Theoretical Calculations of Electrochemical Ammonia Synthesis at Ambient Pressure and Temperature Egill Sk ú lason 1,2 , Thomas Bligaard 1,2 , Jan Rossmeisl 2 , Áshildur Logadóttir 2 , Jens K. Nørskov 2 , Hannes Jónsson 1 1 Science Institute, University of Iceland, Dunhagi 3, VR-II, 107 Reykjavik, Iceland 2 CAMP, NanoDTU, Department of Physics, Building 307, Technical University of Denmark, DK-2800 Lyngby, Denmark
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Theoretical Calculations of Electrochemical AmmoniaSynthesis at Ambient Pressure and Temperature
Egill Skúlason1,2, Thomas Bligaard1,2, Jan Rossmeisl2,Áshildur Logadóttir2, Jens K. Nørskov2 , Hannes Jónsson1
1Science Institute, University of Iceland, Dunhagi 3, VR-II, 107 Reykjavik, Iceland2 CAMP, NanoDTU, Department of Physics, Building 307,
Technical University of Denmark, DK-2800 Lyngby, Denmark
Contents• Ammonia Synthesis (AS)
– Industrial vs. Biological– Why another AS method?– Formation of NH3 in Electrochemical Cell
• Methodology– Density Functional Theory (DFT)– Free Energy and Electrochemical Model
• Results & Discussions– AS in Electrochemical Cell– Stability of Intermediates– Hydrogen Evolution Reaction (HER)– Experiments on Electrochemical AS
• Further Studies & Conclusions
Industrial Ammonia Synthesis
N2 H2NH3
Fe Fe
Haber-Bosch Method
Dissociative Mechanism
N2 + 3H2 → 2NH3
40 x 106 tonnes/year430 ˚C150 atm.
K. Aika, K. Tamura, in Ammonia: Catalysis and Manufacture, (A. Nielsen, Ed.), p.103, Springer, Berlin (1995)
T.H. Rod, A. Logadottir & J.K. Nørskov,J.Chem.Phys.,112, 5343 (2000)
Biological Ammonia Synthesis
Fe
S Ne-
H+
H+
Associative Mechanism
N2 + 8H+ + 8 e- → 2NH3 + H2
16 ATP → 16 ADP + 16 Pi + 5 eV
20 ˚C1 atm.
Nitrogenase
e-
L. Stryer, Biochemistry, 4th ed., W.H. Freeman, New York, (1995)
ΔE = + 0.8 eV
T.H. Rod, A. Logadottir & J.K. Nørskov,J.Chem.Phys.,112, 5343 (2000)
Why another AS method?• The Haber-Bosch process
– High pressure and temperature– Large-scale chemical plants
• Worldwide regulations for NOx emission– Ammonia expected to be a reductant of NOx emitted from
ships and stationary facilities– Ammonia synthesis on a small-scale under mild condition
FeSS
V
NH3
Can we produce ammonia with amechanism similar to that of
Nitrogen fixing enzymes?
H+
Contents• Ammonia Synthesis (AS)
– Industrial vs. Biological– Why another AS method?– Formation of NH3 in Electrochemical Cell
• Methodology– Density Functional Theory (DFT)– Free Energy and Electrochemical Model
• Results & Discussions– AS in Electrochemical Cell– Stability of Intermediates– Hydrogen Evolution Reaction (HER)– Experiments on Electrochemical AS
• Further Studies & Conclusions
Density Functional Theory (DFT)• DFT
– ab initio method– Solves the Schrödinger equation– Hohenberg-Kohn theorem (1964)– Total energy of a quantum mechanical
electron gas is a unique functional ofits density r(r)
– Code: Dacapo from CAMP• Valence electrons
– Expanded in plane wave basis set– Includes periodic boundary conditions
• Vanderbilt pseudopotential– The nucleus– The core electrons
Free Energy CalculationsDFT energy values converted into free energy values:
ΔG = ΔEDFT + Δ(ZPE) - TΔS
ZPE and S: from vibrational frequency calculations in DFTor taken from handbooks of gas phase molecules.