Effect of engine-based thermal aging on surface morphology and performance of Lean NOx traps

A small single-cylinder diesel engine is used to thermally age model (Pt + Rh/Ba/gamma-Al2O3) lean NOx traps (LNTs) under lean/rich cycling at target temperatures of 600 degrees C, 700 degrees C, and 800 degrees C. During an aging cycle, fuel is injected into the exhaust to achieve reproducible exotherms under lean and rich conditions with the average temperature approximating the target temperature. Aging is performed until the cycle-average NOx conversion measured at 400 degrees C is approximately constant. Engine-based NOx conversion decreased by 42% after 60 cycles at 600 degrees C, 36% after 76 cycles at 700 degrees C and 57% after 46 cycles at 800 degrees C. The catalyst samples were removed and characterized by XRD and using a microreactor that allowed controlled measurements of surface area, precious metal size, NOx storage, and reaction rates. Three aging mechanisms responsible for the deactivation of LNTs have been identified: (i) loss of dispersion of the precious metals, (ii) phase transitions in the washcoat materials, and (iii) loss of surface area of the storage component and support. These three mechanisms are accelerated when the aging temperature exceeds 850 degrees C-the gamma to delta transition temperature of Al2O3. Normalization of rates of NO reacted at 400 degrees C to total surface area demonstrates the biggest impact on performance stems from surface area losses rather than from precious metal sintering. (c) 2007 Elsevier B.V. All rights reserved.