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.