Role of NO in Diesel Particulate Emission Control

The effective use of a catalyst to initiate regeneration of a diesel particulate trap has traditionally been based on the concept that the catalyst coated onto the trap adsorbs particulate, and activates oxygen in the exhaust causing initiation of particulate combustion. Reported regeneration temperatures generally lie in the range of 350°C and above. This paper reports on a new mechanism of diesel particulate combustion involving activation of oxygen over a catalyst to form NO₂, which is then capable of adsorbing on diesel particulate trapped in a filter and initiating combustion at lower temperatures. Diesel particulate has been combusted on a wire mesh trap at temperatures as low as 265°C, and this regeneration capability has been maintained over hundreds of hours of operation. However, the most active catalysts for low temperature activation of diesel particulate are also high sulfate producers. In the absence of low sulfur fuel, future direction must concentrate on improvement of catalyst selectivity such that high NO₂ and low SO₃ formation may be achieved to obtain the concurrent goals of low regeneration temperature and low sulfate emissions.