(380d) Pd@CeO2 Core@Shell Nanoparticles: Enhancing Thermal Stability and Activity in Three-Way Automotive Catalysts

Authors: 
Hill, A., University of Michigan
Seo, C. Y., University of Michigan
Schwank, J. W., University of Michigan
Lenert, A., University of Michigan
Increasingly stringent fuel efficiency standards for gasoline combustion engines are resulting in lower exhaust temperatures. Consequently, enhanced three-way catalysts are needed that can achieve light-off at significantly lower temperatures (150˚C) than current systems (~250–300˚C). At the same time, any new catalyst must maintain thermal stability and activity even after prolonged aging at temperatures exceeding 850°C. Both goals may be addressed by a core@shell architecture where a nanoscale metal particle such as palladium is encapsulated by a porous metal oxide shell such as ceria or ceria-zirconia. Counterintuitively, these catalysts exhibit improved performance after severe aging, in contrast to the behavior of commercial catalysts. This improved performance is related to the re-dispersion of metal atoms throughout the porous shell structure. Furthermore, the reducibility exhibited by these oxides allows for the reversible storage and release of oxygen. In this work, the tunability of the metal core size and shell thickness was leveraged to address fundamental questions about the mechanism of palladium transport into the shell nanostructure and the critical distance for effective oxygen donation from the shell to the metal surface. The most active sites in these catalysts are those located at the interface between the noble metal and the metal oxide. These active sites are maximized by encapsulation through core@shell morphology, resulting in appreciable enhancement in catalytic activity. Due to differences in coordination between interfacial and bulk metal oxide, the energy required to produce oxygen vacancies is location dependent. This effect coupled with increasing oxygen transport/mobility limitations further away from metal catalyst sites may give rise to an optimum core shell geometry. The insights gained from this research offer new guidelines for the development of a robust three-way catalyst with low-temperature light-off, thermal stability and activity.