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

The electrochemical reduction of NO3- 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 NO2-, NH3, N2 and small amounts of NO2 and NO. The more efficient cathodes as regards the conversion of NO3- 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: \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$NO_{3}^{-} {\buildrel k_{1} \over \rightarrow} NO_{2}^{-} {\buildrel k_{2} \over \rightarrow} NH_{3}$$\end{document}\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$NO_{2}^{-} {\buildrel k_{3} \over \rightarrow} N_{2}$$\end{document} The rate constants on Sn85Cu15 at −2.0 V for the above reactions were found to be k1=4.9 × 10−4s−1, k2=1.76 × 10−5s−1 and k3=7.66 × 10−3lmol−1s−1. At more negative potential more NO2- 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 NO2- 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 NO2- and NH3 were 4.8 and 2.2 times lower, respectively.