A Kinetic Study of NO Oxidation and NO_x Storage on Pt/Al₂O₃ and Pt/BaO/Al₂O₃

Abstract

Modeling and flow reactor experiments were used to study the kinetics of NO x storage/release on a Pt/BaO/Al2O3 model catalyst. The mechanism for this concept can be divided into four steps: (i) NO to NO2 oxidation on Pt, (ii) NO2 storage on BaO, (iii) NO x release, and (iv) NO x reduction to N2. In this paper, we have focused on the first three steps. From the NO oxidation study on Pt/Al2O3 compared to Pt/BaO/Al2O3, we observed that the presence of BaO decreases the formation of NO2. To test the importance of this step for effective storage, experiments were performed with a Pt/Al2O3 catalyst placed before the Pt/BaO/Al2O3 catalyst. This resulted in increased NO x storage for the combined system compared to the Pt/BaO/Al2O3 case. To resolve the second and third steps, an experimental investigation of NO x storage/release on BaO/Al2O3 was performed using only NO2 and N2 in the gas feed. We propose a kinetic model, which first includes adsorption of NO2, which oxidizes the surface, followed by nitrate formation. Finally, NO3 -BaO−NO2, i.e., Ba(NO3)2, is formed. By using the kinetic parameters from the NO oxidation on Pt/BaO/Al2O3 and the NO x storage on BaO/Al2O3, a kinetic model was constructed to describe NO x storage/release experiments on Pt/BaO/Al2O3. However, the rate for NO x release was increased when Pt was present, and the kinetic model could not accurately describe this phenomenon. Therefore, the mechanism was modified by including a reversible surface spillover step of NO2 between Pt sites and BaO sites. Further, experiments with NO2 exposure followed by a temperature ramp with NO/N2 showed that the desorption behaviors from the BaO/Al2O3 and Pt/BaO/Al2O3 were significantly different, which further supports the spillover mechanism. Finally, the models describing NO x storage on BaO/Al2O3 and on Pt/BaO/Al2O3 were successfully validated with independent experiments.

Adsorption/Desorption Studies of NOx on Well-Mixed Oxides Derived from Co−Mg/Al Hydrotalcite-like Compounds

“…5,6 NO x storage/reduction technology is used in engines that alternately operate under lean-burn and rich-burn conditions. 7,8 Under lean-burn conditions, NO is oxidized and is stored on the catalyst. When the engine is switched to conditions with a stoichiometric air-to-fuel ratio, i.e., rich-burn conditions, NO x species are released and subsequently reduced by hydrocarbons and carbon monoxide.…”

Simulation of lean NOx trap performance with microkinetic chemistry and without mass transfer.

“…The presence of BaCO 3 is assured in any system containing realistic amounts of CO 2 ; on the other hand, we have chosen not to include Ba(OH) 2 , in part because it is less stable than the carbonate, but also because the roles of the two species could probably not be distinguished, given the unvarying levels of CO 2 and H 2 O. The final four species are those that appear in conventional descriptions of NO x storage (Olsson et al, 2001), although other candidates have been proposed (Sedlmair et al, 2003). Because these are true surface species, they need not adhere to any conventional bulk stoichiometry, but the inclusion of the stoichiometric nitrate Ba(NO 3 ) 2 makes it possible to write some overall (global) reactions for the NO x storage process.…”

“…The direct displacement of NO x by CO 2 (Balcon et al, 1999) could have been used instead (or in addition), but this was found not to be advantageous. Reactions (S30)-(S33) constitute a NO x storage mechanism that is similar to that in Olsson et al (2001), but with two principal differences. Specifically, while (S30) and (S33) have exact analogs in that paper, (S31) and (S32) do not.…”

“…The NO x storage model developed by Olsson et al (2001), which was later supplemented with propene-based regeneration kinetics (Olsson et al, 2002), was shown to be quite accurate in predicting NO 2 uptake but could not account for NO uptake (in the presence of O 2 ) on Pt/BaO/Al 2 O 3 (Scotti et al, 2004). This was due to the fact that storage in this model was limited by NO oxidation to NO 2 .…”

“…These include simple control-oriented models (Ketfi-Cherif et al, 2000;Kim et al, 2003;Larsson et al, 1999), global kinetic models (Bhatia et al, 2009;Guthenke et al, 2007;Koci et al, 2004;Koci et al, 2009;Koop and Deutschmann, 2007;Laurent et al, 2003;Olsson et al, 2005), and models attempting a detailed description of the surface chemistry (Lindholm et al, 2008;Olsson et al, 2001;Olsson et al, 2002;Xu et al, 2008). It is no longer practical to give a brief survey of this growing field, which is the subject of a recent comprehensive review (Roy and Baiker, 2009).…”