Specific reactivity of 4d and 5d transition metals supported over CeO2 for ammonia oxidation

Abstract

Optimizing NH₃ oxidation activity and N₂ selectivity remains a challenge in selective catalytic oxidation of ammonia (NH₃-SCO). Herein, a series of 4d (Pd, Ag, Rh) and 5d (Pt) transition metal catalysts were synthesized and loaded over a reducible CeO₂ support and tested for NH₃-SCO reactions. The catalytic results indicated that Pt/CeO₂ catalysts outperformed the other supported metals with superior NH₃ oxidation performance and N₂ selectivity, which was related to the strong metal–support interaction (SMSI) with electron transfer from Pt metals to CeO₂ supports. This could be ascribed to the Pt–O–Ce synergistic effects and the high Ce³⁺ concentration over the Pt/CeO₂ catalysts. The strong interactions between Pt and CeO₂ also improved the reducibility of Pt/CeO₂ and promoted the activation of surface lattice oxygen participating in the NH₃-SCO reaction. The rigorous kinetic studies elucidated the elementary steps for the NH₃-SCO reaction, where surface lattice oxygen activated surface adsorbed NH₃ to form an amide (–NH₂) which is regarded as the rate-determining step of NH₃ oxidation. In situ DRIFTS also confirmed that surface lattice oxygen as a reactant triggered the formation of reactive intermediates of –NH₂, which were consumed to generate N₂ and H₂O at low temperatures.