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      Ta2+-mediated ammonia synthesis from N2 and H2 at ambient temperature

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      Proceedings of the National Academy of Sciences
      Proceedings of the National Academy of Sciences

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          Abstract

          <p id="d3774638e210">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 <sub>2</sub> and H <sub>2</sub> (the “Haber–Bosch” process) at room temperature. Crucial steps were identified for both the forward reactions (i.e., the activation of N <sub>2</sub>) and the backward process (i.e., the Ta <sub>2</sub> <sup>+</sup>-mediated decomposition of NH <sub>3</sub>). The central intermediate for either path corresponds to Ta <sub>2</sub>N <sub>2</sub> <sup>+</sup>, 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 <sub>2</sub> under mild conditions has been identified by state-of-the-art quantum chemical calculations. </p><p class="first" id="d3774638e244">In a full catalytic cycle, bare Ta <sub>2</sub> <sup>+</sup> in the highly diluted gas phase is able to mediate the formation of ammonia in a Haber–Bosch-like process starting from N <sub>2</sub> and H <sub>2</sub> 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 <sub>2</sub>N <sub>2</sub> <sup>+</sup>, 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 <sub>2</sub> <sup>+</sup> with N <sub>2</sub> from the educt side or with two molecules of NH <sub>3</sub> from the product side. In the thermal reaction of Ta <sub>2</sub> <sup>+</sup> with N <sub>2</sub>, 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 <sub>2</sub> <sup>+</sup> to serve as a multipurpose tool is further shown by splitting the single bond of H <sub>2</sub> 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. </p>

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          How a century of ammonia synthesis changed the world

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            Challenges in reduction of dinitrogen by proton and electron transfer.

            Ammonia is an important nutrient for the growth of plants. In industry, ammonia is produced by the energy expensive Haber-Bosch process where dihydrogen and dinitrogen form ammonia at a very high pressure and temperature. In principle one could also reduce dinitrogen upon addition of protons and electrons similar to the mechanism of ammonia production by nitrogenases. Recently, major breakthroughs have taken place in our understanding of biological fixation of dinitrogen, of molecular model systems that can reduce dinitrogen, and in the electrochemical reduction of dinitrogen at heterogeneous surfaces. Yet for efficient reduction of dinitrogen with protons and electrons major hurdles still have to be overcome. In this tutorial review we give an overview of the different catalytic systems, highlight the recent breakthroughs, pinpoint common grounds and discuss the bottlenecks and challenges in catalytic reduction of dinitrogen.
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              Introduction ofn-electron valence states for multireference perturbation theory

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                Author and article information

                Journal
                Proceedings of the National Academy of Sciences
                Proc Natl Acad Sci USA
                Proceedings of the National Academy of Sciences
                0027-8424
                1091-6490
                November 13 2018
                November 13 2018
                November 13 2018
                October 23 2018
                : 115
                : 46
                : 11680-11687
                Article
                10.1073/pnas.1814610115
                6243240
                30352846
                be22911d-5c97-4c9d-ab3b-934ee5f608c2
                © 2018

                Free to read

                http://www.pnas.org/site/misc/userlicense.xhtml

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