Sodium promotion of Pd/gamma-Al2O3 catalysts operated under simulated "three-way" conditions

Sodium promotion strongly enhances the performance of Pd/gamma -Al2O3 catal ysts operated under simulated "three-way" conditions over a range of stoich iometry from fuel rich to fuel lean. The optimally promoted catalyst (7.0% w/w Na) exhibits a similar to 50 degreesC lower light-off temperature than the unpromoted sample when tested under stoichiometric feed conditions. Sod ium promotion is also beneficial under both oxidising and reducing conditio ns. Under reducing conditions the alkali prevents poisoning by hydrocarbon deposits, therefore maintaining high NO conversions. Under oxidising condit ions sodium significantly reduces the formation of N2O (nitrogen selectivit y, S-N2 = 80% over the optimally promoted sample compared to similar to 45% over the unpromoted catalyst at 325 degreesC). The improvement in activity and selectivity is understandable in terms of Na-induced changes in the ad sorption strength of the various reactants on the Pd surface, accompanied b y enhanced NO dissociation. (C) 2001 Academic Press.