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

In a full catalytic cycle, bare Ta in the highly diluted gas phase is able to mediate the formation of ammonia in a Haber-Bosch-like process starting from N and H 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 TaN, 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 with N from the educt side or with two molecules of NH from the product side. In the thermal reaction of Ta with N, 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 to serve as a multipurpose tool is further shown by splitting the single bond of H 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.