New Developments in Lean NOx Catalysis for Gasoline Fueled Passenger Cars in Europe 962047

There is an increasing interest in running gasoline fueled passenger cars lean of stoichiometric air to fuel (A/F) ratio to improve fuel economy. These types of engines will operate at lean A/F ratios during cruising at partial load and return to stoichiometric or even rich conditions when more power is required. The challenge for the engine and catalyst manufacturer is to develop a system which will combine the high activity rates of a state-of-the-art three way catalyst (TWC) with the ability to reduce nitrogen oxides (NO_x) under excess of oxygen. The target is to achieve the future legislation limits (EURO III/IV) in the European Union. Recent developments in automotive pollution control catalysis have shown that the utilization of NO_x adsorption materials is a suitable way for reduction of NO_x emissions of gasoline fueled lean burn engines. However, the primary task for the implementation of this technology in the European market will be to improve the high temperature stability and to decrease the susceptibility to sulfur poisoning.

The present paper outlines results of a recent research and development program to achieve nitrogen oxide reduction under lean burn gasoline engine conditions. Model gas test results as well as engine bench data are used for discussion of the parameters which control NO_x adsorption efficiency under various conditions. It is shown that the NO_x adsorption technology enables an efficient removal of NO_x up to 85% over a wide temperature range even at high NO_x inlet concentrations. The major part of the paper is dedicated to the problem of sulfur poisoning under laboratory and engine operation conditions. The relationship between the uptake of poisons such as sulfur and phosphorus and the activity results is reported. It is demonstrated that relatively small amounts of adsorbed sulfur might cause a severe overproportional suppression of NO_x storage activity. Regarding this, fundamental studies using various physicochemical and spectroscopic methods such as BET-surface area measurements, energy dispersive x-ray analysis (EDX), inductive coupled plasma (ICP) and x-ray fluorescence (XRF) were performed to elucidate the nature of sulfur deposition on the NO_x storage catalyst. Some evidence exists that at short time-on-stream the SO₃ reacts preferably with the NO_x adsorbents by forming surface sulfate species which block off the active sites of the adsorbents for further NO_x adsorption. However, by treating the, sulfur poisoned NOx storage catalysts with hydrogen the equivalent activity of the fresh catalyst can be restored. The results obtained by thermogravimetrical analyses coupled to a mass spectrometer (TG-MS) show that high concentrations of hydrogen bring about high levels of H₂S and little SO₂ whereas low concentrations of hydrogen preferably lead to increased SO₂ emissions at expense of H₂S. Another subject reported in detail is the progress made recently concerning improved sulfur tolerance of the technology by applying a sulfur trap mounted upstream to the NO_x storage catalyst. The concept of the sulfur trap provides the periodical adsorption of SO₃ whenever the exhaust is adjusted lean and the decomposition and emission of the sulfur as SO₂ when the engine management provides the rich spikes. One target was to avoid the generation of H₂S during the rich periods. Aided by the utilization of ion molecular reaction mass spectroscopy (IMR-MS) the general ability of the sulfur trap to function could be confirmed at different engine operating points and temperature levels. It is demonstrated that in the presence of sulfur a significant improvement in NO_x activity retention by additionally applying the sulfur trap is feasible.