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Sulfur poisoning from engine fuel and lube is one of the most recognizable degradation mechanisms of a NOx adsorber catalyst system for diesel emission reduction. Even with the availability of 15 ppm sulfur diesel fuel, NOx adsorber will be deactivated without an effective sulfur management. Two general pathways are currently being explored for sulfur management: (1) the use of a disposable SOx trap that can be replaced or rejuvenated offline periodically, and (2) the use of diesel fuel injection in the exhaust and high temperature de-sulfation approach to remove the sulfur poisons to recover the NOx trapping efficiency. The major concern of the de-sulfation process is the many prolonged high temperature rich cycles that catalyst will encounter during its useful life. It is shown that NOx adsorber catalyst suffers some loss of its trapping capacity upon high temperature lean-rich exposure.

Diesel engines are far more efficient than gasoline engines of comparable size, and emit less greenhouse gases that have been implicated in global warming. In 2000, the US EPA proposed very stringent emissions standards to be introduced in 2007 along with low sulfur (< 15 ppm) diesel fuel. The California Air Resource Board (CARB) has also established the principle that future diesel fueled vehicles should meet the same low emissions standards as gasoline fueled vehicles and the EPA followed suit with its Tier II emissions regulation. Achieving such low emissions cannot be done through engine development and fuel reformulation alone, and requires application of NOx and particulate matter (PM) aftertreatment control devices. There is a widespread consensus that NOx adsorbers and particulate filter are required in order for diesel engines to meet the 2007 emissions regulations for NOx and PM. In this paper, the key exhaust characteristics from an advanced diesel engine are reviewed.

The aim of this paper is to investigate the potential of Lean NOx technology for diesel emission control. In this work, the focus is on the precious metal (low temperature) catalyst. Engelhard optimized the catalyst for cells per square inch (cpsi) and Platinum loading. Effect of various parameters, including, reductant type, catalyst volume, space velocity range and injector locations were investigated both analytically and experimentally at Cummins in search for the optimum system design. Both steady state and transient tests were conducted in this work. The precious metal catalysts have a narrow temperature window, however, with the use of proper reductant and an efficient control strategy (to minimize fuel penalty) cycle conversion efficiencies as high as 40% may be obtained for FTP-75. The analysis tool developed to aid the system design is capable of predicting effects of catalyst temperature, NOx concentration, O2 concentration, space velocity etc. on NOx conversion efficiency.

Many studies suggest that lean-NOx SCR proceeds via oxidation of NO to NO2 by oxygen, followed by the reaction of the NO2 with hydrocarbons. On catalysts that are not very effective in catalyzing the equilibration of NO+O2 and NO2, the rate of N2 formation is substantially higher when the input NOx is NO2 instead of NO. The apparent bifunctional mechanism in the SCR of NOx has prompted the use of mechanically mixed catalyst components, in which one component is used to accelerate the oxidation of NO to NO2, and another component catalyzes the reaction between NO2 and the hydrocarbon. Catalysts that previously were regarded as inactive for NOx reduction could therefore become efficient when mixed with an oxidation catalyst. Preconverting NO to NO2 opens the opportunity for a wider range of SCR catalysts and perhaps improves the durability of these catalysts. This paper describes the use of a non-thermal plasma as an efficient means for selective partial oxidation of NO to NO2.

On vehicles equipped with fuel shut-off, the rhodium in conventional TWC catalyst tends to strongly interact with the washcoat components during the high temperature lean excursions seen during deceleration. Studies were conducted to elucidate the conditions under which rhodium interacts with the alumina washcoat support and the rare earth components used in some of the new “High Tech” catalyst compositions. Technology was developed which decreased the rhodium interaction with these washcoat components which permits the catalyst to operate under a much broader set of engine control conditions. Laboratory hydrothermal and engine dynamometer aging studies were used to demonstrate the feasibility of lowering precious metal usage and especially rhodium content of TWC catalysts without sacrificing “High Tech” performance.