Kinetic study of NO oxidation and NOx storage on Pt/Al2O3 and Pt/BaO/Al2O3

Modeling and flow reactor experiments were used to study the kinetics of NO, 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, release, and (iv) NO, reduction to N-2. 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, storage for the combined system compared to the Pt/BaO/Al2O3 case. To resolve the second and third steps, an experimental investigation of NOx storage/release on BaO/Al2O3 was performed using only NO2 and N-2 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, storage on BaO/Al2O3, a kinetic model was constructed to describe NO, storage/release experiments on Pt/BaO/Al2O3. However, the rate for NO, 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/N-2 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, storage on BaO/Al2O3 and on Pt/BaO/Al2O3 were successfully validated with independent experiments.