Initial Performance of Supported NOx Reduction Catalysts in a Dual-Catalyst System

The initial NO_x reduction activity of several alumina-supported, platinum-group metal catalysts is evaluated in vehicle tests. The experimental vehicle was equipped with two 36 in³ NO_x converters, and a 260 in³ oxidation converter containing a pelleted platinum catalyst, rich carburetion, and exhaust gas recirculation.

The NO_x catalysts included Ru, Ru-Pt, Ru-Pd, mechanical mixtures of Ru and Pt, Pt-Ni, Pt, and Pd. As evaluated over the 1975 FTP, the NO_x reducing effectiveness of these catalysts was: Ru combinations > Pt-Ni > Pt ≈ Pd.

Modulated air injection was used, enabling the NO_x converters to be used as oxidizing converters during vehicle start-up. Operation in this mode greatly reduced HC and CO emissions, with an acceptable increase in NO_x emissions.

In addition, the NO_x reduction performance of all the NO_x catalysts was enhanced by bleeding a small amount of air into the NO_x converters. Minimum NO_x emissions corresponded to air bleed rates of 3-4% of the total engine airflow. The air bleed effect was due to decreased NH₃ formation over Ru, and to increased NO_x activity due to higher catalyst temperatures over Pt.