Hemin (Hm) is an iron macrocycle derived from biological sources often regarded as a model electrocatalyst for the reduction of a number of simple molecules, including nitric oxide, NO,¹ and dioxygen.² This contribution presents the results of cyclic voltammetry and rotating disk electrode, RDE experiments in buffered neutral aqueous solutions aimed at exploring the electrocatalytic activity of Hm adsorbed on glassy carbon (GC) surfaces, Hm|GC, toward hydroxylamine, NH₂OH. This latter species is not only of technological importance, but has been identified as the only significant product of the reduction of nitric oxide (NO) in aqueous solutions of relevance to biological systems.¹ Shown in Figure 1 are dynamic polarization curves recorded at ν = 10 mV/s with a Hm|GC RDE at a fixed bulk concentration of hydroxylamine, c(NH₂OH,¥) = 2.5 mM, for different w (Top Panel, Figure 1), and for rotation rate, i.e. w = 900 rpm for a range of c(NH₂OH,¥) (Bottom Panel in the same figure), using Hm|GC surfaces prepared independently. Close inspection of these results revealed a number of distinctive features:

1. For scans toward negative potentials (see solid lines), NH₂OH reduction commences at ca. -0.35 V, reaching a limiting value at ca. -0.5 V, which extends down to about -0.65 V. At more negative potentials, the current markedly increases and begins to bend over at ca. -0.75 V. This overall behavior is very similar to reported by de Groot et al.¹ for Hm|GC at 960 rpm at a scan rate of 0.5 V/s in 2 mM NH₂OH in the same media.

2. The onset of the process at more positive potentials occurs at ca. -0.35 V, a value that is ca. 0.2 V more negative than the onset of Hm reduction in neat PB and very close to the peak maximum for that redox process. In contrast, the onset of the more negative process coincides almost precisely with the onset of the reduction of ONHm (see grey curve in Panel A) for data collected in PB devoid of NH₂OH in solution.

3. Upon reversing the scan at the negative limit, i.e. -0.9 V (see dotted lines), the currents for the highest c(NH₂OH,¥) examined were found to increase especially in the plateau region, 0.5 - 0.6 V. This behavior may be caused by the full reduction of a Hm adduct, yielding an iron center in the macrocycle displaying much higher reactivity toward NH₂OH reduction than the adduct. One possible explanation for this effect may be found in the slow rate of NH₂OH disproportionation which would render interfacial conditions different during the scans in the negative and positive directions.

4. As c(NH₂OH,¥) is increased keeping w constant, or w is increased for a fixed c(NH₂OH,¥), the current increases, while the overall shape of the curves remains virtually unchanged

5. Except for a very narrow range of negative potentials, the overall currents recorded for c(NH₂OH,¥) ≥ 2 mM during the scan in the positive direction are higher than the negative-going counterparts, whereas, for c(NH₂OH,¥) ≤ 1 mM, portions of the reverse scan have a larger current than at the corresponding potential in the forward scan. Furthermore, this trend is maintained regardless of the value of ω, as illustrated by the data in the bottom panel in this figure collected for c(NH₂OH,¥)= 1 mM over the range 100 ≤ w ≤ 1600 rpm.

Also to be discussed are mathematical models for the Hm|GC-mediated reduction of NH₂OH.

Acknowledgements

This work was supported by a grant from NSF, CHE-1412060

References

1. M. T. de Groot, M. Merkx, A. H. Wonders and M. T. M. Koper, Journal of the American Chemical Society, 127, 7579 (2005).

2. Z. X. Liang, H. Y. Song and S. J. Liao, Journal of Physical Chemistry C, 115, 2604 (2011).