a narrow interval around sto ichiometric composition of the feed gas. Compared to Pd alone, additio n of La or Co caused a widening of the interval under net reducing con ditions. Addition of Co to Pd caused a significant increase in the act ivities for oxidation of CO and C3H6 under stoichiometric conditions. The conversions of CO and C3H6 started at about 100 degrees lower temp eratures over Co-promoted Pd compared to unpromoted Pd. A marked incre ase in the activity for the reduction of NO at transient conditions wa s observed over Co-promoted Pd compared to unpromoted Pd. The catalyst s were characterized by X-ray powder diffraction, scanning electron mi croscopy, and transmission electron microscopy combined with energy-di spersive 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 significan t part of the cobalt is present in forms which can be oxidized and red uced under synthetic car exhaust conditions. These oxidizable/reducibl e cobalt sites are predominantly active for oxidation of CO and C3H6, hence promoting the reduction of NO over Pd by initiating these exothe rmic reactions in the catalyst.