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Selective catalytic reduction (SCR) of NOx by NH3 is under intensive development as a technology to enable diesel engines to meet stringent NOx emission regulations. Cu/zeolite SCR catalysts are leading candidates because of their ability to catalyze NOx reduction at the low temperatures encountered on many diesel vehicles. However, both engine evaluation and laboratory studies indicated that commonly available Cu/zeolite SCR catalysts did not have sufficient thermal stability to maintain performance during the full useful life of a vehicle (with steady-state NOx conversion decreasing ~ 10% over 64 hours of hydrothermal aging at 670 °C). Characterization of aged Cu/zeolite catalysts revealed that the loss of zeolite acidity was the main deactivation mechanism; while the zeolite support maintained its framework structure and surface area after aging. Improvement of the hydrothermal stability of the acid sites resulted in a new generation of SCR catalysts.

A 1-D numerical model describing the ammonia selective catalytic reduction (SCR) de-NOx process has been developed based on data measured on a laboratory microreactor for a vanadia-titania washcoated catalyst system. Kinetics for various NH3-NOx reactions were investigated, as well as those for ammonia, CO and hydrocarbon oxidation. The model has been successfully validated against engine bench measurements, over light-off and ESC tests, under a wide range of conditions, e.g. flow rate, temperature, NO2/NO ratio, and ammonia injection rate. A very good agreement between the experimental data and the model has been achieved. The model has now been used to predict the effect of NO2/NO ratio on NOx conversion, and the effect of different ammonia injection rates on the efficiency of the SCR process.