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/H2 pump-probe experiments at 350 °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 N2 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 O2. 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 H2 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/H2 pump-probe experiments. NO conversion to N2 by decomposition is sustained on clean Pt using excess H2 pump-probe feeds. With excess NO pump-probe feeds NO is converted to N2 and N2O via the sequence of barium nitrate and NO decomposition. Pump-probe experiments with pre-oxidized or pre-nitrated catalyst show that N2 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/H2 feed ratio.