Catalytic performances of ceria and ceria-zirconia materials for the combustion of diesel soot under NOx/O2 and O2. Importance of the cerium precursor salt

Abstract

Two different cerium precursors (Ce(NO3)3·6H2O or (NH4)2Ce(NO3)6) and two different calcination temperatures (500°C or 1000°C) were used in the synthesis by coprecipitation method of CexZr1−xO2 mixed oxides with three different Ce/Zr ratios (Ce0.8Zr0.2O2, Ce0.5Zr0.5O2 and Ce0.2Zr0.8O2) along with pure cerium and zirconium oxides, obtaining a wide range of oxides with very different textural and structural features. These catalysts were tested for soot combustion in loose-contact mode both under NOx/O2 and O2 atmosphere, pointing out that the choice of the (NH4)2Ce(NO3)6 precursor leads to mixed oxides with better catalytic activity in the two atmospheres proved with regard to the Ce(NO3)3·6H2O precursor. Catalysts with ceria-rich compositions showed better catalytic behaviour than the corresponding zirconia-rich compositions, demonstrating that BET surface area is not the only parameter to be considered for this application, since the catalytic activity of the prepared samples is not proportional to surface area of the solids. Important contribution of the NO2-assisted mechanism exists both to initiate and continue soot combustion under loose-contact mode in NOx/O2, when the ceria-based material produces NO2 at low temperatures (425–450°C), being the capability of the catalyst to produce NO2 the most important feature to explain the soot combustion. However, this role becomes minor when the ceria-based material produces NO2 at higher temperatures. Under O2, in a loose-contact situation, a good Zr-insertion into the lattice affects positively to the soot combustion activity.