2011 Annual Meeting

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

Authors

Prasanna R. Dasari - Presenter, University of Houston
Rachel L. Muncrief - Presenter, University of Houston


Ground level ozone produced by the NOx emitted from lean burn and diesel engines is the main driving force for the active research in lean NOx reduction. Several NOx reduction technologies, such as selective catalytic reduction (SCR) using urea, lean NOx 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 NOx 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-H2, CO-NO-H2-H2O, CO-H2O, NO-H2 reaction systems on Pt/BaO/Al2O3 monolithic catalyst have been carried out to evaluate the NOx 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.