(560c) Isotopic Studies of Nox Storage and Reduction Over Pt/BaO/Al2O3 Using Temporal Analysis of Products

NOX Storage and Reduction (NSR) is an emerging technology for NOX emission abatement in lean burn gasoline and diesel engines. The NOX removal process involves storage of NOX on an alkali earth component (Ba, Ca) mediated by precious metals (Pt, Rh) followed by injection of a rich pulse for a shorter duration to reduce the stored NOX. In this study, we employ Temporal Analysis of Products (TAP) experiments with isotopic labeling to elucidate the regeneration chemistry. TAP experiments are carried out isothermally in the Knudsen transport regime, thereby avoiding thermal and mass transport complications typical of atmospheric pressure reactors. TAP experiments are carried out utilizing isotopic species on Pt/BaO/Al2O3 catalysts with varied Pt loadings and dispersions but an equal number of exposed Pt sites. The use of labeled 18O2 and 15NO provided resolution of reaction pathways such as for N2 formation during the regeneration step. The experiments help to elucidate the roles of Pt and the coupling between the precious metal and storage phases.

Materials and Methods

Two Pt/BaO/Al2O3 powder catalysts were used having vastly different Pt loading (wt.%) and dispersion, but equal exposed Pt sites (catalysts provided by BASF Inc.; Table 1). While sample D3 was used as provided, sample B2M was a physical mixture of Pt/BaO/Al2O3 and BaO/Al2O3, the latter of which was added to achieve the requisite BaO sites. The TAP studies involved feeding pulses of the reactant over catalysts over 150°C - 400°C. The feed gas consisted of NO, H2, 15NO (98 atom% 15N, Cambridge Isotope Laboratories) and 18O2 (99 atom% 18O, Icon Isotopes). The use of labeled 15NO enabled the probing of pathways to the N-containing products. Effluent species included H2 (m/e=2), N2 (m/e=28), 15NN (m/e=29), 15N2 (m/e=30), NO (m/e=30), 15NO (m/e=31), O2 (m/e=32), 15N18O (m/e=33), 18OO (m/e=34), 18O2 (m/e=36), and N2O (m/e=44) and were monitored with a quadrupole mass spectrometer. Two kinds of experiments are performed: (i) pulse storage experiments, in which NO (or 15NO) was pulsed with a spacing time of 4s, and (ii) pump-probe NSR experiments in which sequential pulses of NO and H2 (or 15NO and H2) were pulsed over pre-reduced, pre-oxidized or pre-nitrated catalysts with prescribed delay time and spacing time.

Table 1. Physical properties of Pt/BaO/Al2O3 catalyst samples

Sample D3 Sample B2M

Pt (wt%) 2.7 0.28

BaO (wt%) 14.6 16.6

Pt dispersion (%) 3 33

Mass of catalyst (mg) 110 97

Results and Discussion

NO pulse storage experiments were carried out at 250 oC on pre-reduced catalyst to compare NO decomposition and storage differences in the two catalysts. The NO storage capacity of the lower dispersion catalyst (D3)) exceeded that of the higher dispersion catalyst (B2M). This suggests that in the physical mixture, the bulk BaO/Al2O3 does not provide significant NOX storage. Moreover, catalyst D3 had a higher production of N2 during the initial NO pulses than the higher dispersion catalyst (B2M). This suggests that Pt sites on the larger Pt crystallites are more active for NO decomposition.

Similar experiments with labeled 15NO pulsing on pre-nitrated Pt/BaO/Al2O3 catalysts (using unlabeled NO) provide evidence for a dynamic equilibrium between the gas and Pt as well as between Pt and BaO. Moreover, the sustained production of 15N2 over hundreds of pulses and a noted absence of 15NN and N2 products suggest that the 15N2 formation pathway is by decomposition of 15NO on Pt sites freed up by the storage of 15NO on the BaO phase by spillover.

Isotopic pump-probe experiments provide additional evidence of N2 formation on bulk Pt and also direct evidence of spillover processes during storage and reduction. Sequential pulses of 15NO and H2 were fed to a pre-nitrated Pt/BaO/Al2O3 catalyst (using unlabeled NO. The data show the production of N2, 15N2, and 15NN during 15NO and H2 pulses over 400 pump-probe cycles. Unlabeled N2 is produced by reverse NOX spillover from the Ba phase to Pt/BaO interface, where it decomposes to form N2. The principal role of excess H2 is to scavenge oxygen adatoms formed during NO and 15NO decomposition, freeing up Pt sites. The excess hydrogen also reacts with NO or N at the Pt/Ba interfacial region to form NH3, causing a smaller production of N2 during the H2 pulse compared to 15NO pulse. As unlabeled NOX is depleted from the Ba phase, the formation of N2 also declines. The formation of 15N2 is evident from the onset and is sustained throughout the experiment, indicating that decomposition occurs on bulk Pt sites farther away from the Pt/BaO interface. In addition to N2 and 15N2, the mixed product 15NN is formed during 15NO pulse, providing evidence for the reverse spillover of stored NOX from the Ba phase to Pt sites where N and 15N recombine. The production of 15NN decreases at the expense of an increase in 15N2 as unlabeled stored NOX is depleted. Ongoing experimental results will be presented that elucidate the role of the Pt/BaO interface during NSR, such as NOX spillover from Pt to Ba phase and reverse spillover from Ba phase to Pt.

Significance

There is a need for reducing NOX emissions from the exhaust of lean burn gasoline and diesel engines. This work is a step towards gaining a fundamental understanding of the complex catalytic chemistry of NSR through systematic transient experiments and the development of microkinetic models.