(547a) Effects of Rh, CeO2 and Regeneration Conditions During NOx Storage and Reduction On Pt/Rh/BaO/CeO2 Monolith Catalysts

NO_X Storage and Reduction (NSR) is a periodically operated catalytic process for NOx emission abatement in lean burn gasoline and diesel engines. The process involves two sequential steps of storage and reduction on a bifunctional catalyst in the lean NOx trap (LNT). Optimization of the multi-functional catalyst is a crucial element in LNT design.  This is complicated by the existence of H₂O and CO₂ in the feed and the large number of concurrent reactions during the regeneration step, including NO decomposition, NO reduction by various reductants (H₂, CO, HCs), water gas shift (WGS), and hydrocarbon steam reforming (HCSR).  For example, CO₂ is known to be detrimental to NOx storage and can consume feed H₂ by the reverse WGS reaction at higher temperature. 

The experimental system comprises a bench-scale reactor with synthetic feeds mimicking lean burn engine exhaust, utilizing both FTIR and mass spectrometers to measure effluent compositions.  Monolith catalyst samples provided by have different combinations of Pt, Rh, BaO, and CeO2 on alumina washcoat (Table 1).  The catalyst samples were evaluated in terms of NOx storage, NO oxidation activity, and reduction activity, the latter of which included cycle-averaged NOx conversion and product selectivities.  Typical cycling experiments involved 60 s storage feed containing 500 ppm NO, 5% O₂, 5% H₂O, and 5% CO₂ in diluent followed by a 5 to 30 s regeneration feed containing 0.5% H₂, 5% H₂O, and 5% CO₂ in diluent. The four catalysts enable a systematic comparison of NO oxidation activity relevant to NOx storage, the role of CeO₂ on storage, contributions of WGS reactions in affecting to NSR performance, of Rh and/or CeO₂ on WGS activity, among other salient features.

Typical cycling results are shown in Figure 1 which compares the cycle-averaged NOx conversion as a function of catalyst temperature for the four catalysts and for feeds with H₂O, with CO₂ and without both during the lean and rich steps.  The protracted reduction (30 s) ensures that most of the stored NOx is reduced. In the absence of feed H₂O and CO₂ all four catalysts have conversions exceeding 80% over the entire temperature range with the conversion approaching 100% between 300 and 350 ^oC.  An adverse effect of feed CO₂ is evident for all catalysts which results from a reduction in NOx storage on alumina and/or barium phases primarily and a reduction in NO oxidation conversion secondarily. The feed H₂O (250-400 ^oC) reduces the NOx storage, which results in a reduction of NOx conversion. At lower temperature (150-200 ^oC) an improvement effect of H₂O is observed.

The addition of Rh reduces the CO₂ inhibition, due likely to the reverse water gas shift reaction (H₂ + CO₂ ߨ¤ CO + H₂O) since the formed CO does not inhibit NO reduction on Rh to the extent it does on Pt.  CeO₂ also enhances the NOx conversion in the presence of CO₂, again, due to NOx storage enhancement.

Systematic experiments are carried out to determine the optimal operation for each catalyst.  This is accomplished by variation of the regeneration for fixed storage time. Optimal operation is defined in terms of the cycle-averaged NOx and H₂ conversion.  The regeneration of stored NOx on each of the four catalysts is evaluated by a fixed NOx storage method which ensures the same amount of stored NOx.  These ongoing experiments will be described and the roles of Rh and CeO₂ elucidated.

    Table 1. Washcoat compositions of Pt/Rh/BaO/CeO₂/Al₂O₃ catalyst samples.

			Figure 1.  Cycle-averaged NOx conversion for feeds with H2O, with CO2 and without both for catalysts P/B (a), P/BC (b), PR/B (c), and PR/BC (d).