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Abstract
As morphology plays a relevant role in solid/solid catalysis, where the number of contact points is a critical feature in this kind of reaction, three different ceria morphologies have been investigated in this work as soot oxidation catalysts: ceria nanofibers, which can become organized as a catalytic network inside diesel particulate filter channels and thus trap soot particles at several contact points but have a very low specific surface area (4 m2/g); solution combustion synthesis ceria, which has an uncontrolled morphology but a specific surface area of 31 m2/g; and three-dimensional self-assembled (SA) ceria stars, which have both high specific surface area (105 m2/g) and a high availability of contact points. A high microporous volume of 0.03 cm3/g and a finer crystallite size compared to the other morphologies suggested that self-assembled stars could improve their redox cycling capability and their soot oxidation properties. In this comparison, self-assembled stars have shown the best tendency towards soot oxidation, and the temperature of non-catalytic soot oxidation has dropped from 614°C to 403°C in tight and to 552°C in loose contact conditions, respectively. As far as the loose contact results are concerned, this condition being the most realistic and hence the most significant, self-assembled stars have exhibited the lowest T10% onset temperature of this trio (even after ageing), thus proving their higher intrinsic activity. Furthermore, the three-dimensional shape of self-assembled stars may involve more of the soot cake layer than the solution combustion synthesis or nanofibers of ceria and thus enhance the total number of contact points. The results obtained through this work have encouraged our efforts to understand soot oxidation and to transpose these results to real diesel particulate filters.
Highlights
An increasing share of the automobile market has been gained by diesel engines on-board passenger cars, over the last two decades, as they are more fuel economic than gasoline vehicles
The images show three micrometric star assemblies with different sizes and shapes, proving that the residence time in the reactor affects their final size (Figure 3a, 12 h; b, 24 h). This design offers a controlled and repeatable morphology, with a tridimensional shape constituted by individual rods, which offer a concave space for soot intrusion
This behaviour is desirable in the perspective of depositing such SA stars on the surface of the diesel particulate filters (DPF) channels as a carrier for noble metals or other active species: an effective penetration of the soot cake through a relevant portion of the catalytic layer would increase the number of contact points between the soot particles and the catalyst itself, promoting catalyst activity
Summary
An increasing share of the automobile market has been gained by diesel engines on-board passenger cars, over the last two decades, as they are more fuel economic than gasoline vehicles. Particulate matter (PM) is a complex aerosol composed of nanosized carbonaceous particles (called soot) on which soluble hydrocarbons, sulphates and metals adhere through complex filtration and oxidation phenomena. These particulates have diameters that range from a few nanometers to hundreds of nanometers and beyond [1]. The soot particles deposit in the pores of the ceramic walls and progressively form a soot layer on top of the wall, which is called cake [5] The latter generates a drop in pressure across the filter, which becomes unsustainable for the engine; the cake periodically needs to be burned off, in order for the filter to regenerate. Regeneration is currently achieved through the post-injection of fuel from the engine [6,7], which causes a relevant fuel penalty for modern engines
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