(575ba) Tap Studies of Nox Storage and Reduction on Variable Dispersion Pt/BaO/Al2O3 Catalysts

NOX emission from diesel vehicles is a significant environmental problem affecting major urban areas. NOX Storage and Reduction (NSR) is an emerging technology that overcomes the difficulty of accomplishing NOX reduction in a lean exhaust. The NOX removal process involves two stages on a bifunctional catalyst in the lean NOX trap (LNT). The first stage involves storage of NOX on an alkali earth component (Ba, Ca) mediated by precious metal (Pt, Rh). The second stage involves the injection of a rich pulse of shorter exposure to reduce the stored NOX. In this study we employ Temporal Analysis of Products (TAP) experiments and microkinetic mechanistic modeling to further the understanding of NSR. TAP experiments are carried out isothermally in the Knudsen transport regime thereby avoiding complicating thermal and mass limitations encountered in atmospheric pressure studies.

The TAP studies involve feeding reactant pulses to model Pt/BaO/Al2O3 (3 - 30 % Pt dispersion, 2-3 wt.% Pt, 15 wt.% BaO) and Pt/Al2O3 (1.5 wt% Pt with 30 % dispersion) catalysts at ~10-8 torr pressure over the 150 oC - 400 oC temperature range. Two kinds of experiments are performed; storage-pulsing experiment, in which NO is pulsed with spacing time (microsec), and pump-probe experiment in which NO and H2 are sequentially pulsed with a prescribed delay time (Td) and spacing time (Ts). The product gas emerging from the catalyst bed is analyzed by a Quadrupole Mass Spectrometer (QMS) in a separate chamber maintained at ~10-8 torr. The QMS monitors effluents N2, NO, NH3, N2O, H2 and H2O. The feed intensities are selected to ensure Knudsen transport. Post-reaction TPD is performed to quantify the adsorbed species coverages.

During NO pulsing experiments on Pt/Al2O3, NO decomposes forming N2 as the dominant product with O adatoms accumulating on the Pt (reactions 1-3 below), which leads to a decrease in the decomposition rate and corresponding breakthrough of NO. Secondary product N2O achieves a maximum at the NO breakthrough (reaction 4). At lower temperatures adsorbed NO strongly inhibits the reaction. The NO decomposition rate increases with temperature with negligible reaction at 150 oC. In the presence of BaO, NOX is stored as nitrite or nitrate by synergistic transfer of NO and O from Pt to Ba (reactions 5-8). The NOX storage increases as temperature increase with almost no storage at 150 oC.

(1) NO + Pt ↔ NO-Pt

(2) NO-Pt + Pt ↔ N-Pt + O-Pt

(3) 2 N-Pt ↔ N2 + 2 Pt

(4) N-Pt + NO-Pt ↔ N2O + 2 Pt

(5) O-Pt + BaO ↔ BaO2 + Pt

(6) NO + BaO2 ↔ BaO-NO2

(7) BaO-NO2 + NO-Pt ↔ Ba-(NO2)2

(8) Ba-(NO2)2 + 2 O-Pt ↔ Ba-(NO3)2

(9) H2 + 2 Pt ↔ 2 H ? Pt

(10) H ? Pt + NO ? Pt ↔ N ? Pt + HO ? Pt

(11) H-Pt + HO-Pt ↔ H2O + Pt

(12) N? Pt + H ? Pt ↔ NH ? Pt

(13) NH ? Pt + H ? Pt ↔ NH2 ? Pt + Pt

(14) NH2 ? Pt + H ? Pt ↔ NH3 ? Pt + Pt

(15) NH3 ? Pt ↔ NH3 + Pt

NO/H2 pump probe experiments on Pt/Al2O3 with excess H2 indicate the reaction between adsorbed NO and H2 at 150 °C, forming primarily NH3 (reactions 9-15). At this temperature, negligible N2O is formed, confirming that the slow NO bond scission (reaction 2) limits N formation required for N2O. In the presence of BaO, it forms N2 and NH3 in 1:3 ratios without any storage of NO on BaO. At higher temperature, N adatoms combination leads to produce N2 resulting in a decrease in NH3 production. The storage of NOX on BaO increases with temperature.

Ongoing experimental results will be presented that compare the performance of Pt and Pt/Ba catalysts having different Pt loadings and dispersions. In particular, we report results for a series of Pt/Ba catalysts with fixed Pt loading and varied dispersion.