In the case of the other two fuels tested (0.4% and 2.0% S), the
same three activity regimes are evident (Fig. 6b and c). The main
difference in that the progressive increase in sulphur content progressively
suppresses the catalytic activity observed. The only significant
difference in the behaviour is that deNOx activity is
observed at all engine loads, albeit to a very small extent. Furthermore,
a significant sulphur deactivation effect on NO oxidation is
seen only for 2.0% S.
The increase in NO2 concentration is due almost exclusively to
the presence of the catalyst, and, as will be discussed in Section 3.2,
NO oxidation promotes overall deNOx activity across the combined
system. In addition, it is clear from Fig. 6 that, from the point-ofview
of the desired oxidation of NO to NO2, an optimum temperature
exists. This optimum temperature depends on the fuel used
and, thus, temperature control would be advantageous to maximise
the NOx abatement observed.
Clearly the increase of sulphur content in the fuel depresses the
catalytic activity; nevertheless, a significant 15% of NO conversion
to NO2 is achieved at 50% load using the 2.0% S fuel at a GHSV of
40.700 h1 at STP, which corresponds to GHSV of 53.600 h1 at
the reaction temperature (cfr. Fig. 4). For the sake of comparison,
a GHSV of ca. 100.000 h1 is normally employed for light duty
vehicle (DOC or TWC), whereas urea-SCR catalysts work at much
lower GHSVs, the catalyst volume being often three-times that of
TWCs.
With regard to the overall activity of the catalyst, an interesting
aspect of the overall NO oxidation behaviour is illustrated
graphically in Fig. 7: most of the conversion is achieved on the first
catalytic bed, indicating that there is room for significant performance
improvement.