Electrochemical reduction of nitrate ion on various cathodes – reaction kinetics on bronze cathode

Summary

The electrochemical reduction of NO ^-₃ in 0.1 M K₂SO₄ and 0.05 M KNO₃ solution was studied on various electrodes in two different cell configurations, a divided and an undivided one. The products in all cases were NO ^-₂ , NH₃, N₂ and small amounts of NO₂ and NO. The more efficient cathodes as regards the conversion of NO ^-₃ to N₂ were Al and the alloy Sn85Cu15, where the selectivity for nitrogen formation was 43 and 35.3% at −1.8 and −2.0 V, respectively. The kinetic analysis of the experimental results was carried out by numerical solution of the resulted differential equations according to the scheme: $NO_{3}^{-} {\buildrel k_{1} \over \rightarrow} NO_{2}^{-} {\buildrel k_{2} \over \rightarrow} NH_{3}$ $NO_{2}^{-} {\buildrel k_{3} \over \rightarrow} N_{2}$ The rate constants on Sn85Cu15 at −2.0 V for the above reactions were found to be k₁=4.9 × 10⁻⁴ s⁻¹, k₂=1.76 × 10⁻⁵ s⁻¹ and k₃=7.66 × 10⁻³ l mol⁻¹ s⁻¹. At more negative potential more NO ^-₂ ions reduced and converted either to N2 or NH3. The rate constant of reduction of nitrate was almost the same in the region between −1.7 and −2.0 V, because the reaction is limited by the diffusion. In order to oxidize a part of the undesirable byproducts NO ^-₂ and NH₃ at the anode of the cell to nitrate and nitrogen respectively, an undivided cell was used. Comparison between the two cell configurations indicated that, although in the undivided cell the % removal efficiency of nitrate was lower than that in the divided one, the selectivities of NO ^-₂ and NH₃ were 4.8 and 2.2 times lower, respectively.