Fundamental Studies of NOx Adsorber Materials

Charles H. F.  Peden, Pacific Northwest National Laboratory

This talk will be describing some recent results of the multi-year effort at Pacific Northwest National Laboratory to develop a fundamental understanding of the operation of NOx adsorbers. Our initial efforts are focused on understanding the current limitation of both the capture and regeneration modes of model NOx adsorber materials. Specifically, studies are addressing details of the elementary adsorption, reaction and desorption processes, as well as the identity of reaction intermediates and their thermal stabilities on the catalyst surfaces. Efforts are also aimed at understanding the roles of additives and their effect on the mechanism(s) of reaction. In addition, we are focused on optimizing material properties to increase performance and durability over many regeneration cycles.

The fundamentals of NOx uptake/release processes have been studied extensively on catalysts based on BaO on alumina. Studies using vibrational spectroscopic teachniques (IR and Raman) and temperature programmed desorption (TPD) have aimed at understanding the chemical nature of the NOx species formed on/in these catalysts. The formation of different NOx species on the active BaO phase has clearly been shown, however, their assignments were not unambiguous. For example, in TPD studies two distinct NOx desorption features were reported; one at lower temperature (release of NO2), and another at higher temperature (desorption of NO+O2). The origin of these two features, however, has not been adequately explained. In order to understand the processes that take place upon NOx adsorption on these Ba/Al2O3 catalysts in different reactant gas mixtures, and in the subsequent regeneration, we have undertaken studies utilizing a series of spectroscopic techniques. The adsorption of NOx was investigated on 2%, 8%, and 20% BaO/Al2O3 catalysts with or without Pt using a series of in situ spectroscopic techniques: FTIR, TPD, 15N MAS NMR, and TP XRD. TEM has also been used to follow morphological changes during NOx adsorption and desorption processes. The formation of various adsorbed NOx species was followed as a function of sample composition (2%, 8%, and 20% BaO; 0% and 2% Pt), reactant gas mixture composition (NO2, NO+NO2, NO+O2), and catalyst temperature during NOx adsorption.

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