Synergy Between Ceria and Metals (Ag or Cu) in Catalytic Diesel Particulate Filters: Effect of the Metal Content and of the Preparation Method on the Regeneration Performance

The catalytic performance in soot oxidation of lab-scale silicon carbide (SiC) diesel particulate filters (DPFs) washcoated with metal-promoted CeO has been investigated focusing on the amount and the nature of the metal (Ag or Cu), and on the preparation method for the catalytic washcoat deposition. A wide range of metal loads was explored. The metal-promoted washcoat was dispersed onto the filters according to two methods: method (A)--cycles of deposition of metal nitrate after previous deposition of nanometric CeO, up to the achievement of the target load; method (B)--filter dipping into a suspension containing both CeO and metal nitrate in a suitable ratio. Bare Ag DPFs have also been investigated for the sake of comparison. Catalysts were characterized by SEM/EDX, Hg intrusion porosimetry and TPR and O-TPD techniques. All samples show a high metal dispersion and some reduction of support macro-pores. The metal addition improves the catalytic performance of bare ceria basically decreasing the temperature corresponding to the maximum soot oxidation rate. The positive effect increases with increasing metal load. As far as the method A is concerned, a better activity has been found for Cu-promoted catalysts with respect to the corresponding Ag-promoted catalysts with the same amount of metal. Ag-promoted catalysts prepared according to the method B are much more active than all other samples, showing a marked reduction of activation temperature as well. As disclosed by the analysis of redox properties and oxygen mobility, the method B significantly enhances the silver-ceria interface where highly reactive oxygen species (O) are produced, providing the outstanding activity of these catalysts. This same effect is much less evident in the case of Cu-promoted catalysts.