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For the NOx-assisted diesel soot abatement, the trend of decreasing engine-out NOx emission will become a serious threat unless NOx can effectively be utilised. A filter candidate consisting of Pt-containing ceramic foam and a wall-flow monolith configuration (an optimised catalytic filter) is proposed to employ an optimal usage of NOx. By design this configuration is capable to employ two stages of filtration, namely: deep bed filtration on the foam and surface filtration on the wall-flow monolith. At the same time, two modes of soot oxidation reactions will operate. In Pt/ceramic foam multiple NO oxidation to NO2 and trapped diesel soot oxidation by NO2 take place, while part of NO2 is released as NO2-slip and this NO2-slip is subsequently utilised to oxidise trapped soot on a wall-flow monolith. In laboratory tests, this concept is clearly demonstrated that it outperforms the activity of conventional regenerating trap, based on upstream NO-NO2 conversion.

The physical state of a heterogeneous soot oxidation catalyst has a big impact on their performance. Contact between soot and solid catalysts is one of the key parameters for soot oxidation. Catalyst formulations based on low eutectic melting points were studied. Formulation based on cesiumsulfate and vanadiumoxide, became active at their melting point of 320-350 °C. Ceramic foam can act as support for the liquid catalyst as well as a deep filter for the soot. Downstream of a small diesel engine a balance temperature of 350 °C was established with an initial trapping efficiency of 40 %. Stable pressure drop over the system was maintained over a period of more than 24 hours.

We studied the mechanism of NO reduction as well as its selectivity and reactivity in the presence of excess O2. Results show that fuel injection and/or pretreatment are important for ceria catalyst reduction and carbon deposition on the catalyst surface. Oxygen defects of reduced ceria are the key sites for the reduction of NO into N2. The deposited carbon acts as a buffer reductant, i.e., the oxidation of carbon by lattice oxygen recreates oxygen defects to extend the NO reduction time interval. A small amount of NO showed a full conversion into only N2 both on the reduced Zr-La doped ceria and reduced Pt-Zr-La doped ceria. Only when the catalyst is oxidised NO is converted into NO2.