(297f) Studies of NH3 Formation Over Pt/BaO/Al2O3 LNT Monolith In the Presence of Excess Water

Ground level ozone produced by the NO_x emitted from lean burn and diesel engines is the main driving force for the active research in lean NO_x reduction. Several NO_x reduction technologies, such as selective catalytic reduction (SCR) using urea, lean NO_x traps (LNT) etc., are under development. Considerable research is also being done in developing technologies that combine both LNT and SCR after-treatment systems. The LNT/SCR technology utilizes NH₃ produced during the regeneration of the LNT for storage and reaction with NO_x in the downstream SCR. It is therefore, important to understand the NH₃ formation chemistry and kinetics on the LNT catalyst to develop an optimal design and operating strategy for the LNT-SCR system.

The production of NH₃ is known to occur by reaction between NO and H₂ under the rich conditions on precious group metals.  However, in the presence of NO, CO and excess H₂O, two additional routes are possible. The first is by the water-gas shift reaction of CO and H₂O to give H₂, which then reacts with NO to give NH₃. A second pathway is through reaction between NO and CO forming support bound isocyanate (-NCO) species, which are readily hydrolyzed to form NH₃.  The contribution of both pathways is of significance because each proceeds in the presence of H₂O which is obviously in high concentration in the exhaust (5-10%) but without H₂ in the feed, which is typically low in diesel exhaust. While Cant and coworkers [1-2] have conducted systematic studies in which, they studied the reaction of NO, CO, H₂ and H₂O on different precious metal catalysts and also determined the effect of supports on the production of NH₃. However, the major pathway leading to NH₃ is still unclear and the kinetics of NH₃ formation has not been properly determined. 

To this end, comprehensive steady-state experiments of CO-NO, CO-NO-H_2, CO-NO-H₂-H₂O, CO-H₂O, NO-H₂ reaction systems on Pt/BaO/Al₂O₃ monolithic catalyst have been carried out to evaluate the NO_x conversion and product distribution features as a function of the feed composition and temperature. CO reduces NO to N₂, N₂O and NH₃ in the presence of excess water on precious metal catalysts with the product selectivity shifting from N₂O to N₂ and then finally to NH₃ with an increase in the feed concentration of CO, consistent with the stoichiometry. However, an increase in the feed concentration of CO leads to a significant drop in the overall conversion of both NO and CO, which was also the case with the reduction of NO by CO in the absence of water. Differential kinetics measurements showed that the NO+CO reaction is nearly first order with respect to NO and negative first order with respect to CO. This confirms that the strong adsorption of CO on the Pt serves to block sites for adsorption of the other reactants. In addition, support-bound isocyanate (-NCO) species are also expected to form at high CO concentrations in the presence of NO [3]. These isocyanate species readily hydrolyze in the presence of excess water to give NH₃.

The water gas shift (WGS) reaction was observed to be ~ -0.23 order in CO, indicating that CO has a stronger inhibiting effect on the NO+CO reaction than the WGS reaction. This suggests that during the reduction of NO by CO in the presence of a large excess of water, CO is consumed more rapidly by H₂O than NO, thereby producing H₂, so that NO reduction is by reaction with H₂ than with CO.  Furthermore, H₂ reduces NO more selectively to NH₃ than N₂O or N₂ irrespective of the stoichiometric requirements, when the catalyst surface is largely covered by CO. Thus, while the isocyanate route cannot be ruled out, the WGS pathway appears to be the main route for the production of NH₃. The results of additional experiments including the use of a spatially-resolved mass spectrometer (SpaciMS) will be presented to establish phenomenological mechanism and to support the kinetics measurements.

References

1. Dumpelmann R, Cant N.W, Catalysis and Automotive Pollution Control III, 1995, 96: p. 123-135.
2. Cant N. W., Chambers D. C., Journal of Catalysis, 2001, 204: p. 11-22.
3. Granger, P, Journal of Catalysis, 1998, 177: p. 147-151.