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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+

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)

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


Ru(0001) surface


Ammonia SynthesisFree Energy Diagram at 300 K


Ru(0001) surface


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

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

• 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.

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