Electrochemical Approach to Ammonia Synthesis Using Ionic Liquid Based Electrolytes

Ammonia is naturally formed from the conversion of N₂ from the atmosphere.  A subsequent conversion of the produced ammonia promotes the generation of nitrogen-containing biomolecules. This natural process is carried out by anaerobic bacteria utilizing nitrogenase enzymes to catalyze the transformation.  Though biological rates of nitrogen reduction are very slow, ammonia synthesis is carried out by the chemical industry using the Haber-Bosch process, which is a high-pressure, high-temperature, heterogeneous catalysis reaction.  The ammonia synthesis reaction is active over Fe and Ru catalysts, though Fe is normally used industrially due to the lower cost.  Both H₂ and N₂ must dissociate at the catalyst surface (Fe or Ru) before the reaction can proceed.   Typical operating conditions are 350- 500^oC, and 100-200 atm.  Only about 15% ammonia conversion is obtained through the catalyst beds with large recycle, which makes the process very energetically inefficient. As a potential alternative, the electrochemical synthesis of NH₃provides the most direct and theoretically most efficient process.

DFT calculations by Norskov et al. predict that ammonia can be electrochemically synthesized at room temperature on Ru surfaces if protons are supplied at sufficiently negative potentials and hydrogen evolution is suppressed.¹  The ammonia formation reaction can only occur in the absence of site-blocking adsorbates such as water or oxygen since they are a poison for NH₃ catalysts.  Marnellos et al. demonstrated the electrochemical synthesis of ammonia from H₂ and N₂ using high-temperature proton conductors at atmospheric pressures.² However, though they reported that >78% of their electrochemically-pumped H₂ was converted to NH₃, they were limited by very low proton conductivity at 570^oC.  In fact, in order to get electrochemical NH₃ production at greater rates, there is a pressing need to use electrolytes that show greater proton transport at reduced temperatures. Ideally, the temperature range should be high enough for fast kinetics and low enough to reduce NH₃ decomposition, which can greatly enhance the viability of the electrochemical production of NH₃.

The work we are presenting involves the evaluation of the electrochemical synthesis of ammonia in different ionic liquids (IL) using a newly designed electrochemical system able to operate in a wide range of pressures (0 – 3,000 psi) and temperatures (-30 – 400 ºC).   In addition, ionic liquids with different anions (i.e. triflamide and triflate) and cations (i.e.butylmethylpyrrolidinium and butylmethylimidazolium) have also been studied and their physicochemical and electrochemical properties characterized.  As nitrogen is converted to ammonia during the electrochemical synthesis, ammonia solubility in the ionic liquid will play a very important role.  Parameters such as conductivity and diffusion coefficient of the IL will be affected, which will greatly influence the production rate of the overall process.  Consequently, we have carried out the physicochemical and electrochemical characterization of the selected ionic liquids with different ammonia compositions and the influence on the electrosynthesis process.  As part of the obtained results, figure 1 shows cyclic voltammograms of  butylmethylpyrrolidinium triflamide in presence of increasing ammonia compositions.

Acknowledgement

Financial support from the LANL LDRD-DR Program is gratefully acknowledged.

References

1. Rod, T. H.; Logadottir, A.; Norskov, J. K., Ammonia synthesis at low temperatures. Journal of Chemical Physics 2000, 112(12), 5343-5347.

2. Marnellos, G.; Stoukides, M., Ammonia synthesis at atmospheric pressure. SCIENCE 1998, 282 (5386), 98-100.