Abstract
The electrochemical oxidation of ammonia has been studied in detail due to its inherent utility as a fuel cell reactant 1 for the remediation of wastewater systems 2 for sensors 3 and as a hydrogen carrier.4 However, the bulk of the research conducted has been focused on the use of noble metal-based catalysts such as Pt, Pd, Rh and Ir due to their inherent activity towards the ammonia oxidation reaction (AOR).5,6 Electrochemically, the ammonia oxidation reaction on platinum is a complex multistep reaction which tends to be sluggish and suffers from poor faradaic efficiency, with a variety of products being formed. Another notable feature of noble based systems is the tendency to become poisoned, by strongly adsorbed 'N' species.5 These have been shown to build up over time resulting in electrode de-activation.
In nature, transition metals are found at the core of AOR enzymes such as ammonia monooxygenase which suggests that it is possible to perform this reaction with non-noble catalysts.7 With the issues surrounding noble-metal catalysts and following nature as an example, we have begun investigating the oxidation of ammonia with transition-metal based catalysts. To this end our investigations have revealed that it is possible to conduct electrochemical ammonia oxidation to NOx species with faradaic efficiencies of up to 100%, at appreciable rates with no evidence of catalytic poisoning (see fig. 1). This work represents a step forward in our understanding of the electrochemical AOR reaction and may serve as a foundation for future work in this field.
1. M. H. M. T. Assumpção et al., Int. J. Hydrogen Energy, 39, 5148–5152 (2014).
2. L. Marinčić and F. B. Leitz, J. Appl. Electrochem., 8, 333–345 (1978).
3. A. Galdikas et al., Sensors Actuators B Chem., 67, 76–83 (2000).
4. F. Vitse, M. Cooper, and G. G. Botte, J. Power Sources, 142, 18–26 (2005).
5. Z.-F. F. Li, Y. Wang, and G. G. Botte, Electrochim. Acta, 228, 351–360 (2017).
6. S. Johnston, B. H. R. Suryanto, and D. R. MacFarlane, Electrochim. Acta, 297, 778–783 (2019).
7. J. A. Zahn, D. M. Arciero, A. B. Hooper, and A. A. DiSpirito, FEBS Lett., 397, 35–38 (1996).
Fig. 1. (A) Chronoamperometry conducted in 1 M KOH + 0.1 M NH3 over a time of 48 hours and (B) associated yields of NOx.
Figure 1