A size-dependent property of Pt catalyst for the synthesis of NH3 during a gas-switching NOx storage and reduction process using BaO/Pt/γ-Al2O3

Abstract

Nitrogen oxides (NOx) are harmful but potential as an effective nitrogen source because of the reactivity higher than nitrogen (N2) in air. For example, an alternate gas-switching process at a constant temperature lower than 300 °C between the storage of NOx and the reduction by hydrogen (H2) has been proposed for the selective synthesis of valuable ammonia (NH3). In this study, we demonstrated the size-dependent property of platinum (Pt) catalyst for further understanding the activation of H2 and subsequent hydrogenation of stored NOx in the reduction process using systematically deigned BaO/Pt/γ-Al2O3 type nanocomposite catalysts. The hydrogenation rate over γ-Al2O3 almost depended on the resultant size of Pt nanoparticles (NPs) controllable by changing the reaction temperature only as well as the catalytic property in the activation of H2. The formation of N2 was progressed preferentially with a rapid hydrogenation of stored NOx by highly active (smaller) Pt NPs, being not contradictory to the fact that enough but slow hydrogenation of stored NOx was helpful for synthesizing NH3 by activity-controlled (larger) Pt NPs. Consequently, in the presence of enough H2, more than 90% of stored NOx can be recovered as NH3 above 200 °C by using BaO/Pt/γ-Al2O3 containing large Pt NPs. This knowledge is potential for the design of high-performance nanocomposite catalysts to produce starting NH3 for value-added chemicals and CO2 free fuel as strategic chemicals in our future society.