Theoretical Calculations of Electrochemical Ammonia Synthesis at Ambient Pressure and Temperature

Transcription

1 Theoretical Calculations of Electrochemical Ammonia Synthesis at Ambient Pressure and Temperature Egill Skúlason 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, VRII, 107 Reykjavik, Iceland 2 CAMP, NanoDTU, Department of Physics, Building 307, Technical University of Denmark, DK2800 Lyngby, Denmark

2 Contents Ammonia Synthesis (AS) Industrial vs. Biological Why another AS method? Formation of NH 3 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

3 Industrial Ammonia Synthesis HaberBosch Method Dissociative Mechanism N 2 3H 2 2NH 3 40 x 10 6 tonnes/year 430 C 150 atm. N 2 H 2 NH 3 T.H. Rod, A. Logadottir & J.K. Nørskov, J.Chem.Phys.,112, 5343 (2000) Fe Fe K. Aika, K. Tamura, in Ammonia: Catalysis and Manufacture, (A. Nielsen, Ed.), p.103, Springer, Berlin (1995)

4 Biological Ammonia Synthesis Associative Mechanism N2 8H 8 e 2NH3 H2 16 ATP 16 ADP 16 Pi 5 ev ΔE = 0.8 ev 20 C 1 atm. T.H. Rod, A. Logadottir & J.K. Nørskov, J.Chem.Phys.,112, 5343 (2000) H es e N Fe H Nitrogenase L. Stryer, Biochemistry, 4th ed., W.H. Freeman, New York, (1995)

5 Why another AS method? The HaberBosch process High pressure and temperature Largescale chemical plants Worldwide regulations for NO x emission Ammonia expected to be a reductant of NO x emitted from ships and stationary facilities Ammonia synthesis on a smallscale under mild condition

6 Can we produce ammonia with a mechanism similar to that of Nitrogen fixing enzymes? H NH 3 V S S Fe

7 Contents Ammonia Synthesis (AS) Industrial vs. Biological Why another AS method? Formation of NH 3 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

8 Density Functional Theory (DFT) DFT ab initio method Solves the Schrödinger equation HohenbergKohn theorem (1964) Total energy of a quantum mechanical electron gas is a unique functional of its 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

9 Free Energy Calculations DFT energy values converted into free energy values: ΔG = ΔE DFT Δ(ZPE) TΔS ZPE and S: from vibrational frequency calculations in DFT or taken from handbooks of gas phase molecules. The energy levels of a harmonic oscillator: E ν = (ν 1/2)hω ω = (k/m) 1/2 ν = 0, 1, 2, ZPE: E 0 = 1/2hω S = Nk{(βε/(e βε 1)) ln(1e βε )} ~ βε = hcν/kt

10 Electrochemical Model Calculate the free energy at zero potential, ΔG(0). Then the number (n) of electrons (with the elementary charge e), multiplied with the cell potential (U), is added to ΔG(0): ΔG(U) = ΔG(0) nu All states involving an electron will simply be shifted in free energy by nu due to the external potential. Norskov et al, J. Phys. Chem. B, 108 (2004)

11 Contents Ammonia Synthesis (AS) Industrial vs. Biological Why another AS method? Formation of NH 3 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

12 Possible Formation of NH 3 in Electrochemical Cell First Step: N 2 Adsorbs on Surface Electrode e e Cathode Electrolyte 300 K 1 M protons 1 bar H 2 Anode N 2 6H 6e 2NH 3 H 2 2H 2e

13 Possible Formation of NH 3 in Electrochemical Cell Second Step: A Proton is Transferred from the Electrolyte and an Electron from the Cathode to N 2 e e Cathode N 2 6H 6e 2NH 3 Electrolyte 300 K 1 M protons 1 bar H 2 Anode H 2 2H 2e

14 Possible Formation of NH 3 in Electrochemical Cell Last Step: After Adding 6H and 6e to N 2, 2NH 3 is formed e e Cathode N 2 6H 6e 2NH 3 Electrolyte 300 K 1 M protons 1 bar H 2 Anode H 2 2H 2e

15 Ammonia Synthesis Free Energy Diagram at 300 K Associative Mechanism ΔG = ΔE DFT Δ(ZPE) TΔS Ru(0001) surface

16 AS in Electrochemical Cell Free Energy at 300 K and 1.07 V vs. SHE Associative Mechanism ph = 0 ΔG = ΔE DFT Δ(ZPE) TΔS Anode: H 2 2H 2e Cathode: N 2 6H 6e 2NH 3 Over all: N 2 3H 2 2NH 3 ΔG(U) = ΔG(0) eu Ru(0001) surface

17 Stability of Intermediates Ru(0001) surface T = 300 K, ph = 0 ΔG(U) = ΔG(0) eu

18 The Electrochemical System e e Cathode Electrolyte 1 M protons 1 bar H 2 Anode N 2 6H 6e 2NH 3 H 2 2H 2e

19 Hydrogen Coverage on the Electrode Surface e e Cathode Electrolyte 1 M protons 1 bar H 2 Anode N 2 8H 8e 2NH 3 H 2 H 2 2H 2e

20 Hydrogen Evolution on the Electrode Surface e e Cathode Electrolyte 1 M protons 1 bar H 2 Anode N 2 8H 8e 2NH 3 H 2 H 2 2H 2e

21 Hydrogen Evolution on the Electrode Surface e e Cathode Electrolyte 1 M protons 1 bar H 2 Anode N 2 8H 8e 2NH 3 H 2 H 2 2H 2e

22 Experiments on Electrochemical AS 450 C 1 atm. Current efficiency: 72% Potential: 0.48 V (vs. Li /Li) Li /Li: 3.04 V vs. SHE 3/2 H 2 N 3 NH 3 3e 1/2 N 2 3e N 3 Over all: 1/2 N 2 3/2 H 2 NH 3 T. Murakami et al, JACS, 125 (2003)

23 Contents Ammonia Synthesis (AS) Industrial vs. Biological Why another AS method? Formation of NH 3 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

24 Further Studies Finish calculation of the electrochemical ammonia synthesis on a step of the Ru(0001) surface. Understand the hydrogen evolution process and find some ways to stop it in the electrochemical ammonia synthesis.

25 Further Studies Apply external electric potential in the calculations U Metal slab Next image Z axis

26 Further Studies Apply external electric potential in the calculations Calculate the stability of the intermediates U Metal slab Next image Z axis

27 Further Studies Apply external electric potential in the calculations Calculate the stability of the intermediates U Metal slab Next image Z axis

28 Further Studies Electron Density Changes due to External Electric Potential Calculated with DFT U Z axis

29 Conclusions Our electronic structure calculations predict that it could be possible to electrochemically produce ammonia under ambient reaction conditions. Apply a negative voltage on the order of 1.07 V to an electrochemical cell. The HER is needed to be suppressed.