Theoretical Calculations of Electrochemical Ammonia ... · Theoretical Calculations of...

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.

The energy levels of a harmonic oscillator:

Eν = (ν + 1/2)hω ω = (k/m)1/2 ν = 0, 1, 2, …⇒ ZPE: E0 = 1/2hω

S = Nk{(βε/(eβε-1)) - ln(1-e-βε)} βε = hcν/kT~

• Calculate the free energy at zero potential, ΔG(0).

• Then the number (n) of electrons (with the elementarycharge -e), multiplied with the cell potential (U), is addedto ΔG(0):

ΔG(U) = ΔG(0) + nU

• All states involving an electron will simply be shiftedin free energy by nU due to the external potential.

Electrochemical Model

Norskov et al, J. Phys. Chem. B, 108 (2004)

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

AnodeCathode

e- e-

+++++

-----

Electrolyte

N2 + 6H+ + 6e- → 2NH3 H2 ↔ 2H+ + 2e-

Possible Formation of NH3 in Electrochemical Cell

First Step: N2 Adsorbs on Surface Electrode

1 bar H2

1 M protons

300 K

AnodeCathode

e- e-

+++++

-----

H2 ↔ 2H+ + 2e-N2 + 6H+ + 6e- → 2NH3

1 M protons

1 bar H2

Electrolyte

Possible Formation of NH3 in Electrochemical Cell Second Step: A Proton is Transferred from the Electrolyte

and an Electron from the Cathode to N2

300 K

AnodeCathode

e- e-

+++++

-----

H2 ↔ 2H+ + 2e-N2 + 6H+ + 6e- → 2NH3

Electrolyte

1 M protons

1 bar H2

Possible Formation of NH3 in Electrochemical Cell

Last Step: After Adding 6H+ and 6e- to N2, 2NH3 is formed

300 K

Associative Mechanism

Ru(0001) surface

ΔG = ΔEDFT + Δ(ZPE) - TΔS

Ammonia SynthesisFree Energy Diagram at 300 K

Associative Mechanism

Ru(0001) surface

ΔG = ΔEDFT + Δ(ZPE) - TΔS

Anode: H2 ↔ 2H+ + 2e-

Cathode: N2 + 6H+ + 6e- ↔ 2NH3Over all: N2 + 3H2 ↔ 2NH3

ΔG(U) = ΔG(0) + eU

AS in Electrochemical CellFree Energy at 300 K and -1.07 V vs. SHE

pH = 0

Stability of Intermediates

Ru(0001) surfaceT = 300 K, pH = 0

ΔG(U) = ΔG(0) + eU

AnodeCathode

e- e-

+++++

-----

N2 + 6H+ + 6e- ↔ 2NH3 H2 ↔ 2H+ + 2e-

The Electrochemical System

Electrolyte

1 M protons

1 bar H2

AnodeCathode

e- e-

+++++

-----

H2 ↔ 2H+ + 2e-N2 + 8H+ + 8e- ↔ 2NH3 + H2

Electrolyte

1 M protons

1 bar H2

Hydrogen Coverage on theElectrode Surface

AnodeCathode

e- e-

+++++

-----

H2 ↔ 2H+ + 2e-N2 + 8H+ + 8e- ↔ 2NH3 + H2

Electrolyte

1 M protons

1 bar H2

Hydrogen Evolution on theElectrode Surface

AnodeCathode

e- e-

+++++

-----

H2 ↔ 2H+ + 2e-N2 + 8H+ + 8e- ↔ 2NH3 + H2

Electrolyte

1 M protons

1 bar H2

Hydrogen Evolution on theElectrode Surface

Experiments on Electrochemical AS

3/2 H2 + N3- → NH3 + 3e- 1/2 N2 + 3e- → N3-

Over all: 1/2 N2 + 3/2 H2 → NH3T. Murakami et al, JACS, 125 (2003)

450 ˚C1 atm.

Current efficiency: 72%Potential: 0.48 V (vs. Li+/Li)Li+/Li: -3.04 V vs. SHE

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

• Finish calculation of the electrochemical ammoniasynthesis on a step of the Ru(0001) surface.

• Understand the hydrogen evolution process andfind some ways to stop it in the electrochemicalammonia synthesis.

Further Studies

Further Studies

Metal slab

---

-

+++++

+++++

-

Next image

Z- axis

U

• Apply external electric potential in the calculations

Further Studies

Metal slab

---

-

+++++

+++++

-

Next image

Z- axis

U

• Apply external electric potential in the calculations• Calculate the stability of the intermediates

Further Studies• Apply external electric potential in the calculations• Calculate the stability of the intermediates

Metal slab

---

-

+++++

+++++

-

Next image

Z- axis

U

Electron Density Changes due to ExternalElectric Potential Calculated with DFT

U

Z- axis

Further Studies

Conclusions

• Our electronic structure calculations predict thatit could be possible to electrochemically produceammonia under ambient reaction conditions.

• Apply a negative voltage on the order of 1.07 Vto an electrochemical cell.

• The HER is needed to be suppressed.