Investigation of Aging Mechanisms in Lean NOx Traps

Lean NOx traps (LNTs) represent a promising technology for the abatement of NO x under lean conditions. While LNTs have found application on lean-burn gasoline vehicles in Europe, the issue of catalyst durability remains problematic. LNT susceptibility to sulfur poisoning is the single most important factor determining effective catalyst lifetime. The NOx storage element of the catalyst has a greater affinity for S02 than it does for N02, and the resulting sulfate is more stable than the stored nitrate. Although this sulfate can be removed from the catalyst by means of high temperature treatment under rich conditions, the required conditions give rise to deactivation mechanisms such as precious metal sintering, total surface area loss, and solid state reactions between the various oxides present. In order to gain fundamental insights into these deactivation mechanisms, the University of Kentucky Center for Applied Energy Research (UK CAER) will with team with Oak Ridge National Laboratory (Fuels, Engines, and Emissions Research Center), Ford Motor Co. (Scientific Research Laboratory), and Umicore Autocat USA. Given that there is little incentive to employ lean-burn gasoline vehicles in the US market (in view of the development of hybrid gasoline-electric vehicles), this project will focus on lean NOx traps of the sort used on diesel vehicles. While numerous academic studies have been performed using model catalysts of the Pt/(Rh)/BaO/alumina type, few studies have been reported for catalysts aged to any significant degree, and details pertaining to the evolution of catalyst composition and structure with aging (and hence performance deterioration) are lacking. The approach utilized in this project will make use of detailed characterization of model catalysts prior to and after aging, in tandem with measurement of catalyst performance in NOx storage and reduction. In this manner, the evolution of catalyst microstructure upon aging will be related to NOx storage and reduction characteristics. Rather than using poorly characterized proprietary catalysts, or model catalysts of the Pt/Rh/BaO/ Ah03 type (which represent the first generation ofLNTs), we will focus on Pt/Rh/Ce02(-Zr02)/BaO/Ah03 catalysts, representing a model system which more accurately reflects current diesel LNT formulations. The effect of washcoat composition on catalyst aging characteristics will be examined by systematic variation of the concentration of the four main active components, Pt, Rh, Ce02 (or Ce02-Zr02) and BaO. Catalyst aging will be performed on a synthetic gas bench using repeated sulfation-desulfation cycles with periodic measurement of NOx storage capacity and NOx reduction activity. In addition to the use of standard physico-chemical analytical techniques for studying the model catalysts, use will be made of advanced tools for characterizing their NOx storage/reduction and sulfation/desulfation characteristics. These tools will comprise Spatially-resolved capillary-inlet Mass Spectrometry (SpaciMS), in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), and on-line chemical ionization mass spectrometry. The application of SpaciMS, in particular, will permit the dynamic chemistry of the catalysts to be studied; by measuring the gas composition at 6 points along the length of the catalyst, transients occurring during regeneration and desulfation of the fresh and aged catalysts will be monitored with spatial resolution. Further, use of the SpaciMS method will allow catalyst performance to be correlated with residual (non-regenerable) sulfur concentration along the length of the catalyst. Together, the application of these advanced analytical techniques will allow both the static and dynamic chemistry of the model catalysts to be studied. Subsequently the aged catalysts will subjected to detailed chemical and physical analysis in order to correlate NOx adsorption/reduction performance with catalyst composition and structure. The kinetic data will be incorporated in a model with the objective of describing, in quant