A Theoretical Evaluation of Possible Transition Metal Electro-catalysts for N-2 Reduction Page: 2 of 22
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that has been of key importance in supporting the large global population growth over
the past century . Much work has gone into the optimization of this process and today
it is understood in great detail [2, 3, 4, 5, 6, 7, 8, 9, 10]. In the Haber-Bosch process,
nitrogen and hydrogen gas molecules are heated to approximately 400 C, pressurized to
around 150 bar and passed over an Fe-based catalyst to form ammonia [6, 8]:
N2 + 3H2 -- 2NH3 (1)
Although the reaction is exothermic, relatively high temperature is required to make
the reaction kinetics fast. This, however, shifts the equilibrium towards the reactants
resulting in lower conversion. The high pressure is chosen to alleviate this problem,
since it shifts the equilibrium in favor of the products.
The industrial conditions are in remarkable contrast to those in microorganisms
which exist in nature and use the enzyme nitrogenase to produce ammonia from sol-
vated protons, electrons and atmospheric nitrogen under ambient conditions. The active
site in the enzyme is a MoFe7S9N cluster, the FeMo-cofactor, which catalyzes the elec-
N2 + 8H++ 8e- -2NH3 + H2 (2)
While the reaction has AG ~ 0 at pH 7 and standard conditions, at least 16
adenosine triphosphate (ATP) molecules (or approximately 5 eV) [11, 12] are used to
facilitate the reaction. Nitrogenase can thus be viewed as part of an ATP driven elec-
trochemical 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 situ-
ations where ammonia can only be produced in large factories. The protons could come
from water splitting, while the electrons would be driven to the electrode surface by an
applied bias. The reaction mechanism in enzymes is quite different from that of the
industrial synthesis process. In the enzyme, N2 molecules are hydrogenated (associative
mechanism) [15, 16, 17, 18], while in the Haber-Bosch method, the nitrogen and hy-
drogen atoms do not react until the strong N2 triple bond and the H2 bond have been
broken (dissociative mechanism) .
It has been shown that transition metal complexes based on molybdenum can reduce
N2 to ammonia for the artificial process at room temperature and ambient pressure
. The energy input needed for artificial processes is estimated to be as large as
for the biological N2 fixation . For the electro-catalytic N2 reduction, various types
of electrolytes and electrode materials have been tried, but the kinetics are too slow
for practical applications [21, 22, 23, 24, 25, 26, 27, 28]. Little is known about the
mechanism of this process and in most cases hydrogen gas is formed more readily than
hydrogenation of N2.
In the present study, reactions on Ru surfaces were first studied, since this is the
optimal pure metal catalyst for the industrial process . Density functional theory
(DFT) calculations of ammonia formation on both the flat and stepped surface were
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Skulason, Egill; Bligaard, Thomas; Gudmundsdottir, Sigridur; Felix Studt3, Jan a Felix Studt; Rossmeisl, Jan; Abild-Pedersen, Frank et al. A Theoretical Evaluation of Possible Transition Metal Electro-catalysts for N-2 Reduction, article, January 9, 2013; United States. (digital.library.unt.edu/ark:/67531/metadc845199/m1/2/: accessed June 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.