Population increase and technology growth are the main reasons for increasing demand of energy sources and fossil fuels consumption as the main energy source consequently increases. However, these fuels emit large amounts of greenhouse gases to the atmosphere. Moreover, they are not renewable resources. Hydrogen as an energy carrier is attractive because the only product of its combustion is water. However, production and storage of hydrogen raises concerns about costs and safety. Ammonia (NH₃) can be used as a hydrogen source and it requires about 5% of the thermodynamic cell voltage (0.06 V) needed to produce hydrogen from water [1-4].

2NH₃(aq)+6OH^- → N₂(g)+6H₂O+6e^-    E°=-0.77V vs. SHE  (1)

6H₂O+6e^- → 3H₂(g)+6OH^-    E°=-0.83V vs. SHE  (2)

2NH₃(aq) → N₂(g)+3H₂(g)    E°=0.06V (3)

Objectives

In order to decrease the cost of hydrogen production through ammonia electrolysis, the kinetics of the process should be known. While experimental techniques are the primary procedure for elucidating the kinetics especially on monometallic surfaces, theoretical calculation can be fascinating in terms of investigation of intermediates produced during the surface reaction. These intermediates can not be recognized with traditional experimental methods on monometallic, bimetallic or polymetallic surfaces. In this work, density functional theory calculations are performed on four platinum-iridium clusters, Pt_3-xIr_x (x=0-3) to examine ammonia oxidation in alkaline media. The adsorption of NH_3-x(x=0-3) on these clusters and the effect of cluster composition on the adsorption are investigated. The hybrid B3LYP level of theory is used in Gaussian 09 along with the LANL2DZ and 6-311++g basis sets.

Results

Our Preliminary results for the HOMO-LUMO gap of adsorbed species on clusters, combined with activation and dissociation energy calculations of sequential dehydrogenation reactions of NH₃→NH₂→NH→N showed that Ir₃ is more active than Pt₃for ammonia oxidation. Additionally, the combination of iridium and platinum makes a more favorable pathway for the ammonia oxidation reaction. However, experimental analyses indicate improved reactivity on platinum in comparison to iridium in ammonia oxidation. This suggests the possibility of two different mechanisms for ammonia oxidation on platinum and iridium. 

References

[1]        F. Vitse, M. Cooper, and G. G. Botte, "On the use of ammonia electrolysis for hydrogen production," Journal of Power Sources, vol. 142, pp. 18-26, 2005.

[2]        G. G. Botte, "Electrochemical method for providing hydrogen using ammonia and ethanol," U.S. Patent No. 8,221,610 2012.

[3]        G. G. Botte, M. Cooper, and F. Vitse, "Electro-catalysts for the oxidation of ammonia in alkaline media," U.S. Patent No. 7,485,211 2009.

[4]        B. K. Boggs and G. G. Botte, "Optimization of Pt-Ir on carbon fiber paper for the electro-oxidation of ammonia in alkaline media," Electrochimica Acta, vol. 55, pp. 5287-5293, 2010.