Sodium Promotion of Pd/γ-Al2O3 Catalysts Operated under Simulated “Three-Way” Conditions

Sodium promotion strongly enhances the performance of Pd/γ-Al2O3 catalysts operated under simulated “three-way” conditions over a range of stoichiometry from fuel rich to fuel lean. The optimally promoted catalyst (7.0% w/w Na) exhibits a ∼50°C lower light-off temperature than the unpromoted sample when tested under stoichiometric feed conditions. Sodium promotion is also beneficial under both oxidising and reducing conditions. Under reducing conditions the alkali prevents poisoning by hydrocarbon deposits, therefore maintaining high NO conversions. Under oxidising conditions sodium significantly reduces the formation of N2O (nitrogen selectivity, SN2=80% over the optimally promoted sample compared to ∼45% over the unpromoted catalyst at 325°C). The improvement in activity and selectivity is understandable in terms of Na-induced changes in the adsorption strength of the various reactants on the Pd surface, accompanied by enhanced NO dissociation.