The conversion of nitrate into dinitrogen in aqueous electrolytes is key toward reducing the risks of algal blooms and their deleterious effects on fragile ecosystems.¹ As has been pointed out in the literature, this goal could potentially be accomplished using electrochemical means and indeed several groups are searching for electrocatalysts capable of carrying out this complex multielectron transfer process with optimum efficacy and high specificity.¹ Among the most promising strategies being considered is the use of two or more electrocatalysts acting in series, as illustrated more than a decade ago in our research group.² The specific electrode employed in that study consisted of Au nanoparticles dispersed on a glassy carbon surface bearing a monolayer of adsorbed hemin, Hm. As shown therein, polarization of such electrode at sufficiently negative potentials in mildly acidic nitrate solutions containing cadmium ions led to the reduction of NO₃^- on underpotential deposited cadmium on Au to yield nitrite, NO₂^-, which was then subsequently reduced to hydroxylamine, NH₂OH, by the adsorbed macrocycle. More recently, our efforts have been focused on the search of an additional electrocatalyst that might oxidize NH₂OH to N₂. In related studies, the groups of Koper and Feliu have shown that nitrite, NO₂^-, can be reduced to N₂on quasi perfect Pt(100) electrodes in alkaline media.³

The specific strategy developed in our laboratory for acquiring on line mass spectrometric data, described in a previous publication, relies on the use of a submerged laminar-flow jet of circular cross section impinging at normal incidence on the surface of a disk embedded in a plane that contains a concentric porous Teflon disk covered by a porous Teflon membrane permeable to gases.⁴ The promising attributes of this device were demonstrated using the oxidation of hydrazine, N₂H₄, in phosphate buffer, PB, (pH 7) on Au(poly), a process known to yield N₂ as the only product. The instrument calibration was performed using the oxidation of hydrazine, N₂H₄, in 0.1 M phosphate buffer (pH 7) as a standard, a reaction that proceeds exclusively via a four electron transfer to yield N₂ and very well defined limiting currents, i_lim. This approach allows the dependence of the latter on both the bulk concentration of N₂H_4, [N₂H₄], and the rate flow, v_f, and the response of the mass spectrometer (m/e = 28) to be determined experimentally. Under the conditions of our measurements,

i_{lim =} 1.60knFC^oD^2/3v^-5/12v_f^3/4a^-1/2R^3/4 (1)

 where k is a dimensionless empirical constant; n the number of electrons transferred; F, Faraday’s constant; C^o, the bulk concentration in mol/cm³; D, the diffusion coefficient in cm²/s; v, the kinematic viscosity in cm²/s; v_f, the flow rate in cm³/s; a, the internal diameter of a nozzle in cm, and R is the radius of a disk electrode in cm. In agreement with Eq.(1) above, a plot of i_lim vs (see Fig. 1) yielded a straight line with a negligible intercept. Good agreement was also found for a plot of i_lim vs [N₂H₄] (not shown here).

Our presentation focuses on the ability of polycrystalline Au, Au(poly), to oxidize NH₂OH in neutral and slightly alkaline media to yield N₂ and nitrous oxide, N₂O, in relatively high yields, as evidenced by quantitative data collected with our newly developed system for measurements of on line mass spectrometry under forced convection. Shown in Fig. 1 are plots of the differential partial pressure of N₂, N₂O and NO recorded with the mass spectrometer during dynamic polarization measurements (n = 5 mV/s) for a Au(poly) disk electrode in a jet impinging configuration in Ar-purged 0.1 M phosphate buffer (pH 7.01) solutions containing 10 mM NH₂OH, v_f  = 0.4 mL/s.. In fact, preliminary estimates of the Faradaic yields for N₂ and N₂O, based on the data in Fig. 1 yielded values of 17.5 and 14.3%, respectively. In other words, about one third of the current leads to the formation of N-N bonds.

1. Victor Rosca^†, Matteo Duca, Matheus T. de Groot^‡ and Marc T. M. Koper* Nitrogen Cycle Electrocatalysis, Chem. Rev., 2009, 109 (6), pp 2209–2244
2. Youjiang Chen, Huanfeng Zhu, Michelle Rasmussen and Daniel Scherson* J. Phys. Chem. Lett., 2010, 1 (13), pp 1907–1911
3. Imre Treufeld, Adriel Jebin Jacob Jebaraj, Jing Xu, Denis Martins de Godoi, and Daniel Scherson* Anal. Chem., 2012, 84 (12), pp 5175–5179
4. Matteo Duca, Marta C. Figueiredo, Victor Climent, Paramaconi Rodriguez, Juan M. Feliu, and Marc T. M. Koper J. Am. Chem. Soc., 2011, 133 (28), pp 10928–10939