Sulfur-resistant NOx sorbate catalyst for increasing longevity in diesel exhaust

Sorbate or "trap" catalysts are capable of obtaining greater than 90% NOx conversion in lean exhaust over a wide range of operating temperatures. The critical limitation to sorbate catalysts is their susceptibility to sulfur poisoning, which is of particular concern in diesel applications where high sulfur levels exist and exhaust temperatures are typically too low for sulfur removal with thermal techniques. A novel catalyst formulation that exhibits sulfur-resistant characteristics is introduced here. The catalyst sorbs NOx and SO₂. The NOx is reduced and released as N₂, and the sorbed SO₂ in the form of sulfates and sulfites is reduced and released as H₂S and SO₂. The catalyst also oxidizes hydrocarbons and CO. The catalyst system incorporates a dual-chamber design for efficient operation. H₂, available from onboard fuel reformers, is used as the reductant.¹ The catalyst exhibits two beneficial characteristics related to sulfur: the NOx conversion decay rate due to sulfur-poisoning is reduced relative to standard NOx sorbate catalysts, and the sulfur-poisoned NOx sites can be cleansed of sulfur and reactivated by reduction at temperatures in the diesel exhaust range. This paper will show data from tests of the catalyst on a bench scale reactor and a stationary light-duty diesel engine. Studies were conducted with varying engine load, NOx level, and exhaust temperature. All testing was performed under steady-state conditions with No. 2 Diesel.