Isotopic Temporal Analysis of Products study of anaerobic NO reduction by NH3 on Pt catalysts

A Temporal Analysis of Products (TAP) study was carried out of the anaerobic NO + NH3 reaction system on Pt sponge and Pt/Al2O3 catalysts. Temperature, pulse timing, and feed composition were varied to quantify the conversion and product selectivities. Isotopically labeled nitric oxide (15NO) was used to follow reaction pathways to molecular nitrogen and nitrous oxide. A variation in the delay time between sequential pulses of 15NO and NH3 had a significant effect on the reaction path to molecular nitrogen. A mixed feed (no delay time) above the light-off temperature resulted in a product mixture nearly balanced between the mixed product 15NN, and the two single-source products 15N2, and N2. With increased delay time the selectivity of 15NN decreased significantly in favor of 15N2 and N2. The data suggest at least two major routes to nitrogen formation, one involving direct NO decomposition on reduced Pt sites and the other involving reaction between NHx species and NO. There is also kinetic evidence for a slower, third minor route involving an unidentified surface complex, possibly HNOad. The alumina support is shown to suppress this effect of delay time through ammonia adsorption, serving as a source-sink. The trends are interpreted with a mechanistic model constructed from a sequence of elementary steps.

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► Systematic study of anaerobic NO reduction by NH3 on Pt sponge & Pt/Al2O3 catalysts having same exposed Pt area.
► Isotopic pump-probe with NH3 and 15NO identify reaction paths to molecular nitrogen.
► Two pathways identified over range of temperatures and pulse timing for a fixed 15NO + NH3 feed: 15NO decomposition and 15NO + NH3 pathways.
► Reversible uptake of NH3 by alumina support suppresses large shifts in reaction mechanism over range of pulse timing.