Evaluating the Thermodynamics of Electrochemical Ammonia Oxidation for Hydrogen Production
Authors: Daramola Damilola A. , Botte Gerardine G. 
Published: February 2020

Objective

Electrolysis of ammonia on platinum electrodes is a viable technology for wastewater remediation and hydrogen production[1]. Ammonia is oxidized at the anode, in the presence of hydroxide ions, and water is reduced at the cathode. The only products generated are nitrogen and hydrogen.
While the anodic reaction is limited by slow kinetics and catalyst deactivation[2], improvements have been made using bimetallic catalysts[3,4], especially at low concentrations of ammonia. This improved activity by empirical observation indicated a need for more fundamental analysis through a surface chemistry approach to the electrochemical reaction. This analysis will aid in designing improved catalysts and catalytic compositions.
The use of density functional theory for characterizing chemical reactions, especially at the catalyst surface, has yielded valuable insight into the thermodynamics and kinetics of the reactions[5] and this method will be applied for this study.

Methodology

Density Functional Theory as applied in Gaussian 09[6] was used to generate the electronic and thermodynamic properties of the reactants, products and intermediates of ammonia electro-oxidation. Unrestricted spin calculations (due to unpaired electrons) were performed using the hybrid B3LYP functional[7] while the basis sets used were LANL2DZ[8-11] for platinum atoms and 6-311++G** for nitrogen, oxygen, and hydrogen atoms[12,13]. Using the computational hydrogen electrode[14], the effect of an applied potential was evaluated.

Results

The aforementioned species present during ammonia electro-oxidation are of the form: NHx (x = 0 – 3), OHy (y = 1 & 2) and N2Hz (z = 0 – 4) when ammonia electrolysis occurs on platinum below the potential for oxide formation. The adsorption of these species has been previously investigated and characterized using the method above[15,16]. The following trend for adsorption was predicted: N2 < H2O < NH3 < N2H2 < N2H4 < N2H < N2H3 < OH< NH2 < NH < N. However, these calculations have not accounted for the effect of an applied potential, pH or the solvent environment. Using the method of Norskov et al.,[14] the effect of these pertubations, which are essential to electrochemical reactions, have been investigated. These results will be presented at the meeting.

References

[1] F. Vitse, M. Cooper, and G. G. Botte, Journal of Power Sources 142, 18 (2005).
[2] H. Gerischer and A. Mauerer, Journal of Electroanalytical Chemistry 25, 421 (1970).
[3] E. P. Bonnin, E. J. Biddinger, and G. G. Botte, Journal of Power Sources 182(2008).
[4] B. K. Boggs and G. G. Botte, Electrochimica Acta 55(2010).
[5] J. Norskov, T. Bligaard, J. Rossmeisl, and C. Christensen, Nature Chemistry 1, 37 (2009).
[6] M. J. Frisch et al., Gaussian 09, Revision B.01, Gaussian, Inc., 2009.
[7] A. D. Becke, Journal of Chemical Physics 98, 5648 (1993).
[8] T. H. Dunning, Journal of Chemical Physics 90, 1007 (1989).
[9] P. J. Hay and W. R. Wadt, Journal of Chemical Physics 82, 299 (1985).
[10] P. J. Hay and W. R. Wadt, Journal of Chemical Physics 82, 270 (1985).
[11] W. R. Wadt and P. J. Hay, Journal of Chemical Physics 82, 284 (1985).
[12] G. S. Chandler and A. D. McLean, Journal of Chemical Physics 71, 2175 (1979).
[13] A. D. McLean and G. S. Chandler, Journal of Chemical Physics 72, 5639 (1980).
[14] J. Norskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. Kitchin, T. Bligaard, and H. Jonsson, Journal of Physical Chemistry B 108, 17886 (2004).
[15] D. A. Daramola and G. G. Botte, Computational and Theoretical Chemistry 989, 7 (2012).
[16] D. A. Daramola and G. G. Botte, Journal of Colloid and Interface Science 402, 204 (2013).