An Air/Fuel Algorithm to Improve the NOx Conversion of Copper-Based Catalysts

Copper-based catalytic converters can offer significant HC and CO conversion at stoichiometry when close-coupled to the engine. The higher temperatures that result from close-coupling are needed to prevent sulfur poisoning of the catalyst as well as to increase the catalytic activity. However the ability of copper catalysts to convert NOx is very small at stoichiometry. This report summarizes the progress made in improving the NOx conversion and 3-way performance of copper-based catalysts.

An A/F algorithm involving slightly rich operation was developed which significantly increased the NOx conversion of an aged copper catalyst while still utilizing a standard stoichiometric oxygen sensor. The addition of periodic A/F maneuvers further increased the NOx conversion, presumably by manipulating the copper into an oxidation state most active for NOx reduction. Air was injected ahead of a second copper catalyst. Different formulations were used for the reducing and oxidizing beds which maximized the 3-way performance of the base metal system.

Higher levels of sulfur in the fuel had minimal effect on the HC conversion. As long as the catalyst was above 600 °C, the higher sulfur levels had only a marginal effect on the CO conversion. But the sulfur degraded the NOx conversion even at temperatures above 700 °C. The A/F algorithm developed here helped to minimize the effects of the sulfur on the NOx activity.