A comparison of sodium-modified Rh/γ-Al2O3 and Pd/γ-Al2O3 catalysts operated under simulated TWC conditions

Addition of Na to Rh/γ-Al2O3 and Pd/γ-Al2O3 catalysts operated under identical simulated TWC conditions has dramatically different effects in the two cases (although the two metals respond similarly to Na promotion of NOx reduction in the absence of gaseous oxygen). Na addition to rhodium has a detrimental effect as manifested by severe poisoning and decreased nitrogen selectivity over the greater part of the temperature range studied. In contrast, Na promotion significantly improves the overall performance of Pd/γ-Al2O3 catalysts under simulated TWC conditions. This is manifested by a considerable widening of the gas composition window over which palladium delivers high NOx conversion, whilst at the same time exhibiting markedly improved selectivity towards N2 formation. The very different behaviour of the two metals may be understood in terms of a single underlying effect, namely, the electronic influence of sodium ions on the adsorption strength (and hence relative coverages) of the various reactants on the metal surface.