Bridging the Gap in Understanding the Mechanism of NH3 Formation During NO Reduction by CO in the Presence of H2O Over Rh/Al2O3 Catalysts: DFT Study

Abstract

Ammonia (NH3) formation from nitric oxide (NO) and carbon monoxide (CO) in the presence of water is a promising pathway for low-temperature NOx reduction. In this work, density functional theory (DFT) calculations were used to investigate the mechanism of NH3 formation on a Rh(111)/γ-Al2O3(110), in which the Rh(111) and γ-Al2O3(110) were considered separately. NO readily dissociates on the Rh site, forming N*, which reacts with CO to produce the key intermediate of NCO*. Meanwhile, water dissociates more easily on γ-Al2O3(110), providing H* species. The hydrolysis of NCO* can proceed via two pathways: on Rh(111), it forms HNCO, while on γ-Al2O3(110), it forms HNCOH*. Both intermediates subsequently decompose into NH* species, which are then hydrogenated to produce NH3. The Rh pathway is more favorable in both kinetics and thermodynamics, while the support mainly supplies hydrogen through water activation. These results suggest a dual-site mechanism in which Rh drives the main transformation, and γ-Al2O3(110) assists via hydrogen transfer. This work presents the individual roles of Rh and Al2O3 surfaces in NO reduction with CO, producing NH3, which is relevant to catalytic behavior under humid conditions.