Ammonia synthesis at low temperatures

Abstract

Density functional theory ͑DFT͒ calculations of reaction paths and energies for the industrial and the biological catalytic ammonia synthesis processes are compared. The industrial catalyst is modeled by a ruthenium surface, while the active part of the enzyme is modeled by a MoFe 6 S 9 complex. In contrast to the biological process, the industrial process requires high temperatures and pressures to proceed, and an explanation of this important difference is discussed. The possibility of a metal surface catalyzed process running at low temperatures and pressures is addressed, and DFT calculations have been carried out to evaluate its feasibility. The calculations suggest that it might be possible to catalytically produce ammonia from molecular nitrogen at low temperatures and pressures, in particular if energy is fed into the process electrochemically.

A theoretical evaluation of possible transition metal electro-catalysts for N2reduction

“…1 for the at Ru(0001) surface. When hydrogen is added in the form of H 2 molecules, the reaction has signicant thermochemical free energy barriers, as has been noted previously [13]. In the enzyme, however, hydrogen is not entering as H 2 but as protons and electrons, H…”

“…Nitrogenase can thus be viewed as part of an ATP driven electrochemical cell for this reaction. It is conceivable that this process could be emulated in a simpler, man-made system [13,14]. A low-temperature, low-pressure process could make more decentralized ammonia production possible compared with the current situations where ammonia can only be produced in large factories.…”

“…The reaction mechanism in enzymes is quite dierent from that of the industrial synthesis process. In the enzyme, N 2 molecules are hydrogenated (associative mechanism) [15,16,17,18], while in the Haber-Bosch method, the nitrogen and hydrogen atoms do not react until the strong N 2 triple bond and the H 2 bond have been broken (dissociative mechanism) [13].…”

FeMo cofactor of nitrogenase: energetics and local interactions in the protein environment

“…It is therefore not surprising that agreement concerning the mechanism of dinitrogen activation and the evolution of dihydrogen (both general and obligatory) has not yet been reached. Significant insight into this problem may be gained by carrying out ab initio density functional calculations [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33] on a quantum model that faithfully represents the active site of the protein. The caveat is, however, that the active site must not be so large that the calculations become intractable at the high level of theory required to afford reliable energetics.…”

Katalytische Ammoniaksynthese – eine “unendliche Geschichte”?

“…[73] Die Antwort auf die im Titel gestellte Frage kann somit lauten, dass es keine andere praktisch relevante Reaktion gibt, bei der das Gesamtverständnis von Theorie, Modellexperimenten und realer Katalyse so eng verzahnt und so weit entwickelt ist wie bei der Hochdrucksynthese von Ammoniak nach Haber und Bosch. Diese Feststellung gilt mittlerweile nicht nur für das Eisensystem, sondern auch für das Rutheniumsystem und in absehbarer Zukunft auch für das Co-Mo-N-Legierungssystem.…”