TAP study of NOx storage and reduction on Pt/Al2O3 and Pt/Ba/Al2O3

A systematic mechanistic study of NO storage and reduction over Pt/Al2O3 and Pt/BaO/Al2O3 is carried out using Temporal Analysis of Products (TAP). NO pulse and NO/H-2 pump-probe experiments at 350 degrees C on pre-reduced, pre-oxidized, and pre-nitrated catalysts reveal the complex interplay between storage and reduction chemistries and the importance of the Pt/Ba coupling. NO pulsing experiments on both catalysts show that NO decomposes to major product N-2 on clean Pt but the rate declines as oxygen accumulates on the Pt. The storage of NO over Pt/BaO/Al2O3 is an order of magnitude higher than on Pt/Al2O3 showing participation of Ba in the storage even in the absence of gas phase O-2. Either oxygen spillover or transient NO oxidation to NO2 is postulated as the first steps for NO storage on Pt/BaO/Al2O3. The storage on Pt/Ba/Al2O3 commences as soon as Pt-O species are formed. Post-storage H-2 reduction provides evidence that a fraction of NO is not stored in close proximity to Pt and is more difficult to reduce. A closely coupled Pt/Ba interfacial process is corroborated by NO/H-2 pump-probe experiments. NO conversion to N-2 by decomposition is sustained on clean Pt using excess H-2 pump-probe feeds. With excess NO pump-probe feeds NO is converted to N-2 and N2O via the sequence of barium nitrate and NO decomposition. Pump-probe experiments with pre-oxidized or pre-nitrated catalyst show that N, production occurs by the decomposition of NO supplied in a NO pulse or from the decomposition of NOx stored on the Ba. The transient evolution of the two pathways depends on the extent of pre-nitration and the NO/H' feed ratio. (c) 2006 Elsevier B.V. All rights reserved.