Influence of controlled spatial deposition of Pt and Pd in NOx storage-reduction catalysts on their efficiency

Catalysts for NO,, storage and reduction (NSR) were prepared with a twin flame spray pyrolysis (FSP) unit allowing the preferential deposition of Pd and/or Pt nanoparticles on the catalyst's storage (K2CO3) or support (Al2O3) components. The structural properties of the catalysts were characterized by nitrogen adsorption, CO chemisorption combined with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray diffraction, and electron microscopy. Various combinations with Pt and Pd either deposited on the storage component (K2CO3) or the support (Al2O3) were tested by fuel lean/rich cycling exhibiting NO conversion above 90% at 300 degrees C. At 250 degrees C the best performance was achieved when Pt was deposited on K2CO3, whereas Pd was more active when deposited on Al2O3, albeit the activity of Pd to reduce NOx was low. Mixing Pd with Pt, that is depositing both noble metals on the same component (storage component or support), lowered the NO conversion, in some cases even below that of Pt only catalysts. In Pt/Pd/Ba/CeO2 catalysts. where Pd was deposited on the Ba storage component and Pt on the CeO2 support, Pd became active above 300 degrees C showing best reduction performance. At these temperatures Pt/Pd/Ba/CeO2 catalysts where the two noble metals were separated by deposition on the storage and support components showed superior behavior compared to corresponding single noble metal catalysts. (C) 2010 Elsevier B.V. All rights reserved.