PrOx catalysts for the combustion of soot generated in diesel engines: effect of CuO and 3DOM structures

Abstract

PrO_x and CuO/PrO_x catalysts have been prepared with conventional (Ref) and three dimensionally ordered macroporous (3DOM) structures, and the effect of the structure on soot combustion has been studied. It has been demonstrated that the 3DOM structure significantly improves the catalytic combustion of soot with O₂. The activity follows the trend PrO_x-3DOM > CuO/PrO_x-3DOM ∼ CuO/PrO_x-Ref ≫ PrO_x-Ref, which is explained considering two aspects: the production of active oxygen and its transfer from catalyst to soot. FESEM microscopy, N₂ adsorption, Hg porosimetry and He density show that the 3DOM catalysts have ordered macroporosity with pores of 80 nm in diameter, which favors the carbon–catalyst contact. In addition, the 3DOM catalysts present higher surface density of active oxygen (O_ads), which follows the trend CuO/PrO_x-3DOM > PrO_x-3DOM ∼ CuO/PrO_x-Ref > PrO_x-Ref. Consequently, the PrO_x-3DOM catalyst combines a good production of active oxygen and an efficient transfer to soot, making it the most active catalyst to accelerate soot combustion. In contrast, PrO_x-Ref is the least active since it is the least efficient in producing and transferring active oxygen. The impregnation of copper with the conventional support (CuO/PrO_x-Ref) enhances the production and transfer of active oxygen, improving the activity with respect to PrO_x-Ref. However, CuO blocks the porosity of the 3DOM support, hindering the contact with soot. Soot combustion is accelerated in the presence of NO_x due to the production of NO₂. This NO₂, once produced, is mostly readsorbed on the surface of the catalysts producing active oxygen that must be transferred to soot. For this reason, the porosity of the catalysts also plays a relevant role during combustion with NO_x/O₂ because it affects the transfer of active oxygen produced by NO₂ to soot.