(126g) Spatiotemporal Studies of NH3 Formation Over Pt-Rh/BaO/Al2O3 LNT Monolith in the Presence of Excess CO2 and H2o

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) and combined LNT/SCR technology are under development. 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 improved designs and operating strategies for the LNT/SCR catalysts and reactors.

The production of NH₃ is known to occur by reaction between NO and H₂ under the rich conditions on precious group metals (PGM; Pt, Rh).  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 surface H, which then reacts with NO to give NH₃. A second pathway is through reaction between NO and CO forming support bound isocyanate (-NCO) species [1,3], 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 in high concentration in the exhaust (5-10%) but without H₂, which is typically low in diesel exhaust.

In a recent study [4] we carried out steady-state NO reduction by H₂, CO/H₂O on Pt/BaO/Al₂O₃ catalyst. Differential kinetics experiments reveal the existence of CO inhibition for the CO + NO, CO + H₂O, and CO + NO + H₂O systems which was more significant in the absence of H₂O. This suggests an enhancing role of hydrogen formed by water gas shift chemistry. Moreover, the CO inhibition gives a high NH₃ selectivity in the presence of excess water and also during the reduction of NO by H₂ irrespective of the H₂/NO ratio, further supporting the WGS route. The results suggest that during the reduction of NO by CO in the presence of excess water, NH₃ is mainly produced by the reduction of NO by surface hydrogen formed as an intermediate during the WGS reaction. Contribution by the hydrolysis of isocyanate intermediate appears to be secondary under the steady state conditions of this study.

We extend that study to examine the transient generation pathways to ammonia during conditions that mimic the lean NOx trap (LNT), utilizing a Pt-Rh/BaO/Al₂O₃ monolithic catalyst, and CO and H₂ asreductants. A combination of conventional bench-scale reactor studies with FTIR analysis of product gases together with construction of spatio temporal profiles using spatially-resolved mass spectrometry (SpaciMS) for NOx storage and reduction (NSR) are carried out. Our objective is to evaluate the NO_x reduction and product distribution features as a function of the feed composition and reaction temperature (150 - 400 ˚C) in the presence of excess CO₂ (3%) and H₂O (5%).

During the transient studies, the addition of water shows a similar promoting effect on NOx reduction by CO, NH₃ is found to be one of the major products at high CO concentrations and temperatures higher than 250 ˚C due to the poisoning effect of CO being more pronounced at temperatures below 250 ˚C. Furthermore, the addition of excess CO₂ significantly increases the selectivity to NH₃ at the expense of N₂O at all the temperatures studied, though slightly inhibiting the reduction of NOx due to the formation of carbonates on the storage component. In this study the spatio temporal profiles of the reactants and products as a function of feed concentration and reaction temperatures are reported and a mechanism is proposed for the formation of NH₃ during the transient reduction of NOx by CO in the presence of excess CO₂ and H₂O.

References

1. M. L. Unland, Journal of Physical Chemistry, 77 (1973) 1952.
2. T. Szailer, J.H. Kwak, D.H. Kim, J.C. Hanson, C.H.F. Peden, J. Szanyi, Journal of Catalysis, 239, (2006) 51.
3. T. Lesage, C. Verrier, P. Bazin, J. Saussey, M. Daturi, Physical Chemistry Chemical Physics, 5 (2003) 4435.

Prasanna R. Dasari, Rachel Muncrief, Michael P. Harold, Catalysis today, 184 (2012) 43– 53.