INFLUENCE OF SOME POLLUTANTS ON THE DURABILITY OF CORDIERITE FILTERS FOR DIESEL CARS

The durability of cordierite honeycomb structure for automotive emission control is deeply affected by the presence of some pollutants (Na2O, CaO, ZnO, PbO, Fe2O3 and V2O5) on the trap walls during the high-temperature step of filter regeneration. The long-term interaction between cordierite and the single pollutant oxides as well as the interaction between cordierite and some binary pollutant mixtures as a function of temperature have been investigated by differential dilatometry, X-ray diffraction and scanning electron microscopy (SEM). It appears that sodium oxide is the most dangerous pollutant, as it completely destroys cordierite starting from 900-degrees-C; the other pollutants influence the form of the silico-aluminate produced. Significant amounts of glassy phase are also formed. Both sodium silico-aluminate and the glassy phase have a thermal expansion coefficient different from that of cordierite: consequently, during filter regeneration, microcracks may appear, as was confirmed by SEM observation.