Abstract
A combined catalyst system of Ag/Al2O3 and Fe-BEA was tested in a light-duty engine-bench for H2-assisted NH3-SCR. Ag/Al2O3 was used to provide a high low-temperature activity, while Fe-BEA can provide a high activity at higher temperatures without the need to co-feed H2. The catalysts were combined in a sequential dual-brick layout and compared to Ag/Al2O3 and Fe-BEA only. The aim of the study was to investigate the performance of the catalysts in real life exhaust in contrast to synthetic gas. The catalysts were both tested during stationary and transient conditions. Transient testing was carried out with the New European Driving Cycle (NEDC). Laboratory tests were performed to complement the results from engine tests. Ag/Al2O3 showed a lower NOx conversion in stationary engine tests than expected. This was investigated further in a laboratory test set-up and was attributed to deactivation by soot and/or Ag oxidation, low H2 levels and low specific catalyst loading. NO2 increased the catalytic activity at below 250 and above 300°C. For the combined systems, it was preferred to have Fe-BEA in an upstream position of Ag/Al2O3 compared to the opposite. The high engine out NO2/NOx ratio, giving fast-SCR over the Fe-BEA, was believed to be the reason. At low temperature, the activity over the combined systems was higher than that for the individual catalysts showing that there were synergy effects of combining Ag/Al2O3 and Fe-BEA. This was also seen in the transient tests. However, the overall cycle NOx conversion was low due to very demanding conditions with a lot of the NOx being emitted at below 150°C. The order of performance based on an overall conversion during the NEDC was dual-brick with Ag/Al2O3 upstream=Ag/Al2O3 only>dual-brick Fe-BEA upstream>Fe-BEA only. The Ag/Al2O3 containing layouts showed a noticeable NOx conversion from the start of the cycle, i.e. before any NH2 or H2 was dosed. We believe that the NOx conversion seen came from NOx storage on the Ag/Al2O3.
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