(401d) TAP Studies of NOx Reduction Using H2 and NH3

NOx Storage and Reduction (NSR) and Selective Catalytic Reduction (SCR) are two promising technologies for meeting upcoming stringent regulations for lean burn engines. NSR or Lean NOx Trap (LNT) is a cyclic process that involves storage of NOx on an alkali earth component (Ba, Ca) mediated by precious metals (Pt, Rh) followed by a shorter duration NOx reduction using HC, H2 or CO. The SCR uses NH3 as a primary reductant, produced from urea, to reduce NOx. LNT encounters fuel penalty and high cost of precious metals while SCR has a urea/ammonia infrastructure concern. The combined LNT - SCR configuration may partially address the fuel penalty, precious metals and on-board ammonia production concerns. Hence, there is a need to understand the production and consumption of NH3 on the LNT catalyst for various temperatures, so that unused ammonia can be used as NOx reductant to produce N2 in the downstream SCR catalyst. In this study, we used Temporal Analysis of Products (TAP) to gain insight in the NOx reduction by reductants like H2 and NH3; and effect of temperatures on N-H, N-O bond scission.

The current TAP experiments are carried out isothermally in the near Knudsen diffusion regime to avoid any thermal and mass transfer limitations encountered in atmospheric pressure studies. The experiments were done on Pt and Pt/BaO/Al2O3 catalysts in the temperature range 50 oC - 400 oC. The feed gas consisted of NO, H2 and NH3. Effluent species included H2 (m/e=2), NH3 (m/e=16), H2O (m/e=18), N2 (m/e=28), NO (m/e=30) and N2O (m/e=44) were monitored with a quadrupole mass spectrometer. Two kinds of experiments are performed: (i) pulse storage experiments, in which NO was pulsed with a spacing time of ts, and (ii) pump-probe NSR experiments in which sequential pulses of NO and H2 (or NH3) were pulsed over pre-reduced, pre-oxidized or pre-nitrated catalysts with prescribed delay time (td) and spacing time (ts). The following global reactions are studied.

NO + H2 -> 0.5 N2 + H2O

6 NO + 4 NH3 -> 5 N2 + 6 H2O

We used stoichiometric amount of reductants (either H2 or NH3) for NOx reduction. The NOx conversions in presence of H2 or NH3 are not thermodynamically limited in the studied temperature range. There is a clear difference in the NOx conversion in the presence of H2 and NH3. The NOx conversion in the presence of ammonia at temperatures below 190 oC is insignificant and there is a sudden increase in NOx conversion above 190 oC. On the other hand, NOx conversion in presence of H2 starts below 75 oC on Pt and gradually increases to 100% at 200 oC. The above observations combined with the results from our previous work that no NO bond session occurs below 150 oC suggests that first N-H bond scission is the crucial step in the NOx reduction with NH3. Once the first H atom is removed from NH3, adsorbed H and NH2 react with adsorbed NO forming H2O and N2.