Presentation on theme: "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."— Presentation transcript:
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, VR-II, 107 Reykjavik, Iceland 2 CAMP, NanoDTU, Department of Physics, Building 307, Technical University of Denmark, DK-2800 Lyngby, Denmark Introduction Conclusions Acknowledgement We acknowledge support from The Danish Center for Scientific Computing Biomimetic Ammonia Synthesis Analysis of Ammonia Electro-Synthesis over Ruthenium N2N2 H2H2 NH 3 Fe Dissociative Mechanism: N 2 + 3H 2 2NH 3 430 ˚C 150 atm. Associative Mechanism: N 2 + 8H + + 8 e - 2NH 3 + H 2 20 ˚C 1 atm. 16 ATP 16 ADP Expensive !!! References 1.S.R. Tennison, (J.R. Jennings, Ed.) Plenum Press, New York (1991). 2.K. Aika, K. Tamura, (A. Nielsen, Ed.) Springer, Berlin (1995). 3.T.H. Rod, Á. Logadóttir, J.K. Nørskov, J. Chem. Phys., 112, (2000). 4.L. Stryer, (W.H. Freeman, Ed), New York, (1995). 5.B.K. Burgess, D.J. Lowe, Chem. Rev. 96 (1996). 6.J.K. Nørskov, J. Rossmeisl, Á. Logadóttir, L. Lindqvist, J.R. Kitchin, T. Bligaard, H. Jónsson, J. Phys. Chem. B, 108, (2004). 7.E. Skúlason, T. Bligaard, J. Rossmeisl, Á. Logadóttir, J.K. Nørskov, H. Jónsson, in preparation. Anode Cathode + + + + - - - - Electrolyte N 2 + 6H + + 6e - 2NH 3 H 2 2H + + 2e - 1 bar H 2 1 M protons We have been trying to learn from nature how to synthesize ammonia in a milder way for a small-scale production We use DFT calculations and a simple model to describe an electrochemical cell Ru was chosen initially because it is the best pure metal catalyst for ammonia synthesis at industrial conditions Electrochemical ammonia synthesis at ambient conditions gets more likely on Ru steps than on flat Ru(0001) surface However, in all cases the H atoms are the most stable species, hindering reduction of nitrogen Hydrogen evolution Designing or finding a material that favors reduction of nitrogen rather than evolving hydrogen is still an ongoing research Ammonia is one of the chemicals which is produced in the largest quantity worldwide. It is primarily used in the subsequent production of fertilizer. Since the early 1900s, ammonia has been produced with the Haber-Bosch method. Such conditions can only be established in large-scale chemical plants. On the other hand there exist microorganisms which use the enzyme nitrogenase to produce ammonia under ambient conditions. We study these reactions using Density Functional Theory (DFT). Our goal is to find a less costly process for ammonia synthesis under ambient conditions (by learning from nature), e.g. in an electrochemical cell. Ref. [3, 4, 5] Ref. [1, 2, 3] The energy values obtained from DFT calculations are converted into Gibbs Free Energy values: G = E DFT + (ZPE) - T S (1) where the Zero Point Energy (ZPE) and the entropy (S) are either calculated using DFT or taken from handbooks. The free energy is calculated from eq. (1) which gives G(0), the free energy when no bias is applied. Then the number of electrons (n, with the elementary charge -e), multiplied with the cell potential (U), is added to G(0): G(U) = G(0) + nU (2) All states involving an electron will simply be shifted in free energy by nU due to the external potential. Electrochemical Model Ref.  Free Energy Calculations Dissociative Mechanism - Ru(0001) Associative Mechanism - Ru(0001) Ref.  Associative Mechanism - Ru Step Dissociative Mechanism - Ru Step 300K Fe SN e- H+H+ H+H+ Nitrogenase e- FeMoco
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