Key-lock Ceria Catalysts for the Control of Diesel Engine Soot Particulate Emissions

A new concept, referred to as key-lock catalyst, is presented in this article. Soot combustion ceria catalysts were prepared combining hard (polymethylmethacrylate colloidal crystals) and soft (Pluronic F127) templates, tuning the porosity of ceria in different size ranges to match the morphology of soot aggregates. The catalysts porosity was characterized in detail by N-2 adsorption-desorption isotherms and Hg-porosimetry. XRD and H-2-TPR characterization ruled out that differences in activity are related neither with crystallographic nor with redox properties. As a proof of key-lock catalyst concept, an optimum key-lock ceria catalyst was synthesized by combining large macropores (100-300 nm) with mesopores (10-30 nm), because they fit to the large soot aggregates and to primary soot particles sizes, respectively. The best soot combustion activity of the optimum key-lock catalyst is attributed to the optimum transfer of ceria active oxygen from catalyst to soot. The catalytic results confirmed that all ceria catalysts prepared with different porosity oxidize NO to NO2 at the same rate, and the NO2-assisted soot combustion pathway is not affected by tuning ceria porosity. This double-templated synthesis and the key-lock concept opens a new synthesis approach to design noble-metal free soot combustion catalysts based on the highly effective active oxygen mechanism.