Energy and environment are two of the most primary concerns of this century, thus leading to holistic research across numerous disciplines.¹ Ammonia electro-oxidation reaction (AOR) is extremely crucial with regards to energy and environment as it holds spotlight in several fields such as wastewater remediation, analytical chemistry and as a source of renewable energy in direct ammonia fuel cells. In addition, AOR is also useful by being the producer of hydrogen via electrolysis.²

Ammonia has a greater promise compared to other compounds like ethanol and methanol due to its carbon-free nature, well-defined production, storage and transportation network and no net increase in carbon dioxide content of the environment.³ However, inadequate performance due to slow kinetics and expensive nature of electrocatalysts have impeded the utilization of ammonia oxidation technology on a large scale. Consequently, large amount of efforts have been put over the years to understand the mechanism properly which can assist in developing highly active and stable electrocatalysts.

In this study, a powerful in-situ surface enhanced infrared absorption spectroscopy (SEIRAS) technique with the attenuated total reflection (ATR) was used to examine the mechanism and intermediates over Pt-Ir nanofilm deposited on a silicon hemispherical prism. Pt-Ir was chosen as it is the most promising alloy as electrocatalyst for AOR till date.

Intermediates such as NO at 1478-1488 cm^-1 previously found over Pt electrode⁶ and NO₂ at 1331 cm^-1 earlier found with CeO₂-modified platinum catalyst⁷ were also observed in this study with Pt-Ir, thus establishing their presence at higher potential range. Most importantly, a weak band at 2140 cm^-1 representing azide intermediate was also observed for the 1^st time using FTIR, previously only seen using in-situ Raman spectroscopy.⁸

This work, confirming azide intermediate presence at higher potential and affirming the NO and NO₂ for Pt-Ir too, provided much clearer understanding of the mechanism and intermediates within the most renowned Gerischer and Mauerer framework, henceforth opening pathway for further efficient AOR electrocatalyst design in future.

Fig 1. Time-resolved IR spectra of the Pt-Ir nanofilm acquired simultaneously with the linear sweep voltammogram in 0.2 M NH₃-10mM PBS buffer solution (pH 6.96). The reference spectrum was taken at 0.1V.

References

1. Liu, J., Liu, B., Ni, Z., Deng, Y., Zhong, C., & Hu, W., Electrochimica Acta 2014, 150, 146-150.
2. Diaz, L. A., Valenzuela-Muñiz, A., Muthuvel, M., & Botte, G. G., Electrochimica Acta 2013, 89, 413-421.
3. Zhong, C., Hu ,W. B., and Cheng Y. F., Journal of Materials Chemistry A 2013, 10, 3216-3238.
4. Vitse, Frederic, Cooper, M., and Botte, G. G., Journal of Power Sources 2005, 1, 18-26.
5. Satyapal, Sunita, et al. Catalysis Today 2007, 3, 246-256.
6. Matsui, T., Suzuki, S., Katayama, Y., Yamauchi, K., Okanishi, T., Muroyama, H., and Eguchi, K.. Langmuir 2015, 31, 11717-11723.
7. Katayama, Y., Okanishi, T., Muroyama, H., Matsui, T., and Eguchi, K., ACS Catalysis 2016, 3, 2026-2034.
8. Vidal-Iglesias, F. J., Solla-Gullón, J., Pérez, J. M., and Aldaz, A.. Electrochemistry Communications 2006, 8, 102-106.