Controlling catalytically active phases for low-temperature NOx storage and reduction

Abstract

NO$_x$ storage and reduction (NSR) has been widely used for removing NO$_x$ exhaust from lean-burn engines. However, the operation range of NSR has been limited because of the poor activity of NSR catalysts at low temperatures (≤300˚C), especially in urban driving conditions. Since the NSR catalysts are exposed to high temperatures and large amounts of water vapor under the high-speed operation or regeneration condition, it is also essential to achieve an excellent hydrothermal stability. To increase NOx removal efficiency during lean-rich cycle operation, a Cu/CeO$_2$ (CC) catalyst was added to a Pt-BaO/CeO$_2$ (PBC) catalyst. Under lean conditions, the CC showed much better activity for NO oxidation, allowing for faster NO$_x$ storage on PBC. Under rich conditions, H$_2$ was generated in situ on the CC by a water-gas shift reaction then accelerated the reduction of NO$_x$, which had been stored on PBC. These catalytically active phases were anchored on the coordinatively unsaturated penta-coordinate Al$^{3+}$ sites on the activated γ-Al$_2$O$_3$ surface. The catalyst showed a superior low-temperature activity and hydrothermal stability after the hydrothermal aging at 750˚C for 25 h using 10% H$_2$O/air. XRD, HR-TEM, CO chemisorption, N$_2$O chemisorption, and N$_2$ physisorption results showed that catalytically active phases were barely changed even after the hydrothermal aging. CO-TPR and XPS results demonstrated that the catalyst has a superior reducibility of lattice oxygen on CeO$_2$, leading to an excellent low-temperature activity.