(512f) Elucidating the Mechanism of NOx Adsorption and Reduction on Dual-Functional Ag/MgO/Al2O3

Dual-functional Ag/MgO/Al₂O₃ catalyst is developed, which functions as a passive NO_x adsorber at low temperatures (115 ℃) and a NO_x reduction catalyst at higher temperatures. A combination of bench-scale reactor studies and in-situ DRIFTS experiments are used to propose the mechanism for NO_x adsorption and reduction. It is found that NO gets adsorbed in the form of nitrites (NO₂^-) and nitrates (NO₃^-) at low temperatures (Fig. 1). The species (1234 cm^-1) are formed at early times, and the peak intensity of species (1499 and 1324 cm^-1) increases at later times. The intensities of some of the nitrate peaks (1499 cm^-1) are significantly higher over Ag/MgO/Al₂O₃ as compared to Ag/Al₂O₃, MgO, Al₂O₃, and the bare MgO-Al₂O₃ support, thus suggesting the significant role of metal-support interactions in enhancing the amount of NO_x storage over Ag/MgO/Al₂O₃. Adsorption experiments are followed by temperature ramp experiments, and the DRIFTS peaks at 1508, 1373, and 1319 cm^-1 are present even at 500 ℃ (Fig. 2). The presence of H₂ significantly increases the amount of NO_x storage and is explained by the conversion of NO₂^- to NO₃^- species (Fig. 3). Even though the adsorbed species are the same in the presence and absence of O₂, the formation of nitrites is more in the presence of O₂. Moreover, the same adsorbed species are formed with NO and NO₂, which is consistent with reactor experiments where similar adsorption/desorption features are observed in the presence of these two species. Based on the insights obtained, a kinetic model is developed to predict the spatio-temporal profiles of NO and NO₂ over Ag/MgO/Al₂O₃ (Fig. 4). In the full manuscript, the mechanistic insights in the presence of C₃H₆ and NH₃ reductants will be presented, and the effect of replacing Ag with Cu will be discussed.