Evaluation of Three-Way Catalysts - Part III Formation of NH3, Its Suppression by SO2 and Re-Oxidation

This is the third and final communication in this series of laboratory evaluation of three-way catalysts. The effect of inlet NO concentration and temperature on the NH₃ formation over fresh, pulsator-aged and dynamometer-aged three-way catalysts of the current generation has been investigated under temperatures and exhaust compositions of practical interest. In spite of differences in aging procedures employed, both the pulsator and dynamometer-aged catalysts show similar selectivity behavior. The effect of SO₂ in feed-gas on gross NO conversion and NH₃ formation was studied over Pt-Rh and Pt-Rh-Ru types of three-way catalysts. A strong dependence of the gross NO conversion on the SO₂ concentration in exhaust gas mixtures was noted. A simultaneous suppression of gross NO conversion and NH₃ formation, in presence of SO₂ in feed-gas, is attributed to the poisoning of Pt sites on aged three-way catalysts. IR studies of SO₂ and CO adsorption over Pt, Rh and Ru metals suggest that differences in the surface affinity of these metals to SO₂ can be related to their observed activity patterns. Finally, reoxidation of NH₃ (formed over three-way catalysts) on the vehicle-aged oxidation catalyst was studied as a function of catalyst temperature, excess oxygen and inlet NH₃/NO ratio.