Electrochemical determination of ammonia at low concentrations: detection of N2-electroreduction product in alkaline medium.

A catalytic system based on CeO2-supported Pt nanoparticles has been utilized and optimized to pursue electrochemical determination of ammonia (NH3) at fairly low concentrations (detection limit down to 2-3 μmol dm-3 level) through voltammetric oxidation of NH3 in alkaline medium (0.5 mol dm-3 NaOH). Dispersing and supporting of platinum nanoparticles on partially-defective and sub-stoichiometric ceria tends to increase the electrochemically active-surface-area and reproducibility of operation of the Pt-based catalyst. The approach has been found suitable to detect NH3 generated during electroreduction of nitrogen at Fe3P catalyst in the N2-saturated alkaline solution. Here the Pt-based "analytical" electrode has been utilized as the additional working electrode placed in the vicinity of the Fe3P-modified electrode. The analytical measurement does not require medium transfer to another solution because both Fe3P and Pt-based electrodes are operating in the sealed N2-electrolysis cell. Under such conditions, uncontrolled leakage and contamination by ammonia existing in the atmosphere can be avoided. The Pt-based electrode permits to perform blank experiments to comment about purity of electrolytes and nitrogen gas with respect to the presence of residual ammonia and the absence of oxygen and nitrogen oxides. Consequently, reliable determination of ammonia as the N2-electroreduction product following 2 h electrolysis at Fe3P iron phosphide catalyst becomes feasible. The results have been verified against spectrophotometric data. Formation of hydrazine or hydroxylamine as side (to ammonia) electroreduction products can be addressed by performing voltammetric diagnostic experiments with use of electrodes modified with Pt and Rh nanoparticles. Conversion of N2 to NH3 at Fe3P catalyst in 0.5 mol dm-3 NaOH is fairly selective because no appreciable amounts of hydrazine or hydroxylamine have been detected.