The electrochemical reduction of NO 3 ) in 0.1 M K 2 SO 4 and 0.05 M KNO 3 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 ) , NH 3 , N 2 and small amounts of NO 2 and NO. The more efficient cathodes as regards the conversion of NO 3 ) to N 2 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:The rate constants on Sn85Cu15 at )2.0 V for the above reactions were found to be k 1 ¼ 4:9 Â 10 À4 s )1 , k 2 ¼ 1:76 Â 10 À5 s )1 and k 3 ¼ 7:66 Â 10 À3 l mol )1 s )1 . At more negative potential more NO 2 ) ions reduced and converted either to N 2 or NH 3 . 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 NH 3 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 NH 3 were 4.8 and 2.2 times lower, respectively.
The effect of the nature of six metal electrodes (Sn, Bi, Pb, Al, Zn, In) on the rate and the distribution of the products of the electrochemical reduction of nitrate was studied. The product distribution depends on the nature of the metal only quantitatively, while the rate of the reduction was found to be about the same on all metals when the electrolysis was performed at the same rational potential (E(r)), which is the difference between the applied potential and the potential of zero charge of each metal. Based on these results it was concluded that the mechanism of nitrate reduction is the same for all cathodes studied. Additionally, the influence of the initial pH on the rate of the reduction of nitrate and the selectivity of the products on a tin cathode was studied. The rate of the reduction increases linearly with the concentration of hydronium ion in the pH range 0-4, whereas it is not dependent on the pH at higher pH values. The main products at pH > 4 were nitrogen, nitrous oxide, ammonia and nitrite, while at pH 0-4 ammonia and hydroxylamine were mainly formed.
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