Ta ₂ ⁺-mediated ammonia synthesis from N ₂ and H ₂ at ambient temperature

Significance

A combined experimental/computational approach provides deep mechanistic insight into an unprecedented cluster-mediated N−H coupling mimicking the industrially extremely important ammonia synthesis from N ₂ and H ₂ (the “Haber–Bosch” process) at room temperature. Crucial steps were identified for both the forward reactions (i.e., the activation of N ₂) and the backward process (i.e., the Ta ₂ ⁺-mediated decomposition of NH ₃). The central intermediate for either path corresponds to Ta ₂N ₂ ⁺, a four-membered ring with alternating Ta and N atoms. The root cause of tantalum’s ability to bring about nitrogen fixation and its coupling with H ₂ under mild conditions has been identified by state-of-the-art quantum chemical calculations.

Abstract

In a full catalytic cycle, bare Ta ₂ ⁺ in the highly diluted gas phase is able to mediate the formation of ammonia in a Haber–Bosch-like process starting from N ₂ and H ₂ at ambient temperature. This finding is the result of extensive quantum chemical calculations supported by experiments using Fourier transform ion cyclotron resonance MS. The planar Ta ₂N ₂ ⁺, consisting of a four-membered ring of alternating Ta and N atoms, proved to be a key intermediate. It is formed in a highly exothermic process either by the reaction of Ta ₂ ⁺ with N ₂ from the educt side or with two molecules of NH ₃ from the product side. In the thermal reaction of Ta ₂ ⁺ with N ₂, the N≡N triple bond of dinitrogen is entirely broken. A detailed analysis of the frontier orbitals involved in the rate-determining step shows that this unexpected reaction is accomplished by the interplay of vacant and doubly occupied d-orbitals, which serve as both electron acceptors and electron donors during the cleavage of the triple bond of N≡N by the ditantalum center. The ability of Ta ₂ ⁺ to serve as a multipurpose tool is further shown by splitting the single bond of H ₂ in a less exothermic reaction as well. The insight into the microscopic mechanisms obtained may provide guidance for the rational design of polymetallic catalysts to bring about ammonia formation by the activation of molecular nitrogen and hydrogen at ambient conditions.