Ammonia Dehydrogenation on Cobalt Cluster Cations Doped with Niobium

Abstract

Reactivity of bimetallic cobalt-niobium cluster cations, Co _n Nb _m ⁺ (n = 1–4, 6; m = 1–3), toward ammonia was investigated at near thermal energy using a guided ion beam tandem mass spectrometer. The dehydrogenation of NH₃ is dominantly observed in the single-collision reactions of all the bimetallic clusters studied here. Most of the clusters containing one niobium atom, Co _n Nb⁺, exhibit large cross sections for the NH₃ dehydrogenation compared to the corresponding single-element clusters, Co _n+1 ⁺ and Nb _n+1 ⁺. Density functional theory calculations reveal that both the transition states involved in the first and the second cleavages of N–H bonds of NH₃ are significantly stabilized for Co₄Nb⁺, which gives rise to the higher reactivity toward the NH₃ dehydrogenation. The multiple-collision reactions of Co _n Nb⁺ (n = 3, 4, and 6) result in the formation of Co _n NbN₂ ⁺, which can be generated via complete dehydrogenation of two NH₃ molecules, in addition to the formation of Co _n NbN _x H _x ⁺.