A New NOx Reduction Catalyst System for Diesel Engine with High Sulfur Tolerance

A new concept of highly efficient and sulfur tolerant NOx reduction catalyst for diesel engines has been demonstrated. This catalyst has a double-layer construction and is composed of Pt and Rh as the precious metals, oxygen storage materials and some other catalytic components. Periodic lean/rich operation similar to the Lean NOx Trap catalyst was found to improve the highly efficient NOx reduction activity in a laboratory reactor test. As a result, this catalyst has the feature of high NOx reduction in a temperature window from 150°C to 350°C, and of high oxidation activity even in a lower temperature condition for HC and CO. Sulfur poisoning characteristics were also evaluated in a laboratory reactor test. The results showed that the proper desulfation procedure with a relatively low temperature environment enabled the stored sulfur to be reomved, and to maintain the NOx reduction efficiency for asignificantly long time period. In an application of this catalyst to HSDI diesel engines for passenger vehicle, the results of NOx reduction efficiency were demonstrated to be over 80% in steady state conditions under the periodic rich combustion operation. The sulfur tolerance in an actual diesel engine exhaust gas was also evaluated under the condition of using 50ppm and 350ppm sulfur diesel fuel. When applying a suitable rich combustion of the diesel engine, even after poisoning with 350ppm sulfur fuel for a good while, it was possible to remove the stored sulfur and have no deterioration of NOx conversion efficiency.