Ammonia has received much attention as a potential energy storage medium and an alternative fuel for vehicles, in addition to its use as a kind of fertilizer. Recent studies showed that ammonia could be produced by N₂ and H₂O through an electrochemical method, with the advantages of zero CO₂ emission and energy saving compared with the “Haber-Bosch” process.^1-2 However, as the theoretical potentials of hydrogen evolution reaction (HER) and nitrogen reduction reaction (NRR) are very close, H₂ is the dominant product due to the much fast reaction kinetics of the former. For this reason, the experimental studies on NRR mechanism are rather rare. In this study, the NRR mechanism on Au surfaces was first studied by surface enhanced infrared absorption spectroscopy (SEIRAS) technique. Au was selected as its excellent NRR activity but low HER activity.^3-4

An Au nanofilm deposited on the Si prism by a chemical deposition method was used as the working electrode. Its cyclic voltammograms (CV) in a N₂ saturated KOH solution is shown in Figure 1a. In the 1^st segment (cathodic scan, solid red line), the reduction current resulting from the HER and NRR appeared at 0 V and arose sharply starting at -0.1 V. Figure 1b is the corresponding FTIR spectra of the 1^st segment in Figure 1a. The bands at 3300 cm^-1 and 1645 cm^-1 attribute to the O-H stretching and H-O-H bending of water molecules. Those bands intensity increase significantly as a result of the change in the adsorption configuration of water molecules upon decreasing the potential. The bands at 1450 cm^-1, 1298 cm^-1 and 1109 cm^-1 attribute to the H-N-H bending, -NH₂ wagging and N-N stretching of adsorbed N₂H_y species, respectively. Those bands started to appear at potentials below 0 V and increased with potential decreasing. This result indicates that the nitrogen reduction reaction on Au surfaces follows an associative mechanism, and the N≡N bonds in N₂ tend to break simultaneously with the hydrogen addition. Similar spectroscopic studies on Pt surfaces will be also discussed.

Figure 1. a) Cyclic voltammograms of an Au film electrode deposited on the Si prism in a N₂-saturated 0.1M KOH aqueous solution; potential scan rate: 2.5 mV s^-1. b) FTIR spectra during the 1^st segment from 0.4 V to -0.5 V on the Au film electrode in a N₂-saturaed 0.1 M KOH solution. The reference spectrum was taken at 0.4 V.

References

1. Lan, R.; Irvine, J. T. S.; Tao, S. W. Sci Rep 2013, 3, 1145..
2. Renner, J. N., Greenlee, L. F., Ayres, K. E., Herring, A. M. The Electrochemical Society Interface 2015, 24, 7, 51-57.
3. Bao, D.; Zhang, Q.; Meng, F. L.; Zhong, H. X.; Shi, M. M.; Zhang, Y.; Yan, J. M.; Jiang, Q.; Zhang, X. B. Adv. Mater. 2017, 29, 1604799.
4. Shi, M. M.; Bao, D.; Wulan, B. R.; Li, Y. H.; Zhang, Y. F.; Yan, J. M.; Jiang, Q. Adv. Mater. 2017, 29, 1606550.