Evaluating the Thermodynamics of Electrochemical Ammonia Oxidation for Hydrogen Production

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: NH_x (x = 0 – 3), OH_y (y = 1 & 2)  and N₂H_z (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

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[5]  J. Norskov, T. Bligaard, J. Rossmeisl, and C. Christensen, Nature Chemistry 1, 37 (2009).
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[16]  D. A. Daramola and G. G. Botte, Journal of Colloid and Interface Science 402, 204 (2013).