The NOx storage and Reduction (NSR) is emerging as a promising technology for NOx emission abatement for lean burn and diesel engines. The NOx removal process involves two stages on a bifunctional supported catalyst. The first stage involves storage of NOx on an alkali earth component (such as Ba, K) mediated by a precious metal (Pt, Rh). The second stage involves purging by a shorter exposure of rich pulse when an unacceptable amount of NOx is released. Elucidation of the synergistic roles played by both alkali compound and precious metal is key in improving the efficiency of the NSR process. In the current study we employ Temporal Analysis of Products (TAP) to probe the catalytic chemistry. The transient TAP protocols (pulsing, pump-probing) [1] are well suited for the current study since NSR is an inherently transient catalytic process. Moreover, TAP experiments to a large extent operate in the Knudsen regime and are isothermal, thereby simplifying the mechanistic analysis and kinetic parameter estimation. To this end, we seek to develop a mechanism by conducting systematic TAP experiments together with mathematical modeling and kinetic parameter estimation.

We investigate the mechanistic details of NOx storage and reduction over Pt/Al2O3 and Pt/BaO/Al2O3 powders at 350 °C. Storage experiments involve feeding a series of NO pulses over the catalyst and quantifying the effluent flows of product species N2, NO and N2O. NO decomposes over Pt surface forming N2 as the dominant product with oxygen adsorbed on its surface. As the Pt surface (in case of Pt/Al2O3) is saturated with oxygen adatoms, a breakthrough of NO occurs. N2O formation achieves a maximum at the NO breakthrough point. Storage over Pt/BaO/Al2O3 shows similar behavior except the NO breakthrough is more gradual indicating storage occurring over the Ba component. The storage is an order of magnitude higher in case of Pt/Ba compared to Pt only catalyst. Storage and reduction experiments involve feeding a series of alternating NO and H2 pulses, mimicking the NSR cycling. These pump-probe experiments on the Pt/BaO/Al2O3 catalyst uncover several interesting features that are sensitive to the relative amounts of NO and H2. For feed NO:H2 > 2 over a clean Pt/Ba surface (pre-reduced), the NO decomposes to form N2 and oxygen accumulates on the Pt. The H2 pulse reduces the Pt surface completely, as seen by steady level of N2 formed during NO pulses. For feed NO:H2 < 1, H2 is not sufficient to completely reduce the surface for the next NO pulse to decompose and form N2. As a result, oxygen poisons the surface, albeit gradually, and NO storage occurs. The results suggest one of two mechanisms. The first involves oxygen spillover from the Pt to Ba storage component at Pt/Ba interface. The second mechanism involves the transient production of NO2 due to the reaction between the pulsed NO and oxygen adatoms. The NO2 can then transport via the gas phase to storage sites in close proximity to or removed from the Pt/Ba interface. Oxygen adsorbed over Pt seems important for NO storage, as observed for pump-probe over pre-oxidized and pre-nitrated as compared to pre-reduced catalyst.

The mechanistic pathway for NO storage and reduction can be explained by the model presented in our previous work [1] and work reported in literature [2, 3]. We estimate the kinetic parameters based on NO storage on Pt/BaO/Al2O3 and its reduction with H2. Starting parameters were chosen from literature and others were fitted with optimization. The oxygen adsorption coefficients as determined from literature [2, 4] shows very little desorption at the temperatures involved validating the mechanistic model of oxygen poisoning and eventual NO breakthrough. Initial kinetic modeling reveals that N2 is formed from NO adsorbing on Pt and decomposing during storage. During reduction, NO is supplied from the storage component and reacts at the interface to form N2. Modeling also shows the role played by the oxygen spillover mechanism for NO storage which is manifested in the gradual breakthrough of NO over Pt/Ba catalyst. NO oxidation to NO2 affects the N2 production rate at initial pulse numbers. As O2 on Pt surface accumulates, the model predicts rise in NO2 production. NO disproportionation to N2O which results in its formation after NO breakthrough is validated. We are currently in the process of modeling the mechanistic route NO takes over Pt surface for its storage and the mechanism of its reduction based on the experimental TAP data. This with other features of the mechanism will be presented.

1. Kabin, K., P. Khanna, R.L. Muncrief, V. Medhekar, and M.P. Harold, Monolith and TAP reactor studies of NOx storage on Pt/BaO/Al2O3: Elucidating the mechanistic pathways and roles of Pt. Catalysis Today, 2006. In Press.

2. Olsson, L., B. Westerberg, H. Persson, E. Fridell, M. Skoglundh, and B. Andersson, A kinetic study of oxygen adsorption/desorption and NO oxidation over Pt/Al2O3 catalysts. Journal of Physical Chemistry B, 1999. 103: p. 10433-10439.

3. Nova, I., L. Castoldi, L. Lietti, E. Tronconi, and P. Forzatti, On the dynamic behavior of "NOx-storage/reduction" Pt-Ba/Al2O3 catalyst. Catalysis Today, 2002. 75: p. 431-437.

4. Olsson, L., H. Persson, E. Fridell, M. Skoglundh, and B. Andersson, A kinetic study of NO oxidation and NOx storage on Pt/Al2O3 and Pt/BaO/Al2O3. Journal of Physical Chemistry B, 2001. 105: p. 6895-6906.