It has previously been shown that an Ag/CZA catalyst can simultaneously remove NOx and soot from an oxygen-rich exhaust gas at low temperatures, by utilising the N2O generated preferentially during incomplete NOx reduction. Here, we examine the effect of reformulating the catalyst to include potassium, which is a known promoter of soot combustion. On including 2 wt% K, NOx-reduction occurs both in the absence and presence of soot, but the N2O formed does not play a part in the oxidation of soot. At higher K loadings (5, 10 and 15 wt%), NOx reduction is almost completely disabled, and only contributes to the activity of the catalyst containing 5 wt% K when tested in the presence of soot. At a loading of 20 wt% K, the potassium phase segregates, leaving NO and NH3 adsorption sites exposed. In the absence of soot, this catalyst can remove NOx by reduction on the Ag/CZA component and through nitration of the potassium phase. Although the presence of potassium lowers the onset temperature for soot oxidation to within the range of NOx reduction over Ag/CZA, the mobile K species prevents the desirable C+N2O reaction.

Ag/CeO2-ZrO2-Al2O3, a known catalyst for the simultaneous removal of NOx and soot, was modified by the addition of K, and was prepared using various techniques: wet impregnation, incipient wetness, and chemical vapor impregnation at different temperatures. The effect of the preparation method on catalyst activity was studied. It was found that catalysts prepared via wet impregnation, incipient wetness, and chemical vapor impregnation at 80 °C were able to utilize in situ formed N2O at low temperatures, to simultaneously remove NOx and soot. The difference in preparation method affected the catalyst’s ability to produce and use N2O as an oxidant for soot. The temperature at which chemical vapor impregnation was performed greatly influenced the catalyst’s ability to oxidize soot. The introduction of K to the Ag/CeO2-ZrO2-Al2O3 vastly improved the soot oxidation activity, particularly for the catalyst prepared via wet impregnation. However, the incorporation of K had an adverse effect on the reduction of NOx.

One of the outstanding challenges in diesel exhaust catalysis is to integrate oxidation chemistry, soot filtration and NOx reduction in a single aftertreatment unit, while avoiding the need for fuel injection to regenerate the filter. Here we show that destruction of trapped soot can be initiated catalytically at 200 degrees C when its oxidation is coupled with non-selective NOx reduction (using NH3 as reductant), which acts as an in-situ source of N2O. In laboratory tests over an extended temperature range (up to 800 degrees C), using supported silver as a catalyst for both non-selective NOx-reduction and soot oxidation, the conversion of immobilised soot to CO2 can be resolved into four consecutive steps as the temperature rises: catalysed oxidation by N2O; non-catalysed oxidation by NO2; catalysed oxidation by O-2; non-catalysed oxidation by O-2. Initial engine tests indicate that the critical first step (C + N2O) can be replicated in a diesel exhaust.