Electrochemical Reduction of Nitrate on Pyrolytic Graphite-Supported Cu and Pd–Cu Electrocatalysts

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Abstract
The electrochemical reduction of nitrate and nitrite on Cu- and Pd–Cu-modified pyrolytic graphite electrodes was studied in neutral and alkaline media. The beneficial effect of Pd on the intrinsic electrocatalytic performance of the modified graphite electrode was demonstrated. In fact, the presence of Pd on the catalytic surface causes a positive potential shift in the onset of nitrate reduction current and a remarkable increase of the faradaic current. The activation energy of the nitrate electroreduction was estimated for the Cu/graphite and Pd–Cu/graphite electrodes at about 32 and 18kJmol−118kJmol−1 , respectively. The reaction rate constants were equal to 0.75×10−40.75×10−4 and 2.23×10−4s−12.23×10−4s−1 for the Cu and Pd–Cu surface, respectively, reflecting a more facile electron-transfer process on the latter surface. The beneficial effect of Pd on the catalytic stability is probably related to the existence of a protective Pd layer enveloping the Cu core in the electrode surface structure. The selectivity of the modified electrodes depends on the electrolysis time and potential. In neutral medium, the nitrate electroreduction mainly led to the formation of nitrite at −1.1V−1.1V and ammonia at −1.5V−1.5V vs Ag/AgCl. The reaction selectivity was also studied in alkaline medium at −1.1V−1.1V for the Pd–Cu/graphite electrode only. In this case, the maximal selectivity for the production of N2N2 was 70% and was achieved with a surface composition of 95 atom % Pd–5 atom % Cu.

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