Ensuring prolonged life for car catalytic converters
The performance and lifetime of nitrogen oxides (NOx) storage catalytic converters as well as engine fuel economy could be significantly improved using sulphur-free fuels. Over the last few years a clear trend has been witnessed towards a lower sulphur content of diesel fuels. However, it would be both technologically and economically difficult to achieve a sulphur content below 10ppm. On the other hand, this extremely low sulphur content provided the NANOSTRAP project partners the option of using disposable or ex situ regenerable sulphur oxides (SOx) traps. Even if these SOx traps are not efficient enough to remove all sulphur compounds from diesel engine emissions, significant benefits can still be derived from the combination of SOx/NOx traps. By trapping the SOx of the exhaust gas before passing through the NOx trap the permanent deactivation of NOx storage catalyst due to sulphur poisoning could be overcome. For this purpose, different sulphur trap materials were prepared at the laboratories of Venezia Technologie SpA. One of the main objectives was to develop the sulphur traps without the use of noble metals. Due to the low sulphur content of the exhaust gases in combination with their high velocities, very active surface sites able to absorb and oxidise SOx were necessary. The sulphur trap materials prepared contained cerium (Ce) as an oxygen storage component together with copper (Cu) and manganese (Mn) as oxidation components. These materials absorb SOx under lean conditions and desorb the accumulated SOx under rich conditions. To evaluate the SOx trapping efficiency of these materials, their dynamic behaviour was investigated under non-isothermal loading conditions. The NANOSTRAP project partners aimed to approach the conditions in a car, where the exhaust temperature is unstable. While analysing in detail the dependence of the SOx trapping efficiency on temperature, strong variations between materials prepared by different methods became obvious. Venezia Technologie SpA therefore advocates further research before the high performance offered by materials containing noble metals used to promote the oxidation of SOx can be attained.
