Size-dependent properties of Pt catalysts for the synthesis of NH3 during a gas-switching NOx storage and reduction process using BaO/Pt/γ-Al2O3

Abstract

Nitrogen oxides (NO_x), though harmful, serve as an effective nitrogen source owing to their higher reactivity compared to atmospheric nitrogen (N₂). For example, an alternate gas-switching process has been proposed in which NO_x is stored and then reduced by hydrogen (H₂) at a constant temperature below 300 °C, enabling the selective synthesis of valuable ammonia (NH₃). In this study, we demonstrated the size-dependent properties of a platinum (Pt) catalyst for further understanding the activation of H₂ and subsequent hydrogenation of stored NO_x in the reduction process using systematically designed BaO/Pt/γ-Al₂O₃ type nanocomposite catalysts. The hydrogenation rate over γ-Al₂O₃ was governed almost completely by the size of the resulting Pt nanoparticles (NPs), which could be tuned solely by adjusting the reaction temperature; the catalytic properties in activation of H₂ followed the same trend. The formation of N₂ progressed preferentially with the rapid hydrogenation of stored NO_x by highly active (smaller) Pt NPs, which is not contradictory to the fact that sufficient but slow hydrogenation of stored NO_x was helpful for the synthesis of NH₃ by activity-controlled (larger) Pt NPs. Consequently, in the presence of sufficient H₂, more than 90% of stored NO_x can be recovered as NH₃ above 200 °C by using BaO/Pt/γ-Al₂O₃ containing large Pt NPs. This knowledge has potential for the design of high-performance nanocomposite catalysts to produce starting NH₃ for value-added chemicals and CO₂-free fuels as strategic chemicals in the future society.